PLEOMORPHISM IN THE OPPORTUNISTIC FISH PATHOGEN Aeromonas hydrophila FOLLOWING OSMOTIC CHANGES IN GROWTH CONDITIONS

Aeromonas hydrophila, a freshwater, Gram-negative, motile, rod-shaped bacterium ubiquitous in the aquatic environments throughout the world, is the causative agent of motile Aeromonas septicemia (MAS) in fish. Virulent A. hydrophila has severely impacted catfish farming in the southeastern United States for over a decade. Despite the extensive economic losses that this pathogen inflicts on the aquaculture industry, its ecological attributes are poorly understood. Analysis of farm-level risk factors associated with MAS outbreaks in farmed catfish in Alabama indicated that the use of salt (sodium chloride, NaCl) significantly lowers the odds of A. hydrophila infections. Exposure of bacteria to stressful environmental situations such as high salinity may lead to modification of physiological and phenotypical characteristics. Sodium chloride can inhibit bacterial growth and/or attachment and have beneficial effects on the fish host health.

The objective of this study was to describe the adaptive morphological and structural changes that A. hydrophila cells undergo in response to different salinity conditions. Multiple strains of this pathogen were grown in tryptic soy broth medium with varying salt concentrations (5, 15, and 45 g/L) at 28 ? and monitored for 14 days. Specimens of bacterial cultures were processed for light and scanning electron microscopy at different times during osmotic stress. Analysis of stressed A. hydrophila cells revealed the presence of filamentous morphotypes under the highest (45 g/L) salt concentration. The length of some elongated cells exceeded 15 μm. However, normal short rod cells ranging from 1.0 to 3.0 μm in length prevailed in cultures with 5 and 15 g/L NaCl. This study suggests that exposure of A. hydrophila to osmolarity stressful conditions leads to generation of an elongated morphotype, which allow bacterial cells to cope with the adverse conditions, prolong survival, and repopulate in post-stress favorable environments. Moreover, viability, culturability, biofilm formation, and the virulence potential of A. hydrophila cells were assessed throughout the study and the results will be presented. The findings of this study would help understand the mechanism of A. hydrophila survival in catfish ponds and facilitate research on prevention and control of the recurring MAS outbreaks.