Effect of NaCl and nafcillin on penicillin-binding protein 2a and heterogeneous expression of methicillin resistance in Staphylococcus aureus
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1Medical Service, San Francisco General Hospital, California 94110.
Abstract:
Expression of methicillin resistance in heterogeneous strains of Staphylococcus aureus is enhanced by 2 to 5% NaCl in the medium and by selection with beta-lactam antibiotics. Resistance is associated with production of a penicillin-binding protein (PBP), PBP 2a, with low affinity for binding beta-lactam antibiotics. Therefore, the effects of NaCl and nafcillin on amounts of PBP 2a produced and its binding affinity were examined and correlated with expression of resistance. Nafcillin-triggered autolysis also was examined. No relationships between the level of resistance expressed and (i) relative amounts of PBP 2a, (ii) inducibility of PBP 2a by nafcillin, or (iii) binding affinity of nafcillin for PBP 2a were found. A protective effect of NaCl for the susceptible subpopulation, corresponding to inhibition of autolysis, was observed for heterogeneous strains. Even in the absence of NaCl, highly resistant cells were relatively tolerant to nafcillin-triggered autolysis. These results support the hypothesis that high levels of resistance require an additional factor besides PBP 2a. This factor may act within the autolytic pathway.
Insights
High salt concentrations and beta-lactam antibiotics enhance methicillin resistance in Staphylococcus aureus. However, the study found no direct link between penicillin-binding protein 2a (PBP 2a) levels and resistance, suggesting other factors are involved.
Area of Science:
- Microbiology
- Molecular Biology
- Antibiotic Resistance
Background:
- Methicillin resistance in Staphylococcus aureus (MRSA) is a significant clinical concern.
- Heterogeneous MRSA strains exhibit variable resistance levels.
- Penicillin-binding protein 2a (PBP 2a) is a key determinant of beta-lactam resistance.
Purpose of the Study:
- To investigate the influence of NaCl and nafcillin on PBP 2a expression and binding affinity in MRSA.
- To correlate these factors with the expression of methicillin resistance.
- To examine the role of autolysis in nafcillin-induced resistance.
Main Methods:
- Culturing heterogeneous MRSA strains in media with varying NaCl concentrations (0-5%).
- Exposure to nafcillin to assess resistance induction.
- Quantification of PBP 2a levels and determination of nafcillin binding affinity.
- Analysis of nafcillin-triggered autolysis in susceptible and resistant subpopulations.
Main Results:
- NaCl (2-5%) and beta-lactam antibiotics enhance MRSA expression.
- No correlation found between PBP 2a levels, inducibility, or binding affinity and the level of resistance.
- NaCl demonstrated a protective effect against autolysis in susceptible subpopulations.
- Highly resistant cells showed tolerance to nafcillin-induced autolysis, even without NaCl.
Conclusions:
- PBP 2a alone does not fully explain high-level methicillin resistance in MRSA.
- An additional factor, potentially involved in the autolytic pathway, is likely required for high-level resistance.
- NaCl may protect susceptible MRSA subpopulations by inhibiting autolysis.