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Gut microbiota promotes host resistance to low-temperature stress by stimulating its arginine and proline metabolism pathway in adult Bactrocera dorsalis

['Muhammad Fahim Raza', 'State Key Laboratory Of Agricultural Microbiology', 'Key Laboratory Of Horticultural Plant Biology', 'Moe', 'China-Australia Joint Research Centre For Horticultural', 'Urban Pests', 'Institute Of Urban', 'Horticultural Entomology', 'College Of Plant Science', 'Technology']
Date: None

Gut symbiotic bacteria have a substantial impact on host physiology and ecology. However, the contribution of gut microbes to host fitness during long-term low-temperature stress is still unclear. This study examined the role of gut microbiota in host low-temperature stress resistance at molecular and biochemical levels in the oriental fruit fly Bactrocera dorsalis. The results showed that after the gut bacteria of flies were removed via antibiotic treatment, the median survival time was significantly decreased to approximately 68% of that in conventional flies following exposure to a temperature stress of 10°C. Furthermore, we found that Klebsiella michiganensis BD177 is a key symbiotic bacterium, whose recolonization in antibiotic treated (ABX) flies significantly extended the median survival time to 160% of that in the ABX control, and restored their lifespan to the level of conventional flies. Notably, the relative levels of proline and arginine metabolites were significantly downregulated by 34- and 10-fold, respectively, in ABX flies compared with those in the hemolymph of conventional flies after exposure to a temperature stress of 10°C whereas recolonization of ABX flies by K. michiganensis BD177 significantly upregulated the levels of proline and arginine by 13- and 10- fold, respectively, compared with those found in the hemolymph of ABX flies. qPCR analysis also confirmed that K. michiganensis-recolonized flies significantly stimulated the expression of transcripts from the arginine and proline metabolism pathway compared with the ABX controls, and RNAi mediated silencing of two key genes Pro-C and ASS significantly reduced the survival time of conventional flies, postexposure low-temperature stress. We show that microinjection of L-arginine and L-proline into ABX flies significantly increased their survival time following exposure to temperature stress of 10°C. Transmission electron microscopy (TEM) analysis further revealed that low-temperature stress caused severe destruction in cristae structures and thus resulted in abnormal circular shapes of mitochondria in ABX flies gut, while the recolonization of live K. michiganensis helped the ABX flies to maintain mitochondrial functionality to a normal status, which is important for the arginine and proline induction. Our results suggest that gut microbiota plays a vital role in promoting the host resistance to low-temperature stress in B. dorsalis by stimulating its arginine and proline metabolism pathway.

The physiological mechanisms by which insects survive at low temperatures have not yet been discovered. In addition, there are no reports to verify the role of gut symbionts in the B. dorsalis biology during low-temperature stress. By combining transcriptomic and metabolomic approaches, we found that the presence of gut symbionts, especially K. michiganensis BD177, helps the host B. dorsalis to elevate the levels of particular cryoprotectant transcripts and metabolites, which suggests that the gut symbiont K. michiganensis BD177 stimulates the host arginine and proline metabolism pathway to promote its resistance to long-term low-temperature stress via influencing the mitochondrial functionality. Finally, the methods of this study are relevant for investigating the underpinnings of other host-microbe interactions during environmental stress.

Herein, we examined the role of gut microbiota on host resistance to long-term low-temperature stress in insects. We found that removal of the gut microbiota significantly decreased the survival time of adult B. dorsalis following exposure to low-temperature stress, and reinfection of a key gut symbiont, K. michiganensis BD177, in the ABX flies restored the lifespan under low-temperature stress by stimulating the arginine and proline metabolism pathway in the host. These results suggest that the B. dorsalis gut microbiota helps host to increase temperature stress resistance via modulation of the arginine and proline metabolism pathway.

Bactrocera dorsalis (Diptera: Tephritidae) is one of the most invasive, polyphagous and multivoltine members of the Tephritidae family; this species causes substantial loss of cultivated crops throughout most of Asia and damages more than 250 host fruits and vegetables worldwide [ 27 ]. Survival under different temperature regimes is a critical factor to consider when B. dorsalis is accidently introduced into new areas, and this factor likely plays an important role in the invasion process, i.e., overcoming a basic population establishment barrier. The population expansion of B. dorsalis indicates that it possesses high invasive potential due to its broad host range, relatively broad climate tolerance, and dispersal capacity [ 28 , 29 ], posing considerable challenges to its effective management. Previous studies based on high-throughput pyrosequencing of the 16S rRNA gene and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) fingerprinting have identified the prevalence of microbial communities occupying the gut and the reproductive organs of B. dorsalis [ 30 , 31 ]. Enterobacteriaceae was identified as the predominant family, and many cultivable species were isolated from the gut of the host fly, such as Enterobacter cloacae, K. oxytoca, Morganella sp., Providencia rettgerii and C. freundii [ 24 , 31 ], indicating that they may play an important role in promoting host fitness under stressful conditions. The gut symbiont ‘Citrobacter sp.’ of B. dorsalis has been reported to increase the host resistance against an organophosphate insecticide [ 24 ]. In our previous study, we determined that the intestinal probiotic K. oxytoca restored the ecological fitness of B. dorsalis following a decline induced by irradiation [ 32 ].

