Vibrio parahaemolyticus is a curved, rod-shaped Gram-negative bacterium. It is non-spore forming and has a high motility rate due to its polar flagellum. Through a mechanism known as swarming, these microorganisms migrate across semi-solid surfaces [1] with the help of their lateral flagella. Across the world, inshore marine waters are densely populated with V. parahaemolyticus which is particularly common in estuarine marine water. Research has shown that V. parahaemolyticus is seasonal [2] and thrives well in warmer conditions. For example, the bacteria could not be detected during the winter (November–March) in Chesapeake Bay seawater [2]. On the other hand, V. parahaemolyticus begins to multiply when there is an increase in temperature [2]. This could be a result of the microorganism somehow being reintroduced into the sea water or its emergence from marine sediments in which it could have survived throughout the winter.
Temperatures ranging from 35 to 39 °C [3] are the optimal conditions for the growth of V. parahaemolyticus. It has a generation time of less than 20 min, although it can double in as little as 5 min [4] under certain conditions. As a result, V. parahaemolyticus is most commonly observed in the warm season as a mesophilic bacterium causing food-borne outbreaks which peak in summer [5, 6], the levels of V. parahaemolyticus found in freshly harvested seafood tend to be rather lower than the predicted infection doses [7]. However, the ability of the bacterium to multiply very rapidly at suitable temperatures means that its presence in food is often enough to cause disease.
Vibrio parahaemolyticus has one very important requirement to live and multiply that is salinity. V. parahaemolyticus typically encounters salinity concentrations in the marine environment ranging between 0.8 and 3 % [8]. It requires optimal salinity levels between 1 and 3 %, but laboratory studies have shown that V. parahaemolyticus can thrive in between 0.5 and 10 % sodium chloride concentrations.
Vibrio parahaemolyticus isolates were found to survive even in 300 mM magnesium (e.g. in severely polluted coastal waters in some parts of India)—a level considered toxic to many other microorganisms [9]. It’s survival under such wide-ranging conditions may be due to its ability to utilize magnesium. A 5.5 kb plasmid is said to carry the genes responsible for the bacterium’s high resistance to high magnesium concentrations [9]. Injured or thermally treated V. parahaemolyticus cells display an increased uptake of magnesium suggesting a possible increased requirement for magnesium not only for the stability and repair [10] of the ribosome, but also of the cell membrane. To sum up, V. parahaemolyticus’s ability to survive in high concentrations of magnesium or other metal ions allows it to out-compete other basic seawater flora in terms of survival and growth in such drastic environmental conditions.
The giant freshwater prawn Macrobrachium rosenbergii is a freshwater aquatic organism. The optimal temperature range for Macrobrachium rosenbergii larvae to survive is 28 to 31 °C. Observations have shown that a salinity of 10 parts per thousand (ppt) is ideal for freshwater prawn hatcheries (http://www.fao.org/docrep/005/y4100e/y4100e04.htm#P193_35649). While calcium is important for the formation of the prawn exoskeleton (http://www.thefishsite.com/articles/464/moulting-and-behaviour-changes-in-freshwater-prawn), the crucial element for this species is a favourable condition for the survival of its larvae.
Various reports have suggested that magnesium is an important component of the environment for prawn survival particularly for juvenile prawns [11]. A recent article [12] describing the effects of salinity through the use of artificial sea water clearly explains the importance of magnesium in the survival amounts of post larvae. Taking an example, the effect of an environment that is acidic due to the presence of aluminium could not hinder the survival stages of post larvae due to the presence of increased levels of magnesium ions (Mg2+) [13]. The characteristics of water which are good for prawn hatcheries are said to be 10–27 parts per million (ppm) of magnesium in fresh water, 1250–1345 ppm Mg in seawater and 460–540 ppm Mg in brackish water (http://www.fao.org/docrep/005/y4100e/y4100e04.htm#P193_35649). These features and conditions show how important magnesium ion is for the survival of larvae which undergo a very critical moulting stage before reaching the post-larval stage.
Most Vibrio species have adapted to aquatic organisms and caused severe infections on consumption by humans. V. parahaemolyticus has several virulence, pathogenicity and antibiotic resistance factors which show that it can survive well in aquatic organisms, especially the giant freshwater prawn, M. rosenbergii [14].
Detailed studies of the growth conditions of M. rosenbergii in the environment can help us to correlate the respective levels of adaptability of V. parahaemolyticus to M. rosenbergii. Studies have shown that M. rosenbergii can survive in a range of different media compositions with varying proportions of NaCl, KCl and MgCl2 + MgSO4 [5]. However, the fertilization envelope of shrimp eggs was observed to grow thin when there is a depletion of calcium and magnesium [15]. Early-stage embryos were shown to require optimal levels of medium containing MgCl2 + MgSO4 for their proper development [16].
The role of magnesium ion in the normal hatching rate of larvae has not been shown to be significant [16]. However, the importance of magnesium in survival mechanisms was observed [12] as explained earlier. Perhaps the most interesting similarity of V. parahaemolyticus to prawn is its unusually good tolerance levels to high concentrations of magnesium and its growth capability under iron-limiting conditions—both of which are quite a match to the conditions of prawn larvae survival.
In addition, another important factor is the N-acetyl glucosamine binding protein (GbpA) reported in Vibrio cholerae [17, 18] to have the property to bind to epithelial cell surfaces and chitin of the host surface. An in vitro study in 1996 presented how cell associated N-acetyl D-glucosamine specific haemagglutinin of Vibrio cholerae O1 showed adhesive characteristics to the rabbit intestinal epithelial cells [19]. In 2008, gbpA gene of V. cholerae was studied in specific with mucin for its co-operative levels of gene expression ultimately giving way to intestinal colonization and infection by the bacterium [20]. In infant mouse models it was observed that a deletion in the adhesion gbpA portrayed a deficit in the intestinal colonization [21, 22]. The importance of gbpA in the intestinal colonization of V. cholerae was reported by a study along with several other colonizing factors [23]. Our study aims at checking the levels of bacterial gbpA gene expression in the presence of the host carapace and commercial chitin at different magnesium environment concentrations. This study could help researchers to consider environment as an indispensable factor in host-pathogen studies, not only in seafood industries, but even in omics studies.