The Journal of Medical Microbiology, Vol 8, Issue 1 137-147, Copyright © 1975 by Society for General Microbiology
Effect of plasmid carriage on the virulence of staphylococcus aureus
R. W. Lacey and I. Chopra
The possession of any of eight different plasmids by Staphylococcus aureus
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.
