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 ,ug of methicillin per ml was sufficient to effectively compete with [3H]penicillin for the PBPs of 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 /Lg/ml at pH 7.0 to 0.8 ,ug/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.Extensive studies on the nature of methicillin resistance in Staphylococcus aureus have led to the recognition of several pleimorphic properties in these mutants. These include reduced susceptibility to lysostaphin (17), deficiency in protein A (23), and a change in the net surface charge of the cells (9). In addition, variation in several growth parameters was found to have profound influence on the expression of methicillin resistance. Specifically, growth of resistant mutants at low pH (16,20) or in the presence of ethylenediaminetetraacetate (19) was shown to result in suppression of methicillin resistance. Furthermore, it was reported that, with at least some of the methicillin-resistant isolates, bacterial cultures appeared to be unable to fully express resistance if the culture was passed in drug-free medium and subsequently plated on the surface of agar plates containing methicillin. Addition of certain supplements to the agar such as high concentrations of sucrose or sodium chloride, or incubation of the plates at temperatures suboptimal for growth (30°C instead of 37°C), resulted in the recovery of an increased proportion of the population behaving as phenotypically methicillin-resistant bacteria (1, and for review 16). Serial passage in the presence of beta-lactams had the same effect (18), and the term "heterogeneity" was used to refer to this phenomenon.Biochemical studies on amino acid, amino sugar, and teichoic acid composition (24) failed to detect significant chemical differences between the methicillin-susceptible (MS) and -resistant (MR) staphylococci. These studies have led to an extensive characterization of the physiology of MR staphylococci. Nevertheless, the biochemical bas...

Antibiotic Resistance in the Environment, with Particular Reference to MRSA

“…Resistance is encoded by the mecA gene (Ito et al, 1999), mecA encodes an attenuated penicillin binding protein (PBP2' or PBP2a) which has a lower affinity for penicillin and other β-lactams than the innate PBP's (Hartman and Tomasz, 1981), hence interfering with antimicrobial activity. There are 6 basic described SCCmec types; additional resistance genes may be present or absent depending on the type.…”

Methicillin-resistant staphylococci

“…Homogeneous strains may appear fully susceptible to beta-lactam antibiotics under these conditions (79,80).…”

Occurrence of a beta-lactam-inducible penicillin-binding protein in methicillin-resistant staphylococci

“…In MRS, production of altered PBPs with extremely low affinity for ,3-lactams (14,16,17) or an increase of an altered PBP-3 (6) has been described. However, the double-zone phenomenon occasionally detected on a disk diffusion susceptibility test of P-lactams, such as methicillin, nafcillin (24), or imipenem (4), for MRS cannot be explained by the decreased affinities of PBPs for ,-lactams.…”