The gut microbiota provides nutrients to the host, such as amino acids, essential vitamins, carbon, and nitrogen compounds [ 20 , 21 ]. Enterobacteriaceae species in the digestive tract of fruit flies are involved in nitrogen fixation [ 22 ], temporal host range expansion [ 23 ], reproductive success [ 21 ], detoxification [ 24 ], protection against pathogens and promoting host immunity [ 25 ]. The addition of the gut bacterial species K. oxytoca to the postirradiation diet resulted in improved sterile male competitiveness in Mediterranean fruit flies, and colonization of this strain in the host gut functions to suppress the proliferation of pathogenic strain Pseudomonas [ 26 ].

Animals are inevitably challenged by adverse environmental conditions, such as extreme temperature, toxic substances or pathogen infections, ultraviolet (UV) radiation, insecticide exposure and oxidative stress [ 1 , 2 ]. Environmental stress has a direct effect on metabolism and cellular processes that affect individual survival [ 3 – 5 ]. Among the abiotic factors affecting the physiology of terrestrial organisms, temperature is certainly the most critical. Extreme temperature can be detrimental to ectotherms or insect populations either through direct effects, which include temperature-induced cellular or tissue injury, functional restrictions or mortality and/or through indirect effects, e.g., activity constraints [ 6 – 9 ]. However, some invasive insect pests of fruit crops exhibit a unique form of phenotypic response [ 10 , 11 ] to mild or even extreme temperature conditions [ 12 , 13 ] or a rapid adaptation of thermal traits [ 14 , 15 ] that may facilitate the survival of those introduced or alien species in novel environments. Unpredictable climatic conditions, sudden cold spells or milder winters are among the explanatory factors involved in the reduced mortality [ 16 ], distribution and dynamics of many insect pest species [ 17 ]. The mechanisms underlying cold acclimation responses are the main focus of much research in insect species [ 18 , 19 ].

Moreover, to define the mitochondria functionality and metabolic mechanisms that promote host survival during cold stress, we measured the amount of ATP in the gut tissues of these flies. We found that ATP concentration was significantly reduced by 78.64% in the gut tissues of ABX flies compared with the conventional flies 5 dpe to 10°C ( Fig 8F , P<0.05). However, after the feeding of live K. michiganensis to ABX flies, ATP concentration in the gut tissues was reestablished and significantly increased by 377% compared with those in the ABX flies ( Fig 8F ).

Representative TEM images of gut tissues of conventional (A), ABX (B), K. michiganensis (C) reinfected flies 5 dpe to 10°C. The boxed areas were enlarged in the bottom panels. Scale bars, 1 μm (5000×, top) and 500 nm (9600×, bottom). (D) Percentage of distorted mitochondria in gut tissues of conventional, ABX, K. michiganensis reinfected flies; percent distorted mitochondria observed in Conventional were 5.027 ± 0.9992%; K. michiganensis: 14.95 ± 1.709% and ABX flies: 77.99 ± 1.838%, P<0.0001. (E) Comparison of the number of mitochondrial cristae in gut tissues of conventional, ABX, K. michiganensis reinfected flies n = 20. (F) Comparison of ATP levels in the conventional, ABX, K. michiganensis reinfected flies gut tissues 5 dpe to 10°C n = 4. One-way analysis of variance (ANOVA) and Tukey’s test were performed (P<0.05; and not significant denoted by ns, P>0.05) for the analysis of D, E and F figures.

To investigate a link between gut bacteria and mitochondria activity for the arginine and proline induction to promote host resistance during low-temperature stress, we observed gut tissues of conventional, K. michiganensis-reinfected or ABX flies 5 dpe to 10°C, through transmission electron microscopy (TEM). The results showed that in the absence of gut bacteria (ABX flies), low-temperature stress caused severe destruction in cristae structure and thus resulted in abnormal circular shapes of mitochondria compared with conventional flies ( Fig 8A–8C ). On the other hand, K. michiganensis inoculation helped the host to maintain mitochondrial morphology, clearly different from that of ABX flies following exposure to low-temperature stress. Furthermore, mitochondrial morphology of ABX flies were severely distorted compared with conventional or K. michiganensis inoculated flies ( Fig 8D ; percentage of distorted mitochondria in Conventional: 5.027 ± 0.9992%; K. michiganensis: 14.95 ± 1.709%; ABX flies: 77.99 ± 1.838%, P<0.0001). Number of cristae in ABX flies were also significantly reduced by 71% than that of K. michiganensis fed flies ( Fig 8E ). These experimental observations led us to conclude that, once inoculated, K. michiganensis might help the ABX flies to maintain a dynamic equilibrium in mitochondria and improves its metabolic functions that might induce arginine and proline to the levels sufficient to promote host resistance to low-temperature stress.

