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Methicillin-resistant strains of Staphylococcus aureus: relation between expression of resistance and phagocytosis by
Abstract:
Virulent strains of staphylococci are known to resist phagocytic destruction better than avirulent strains. In this context, in vitro elimination by human polymorphonuclear leukocytes of eight methicillin-resistant strains of Staphylococcus aureus of unknown virulence was studied. After incubation for 1, 3, 5, or 24 hr in a modified phagocytic assay, the methicillin-resistant strains survived as well as other virulent but methicillin-sensitive strains of S. aureus. Highly resistant subpopulations were obtained from three parent strains, and a methicillin-sensitive revertant subpopulation from one resistant parent strain. All subpopulations were eliminated to the same extent as were the moderately resistant parent strains. When the phagocytic assay was performed in the presence of 25 microgram of methicillin/ml, only the methicillin-sensitive strains and the sensitive subpopulation derived from one resistant parent strain were eliminated after incubation for 24 hr. These in vitro data are further evidence against the use of methicillin in infections due to these organisms.
Insights
Methicillin-resistant Staphylococcus aureus (MRSA) strains showed resistance to human immune cell destruction. The antibiotic methicillin was ineffective against these resistant strains in vitro.
Area of Science:
- Microbiology
- Immunology
- Infectious Diseases
Background:
- Virulent staphylococci strains exhibit enhanced resistance to phagocytic destruction compared to avirulent strains.
- Methicillin-resistant Staphylococcus aureus (MRSA) poses a significant public health threat due to its resistance to antibiotics.
Purpose of the Study:
- To investigate the in vitro elimination of eight MRSA strains by human polymorphonuclear leukocytes (PMNs).
- To assess the impact of methicillin on the phagocytic clearance of MRSA strains and their subpopulations.
Main Methods:
- A modified phagocytic assay was employed to evaluate the survival of MRSA strains after incubation with human PMNs for varying durations (1, 3, 5, and 24 hours).
- Resistant and sensitive subpopulations of MRSA were generated and tested in the phagocytic assay, with and without the presence of methicillin.
Main Results:
- MRSA strains demonstrated comparable survival rates to virulent methicillin-sensitive Staphylococcus aureus strains when incubated with PMNs.
- No significant difference in elimination was observed between parent MRSA strains and their highly resistant or sensitive subpopulations.
- Methicillin treatment (25 µg/ml) only facilitated the elimination of methicillin-sensitive strains and sensitive subpopulations, not the resistant MRSA strains.
Conclusions:
- MRSA strains exhibit significant resistance to phagocytic destruction by human PMNs in vitro.
- The antibiotic methicillin is ineffective in enhancing the clearance of MRSA by PMNs.
- These findings challenge the utility of methicillin for treating infections caused by these MRSA strains.