Starting from a clinical isolate of Serratia marcescens that produced a chromosomally encoded AmpC beta-lactamase inducibly, we isolated by stepwise selection two laboratory mutants that showed high levels of resistance to some cephalosporins. The 98R mutant apparently overproduced the unaltered beta-lactamase constitutively, but the 520R mutant produced an altered enzyme, also constitutively. ...

With respect to the selection of beta-lactam-resistant variants marked discrepancies between the recently developed cephalosporin HR 810 and other recently developed cephalosporins could be observed: beta-lactam resistant subpopulations did not emerge during a 16-hour culture in the presence of the 20-fold minimal inhibitory concentration in a clinical Enterobacter cloacae isolate (2240/81) in ...