Alternative Activity of -Lactam Antibiotics against Methicillin- and Cephem-Resistant Staphylococcus aureus in the Presence of Respiratory Tract Mucus

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