Transcript changes in Vibrio cholerae in response to salt stress

Effect of citric acid on cell membrane structure and function of Issatchenkia terricola WJL-G4

AIMS

This study aimed to investigate the changes of cell membrane structure and function of Issatchenkia terricola under citric acid by performing physiological analysis.

METHODS AND RESULTS

The membrane integrity, surface hydrophobicity, structure, fluidity, apoptosis, and fatty acid methyl esters composition of I. terricola WJL-G4 cells were determined by propidium iodide staining, microbial adhesion to hydrocarbon test, transmission electron microscopy analysis, fluorescence anisotropy, flow cytometry, and gas chromatography-mass, respectively. The results showed that with the increasing of citric acid concentrations, the cell vitality, membrane integrity, and fluidity of I. terricola reduced; meanwhile, apoptosis rate, membrane permeable, hydrophobicity, and ergosterol contents augmented significantly. Compared to control, the activities of Na+, K+-ATPase, and Ca2+, Mg2+-ATPase increased by 3.73-fold and 6.70-fold, respectively, when citric acid concentration increased to 20 g l-1. The cells cracked and their cytoplasm effused when the citric acid concentration reached 80 g l-1.

CONCLUSIONS

I. terricola could successfully adjust its membrane structure and function below 60 g l-1 of citric acid. However, for citric acid concentrations above 80 g l-1, its structure and function were dramatically changed, which might result in reduced functionality.

Survival Characteristics and Transcriptomic Analyses Reveal the Adaptive Response of the Aquatic Pathogen Non-O1/O139 Vibrio cholerae to Starvation Stress

Non-O1/O139 Vibrio cholerae is a pathogen of various aquatic organisms but requires major self-regulation to overcome environmental stress in the aquatic environment. However, its survival strategies under environmental stress are not well understood. The objective of this study was to describe the survival characteristics and changes in expression of stress resistance-related genes of non-O1/O139 V. cholerae after 6 months of starvation at room temperature. The results demonstrated that starved cells were still viable, exhibited shortened rods and shrinking surface, and maintained virulence to Macrobrachium rosenbergii. To investigate the changes in gene expression in non-O1/O139 V. cholerae under starvation stress, especially those involved in stress resistance, transcriptome profiles of starved and wild-type cells were determined. The differentially expressed genes (DEGs) in starved cells were identified, including 191 upregulated genes and 180 downregulated genes. Among these DEGs, the well-known stress resistance-related genes were upregulated significantly, including rpoS, rpoD, rpoN, rpoE, uspA, uspC, cspD, hslJ, etc. Gene Ontology (GO) analysis of the DEGs demonstrated that environmental adaptation-related categories, such as response to stimulus and signal transduction, were upregulated significantly in the starved cells, while cell motility was downregulated significantly. These DEGs were also enriched into 54 KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways, including biofilm formation, two-component system, quorum sensing, flagellar assembly, bacterial chemotaxis stress resistance-related pathways, etc. The potential existence of long-starved non-O1/O139 V. cholerae bacteria in the aquatic environment may raise new concerns about this devastating pathogen in aquaculture.

IMPORTANCE Non-O1/O139 V. cholerae is a causal agent of vibriosis that can be subject to nutrient insufficiency and cause high rates of mortality in aquatic animals. However, its molecular mechanisms of survival in response to starvation stress have been investigated only partially. Here, we demonstrate that under starvation stress, non-O1/O139 V. cholerae can survive over the long term and cause disease by dwarfing of the cell structure, upregulation of a series of stress resistance-related genes, and downregulation of flagellum assembly-related genes. This knowledge can help the development of intervention strategies to control non-O1/O139 V. cholerae infection in aquaculture.

Effects of NaCl Concentration on the Behavior of Vibrio brasiliensis and Transcriptome Analysis

The growth of Vibrio bacteria is affected by environmental conditions, and unfavorable conditions will produce different degrees of stress on Vibrio. The cells respond to the stress on the bacteria through changes in biological characteristics and transcriptomes. To study the effect of NaCl concentration on Vibrio brasiliensis, we have determined the biological characteristics of the 0%, 1%, 2%, 3%, 5%, and 7% NaCl concentrations cultured V. brasiliensis to research the salt stress to bacteria. We found that the biological properties of V. brasiliensis cultured with different NaCl concentrations were different, and the expression of outer membrane proteins of V. brasiliensis changed when it was grown under different NaCl concentrations. When bacteria cultured in higher NaCl concentrations (3%, 5% and 7% NaCl), the sodium-type flagellar protein MotY was found. Finally, the transcriptome analysis of V. brasiliensis cultured with 0% NaCl and 7% NaCl was carried out to find out the differentially expressed genes. We found that the same gene have opposite up-regulated and down-regulated expression in two treatments, indicating that these types of genes are regulated different in low and high osmotic stress.

