Morphological changes of Aeromonas hydrophila in response to osmotic stress

The adaptive response of bacteria to stressful environmental situations may lead to a modification of physiological and phenotypical characteristics, including morphology. The aim of this study was the analysis of the ultrastructural changes in Aeromonas hydrophila exposed to different NaCl concentrations (1.7%, 3.4%, 6%) at 4 and 24 8C for 188 days. Bacterial cultures were processed for scanning and transmission electron microscopy, and specimens were analysed at different times during osmotic stress. SEM reveals the presence of three predominant morphotypes: rod, filamentous and spherical forms, depending on the time and culture conditions. Normal rod cells prevail in 1.7% NaCl growth conditions, maintaining high rates until the end of the trial at 4 8C. The most favourable conditions for the elongated morphotype are 3.4% NaCl at 4 8C. Spherical forms appear later, increase with time and are the prevalent population at the end of the trial at 24 8C, in all culture conditions. TEM reveals the presence of normal, necrotic-like and apoptotic-like forms; these latter forms increase with time according to salt concentration and temperature. Initially, a detachment of the external membrane appears, with cytoplasmic clumping into small, dense masses; as the process continues, both these features become more evident with increasing salt concentrations. This behaviour has been compared to that of eukaryotic cells undergoing growth factor deprivation-induced apoptosis. Occasionally, surface blebs are observed. In conclusion, the study suggests that the exposure of A. hydrophila to stressful conditions (osmolarity, temperature and nutrients) leads to the generation of varying morphotypes, which promote cell survival in adverse conditions and a rapid repopulation in post-stress environments.