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 hi ghly methicillin-resistant Staphylococcus aureus strain identified a n ew insertion site located on the SmaI B-fragment of the chromosome tha t reduced the minimal inhibitory concentration of the parent (1600 mu g/ml) to 25-50 mu g/ml in the mutant and caused heterogeneous expressi on of resistance and abnormality in peptidoglycan composition (absence of the unsubstituted pentapeptide and incorporation of alanylglutamat e- and alanylisoglutamate-containing muropeptides). There was an accum ulation of large amounts of the UDP-linked muramyl dipeptide in the cy toplasmic wall precursor pool of the mutant. Reduced (heterogeneous) a ntibiotic resistance and all the biochemical abnormalities were reprod uced in genetic backcrosses by transduction with phage 80 alpha. Mutan t RUSA235 appears to be impaired in the biosynthesis of the staphyloco ccal cell wall precursor muropeptide before the lysine addition step. We propose to provisionally call the gene inactivated in this mutant f emF.