Relationship between Agglutinative Properties of Aeromonas salmonicida Strains Isolated from Fish in Japan and Their Resistance to Mechanisms of Host Defense

Abstract

Phenotypic variation in spontaneous agglutination of Aeromonas salmonicida was investigated using 114 strains isolated from naturally epizootics of furunculosis in salmonid fish in Japan from 1967 through 1982. These strains were categorized into three different types: an agglutinating (AG) type which produced cell flocks resulting in precipitation and loss of turbidity in cell suspensions; a nonagglutinating (NAG) type, representing no loss in turbidity; and an intermediate (IM), weakly autoagglutinating type. Based on the occurrence of the AG, NAG and IM types determined in 1978 and again 1982, it was shown that a transition occurs progressing from the AG to the NAG via the IM type. The strains involved in the AG and NAG types were virulent and avirulent, respectively, based on LD₅₀ determination in rainbow trout (Salmo gairdneri). The NAG type was much less susceptible than the AG to the bactericidal activity of fresh normal rainbow trout serum, to immune bacteriolysis and to phagocytosis by cells from pertioneal exudate. When growth properties of AG and NAG types were compared under mixed culture conditions, the NAG type predominated in the culture broth 2-5 days after inoculation. In contrast, when rainbow trout were inoculated intraperitoneally with cells of both types, the NAG-type cells were selectively eliminated from the kidneys within a period of a few days. The AG-type cells, however, greatly proliferated in the kidneys and brought about clinical furunculosis. These results indicate that the AG type possesses a capacity to escape the defense mechanisms of the host by resisting the bactericidal activity of serum and phagocytosis by leucocytes.