Finally, we tested whether injection of the main constituents of the arginine and proline metabolism pathway could play their role as cryoprotectants to improve the survival of ABX flies at 10°C. The results showed that the median survival times of ABX flies were significantly increased after individual microinjection of L-arginine (10 days) and L-proline (11 days) compared with that of the ABX control (6 days) injected with water, representing 67% and 83% increase, respectively ( Fig 7 ), confirming their role in increasing the host’s low-temperature stress resistance. Furthermore, we evaluated the synergistic application of L-arginine + L-proline to the ABX flies’ survival. The results showed that after injecting the combination of L-arginine + L-proline, the median survival time of ABX flies was considerably increased by 150% (15 days) compared with the ABX control ( Fig 7 ), suggesting the synergistic potential of these amino compounds under cold stress. Despite being the upregulation of fatty acid molecules or enzymes in both ‘omics’ approaches, we did not find the improvement in host low-temperature stress resistance after microinjection of saturated or unsaturated fatty acids ( S4A and S4B Fig ). To understand whether fat synthesis or storage consumptions are also responsible for the survival mechanisms, TG levels were assessed in the fly fat body tissue among conventional, K. michiganensis-reinfected or ABX treatments. However, we did not find significant differences in the TG levels among those different treatments (P>0.05, S4C Fig ), indicating that fatty acid molecules might not be involved in the host resistance to cold stress.

According to the previous experiments, we found that two key genes Pro-C and ASS silencing considerably reduced the median survivals of flies ( Fig 6E ). Next, we made the combination of dsPro-C and dsASS to silence these genes and evaluated the synergistic effect on the survival of conventional flies under low-temperature stress. The results showed that injection of dsPro-C + dsASS significantly decreased the Pro-C and ASS mRNA levels in conventional flies ( Fig 6F ), and as a result their median survival was markedly reduced by 67% postexposure to low-temperature stress of 10°C ( Fig 6G ). These results suggest that a combined decrease in Pro-C and ASS mRNA levels might cause a highly significant reduction in the survival time of flies during low-temperature stress.

Gene silencing efficiency in conventional B. dorsalis (n = 10 in each repeat) injected with dsEgfp as control treatment or dsPro-C (A), dsASS (B), dsArg (C), dsODC (D). Survival curves of conventional flies (n = 59–61 in each condition) injected with dsEgfp or dsPro-C, dsASS, dsArg, dsODC (E) postexposure to 10°C. Combined effect of dsPro-C + dsASS on gene silencing efficiency in conventional flies (n = 10 in each repeat), control group was injected with dsEgfp (F). Survival curves of conventional flies postexposure to low-temperature stress (n = 58–59 in each condition) after dsPro-C + dsASS injection (G). Experiments were performed in three biological replicates. Error bars indicate mean with SE. Significant difference was determined by Student’s t test at P<0.05 in dsRNA treatment. Survival curves of conventional flies were analyzed by log-rank analysis at P<0.05.

To validate the role of arginine and proline genes on the survival of flies under low-temperature stress treatment, we silenced four key genes (ASS, Pro-C, Arg and ODC) by injection of doublestranded RNA (dsRNA) to conventional flies. The dsRNA injection significantly reduced the mRNA levels of all four genes ( Fig 6 ). The dsPro-C and dsASS silencing significantly decreased the Pro-C and ASS mRNA levels by 54% and 50% ( Fig 6A and 6B ) in conventional flies, respectively. After silencing the Pro-C and ASS genes, flies were subjected to low-temperature stress of 10°C and their median survivals were significantly reduced by 57% and 36%, respectively ( Fig 6E ). The dsODC and dsArg treatment also significantly reduced the mRNA levels of ODC and Arg of conventional flies ( Fig 6C and 6D ) but their median survivals were only reduced by 14% and 7%, respectively ( Fig 6E ). These data suggest that ASS and Pro-C, which are required for the synthesis of arginine and proline, are important for the survival of flies under a low-temperature stress environment.

qPCR analysis to assess relative expression levels of genes known to be required for the arginine and proline metabolism at 0 day, and stimulation after 3 days and 5 days postexposure to 10°C in conventional, K. michiganensis-reinfected and ABX flies. One-way analysis of variance (ANOVA) and Tukey’s test were performed. Statistical significance was indicated as follows: ***, P<0.0001; **, P<0.001; *, P<0.05; P>0.05 denoted as nonsignificant (ns). * Arginase (Arg), Ornithine aminotransferase (Oat), Argininosuccinate Synthase (ASS), Pyrroline-5-carboxylate reductase (Pro-C), Ornithine decarboxylase (ODC). Y-axis denotes time: 0 days ( 27 ± 1°C or before exposure to 10°C), 3 days and 5 days postexposure to 10°C.

