Loss of virulence during culture of Aeromonas salmonicida at high temperature

Abstract

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.

A bath challenge model for furunculosis in rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L.

“…The challenge stain was characterized and compared to an avirulent strain (1102). Autoagglutination, haemagglutination, serum resistance and adhesion (propereties displayed by strain 184/86) have been attributed to the presence of the A-layer by several workers (Evenberg, Van Boxtel, Lugtenberg, Schurer, Blommaert & Bootsma 1982;Evenberg & Lugtenberg 1982;Ishiguro, Kay, Ainsworth, Chamberlain, Austen, Buckley & Trust 1981;Kay, Buckley, Ishiguro, Phipps, Monete & Trust 1981;Munn, Ishiguro, Kay & Trust 1982;Trust, Canotice & Atkinson 1980;Udey & Fryer 1978). The A layer has been shown to be hydrophobic in nature (Phipps, Trust, Ishiguro & Kay 1983) and Parker & Munn (1984) demonstrated that Aeromonas salmonicida possessing the A-protein were more hydrophobic than strains devoid of this protein.…”

Aeromonas salmonicida: determination of an antigen associated with protective immunity and evaluation of an experimental bacterin

“…Fuller, Pilcher & Fryer (1977) described an extracellular leucocytolytic factor isolated from virulent strains of A. salmonicida, and an extracellular protein which was immunogenic and afforded protection was described by Cipriano (1982). However, the rough-smooth transformation of A. salmonicida readily occurs on certain artificial media (Ishiguro, Kay, Ainsworth, Chamberlain, Austen, Buckley & Trust 1981;Evenberg, Van Boxtel, Lugtenberg, Schurer, Blommaert & Bootsma 1982). The rough variant, which autoagglutinates in saline, is highly virulent and possesses an additional outer layer on the cells (Udey & Fryer 1978).…”

Development and Application of a Quantitative Real‐time Polymerase Chain Reaction Assay for the Detection of Aeromonas salmonicida

“…This gene encodes the subunit protein (A-protein) of the paracrystalline surface array (A-layer) produced by this organism. The A-layer enhances the virulence of A. salmonicida (Ishiguro et al 1981) and protects A. salmonicida from the bactericidal activity of both immune and nonimmune sera (Munn et al 1982). Because vapA gene is a virulence factor of A. salmonicida, PCR analysis with oligonucleotide primers directed at vapA can be used to provide information on the potential virulence of this strain.…”