Genome-Based Classification of Strain 16-SW-7, a Marine Bacterium Capable of Converting B Red Blood Cells, as Pseudoalteromonas distincta and Proposal to Reclassify Pseudoalteromonas paragorgicola as a Later Heterotypic Synonym of Pseudoalteromonas distincta

A strictly aerobic, Gram-stain-negative, rod-shaped, and motile bacterium, designated strain 16-SW-7, isolated from a seawater sample, was investigated in detail due to its ability to produce a unique α-galactosidase converting B red blood cells into the universal type blood cells. The phylogenetic analysis based on 16S rRNA gene sequences revealed that the strain 16-SW-7 is a member of the Gammaproteobacteria genus Pseudoalteromonas. The closest relatives of the environmental isolate were Pseudoalteromonas distincta KMM 638T and Pseudoalteromonas paragorgicola KMM 3548T, with the plural paralogous 16S rRNA genes of 99.87-100% similarity. The strain 16-SW-7 grew with 1-10% NaCl and at 4-34°C, and hydrolyzed casein, gelatin, tyrosine, and DNA. The genomic DNA G+C content was 39.3 mol%. The prevalent fatty acids were C16:1 ω7c, C16:0, C17:1 ω8c, C18:1 ω7c, C17:0, and C12:0 3-OH. The polar lipid profile was characterized by the presence of phosphatidylethanolamine, phosphatidylglycerol, two unidentified amino lipids, and three unidentified lipids. The major respiratory quinone was Q-8. The finished genome of the strain 16-SW-7 (GenBank assembly accession number: GCA_005877035.1) has a size of 4,531,445 bp and comprises two circular chromosomes L1 and S1, deposited in the GenBank under the accession numbers CP040558 and CP040559, respectively. The strain 16-SW-7 has the ANI values of 98.2% with KMM 638T and KMM 3548T and the DDH values of 84.4 and 83.5%, respectively, indicating clearly that the three strains belonged to a single species. According to phylogenetic evidence and similarity for the chemotaxonomic and genotypic properties, the strain 16-SW-7 (= KCTC 52772 = KMM 701) represents a novel member of the species Pseudoalteromonas distincta. Also, we have proposed to reclassify Pseudoalteromonas paragorgicola as a later heterotypic synonym of P. distincta based on the rules of priority with the emendation of the species.

Cell Wall Biology of Vibrio cholerae

Most bacteria are protected from environmental offenses by a cell wall consisting of strong yet elastic peptidoglycan. The cell wall is essential for preserving bacterial morphology and viability, and thus the enzymes involved in the production and turnover of peptidoglycan have become preferred targets for many of our most successful antibiotics. In the past decades, Vibrio cholerae, the gram-negative pathogen causing the diarrheal disease cholera, has become a major model for understanding cell wall genetics, biochemistry, and physiology. More than 100 articles have shed light on novel cell wall genetic determinants, regulatory links, and adaptive mechanisms. Here we provide the first comprehensive review of V. cholerae's cell wall biology and genetics. Special emphasis is placed on the similarities and differences with Escherichia coli, the paradigm for understanding cell wall metabolism and chemical structure in gram-negative bacteria.

Comparative Genomics and Transcriptomics Analyses Reveal a Unique Environmental Adaptability of Vibrio fujianensis

The genus Vibrio is ubiquitous in marine environments and uses numerous evolutionary characteristics and survival strategies in order to occupy its niche. Here, a newly identified species, Vibrio fujianensis, was deeply explored to reveal a unique environmental adaptability. V. fujianensis type strain FJ201301^T shared 817 core genes with the Vibrio species in the population genomic analysis, but possessed unique genes of its own. In addition, V. fujianensis FJ201301^T was predicated to carry 106 virulence-related factors, several of which were mostly found in other pathogenic Vibrio species. Moreover, a comparative transcriptome analysis between the low-salt (1% NaCl) and high-salt (8% NaCl) condition was conducted to identify the genes involved in salt tolerance. A total of 913 unigenes were found to be differentially expressed. In a high-salt condition, 577 genes were significantly upregulated, whereas 336 unigenes were significantly downregulated. Notably, differentially expressed genes have a significant association with ribosome structural component and ribosome metabolism, which may play a role in salt tolerance. Transcriptional changes in ribosome genes indicate that V. fujianensis may have gained a predominant advantage in order to adapt to the changing environment. In conclusion, to survive in adversity, V. fujianensis has enhanced its environmental adaptability and developed various strategies to fill its niche.