To confirm the response of the arginine and proline metabolic pathway to low-temperature stress, we used qPCR to assess the changes in the expression profiles of five important genes chosen on the basis of their roles in the development and survival of insect hosts under stressful conditions [ 38 , 39 ]. The results showed that K. michiganensis reinfection strikingly induced the expression of argininosuccinate synthase (ASS), a precursor enzyme of arginine synthesis, by 14.70-fold 3 dpe to 10°C, with a peak of 194.51-fold increase compared with that in ABX flies 5 dpe to 10°C ( Fig 5 ). K. michiganensis reinfection also significantly enhanced the mRNA levels of pyrroline-5-carboxylate reductase (Pro-C) and ornithine aminotransferase (Oat), which are required for the synthesis of proline, by 2.52- and 14.01-fold compared with the levels in ABX flies, respectively, 5 dpe to 10°C. Consistent with these observations, increased levels of arginine and proline were found in the hemolymph of K. michiganensis - reinfected flies than those in ABX flies. Similarly, 5 dpe to 10°C, the mRNA levels of other enzymes were also significantly enhanced in K. michiganensis - reinfected flies than that of ABX counterparts, e.g., arginase (Arg), required for catabolism of arginine to ornithine by 8.71-fold, and ornithine decarboxylase (ODC), required for the synthesis of putrescine by 5.26-fold, a likely reflection of the increased low-temperature resistance of the host induced by the gut bacteria. However, before the flies were exposed to low-temperature (0 dpe), the expression levels of these genes were not significantly different among different treatments, with only Oat expression being significant at 0 dpe, suggesting that in the presence of gut bacteria such as K. michiganensis, the host stimulates these genes only following exposure to low-temperature stress ( Fig 5 ). Importantly, there were no significant differences in the transcript levels of these genes between conventional and K. michiganensis - reinfected flies before or after exposure to 10°C, suggesting that K. michiganensis reinfection of ABX flies could upregulate the levels of these genes to the levels found in conventional flies. Taken together, our data demonstrate that K. michiganensis reinfection helps the host to significantly stimulate the expression of these genes, which might account for enhanced low-temperature resistance.

To emphasize our efforts on a smaller subset of highly expressed genes and metabolites, we pooled significant overlapping pathways with those enriched in both the transcriptomic and metabolomic approaches as described by MacMillan et al. [ 34 ]. Among the top enriched pathways, only one pathway, ‘arginine and proline metabolism,’ in K. michiganensis - reinfected vs. ABX and conventional vs. ABX flies was shared across both ‘omics’ approaches. The observation that arginine and proline metabolism genes and metabolites were strongly enriched in both datasets fits with our current understanding of low-temperature stress resistance, as proline and arginine are specially used by mitochondria as a metabolic fuel and/or are involved as cryoprotectant or osmotic protectant of insect tissues under low-temperature exposure [ 35 – 37 ]. These findings suggest that this pathway plays an important role in improving the survival of flies under low-temperature stress.

To assess the effect of gut microbiota on host transcriptome at 10°C, we employed RNA sequencing to identify the differentially expressed genes (DEGs). A total of 10,760 (7737 up- and 3023 downregulated) DEGs between K. michiganensis-reinfected vs. ABX samples and 18,576 (11,507 up- and 7069 downregulated genes) DEGs in conventional vs. ABX groups were identified. Several DEGs encoding heat shock proteins (HSPs), zinc finger proteins (ZFPs) and serine/threonine protein kinase (STKs) were enriched in the current study ( S2A and S2B Data ). To confirm the expression profiles identified from RNA-seq data, we performed qRT-PCR on several randomly selected DEGs. The comparative analysis revealed that RT-qPCR-detected expression patterns supported the RNA-seq results, with consistent results in both comparisons, i.e., conventional vs. ABX ( S3A Fig ) and K. michiganensis vs. ABX groups ( S3B Fig ). Importantly, 36 of the 146 (P<0.05, 25% DEG:TEG) genes in K. michiganensis-reinfected vs. ABX flies and 56 of the 146 genes (P<0.005, 38% DEG:TEG) in conventional vs. ABX flies that mapped to the ‘arginine and proline metabolism pathway’ were significantly upregulated 5 dpe to 10°C ( S2C and S2D Data ).

In the present study, histidine was the most significantly upregulated compound in K. michiganensis - reinfected flies (18-fold), while in conventional flies, it was upregulated by 31-fold compared to that of ABX flies ( S1C and S1D Data ), suggesting that this metabolite is absorbed by the host under low-temperature stress. The main constituents of arginine and proline metabolism were significantly enriched in the hemolymph of conventional or K. michiganensis - reinfected flies compared with those in ABX control ( Fig 4B–4D ). Notably, metabolites, mainly including proline, arginine, ornithine and putrescine, were significantly downregulated by 34-, 10-, 5- and 4-fold in ABX flies compared with conventional flies, respectively, 5 dpe to 10°C ( S1C Data ). Reinfection of ABX flies with K. michiganensis upregulated the relative levels of proline, arginine, ornithine and putrescine by 13-, 10-, 5- and 4-fold the levels found in ABX flies, respectively, 5 dpe to 10°C ( S1D Data ). These findings suggest that cryoprotectants of arginine and proline metabolism elevated by the host in the presence of gut symbionts, in particular K. michiganensis, play a role in low-temperature stress resistance. Most of the differentially expressed metabolites (DEMs) were assigned to metabolic (amino acid, fatty acid synthesis) reference pathways, with arginine and proline metabolism being the top enriched pathway ( S1E and S1F Data ).

(A) Results of PCA based on 1548 metabolites detected from hemolymph of ABX (n = 1–6, red), CK (conventional) (n = 7–12, green) and K. michiganensis-reinfected (n = 13–18, blue) flies showing different clustering groups (95% confidence regions). (B) Volcano plots to infer the overall distribution of 201 differentially expressed metabolites in conventional vs. ABX fly hemolymph, (C) 202 differentially expressed metabolites in K. michiganensis-reinfected vs. ABX fly hemolymph. (D) Unsupervised hierarchical clustering heat map of the 25 metabolites that contributed most to the separation of hemolymph across different samples of ABX (n = 6), conventional (n = 6) and K. michiganensis-reinfected flies (n = 6). Each column represents one sample. Colors specify the relative concentration of each metabolite (minimum–maximum). The tree on the left illustrates a dendrogram of clustering (Ward’s method).

