Effects of NaCl Concentrations on Growth Patterns, Phenotypes Associated With Virulence, and Energy Metabolism in Escherichia coli BW25113

According to the sit-and-wait hypothesis, long-term environmental survival is positively correlated with increased bacterial pathogenicity because high durability reduces the dependence of transmission on host mobility. Many indirectly transmitted bacterial pathogens, such as Mycobacterium tuberculosis and Burkhoderia pseudomallei, have high durability in the external environment and are highly virulent. It is possible that abiotic stresses may activate certain pathways or the expressions of certain genes, which might contribute to bacterial durability and virulence, synergistically. Therefore, exploring how bacterial phenotypes change in response to environmental stresses is important for understanding their potentials in host infections. In this study, we investigated the effects of different concentrations of salt (sodium chloride, NaCl), on survival ability, phenotypes associated with virulence, and energy metabolism of the lab strain Escherichia coli BW25113. In particular, we investigated how NaCl concentrations influenced growth patterns, biofilm formation, oxidative stress resistance, and motile ability. In terms of energy metabolism that is central to bacterial survival, glucose consumption, glycogen accumulation, and trehalose content were measured in order to understand their roles in dealing with the fluctuation of osmolarity. According to the results, trehalose is preferred than glycogen at high NaCl concentration. In order to dissect the molecular mechanisms of NaCl effects on trehalose metabolism, we further checked how the impairment of trehalose synthesis pathway (otsBA operon) via single-gene mutants influenced E. coli durability and virulence under salt stress. After that, we compared the transcriptomes of E. coli cultured at different NaCl concentrations, through which differentially expressed genes (DEGs) and differential pathways with statistical significance were identified, which provided molecular insights into E. coli responses to NaCl concentrations. In sum, this study explored the in vitro effects of NaCl concentrations on E. coli from a variety of aspects and aimed to facilitate our understanding of bacterial physiological changes under salt stress, which might help clarify the linkages between bacterial durability and virulence outside hosts under environmental stresses.

Effects of Long-Term Exposure to Increased Salinity on the Amphibian Skin Bacterium Erwinia toletana

Amphibian's skin bacterial community may help them to cope with several types of environmental perturbations, including osmotic stress caused by increased salinity. This work assessed whether an amphibian skin bacterium could increase its tolerance to NaCl after a long-term exposure to this salt. A strain of Erwinia toletana, isolated from the skin of Pelophylax perezi, was exposed to two salinity scenarios (with 18 g/L of NaCl): (1) long-term exposure (for 46 days; Et-NaCl), and (2) long-term exposure followed by a recovery period (exposure for 30 days to NaCl and then to LB medium for 16 days; Et-R). After exposure, the sensitivity of E. toletana clonal populations to NaCl was assessed by exposing them to 6 NaCl concentrations (LB medium spiked with NaCl) plus a control (LB medium). Genotypic alterations were assessed by PCR-based molecular typing method (BOX-PCR). The results showed that tolerance of E. toletana to NaCl slightly increased after the long-term exposure, EC₅₀ for growth were: 22.5 g/L (8.64-36.4) for Et-LB; 30.3 g/L (23.2-37.4) for Et-NaCl; and 26.1 g/L (19.332.9) for Et-R. Differences in metabolic activity were observed between Et-LB and Et-R and between Et-NaCl and Et-R, suggesting the use of different substrates by this bacterium when exposed to salinized environments. NaCl-induced genotypic alterations were not detected. This work suggests that E. toletana exposed to low levels of salinity, activate different metabolic pathways to cope with osmotic stress. These findings may be further explored to be used in bioaugmentation procedures through the supplementation with this bacterium of the skin microbiome of natural populations of amphibians exposed to salinization.

