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Effect of plasmid carriage on the virulence of staphylococcus aureus
Abstract:
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.
Insights
Staphylococcus aureus plasmid carriage alters bacterial growth and significantly reduces virulence, suggesting plasmids may contribute to decreased staphylococcal sepsis incidence.
Area of Science:
- Microbiology
- Molecular Biology
- Infectious Diseases
Background:
- Plasmids are extrachromosomal DNA elements frequently found in bacteria.
- Plasmids can confer various traits to host bacteria, including antibiotic resistance and altered virulence.
- The impact of plasmid carriage on Staphylococcus aureus growth and virulence is not fully understood.
Purpose of the Study:
- To investigate the effects of plasmid carriage on the growth kinetics and virulence of Staphylococcus aureus.
- To determine if plasmid-mediated changes in virulence persist after plasmid loss.
- To explore potential mechanisms underlying plasmid-induced virulence reduction.
Main Methods:
- Staphylococcus aureus strain 649 was engineered to harbor different plasmids, creating strain 649MR.
- Growth kinetics, including lag period and exponential doubling time, were measured.
- Virulence was assessed using a 10-day chick embryo model.
- Soluble product formation and cell surface characteristics were analyzed.
Main Results:
- Plasmids altered growth kinetics, increasing exponential doubling time (8-25%) and modifying lag phase duration.
- Strain 649MR exhibited significantly reduced virulence in chick embryos compared to plasmid-negative strain 649N.
- Reduced virulence persisted even after plasmid loss.
- Tetracycline resistance plasmids moderately reduced virulence, while chromosomal resistance had minimal effect.
- Virulence decrease was not linked to reduced soluble products but possibly to cell surface alterations.
Conclusions:
- Plasmid carriage in Staphylococcus aureus affects bacterial growth and significantly attenuates virulence.
- The observed avirulence can be a persistent trait, independent of continued plasmid presence.
- Plasmid-mediated virulence reduction may involve cell surface modifications.
- Loss of virulence due to plasmid carriage could contribute to a lower incidence of staphylococcal sepsis.