For a more in-depth understanding of the mechanism whereby the gut microbiota promotes host resistance to low-temperature stress, we compared the metabolomics of the hemolymph for the conventional flies, ABX flies reinfected with K. michiganensis (both termed as ‘nonABX flies’) and ABX flies. Overall, several metabolites displayed considerable differences across the nonABX and ABX fly hemolymph, including amino acids, multiple lipids and their derivatives, and carbohydrates 5 dpe to 10°C ( S1A and S1B Data ). In the principal component analysis (PCA) of these three hemolymph metabolite groups, PC1 explained 36.03% of the variance in expression and clustered conventional and K. michiganensis-reinfected flies separately from ABX flies, indicating that the gut microbiota has a remarkable effect on the hemolymph metabolomic profile at 10°C ( Fig 4A ).

Furthermore, we investigated the physiological mechanism, e.g., the major nutrients level in the hemolymph, in the presence or absence of gut microbes. The results showed that major nutrient levels, i.e., total amino acid (AA) ( Fig 3A ) and lipids ( Fig 3B ), were significantly decreased (P<0.05) by 69.67% and 85.43% in the hemolymph of ABX flies compared with those in conventional flies 5 dpe to 10°C, respectively. Surprisingly, the sugar levels ( Fig 3C ) were not significantly different between these two treatments (P>0.05). After the feeding of live K. michiganensis to ABX flies, the total AA ( Fig 3A , P<0.05) and lipid ( Fig 3B , P<0.05) levels were restored and significantly enhanced by 170% and 300% in the hemolymph compared with those in their ABX counterparts, respectively. There were no significant differences in the major nutrient levels (AAs and lipids) between ABX flies reinfected with K. michiganensis and conventional flies ( Fig 3 ).

Since a single reinfection of live K. michiganensis was sufficient to rescue the ABX host survival at 10°C, we next investigated whether, once inoculated, the host is able to maintain an adequate supply of gut microbes to benefit health throughout life postexposure to 10°C ( S2B and S2C Fig ). The results showed that after 10 dpe to 10°C, the average number of cultivated microbial communities resulting from colony forming units (CFUs) in ABX control flies fed a sterile liquid diet were 7.667 x 10 5 ± 2.028 x10 5 CFUs gut -1 (mean ± SE of 10 individual flies), which represents approximately 96% decrease versus that of live K. michiganensis-reinfected flies, which was 1.937 x 10 7 ± 3.014 x 10 6 CFUs gut -1 (P<0.005, S2C Fig ). Importantly, CFUs of flies after 10 dpe to 10°C; the average numbers of cultivated microbial communities resulting from CFUs in ABX flies fed with all other tested strains showed a significant decrease compared with live K. michiganensis-reinfected flies (P<0.05, S2C Fig ), suggesting that, once inoculated, K. michiganensis can sustain in the ABX flies gut under low-temperature stress environment than that of all other tested strains. In contrast, there was no significant difference in the average number of cultivated microbial communities between conventional (1.98 x 10 7 ± 2.38 x 10 6 CFUs gut -1 ) and live K. michiganensis-reinfected flies (1.937 x 10 7 ± 3.014 x 10 6 CFUs gut -1 ) (P>0.05, S2C Fig ). These results suggest that the survival of flies under low-temperature stress is influenced by the abundance of associated bacteria. Next, qPCR analysis also revealed that the microbial load of K. michiganensis, as well as total bacteria and Enterobacteriaceae in the gut homogenates, was maintained at two different time intervals during the fly survival trial (10 dpe and 20 dpe to 10°C) after feeding live K. michiganensis. ( S2D–S2F Fig ). Collectively, the microbial load in the gut of ABX flies reinfected with live K. michiganensis was similar to that of conventional flies (P>0.05) and significantly higher (P<0.005) than that of ABX control, suggesting that live K. michiganensis reinfection can form a sustained colony in the ABX flies gut that, in turn, improves host resistance to low-temperature stress.

A single dose reinfection or recurring supply with heat-killed (HK) K. michiganensis equivalent to the levels of live K. michiganensis (10 8 CFU/ml) showed no significant effect on the survival time of ABX flies ( Fig 2A , P>0.05) or in another independent longevity test ( Fig 2B , P>0.05) compared with ABX control postexposure to 10°C. Moreover, increasing the quantity of HK K. michiganensis (10 9 CFU/ml) given to ABX flies throughout life did not even rescued the median lifespan (5.5 days) over the axenic control (5 days), and to a significantly lesser extent (P < 0.0001) than that of live K. michiganensis inoculation ( Fig 2A ). Taken together, our results show that survival of flies cannot be mimicked by HK K. michiganensis, suggesting that microbes may stimulate indirect processes or host metabolism, rather than directly providing nutritional benefits to the host during low-temperature stress.

