SUMMARY: Mesophilic strains of the genus were found to cross-agglutinate extensively, and much of this appears to be due to common rough somatic antigens. The psychrophilic strains also cross-agglutinated to some extent. There was little cross-agglutination between mesophils and psychrophils. Neither mesophils nor psychrophils showed clear-cut antigenic subgroups. Many mesophilic strains, whether isolated from naturally-occurring cases of infection or from water, were found to be virulent for experimental animals. The most virulent were strains from two cases of human infection, which had an LD50 dose for guinea-pigs of . 5 x 10 viable organisms. The virulent strains did not form a homogeneous antigenic group. Cultures of mesophils contained an endotoxin, but no exotoxin was found. The experimental disease may be an acute, rapidly fatal septicaemia, or a more chronic disease with multiple abscess formation like that found in natural infections; occasionally a local abscess with subsequent recovery was the only result of the injection of cultures.


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