Reduced methicillin resistance in a new Staphylococcus aureus transposon mutant that incorporates muramyl dipeptides into the cell wall peptidoglycan.

Screening of a new Tn551 library constructed in the background of a highly methicillin-resistant Staphylococcus aureus strain identified a new insertion site located on the SmaI B-fragment of the chromosome that reduced the minimal inhibitory concentration of the parent (1600 micrograms/ml) to 25-50 micrograms/ml in the mutant and caused heterogeneous expression of resistance and abnormality in peptidoglycan composition (absence of the unsubstituted pentapeptide and incorporation of alanylglutamate- and alanylisoglutamate-containing muropeptides). There was an accumulation of large amounts of the UDP-linked muramyl dipeptide in the cytoplasmic wall precursor pool of the mutant. Reduced (heterogeneous) antibiotic resistance and all the biochemical abnormalities were reproduced in genetic backcrosses by transduction with phage 80 alpha. Mutant RUSA235 appears to be impaired in the biosynthesis of the staphylococcal cell wall precursor muropeptide before the lysine addition step. We propose to provisionally call the gene inactivated in this mutant femF.