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Altered penicillin-binding proteins in methicillin-resistant strains of Staphylococcus aureus
Abstract:
The penicillin-binding proteins (PBPs) of a methicillin-resistant (MR) and a methicillin-susceptible (MS) Staphylococcus aureus were compared by various approaches involving the use of high-specific-activity [3H]penicillin as a reagent. The MR and MS strains were found to contain PBPs of the same number and electrophoretic mobilities. However, saturation of PBPs 1, 2, and 3 by methicillin in the MR strain required the use of several thousands of micrograms of antibiotic per milliliter, whereas 0.2 to 0.4 micrograms of methicillin per ml was sufficient to effectively compete with [3H]penicillin for the PBPs for the MS strain. Additional experiments indicate that these differences most likely reflect a greatly decreased affinity of the PBPs of the MR strain as compared to those of the MS strain. Shift of the pH of the culture medium of the MR strain from pH 7.0 to 5.2 resulted in an immediate drop in phenotypic resistance to methicillin (from a minimal inhibitory concentration value of 3,200 micrograms/ml at pH 7.0 to 0.8 microgram/ml at pH 5.2). Examination of the methicillin affinities of PBPs in MR bacteria grown at pH 5.2 showed the presence of the same low-affinity PBPs as in bacteria grown at pH 7.0. Thus, the pH-dependent resensitization to methicillin cannot be explained by a parallel increase in the antibiotic affinities of the PBPs.
Insights
Methicillin-resistant Staphylococcus aureus (MRSA) has penicillin-binding proteins (PBPs) with significantly lower affinity for methicillin compared to susceptible strains. Lowering the culture pH reduces MRSA resistance without altering PBP affinity.
Area of Science:
- Microbiology
- Molecular Biology
- Antibiotic Resistance
Background:
- Methicillin resistance in Staphylococcus aureus is a major public health concern.
- Penicillin-binding proteins (PBPs) are the primary targets of beta-lactam antibiotics like methicillin.
- Understanding the molecular basis of methicillin resistance is crucial for developing effective treatments.
Purpose of the Study:
- To compare the penicillin-binding proteins (PBPs) of methicillin-resistant (MR) and methicillin-susceptible (MS) Staphylococcus aureus.
- To investigate the mechanism underlying the pH-dependent modulation of methicillin resistance in MRSA.
Main Methods:
- Comparative analysis of PBPs from MR and MS Staphylococcus aureus strains using high-specific-activity [3H]penicillin.
- Determination of methicillin saturation levels and PBP affinities in both strains.
- Assessment of phenotypic methicillin resistance at different culture pH levels.
Main Results:
- MR and MS strains possess PBPs with similar numbers and electrophoretic mobilities.
- MRSA PBPs exhibit significantly decreased affinity for methicillin compared to those in MSSA.
- Lowering the culture pH from 7.0 to 5.2 drastically reduces the minimal inhibitory concentration (MIC) of methicillin for MRSA.
- This pH-induced resensitization to methicillin occurs without a corresponding increase in PBP affinity for the antibiotic.
Conclusions:
- The reduced affinity of PBPs for methicillin is a key factor in methicillin resistance in Staphylococcus aureus.
- Phenotypic methicillin resistance in MRSA can be modulated by environmental factors such as pH.
- The pH-dependent resensitization mechanism is independent of changes in PBP-methicillin binding affinity, suggesting alternative regulatory pathways.