Alternative activity of beta-lactam antibiotics against methicillin- and cephem-resistant Staphylococcus aureus in the presence of respiratory tract mucus.

Abstract

Methicillin- and cephem-resistant Staphylococcus aureus (MRSA) strains were found to be unable to grow in sputum from patients treated with a beta-lactam antibiotic. Sputum containing no beta-lactams also exhibited bactericidal activity when the MRSA strain was pretreated with a subinhibitory concentration of a beta-lactam; however, lysozyme-free sputum was inactive. Peptidoglycan of MRSA grown in the presence of cefazolin was susceptible to lysis by respiratory mucus. Hen albumen lysozyme contained the same activity against beta-lactam-treated MRSA. The median minimum inhibitory concentration of cefazolin for 45 S. aureus strains (including 20 MRSA strains) shifted from approximately 50 micrograms/ml to approximately 0.78 microgram/ml by addition of 1 mg/ml of hen albumen lysozyme to susceptibility-testing media. Such lysozyme-dependent antistaphylococcal activity was manifested specifically with beta-lactams (9 penicillins, 10 cephalosporins, 3 cephamycins, 1 oxacephem, and 1 carbapenem).