Ecological Significance of Luminescence in Vibrio cholerae: Occurrence, Structure, Expression, and Function

Abstract

Several Vibrio species are bioluminescent, including Vibrio cholerae. Analysis of 224 non-pathogenic V. cholerae isolates collected from the Chesapeake Bay, MD, revealed that 52% were luminescent, and 58% of the isolates harbor the luxA gene. A significant association of luxA to the gene stn (r = 0.40) was observed and luminescent strains were found to have a significant association with sample fraction and time of sampling, especially from the observed interaction of these two traits. In contrast, 334 non-pathogenic V. cholerae strains isolated from two rural provinces in Bangladesh, yielded 21 luminescent (6.3%) and 35 luxA+ (10.5%) isolates. None (0%) of 48 laboratory reference pathogenic strains from various geographic locations or 222 environmental and clinical isolated strains of V. cholerae O1 or O139 from Bangladesh were luminescent or harbored the lux operon. To improve success of isolation of V. cholerae from environmental samples, two colony blot hybridization methods were developed. Specificity of two probes was confirmed, using laboratory reference strains, in addition to environmental and clinical isolates, and sensitivity of the probes was confirmed using water samples into which V. cholerae had been inoculated. The lux operon of V. cholerae was sequenced and its chromosomal location determined. The operon organization is most similar to that of Shewanella hanedai and the non-luxF Photobacterium leiognathi. Sequence analysis revealed that the V. cholerae lux operon is most similar in its genomic sequence to V. harveyi and Photorhadbus luminescens and it most likely originated from a common Vibrionaceae ancestor. Using a new bioassay method that measures expression level, several classes of defective luminescent V. cholerae were identified and characterized, including one class previously termed dark, or K variants, in V. harveyi. Multiple causes of the defects were identified, indicating several levels of luminescence control in V. cholerae, in addition to autoinduction and lux repression. Using luxA mutants, luminescence was implicated in conveying competitive advantage in growth under microaerophilic conditions, DNA repair by photoreactivation, and neutralization of reactive oxidative species. These results demonstrate that bioluminescence is a frequently occurring trait in non-pathogenic V. cholerae, the expression of which gives a selective advantage in specific habitats.