Mechanisms of resistance to beta-lactam antibiotics in Staphylococcus aureus.

In vitro, Staphylococcus aureus expresses resistance to beta-lactam antibiotics by beta-lactamase production, methicillin resistance, penicillin tolerance, and deficiency in penicillin binding protein number 3. The clinical significance of all these phenomena is not fully defined. Methicillin resistance is due to a large chromosomal linkage group. The reason for the very high proportion of cultures that produce beta-lactamase is still enigmatic. Selection pressure by penicillins may not be the entire explanation. Plasmids determining beta-lactamase have spread between cultures in vivo. Staphylococcal plasmids can transfer between cells in vitro by transduction, transformation and conjugation. The latter is likely to be the most efficient, occurring without death of the donor nor need to protect the recipient from lysis by normal bacteriophage particles. Conjugation in S. aureus requires bacteriophage genome to be present in either the donor or recipient. We have recently identified distinct genes with donor and recipient function in typing phage 55. Staphylococcal beta-lactamase varies in serotype, extra-cellularity, amount, control of production and substrate specificity. The latter has rarely been reported because it is not detected by routine sensitivity testing for methicillin or penicillin resistance. Of the isoxazolylpenicillins, flucloxacillin was found generally to be most labile to beta-lactamase, and dicloxacillin the most stable with cloxacillin intermediate. However, a few exceptions occurred, and it may be prudent to test specific sensitivity in patients infected with staphylococci. Even methicillin may sometimes be hydrolysed to a significant extent.