The possession of any of eight different plasmids by Staphylococcus aurew strain 649-either singly or simultaneously (in no. 649MR)-caused changes in growth kinetics. Six of the plasmids caused an increase in exponential doubling time (by 8-25%), and most also altered the duration of the lag period. Strain 649MR was significantly less virulent for 10-day chick embryos than the corresponding plasmid-negative culture (no. 649N). The avirulence persisted even after loss of the plasmids from no. 649MR. The presence of a single plasmid specifying tetracycline resistance produced a moderate reduction in virulence, but chromosomal tetracycline resistance had an insignificant effect on it. The decrease in virulence could not be attributed to reduced formation of soluble products. It probably resulted from alterations in the cell surface, but membrane-polypeptide profiles of virulent and avirulent cells lacking plasmids were similar. Survival of strains 649MR and 649N on glass was identical. Therefore, reduction in the incidence of staphylococcal sepsis may be due in part to loss of virulence that has resulted from plasmid carriage.


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