Bacteria Remediate the Effects of Food Additives on Intestinal Function in an in vitro Model of the Gastrointestinal Tract

As the site of nutrient absorption, the small intestine is continuously exposed to preservatives and additives present in consumed food. While the effects of diet on the lower gastrointestinal tract are widely studied, the effects of food additives on the small intestinal epithelium and microbiota are less clearly understood. The goal of this work was to develop and establish a physiologically relevant model of the upper gastrointestinal tract to study the complex interactions between food additives, individual bacterial species, and intestinal function. To achieve this, an in vitro model incorporating simulated digestion, human intestinal epithelial cells, and the commensal, Gram-positive Lactobacillus rhamnosus, or the opportunistic, Gram-negative Escherichia coli was developed. This model was used to assess intestinal permeability and alkaline phosphatase activity following exposure to high glucose (HG), salt, emulsifier (TWEEN 20), food (milk chocolate candies) or chemical grade titanium dioxide nanoparticles (TiO₂-NP), and food (whole wheat bread) or chemical grade gluten. It was found that HG increased intestinal permeability, the presence of bacteria remediated the negative effects of HG on intestinal permeability, and a decrease in permeability and IAP activity was observed with increasing concentration of TWEEN 20 both in the presence and absence of bacteria. While L. rhamnosus influenced the activity of intestinal alkaline phosphatase and tight junction protein distribution, E. coli produced indole to reinstate intestinal permeability. The source of TiO₂ and gluten led to altered impacts on permeability and IAP activity. The growth of E. coli and L. rhamnosus was found to depend on the type of food additive used. Overall, the presence of bacteria in the in vitro model influenced the effects of food additives on intestinal function, suggesting a complex association between diet and upper GI microbiota. This model provides a method to study small intestinal function and host-microbe interactions in vitro in both healthy and diseased conditions.

The effect of the rpoSam allele on gene expression and stress resistance in Escherichia coli

The RNA polymerase associated with RpoS transcribes many genes related to stationary phase and stress survival in Escherichia coli. The DNA sequence of rpoS exhibits a high degree of polymorphism. A C to T transition at position 99 of the rpoS ORF, which results in a premature amber stop codon often found in E. coli strains. The rpoSam mutant expresses a truncated and partially functional RpoS protein. Here, we present new evidence regarding rpoS polymorphism in common laboratory E. coli strains. One out of the six tested strains carries the rpoSam allele, but expressed a full-length RpoS protein owing to the presence of an amber supressor mutation. The rpoSam allele was transferred to a non-suppressor background and tested for RpoS level, stress resistance and for the expression of RpoS and sigma70-dependent genes. Overall, the rpoSam strain displayed an intermediate phenotype regarding stress resistance and the expression of σ(S)-dependent genes when compared to the wild-type rpoS(+) strain and to the rpoS null mutant. Surprisingly, overexpression of rpoSam had a differential effect on the expression of the σ(70)-dependent genes phoA and lacZ that, respectively, encode the enzymes alkaline phosphatase and β-galactosidase. The former was enhanced while the latter was inhibited by high levels of RpoSam.

Resistant traits in digital organisms do not revert preselection status despite extended deselection: implications to microbial antibiotics resistance

Antibiotics resistance is a serious biomedical issue as formally susceptible organisms gain resistance under its selective pressure. There have been contradictory results regarding the prevalence of resistance following withdrawal and disuse of the specific antibiotics. Here, we use experimental evolution in “digital organisms” to examine the rate of gain and loss of resistance under the assumption that there is no fitness cost for maintaining resistance. Our results show that selective pressure is likely to result in maximum resistance with respect to the selective pressure. During deselection as a result of disuse of the specific antibiotics, a large initial loss and prolonged stabilization of resistance are observed, but resistance is not lost to the stage of preselection. This suggests that a pool of partial persists organisms persist long after withdrawal of selective pressure at a relatively constant proportion. Hence, contradictory results regarding the prevalence of resistance following withdrawal and disuse of the specific antibiotics may be a statistical variation about constant proportion. Our results also show that subsequent reintroduction of the same selective pressure results in rapid regain of maximal resistance. Thus, our simulation results suggest that complete elimination of specific antibiotics resistance is unlikely after the disuse of antibiotics once a resistant pool of microorganisms has been established.