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

Suma Tiruvayipati, Subha Bhassu

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. 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. 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. 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.