Antimetabolite sensitivity and magnesium uptake by thermally stressed Vibrio parahaemolyticus

Environmental magnesium ion affects global gene expression, motility, biofilm formation and virulence of Vibrio parahaemolyticus

Vibrio parahaemolyticus is widely distributed in marine ecosystems. Magnesium ion (Mg2+) is the second most abundant metal cation in seawater, and plays important roles in the growth and gene expression of V. parahaemolyticus, but lacks the detailed mechanisms. In this study, the RNA sequencing data demonstrated that a total of 1494 genes was significantly regulated by Mg2+. The majority of the genes associated with lateral flagella, exopolysaccharide, type III secretion system 2, type VI secretion system (T6SS) 1, T6SS2, and thermostable direct hemolysin were downregulated. A total of 18 genes that may be involved in c-di-GMP metabolism and more than 80 genes encoding putative regulators were also significantly and differentially expressed in response to Mg2+, indicating that the adaptation process to Mg2+ stress may be strictly regulated by complex regulatory networks. In addition, Mg2+ promoted the proliferative speed, swimming motility and cell adhesion of V. parahaemolyticus, but inhibited the swarming motility, biofilm formation, and c-di-GMP production. However, Mg2+ had no effect on the production of capsular polysaccharide and cytoxicity against HeLa cells. Therefore, Mg2+ had a comprehensive impact on the physiology and gene expression of V. parahaemolyticus.

Host, pathogen and environment: a bacterial gbpA gene expression study in response to magnesium environment and presence of prawn carapace and commercial chitin

Background

Vibrio parahaemolyticus is a Gram-negative halophilic bacterium which is found largely in estuarine and coastal waters. The bacteria has been a main focus in gastro-intestinal infections caused primarily due to the consumption of contaminated seafood. It was shown to survive in magnesium concentrations as high as 300 mM which are toxic to various other micro-organisms. Several genes of V. parahaemolyticus were studied, among which gbpA (N-acetyl glucosamine binding protein) was reported in Vibrio cholerae.

Methods

The current study investigates the V. parahaemolyticus gbpA gene expression at different concentrations of magnesium sulfate heptahydrate (MgSO₄·7H₂O, chosen as the magnesium environment), in the presence of the host’s (prawn) carapace and the mimicked carapace [commercial chitin flakes (Sigma)]. The concentrations of MgSO₄·7H₂O utilized were approximately 0, 1, 75, 137, 225 and 300 mM. These were selected based upon the survival conditions required by prawn and bacteria, respectively. 0.05 gm/3 ml of carapace (by dry weight) and commercial chitin flakes were used in the experiments. Bacterial count was performed for the biological triplicates for the 3 experimental setups. The genome of Vibrio parahaemolyticus PCV08-7 (VPPCV08-7) was used as a reference, based on whose translated gbpA gene the probable protein-chemical interactions were determined on the STITCH database.

Results

The GbpA protein was shown to interact with chitin on the STITCH database. In our experiments, the gbpA showed lower gene expression levels at different MgSO₄·7H₂O concentrations in the presence of chitin and carapace, than with the presence of only MgSO₄•7H₂O. In addition, the bacterial count for various concentrations of magnesium used, revealed a distinct decrease in bacterial count within and among each of the three experimental setups.

Conclusion

In the presence of only magnesium, an increase in the gbpA expression with neither chitin nor carapace and vice versa supported by the results from the bacterial counts could help further studies to prove that the moulting phase of prawns may trigger increased expression of the V. parahaemolyticus gbpA gene.

Electronic supplementary material

The online version of this article (doi:10.1186/s13099-016-0105-5) contains supplementary material, which is available to authorized users.

Host, pathogen and the environment: the case of Macrobrachium rosenbergii, Vibrio parahaemolyticus and magnesium

Macrobrachium rosenbergii is well-known as the giant freshwater prawn, and is a commercially significant source of seafood. Its production can be affected by various bacterial contaminations. Among which, the genus Vibrio shows a higher prevalence in aquatic organisms, especially M. rosenbergii, causing food-borne illnesses. Vibrio parahaemolyticus, a species of Vibrio is reported as the main causative of the early mortality syndrome. Vibrio parahaemolyticus infection in M. rosenbergii was studied previously in relation to the prawn's differentially expressed immune genes. In the current review, we will discuss the growth conditions for both V. parahaemolyticus and M. rosenbergii and highlight the role of magnesium in common, which need to be fully understood. Till date, there has not been much research on this aspect of magnesium. We postulate a model that screens a magnesium-dependent pathway which probably might take effect in connection with N-acetylglucosamine binding protein and chitin from V. parahaemolyticus and M. rosenbergii, respectively. Further studies on magnesium as an environment for V. parahaemolyticus and M. rosenbergii interaction studies will provide seafood industry with completely new strategies to employ and to avoid seafood related contaminations.

Vibrio parahaemolyticus growth under low-iron conditions and survival under high-magnesium conditions

Since 1996, Vibrio parahaemolyticus serotype O3:K6 and closely related strains have been associated with an increased incidence of V. parahaemolyticus gastroenteritis worldwide, suggesting the emergence of strains with enhanced abilities to cause disease. One hypothesis for the recent emergence of V. parahaemolyticus O3:K6 and related strains is an enhanced capacity for environmental survival relative to other strains, which might result in increased human exposure to these organisms. Therefore, the objective of this study was to test the hypothesis that survival or growth characteristics of clinical V. parahaemolyticus isolates differ from those of nonclinical isolates under different environmental conditions. Twenty-six V. parahaemolyticus isolates selected to represent either clinical or food sources were monitored for either survival following exposure to high magnesium (300 mM) or growth under iron-limited conditions. Isolates in each category (clinical or food) differed widely in survival capabilities following 24 h of exposure to 300 mM Mg2+. Although 4 of 15 clinical isolates grew better at approximately 0.96 microM Fe2+ (iron-limited conditions) than at 50 microM Fe2+ (iron-rich conditions), as an entire group clinical isolates in this study were not more effective at growing under iron-limited conditions than were strains not associated with disease. Within the diverse collection of strains examined in these experiments, neither growth characteristics in low-iron environments nor survival capabilities following exposure to high magnesium concentrations were uniformly different between clinical and nonclinical V. parahaemolyticus isolates. Therefore, neither phenotypic characteristic can be used to reliably differentiate potentially pathogenic V. parahaemolyticus strains.

