Loss of virulence during culture of Aeromonas salmonicida at high temperature.

The effect of growth temperature on the loss of virulence of the fish pathogen Aeromonas salmonicida was investigated. Three virulent strains were grown in Trypticase soy broth at temperatures ranging from 22 to 30 degrees C. Growth at a higher-than-optimal temperature (26 to 27 degrees C for the three strains studied) resulted in the selection of spontaneous attenuated derivatives in the initial bacterial population. For example, virulent bacteria represented less than 10% of the population of a culture grown at 30 degrees C, and attenuated derivatives were easily isolated by streaking the culture on solid medium and picking single colonies. Virulent strains autoaggregated during growth and possessed a cell wall layer (A-layer) external to the outer membrane, as previously described. Attenuated strains did not autoaggregate and did not possess the A-layer. The A-layer apparently shielded bacteriophage receptors and a mannose-specific yeast agglutinin located in the outer membrane. Thus, virulent strains exhibited impaired adsorption of phages, whereas attenuated strains were phage sensitive. Furthermore, attenuated strains agglutinated yeast cells but virulent strains did not. The attenuated strains had higher maximum growth temperatures than their virulent parent strains, and this accounts for their selection at high temperatures. It is proposed that the A-layer contributes significantly to the physical properties of the A. salmonicida cell envelope and that these physical properties of the A. salmonicida cell envelope and that these physical change upon loss of the A-layer to permit growth at a higher-than-usual temperature.