(A) Recolonization of ABX flies by live K. michiganensis (10 8 CFU/ml) extends survival (log-rank test, P<0.0001). ABX flies reinfected with HK K. michiganensis (provided a single dose and recurring quantity of 10 8 CFU/ml) did not promotes survival (log-rank test, P>0.05) compared to the ABX flies under temperature stress of 10°C. The median survival time was not significantly different between ABX flies reinfected with an increased HK K. michiganensis dose (5.5 days) (recurring supply of 10 9 CFU/ml at every food change throughout life) and ABX flies (5 days), under temperature stress of 10°C (n = 58–61 for each condition, log-rank test, P>0.05). (B) The scatter diagram of an independent trial of longevity of conventional flies, ABX flies fed with live K. michiganensis, single dose or recurring supply of HK K. michiganensis and the ABX control under temperature stress of 10°C (n = 60 for each condition, one-way ANOVA followed by Tukey’s multiple comparison test was performed with a significant difference at ***P<0.0001, ** P<0.001, ns P>0.05). *A+ strain name (ABX fly reinfection with above gut bacterial strains). In figure B, K. m indicates K. michiganensis.

In trial 2, ABX flies were reinfected with all isolated gut bacterial species to determine whether the key gut bacteria strain could restore the ecological fitness parameter of host’s survival, postexposure to 10°C. The results showed that ABX flies reinfected with live K. michiganensis BD177 significantly extended the median survival time to 13 days from 5 days in ABX counterparts, which represents an approximately 160% increase (P<0.0001, Fig 2A ). The median survival time was also significantly increased in ABX flies recolonized by major gut symbionts such as E. soli and C. koseri (11 days), E. tabaci (9 days), and E. hormaechei (8.5 days) compared with 5 days in ABX control but to a lower extent than that achieved with K. michiganensis reinfection ( S2A Fig ). Moreover, other minor gut strains extended the median survival time, including A. radioresistens and P. alcalifaciens (8 days), Leclercia adecarboxylata (7.5 days), Enterococcus faecium and P. vermicola (7 days), P. rettgeri (6.5 days) and A. bereziniae (6 days), beyond 5 days in the ABX control but to a significantly lesser extent than that achieved in K. michiganensis-reinfected flies ( S2A Fig ). In contrast, survival was unaffected after the recolonization of ABX flies by Lactococcus garvieae, Serratia marcescens and K. ascorbata compared with that in the ABX control ( S2A Fig ). Importantly, there was no significant difference in the median survival time between conventional (17.5 days) and K. michiganensis-reinfected (13 days) flies (P = 0.1275, Fig 2A ), while all other tested strains exhibited a significantly decreased median survival time compared with that of conventional flies (P<0.05, S2A Fig ), suggesting that K. michiganensis is a key gut symbiont that has greater effect on the restoration of the survival time of ABX flies, similar to the outcome achieved by conventional flies.

To find key symbiotic bacteria to improve the host survival under low-temperature stress, we used a culture-dependent approach and identified 223 and 217 isolates from conventional fly guts 5 days postexposure (5 dpe) to 28°C and 10°C, respectively. In total, 15 representative bacterial species were identified. The most prevalent members of Enterobacteriaceae, such as Enterobacter spp., K. michiganensis, C. koseri, Kluyvera ascorbata, and Providencia spp., as well as minor bacterial species such as Acinetobacter spp. were identified from B. dorsalis gut ( S1B Table ). The relative abundances of four major strains—C. koseri, K. michiganensis, E. soli and E. hormaechei—in conventional flies postexposure to 10°C slightly decreased, although the differences were nonsignificant, by 28.57%, 12.12%, 10.53% and 9.09%, respectively, compared with those in conventional flies postexposure to 28°C ( S1B Table ).

Generation of ABX flies (A- C). The efficacy of elimination of gut bacteria confirmed by culturing ABX (A) and conventional (B) fly (n = 10) gut homogenates on LB agar plates, and (C) performing qPCR analysis on the fly gut (n = 15) using universal gene primers. Three biological repeats were conducted. Error bars indicate mean with SE. Significant difference was determined by Student’s t test at P<0.0001. (D) Survival curves of conventional and ABX B. dorsalis by log-rank analysis postexposure to 10°C (individual = 59–61 in each treatment); the median survival time of ABX flies was significantly reduced to 6 days from 19 days in conventional flies, *** P<0.0001.

ABX flies were generated via treatment with oral antibiotics. This treatment did not influence adult survival under normal rearing temperature (27 ± 1°C) ( S1A Fig ) [ 33 ]. The efficacy of elimination of the gut microbiota was confirmed by plating gut homogenates onto Luria–Bertani (LB) agar plates ( Fig 1A and 1B ) and performing qPCR analysis ( Fig 1C , P = 0.0002) using total gut bacteria gene primers ( S1A Table ). To investigate a possible role of gut microbiota in host resistance to temperature stress, ABX and conventional flies were subjected to different long-term low- to mild-temperatures stresses of 5°C ( S1B Fig ), 7.5°C ( S1C Fig ), 10°C ( Fig 1D ) and 15°C ( S1D Fig ); then, 10°C was chosen for the subsequent studies because this was the lowest temperature at which we found a significant difference in the median survival time between conventional and ABX flies ( Fig 1D ). In trial 1, the results showed that after removing the gut bacteria via antibiotic-treatment, the median survival time of ABX flies was significantly reduced to 6 days from 19 days in conventional flies (P<0.0001, log rank test) following exposure to continuous low-temperature (10°C), which represents an approximately 68% decrease ( Fig 1D ), suggesting that an improvement in the host lifespan at 10°C may be linked with the intestinal microbial community.

