Induced resistance to infection of lobsters Homarus americanus by Aerococcus viridans (var.) homari, the bacterium causing gaffkemia

Abstract

A vaccine composed of steam sterilized (autoclaved) cells of a virulent strain of Aerococcus viridans (var.) homari was effective in protecting lobsters Homarus americanus against gaffkemia. At 15°C the heat-killed vaccines (HKV) at concentrations between 1 and 5 × 107 particles kg -1 lobster body wt induced maximal protection in induction periods ranging from 7 to 11 d. Protection was substantial over the course of a 30 d post-induction trial period. Spring-caught lobsters (i.e. those more fully rehabilitated following ecdysis) gained more protection (LD50 = 1.9 × 104) from the vaccination than did those caught in the late fall-early winter period (lobsters that were not yet fully recovered from ecdysis) (LD50 = 3.2 × 103). The protection offered by the HK vaccine was comparable to that induced by a vaccine produced by incubating the pathogen with low concentrations (2 μg ml-1) of the antibiotic vancomycin. The bacterins produced by both methods exhibited similar new properties: (1) agglutination at low titres by lobster hemolymph serum, suggesting an impaired capsule layer, and (2) increased permeability to the large Alcian Blue molecule. With both vaccines, the protection may be a direct result of increased exposure to intact bacterial cell structures by the lobster defences, an exposure which otherwise would be prevented by an intact capsule.