Methicillin resistance in Staphylococcus epidermidis

Abstract

The penicillin‐binding proteins (PBP) of a methicillin‐resistant strain of Staphylococcus epidermidis, 100604 p⁺ m⁺ and a non‐isogenic sensitive strain, p⁻m⁻ were characterised. The presence of a novel PBP, produced by the methicillin‐resistant strain of S. epidermidis, with an M_r identical to that of PBP2′ in Staphylococcus aureus 13136 p⁻m⁺, was revealed by sodium dodecyl sulphate/polyacrylamide gel electrophoresis and subsequent fluorography of solubilised membrane proteins isolated from cells labelled with [³H]benzylpenicillin. This novel PBP was only detected in cells which had been grown at 30°C, in media containing β‐lactam antibiotic and 5% NaCl. The sensitivity of an attachment transpeptidation reaction measured under non‐growing conditions in the sensitive and resistant strains indicated that the novel PBP catalysed this reaction. The similarity of radiolabelled peptides resulting from partial proteolytic digestion of the novel PBP in S. epidermidis 100604 p⁺m⁺ and from PBP2′ in S. aureus 13136 p⁺m⁺ lends support to the theory that the additional DNA encoding PBP2′ in S. aureus and the same protein in S. epidermidis has been passed to both species from an unknown source. Studies of the development and loss of resistance of attachment transpeptidase activity, and the appearance and disappearance of the novel protein when cultures of the resistant strain were transferred from conditions allowing the expression of resistance to those not allowing such expression and vice‐versa, indicated that there was a strong correlation between the presence of PBP2′ and the degree of resistance of the attachment transpeptidation reaction and that the production of this protein was affected by temperature at a regulatory or genetic level. Studies on the induction and loss of β‐lactamase activity and of the novel PBP when the resistant strain was grown in the presence or absence of β‐lactam antibiotics at either 40°C or 30°C suggests that there is little relationship between the production of this enzyme and of PBP2′ other than the fact that β‐lactam antibiotics are common inducers of both.