Discussion

Gut microbiota provides a wide range of beneficial effects for their hosts, including vitamins, energy metabolism, reproduction, immune function, and longer lifespan in unfavorable environments [21, 24, 25, 40]. In our previous study, we found that Enterobacteriaceae is the prevalent community in the digestive tract of B. dorsalis [31]. In particular, Klebsiella was identified as the dominant symbiotic bacterial genus in the B. dorsalis gut [24], suggesting that those bacterial symbionts may contribute to the biology of the host fly species. Pieterse and colleagues found that B. dorsalis adults showed a slight but statistically significant expansion in survival after exposure to a low hardening temperature of 10°C [41], but the role of gut symbionts on the host biology during low-temperature stress has not previously been described. In this study, we found that the gut microbiota of B. dorsalis functions to promote host resistance to temperature stress, as gut bacteria-deprived flies do not live as long as conventional flies at 10°C.

Furthermore, through the recolonization of ABX flies with dominant members of B. dorsalis indigenous gut microbiota, a key gut symbiont, K. michiganensis BD177, was found to be capable of promoting the survival time of the ABX flies to the level of the conventional flies during low-temperature stress of 10°C. Our data also supports the idea that only K. michiganensis can form a sustained colony in the ABX flies gut equivalent to the levels found in conventional flies and/or significantly higher than that of all other tested strain, suggesting that K. michiganensis quantity is influential to promote host resistance ability to low-temperature stress. In future, it may be interesting to determine if microbial quantity impacts fly behavior, physiology, or survival during stressed environment, particularly under multi-microbial experimental conditions. The results of this study are in accordance with the findings of previous studies. For example, it has been shown that stress caused by irradiation significantly reduced the levels of the main gut flora ‘Enterobacteriaceae’ that resulted in the decline of various aspects of ecological fitness in sterile Ceratitis capitata [26] and B. dorsalis [32]. However, the addition of the gut symbiont K. oxytoca to the postirradiation diet restored the survival and competitiveness of sterile C. capitata males and resulted in decreased levels of potentially pathogenic Pseudomonas sp. [26]. In our previous study, we found that gut-symbiont probiotics K. oxytoca reversed the radiation-induced host fitness decline, including survival time of B. dorsalis [32]. However, our recent genome wide sequencing and phylogenetic analysis showed that the Klebsiella sp. BD177, which was isolated from the B. dorsalis gut and previously identified as K. oxytoca on the basis of 16S RNA sequencing [32], has 97.74% ANIm with the reference genome of K. michiganensis DSM 25444 (= W14) (Accession: PRJNA388837) but only 92.20% ANIm with the K. oxytoca ATCC 13182 reference genome; thus, it had been classified as K. michiganensis BD177 (Accession: PRJNA602959), which was separated and independent from K. oxytoca according to average nucleotide identity (ANI), a genomic gold standard for bacteria species identification [42]. After cleaning the gut bacteria of Drosophila melanogaster, reinfection with a live gut symbiont Issatchenkia orientalis has also been demonstrated to promote the lifespan of nutritionally stressed flies [43]. More recent studies show that HK gut microbes can promote the lifespan of nutritionally deprived axenic Drosophila in repeated and dose-dependent manner at normal room temperature [43–45]. In contrast, our study show that increasing the quantity of HK K. michiganensis (recurring) did not promote the ABX flies survival to the levels of live K. michiganensis inoculated or conventional flies, suggesting that gut microbes may act as a source to enhance host metabolism, rather than providing direct nutritional benefits following exposure to low-temperature stress. This contradiction might be due to the different environmental conditions, e.g., different temperature conditions. In future research, the use of HK microbe supplementation may help fully describe microbial influences, i.e., to identify a direct nutritional contribution of microbes to the host under different stressed environmental conditions.

To the best of our knowledge, few studies have employed the symbiotic role of Klebsiella genus strains to explore the host fitness under stressed conditions. For example, Behar and colleagues suggested that inoculation of K. oxytoca to antibiotic-treated medflies contributes to the host nitrogen and carbon metabolism, copulatory success and development [46]. The gut symbiotic species K. oxytoca has also been reported to encode for the nitrogen fixation pathway by converting N2 to ammonia [47].

The gut microbiota not only provides essential nutrients to the hosts [20, 21] but also aid insect resistance to environmental stress via regulating host-signaling pathways and metabolism [24, 48]. An example of Drosophila host-microbe interaction is that the activity of pyrroloquinoline quinone-dependent alcohol dehydrogenase (PQQ-ADH) of a commensal bacterium, Acetobacter pomorum, stimulates the host insulin/insulin-like growth factor signaling (IIS) pathway to regulate homeostasis programs monitoring intestinal stem cell activity, body size, energy metabolism and development rate [48]. Similarly, after deprivation of a key gut symbiont ‘Citrobacter sp.’ via antibiotic treatment in a resistant B. dorsalis strain, the host showed a decreased level of trichlorfon resistance [24]. Furthermore, recolonization of the susceptible B. dorsalis strain with ‘Citrobacter sp.’ increased the host resistance by degrading the highly toxic trichlorfon insecticide into less toxic chloral hydrate and dimethyl phosphite compounds and possibly via stimulation of organophosphorus hydrolase (OPH-like) genes [24].

