Systematic assessment of chlorine tolerance mechanism in a potent biofilm-forming marine bacterium Halomonas boliviensis

Bacterial colonization on submerged surfaces is a fundamental phenomenon associated with biofouling in the marine environment. To control such bio-growth, chlorination is the most commonly practiced strategy in maritime activities. Although chlorination is found to have wide-spectrum biocidal efficacy, it is unsuccessful against some microorganisms. Chlorine-resistant marine bacteria have attracted much attention as their existence enhances microfouling progression and could be a threat to industries that use seawater for cooling purposes. In this study, a chlorine-resistant bacterium that was isolated from the early-stage marine biofilm was in vitro investigated for biofilm-forming potential and chlorine tolerance. The bacterial isolate was identified phylogenetically as Halomonas boliviensis and its chlorine tolerance response was compared with a chlorine-sensitive bacterium (Pseudoalteromonas espejiana), which is taken as reference bacterium. Experiments on chlorine exposure revealed that H. boliviensis could mount an unusual response against chlorine and therefore even at long-term chlorine exposure (for 4–8 h) at 8 mg l−1 of residual chlorine concentration, elicited mere ≤ 1-fold reduction in the viable count. Chlorine-resistant mechanisms of H. boliviensis was ascertained by analyzing cellular morphology, cell membrane destruction, extracellular polymeric substances (EPS), aggregation ability and superoxide dismutase (SOD) pattern. In contrast to P. espejiana, over-production in biosynthesis of capsular EPS, induction in aggregation ability and increase in SOD level were observed and these could contribute to chlorine tolerance stratagem in H. boliviensis. Microscopic analyes revealed that both isolates form strong biofilms on various artificial surfaces. Scientific insight into such contrasting studies on marine bacteria is rather infrequent as well as important from environment perspective and would help in designing efficient antifouling strategy.