Epidemiology, pathogenesis, and prevention of foodborne Vibrio parahaemolyticus infections

Since its discovery about 50 years ago, Vibrio parahaemolyticus has been implicated as a major cause of foodborne illness around the globe. V. parahaemolyticus is a natural inhabitant of marine waters. Human infections are most commonly associated with the consumption of raw, undercooked or contaminated shellfish. A few individual V. parahaemolyticus virulence factors, including the thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH), have been investigated in depth, yet a comprehensive understanding of this organism's ability to cause disease remains unclear. Since 1996, serotype O3:K6 strains have been associated with an increased incidence of gastroenteritis in India and in Southeast Asia, and with large-scale foodborne outbreaks in the United States (US). In light of the emerging status of pathogenic V. parahaemolyticus, the US Food and Drug Administration conducted a microbial risk assessment to characterize the risk of contracting V. parahaemolyticus infections from consuming raw oysters. This review summarizes epidemiological findings, discusses recognized and putative V. parahaemolyticus virulence factors and pathogenicity mechanisms, and describes strategies for preventing V. parahaemolyticus infections.

Response of heat-shocked Vibrio parahaemolyticus to subsequent physical and chemical stresses

Vibrio parahaemolyticus foodborne strains 405, 556, and 690 and a V. parahaemolyticus chopping board isolate were heat shocked at 42 degrees C for 15, 30, or 45 min. Heat shock, regardless of heating periods tested, caused an increased demand for NaCl during recovery from heat injury. Further study with strain 690 and the chopping board isolate also revealed that heat shock generally increased the survival of the test organism during subsequent exposure to 47 degrees C, 20 ppm H202, and 8% ethanol and reduced the tolerance of the test organism to low temperatures (5 and -18 degrees C). The extent of the heat shock response of V. parahaemolyticus varied with strain and the duration of treatment. Furthermore, heat shock treatments in the present study caused the leakage of nucleic acids from V. parahaemolyticus cells. This effect was most pronounced with cells heat shocked at 42 degrees C for 45 min.

Thermal injury of Yersinia enterocolitica

Procedures were developed to evaluate thermal injury to three strains of Yersinia enterocolitica (serotypes 0:3, 0:8, and 0:17). Serotype 0:17 (atypical strain) was more sensitive to bile salts no. 3 (BS) and to sublethal heat treatment than the typical strains, 0:3 and 0:8. When the 0:3, 0:8, and 0:17 serotypes were thermally stressed in 0.1 M PO4 buffer, pH 7.0, at 47 degrees C for 70, 60, and 12 min, respectively, greater than 99% of the total viable cell population was injured. Injury was determined by the ability of cells to form colonies on brain heart infusion (BHI) agar, but not on Trypticase soy agar (TSA) plus 0.6% BS for serotypes 0:3 and 0:8 and TSA plus 0.16% BS for 0:17. Heat injury of serotype 0:17 cells for 15 min in 0.1 M PO4 buffer caused an approximate 1,000-fold reduction in cell numbers on selective media as compared with cells heated in pork infusion (PI), BHI broth, and 10% nonfat dry milk (NFDM). The extended lag and resuscitation period in BHI broth was 2.5 times greater for 0:17 cells injured in 0.1 M PO4 than for cells injured in BHI or PI. The rate and extent of repair of Y. enterocolitica 0:17 cells in three recovery media were directly related to the heating menstruum used for injury. The use of metabolic inhibitors demonstrated that ribonucleic acid synthesis was required for repair, whereas deoxyribonucleic, cell wall, and protein synthesis were not necessary for recovery of 0:17 cells injured in 0.1 M PO4 buffer, BHI, or PI. Inhibition of respiration by 2,4-dinitrophenol slowed repair only for 0:17 cells injured in 0.1 M PO4 buffer, not for cells injured in PI or BHI.

Recovery of chill-stressed Vibrio parahaemolyticus from oysters with enrichment broths supplemented with magnesium and iron salts

The effects of magnesium and iron salts on the recovery and growth of chill-stressed cells of Vibrio parahaemolyticus were studied. Supplementation of glucose salt Teepol (GST) broth with 20 to 100 mM of Mg2+ significantly (P less than or equal to 0.05) increased the number of cells recovered from oyster homogenate stored at 3 degrees C. Populations detected with supplemented GST were comparable to those obtained with Horie arabinose ethyl violet (HAE) broth, with or without Mg2+. Recovery of V. parahaemolyticus from homogenates stored at -18 degrees C was also improved when enrichment broths supplemented with Mg2+ were used. Ferric iron (added as FeCl3) at 240 microM in GST and 240 or 960 microM in HAE significantly enhanced the extent of recovery of chilled cells. Ferrous iron was generally less effective. Teepol did not influence the growth of nonchilled cells, but significantly reduced the viable population in suspensions of chilled cells when used at a level of 0.4% in GST. The relatively high pH (9.0) of HAE caused a significant reduction in the number of viable, chill-stressed cells of V. parahaemolyticus. The overall results indicated that HAE broth is superior to GST for recovering V. parahaemolyticus from refrigerated and frozen oyster homogenates.