Vibrio cholerae OmpR Represses the ToxR Regulon in Response to Membrane Intercalating Agents That Are Prevalent in the Human Gastrointestinal Tract

Multidrug efflux systems belonging to the resistance-nodulation-division (RND) superfamily are ubiquitous in Gram-negative bacteria. RND efflux systems are often associated with multiple antimicrobial resistance and also contribute to the expression of diverse bacterial phenotypes including virulence, as documented in the intestinal pathogen Vibrio cholerae, the causative agent of the severe diarrheal disease cholera. Transcriptomic studies with RND efflux-negative V. cholerae suggested that RND-mediated efflux was required for homeostasis, as loss of RND efflux resulted in the activation of transcriptional regulators, including multiple environmental sensing systems. In this report, we investigated six RND efflux-responsive regulatory genes for contributions to V. cholerae virulence factor production. Our data showed that the V. cholerae gene VC2714, encoding a homolog of Escherichia coli OmpR, was a virulence repressor. The expression of ompR was elevated in an RND-null mutant, and ompR deletion partially restored virulence factor production in the RND-negative background. Virulence inhibitory activity in the RND-negative background resulted from OmpR repression of the key ToxR regulon virulence activator aphB, and ompR overexpression in wild-type cells also repressed virulence through aphB We further show that ompR expression was not altered by changes in osmolarity but instead was induced by membrane-intercalating agents that are prevalent in the host gastrointestinal tract and which are substrates of the V. cholerae RND efflux systems. Our collective results indicate that V. cholerae ompR is an aphB repressor and regulates the expression of the ToxR virulence regulon in response to novel environmental cues.

Zygosaccharomyces rouxii Combats Salt Stress by Maintaining Cell Membrane Structure and Functionality

Zygosaccharomyces rouxii is an important yeast that is required in the food fermentation process due to its high salt tolerance. In this study, the responses and resistance strategies of Z. rouxii against salt stress were investigated by performing physiological analysis at membrane level. The results showed that under salt stress, cell integrity was destroyed, and the cell wall was ruptured, which was accompanied by intracellular substance spillover. With an increase of salt concentrations, intracellular Na+ content increased slightly, whereas intracellular K+ content decreased significantly, which caused the increase of the intracellular Na+/K+ ratio. In addition, in response to salt stress, the activity of Na+/K+-ATPase increased from 0.54 to 2.14 μmol/mg protein, and the ergosterol content increased to 2.42-fold to maintain membrane stability. Analysis of cell membrane fluidity and fatty acid composition showed that cell membrane fluidity decreased and unsaturated fatty acid proportions increased, leading to a 101.21% rise in the unsaturated/saturated fatty acid ratio. The results presented in this study offer guidance in understanding the salt tolerance mechanism of Z. rouxii, and in developing new strategies to increase the industrial utilization of this species under salt stress.

The Outer Membrane Protein OmpW Enhanced V. cholerae Growth in Hypersaline Conditions by Transporting Carnitine

Pathogenic marine bacteria are found in environments and food sources with high salt concentrations, which the bacteria must effectively manage for their survival. Several mechanisms, such as the transport of ions and compatible solutes as well as changes in aerobic and anaerobic respiration, confer salt tolerance to bacteria. In this study, we found that the outer membrane protein OmpW was related to salt stress in Vibrio cholerae and that ompW gene transcription and expression were up-regulated in cultures containing high NaCl concentrations. Deletion of ompW resulted in reduced V. cholerae growth in hypersaline culture conditions. Supplements of the compatible solutes betaine, L-carnitine, or L-lysine enhanced the growth of V. cholerae in hypersaline media. Supplements of betaine or L-lysine had the same growth enhancement effect on the ompW-deletion mutant cultured in hypersaline media, whereas L-carnitine supplementation did not restore mutant growth. In addition, the uptake of L-carnitine was decreased in the ompW-deletion mutant. Our study showed that among the multiplex factors that enhance the hypersaline tolerance of V. cholerae, OmpW also plays a role by transporting L-carnitine.

Comparative Genomics of cpn60-Defined Enterococcus hirae Ecotypes and Relationship of Gene Content Differences to Competitive Fitness

Natural microbial communities undergo selection-driven succession with changes in environmental conditions and available nutrients. In a previous study of the pig faecal Enterococcus community, we demonstrated that cpn60 universal target (UT) sequences could resolve phenotypically and genotypically distinct ecotypes of Enterococcus spp. that emerged over time in the faecal microbiome of growing pigs. In this study, we characterized genomic diversity in the identified Enterococcus hirae ecotypes in order to define further the nature and degree of genome content differences between taxa resolved by cpn60 UT sequences. Genome sequences for six representative isolates (two from each of three ecotypes) were compared. Differences in phosphotransferase systems and amino acid metabolism pathways for glutamine, proline and selenocysteine were observed. Differences in the lac family phosphotransferase system corresponded to lactose utilization phenotypes of the isolates. Competitive fitness of the E. hirae ecotypes was evaluated by in vitro growth competition assays in pig faecal extract medium. Isolates from E. hirae-1 and E. hirae-2 ecotypes were able to out-compete isolates from the E. hirae-3 ecotype, consistent with the relatively low abundance of E. hirae-3 relative to E. hirae-1 and E. hirae-2 previously observed in the pig faecal microbiome, and with observed differences between the ecotypes in gene content related to biosynthetic capacity. Results of this study provide a genomic basis for the definition of ecotypes within E. hirae and confirm the utility of the cpn60 UT sequence for high-resolution profiling of complex microbial communities.