Induction of resistance in Staphylococcus aureus and Klebsiella pneumoniae by exposure to cephalothin and cefoxitin.

Six strains each of Staphylococcus aureus and Klebsiella pneumoniae were exposed to subinhibitory, but serially incremental, concentrations of cephalothin and cefoxitin in a totally defined liquid culture medium. After exposure to cephalothin, the level of resistance to both drugs increased; the gain was greater against cephalothin and among the strains of K. pneumoniae. After exposure to cefoxitin, resistance to both drugs developed; this resistance was greater against cefoxitin and among the strains of K. pneumoniae. Acquired resistance was not lost after serial subculture in drug-free medium. Qualitative tests revealed no cephalosporinase activity in the resistant straphylococci and in four of the strains of K. pneumoniae. Two strains of K. pneumoniae, which had been isolated from a patient before and after failure of treatment with cephalothin, respectively, had no demonstrable cephalosporinase activity as isolated but developed activity after exposure to cephalothin in vitro.