We tested the possible pathogenicity of 27 bacteria that are budding or prosthecate or both in mice and guinea pigs. The strains tested belong to the following taxa: (12 strains), “” (4 strains), (2 strains), (2 strains), (1 strain), “” (1 strain), (2 strains), (1 strain), and (1 strain); 1 strain was unnamed. Mice injected intraveneously with a 0.5-ml suspension containing up to 10 colony-forming units per ml survived for 7 days without macroscopic changes, abscesses, or lesions. Four , one “” strain, and the unnamed strain caused slight spleen enlargement and in some cases a mottled kidney appearance; however, the organs were histologically normal, and the changes observed were considered to be signs of host reaction rather than pathological changes. Subcutaneous injections also did not result in disease. Two strains could be reisolated from mice after 21 days; none of the other strains could be recovered after 7 days. Lethal endotoxic activity was not detected with any of the strains when cell sonic extracts were injected intraveneously. All strains caused antibody formation, and agglutination dilution titers ranged from 1:2 to 1:64. Serological cross-reactions with six strains indicated the presence of distinct taxonomic groups. Hyphomicrobia from soil cross-reacted with each other but not with hyphomicrobia from aquatic sources. Immunosuppressive pretreatment with Natulan did not render the mice more susceptible. Other possible indicators of pathogenicity (i.e., mannitol utilization and β-hemolysis) were not observed in the 27 strains. We concluded that the 27 bacteria tested are nonpathogenic for mice or guinea pigs under the conditions used.


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