By combining transcriptomic and metabolomic approaches, we found that gut symbionts, particularly K. michiganensis BD177, help the host B. dorsalis upregulate the levels of ‘cryoprotectant’ transcripts and metabolites, which increases its resistance to long-term low-temperature stress by stimulating the host arginine and proline metabolism pathway. The accumulation of small cytoprotectant molecules is one of the hallmarks in insect adaptation to low-temperature [49]. In insect low-temperature stress resistance, these molecules, also known as ‘cryoprotectants’ are mostly sugars, polyols [50] and free AAs [51]. Elevated levels of arginine, proline, ornithine and putrescine in nonABX flies, especially in ABX flies reinfected by the key gut symbiont K. michiganensis, suggest that the presence of gut microbiota can stimulate the host metabolism to produce these cryoprotectants, which, in turn, promotes host survival during low-temperature stress.

In fact, other studies have shown that arginine and proline metabolic products are important for the growth and development of Drosophila larvae and adults under a cold stress environment [35, 36, 49]. Misener et al. suggested that proline has a special role in dipterans, accumulating early in cold response and later may provide important energy reserves to maintain ATP levels [35]. In the current study, we found that RNAi mediated silencing of two key genes, i.e., Pro-C and ASS, which are precursor enzymes required for proline and arginine synthesis, significantly reduced the survival time of conventional flies following exposure to low-temperature stress. Combined effect of dsPro-C + dsASS further confirmed that median survival time of conventional flies was considerably reduced compared with that of control (dsEgfp) and individual dsRNA applications. These findings demonstrate that dsRNA targeting Pro-C or ASS may result in the low production of proline and arginine molecules which in turn decrease the survival time of flies. Drosophila transcript displayed a higher level of Pro-C expression in response to low-temperature stress, which shows that a large amount of proline is essential for Drosophila metabolism and survivorship during temperature stress [35]. Diet supplemented with proline conferred partial freezing tolerance by D. melanogaster larvae under winter acclimation [36]. Kostal and his colleagues also showed that proline and arginine-augmented diets can improve the freeze tolerance of Drosophila adults to 42.1% and 50.6%, respectively [49]. In agreement with previous studies, our data demonstrate that the individual or combined injections of L-proline and L-arginine to ABX flies can increase the survival time postexposure to low-temperature. Two possible mechanisms by which high concentrations of arginine and proline might stimulate low-temperature resistance could be as follows; first, proline could reduce the proteins partial unfolding and prevent membrane fusions during low temperature [49]. Second, proline and arginine both are unique among amino compounds in their capability to form supramolecular combinations that possibly bind partially unfolded proteins and cause hindrance in their aggregation under increasing freeze dehydration [49].

Arginine and proline synthesis, which requires mitochondrial activity, may suggest that mitochondrial activity can be maintained even at low temperature in insects [35–37]. Mitochondria are recycled in a dynamic equilibrium between opposing processes of fusion and fission. These processes are needed to control cell quality by substituting defective mitochondria with new and healthy mitochondria during high levels of cellular stress [52]. Herein, our results showed that, once inoculated, K. michiganensis aids the ABX flies in the maintenance of mitochondrial dynamic equilibrium that might restored both arginine and proline to the normal levels. Previous studies have also shown that cristae formations increase the surface area of the mitochondrial inner membrane and plays key roles in maintaining ATP production [53]. Any destruction to the cristae structure results in defective ATP production [54, 55]. In agreement with these findings, our data further confirmed that due to the destruction of mitochondrial cristae structure in ABX flies, the ATP concentration was significantly decreased than that of K. michiganensis-reinfected flies. However, we are currently unable to say how gut microbiota are able to maintain healthy mitochondria during cold stress. Thus, the connection between the gut bacteria and the mitochondria dynamic equilibrium, e.g., fission and fusion, during environmental stresses should be studied in the future. It might also be interesting to investigate the mitochondrial morphology after the injection of highly expressed amino acids, e.g., L-proline and L-arginine and/or by silencing their corresponding genes in the future. Several antibiotics have been reported to be toxic to mitochondria in other animals [56]. It might be valuable to conduct cold stress studies using a truly axenic or germ-free flies, without using antibiotic treatments. We were unable to perform a low-temperature challenge on truly axenic or germ-free flies, however, our study still provides the clear effects of K. michiganensis reinfection that rescues the survival of ABX flies and helps the host to maintain mitochondrial morphology at low-temperature stress, suggesting that the reduced median survival time of ABX flies, which were generated via treatment with oral antibiotics cocktails as most researchers did in this research field [33], at low- temperature resulted from elimination of the gut microbiota, not from their toxic effects to mitochondria. Until this theory has been tested further, we can conclude that in the presence of gut microbes, these charged and polar amino acids, in particular the modulation of the host’s arginine and proline metabolism via influencing mitochondrial functionality, are central to the acquisition of long-term low-temperature stress resistance.

[1] Url: https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1008441

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