Different mechanisms of resistance to latamoxef (moxalactam) in Serratia marcescens

Three pairs of latamoxef (moxalactam)-resistant and -sensitive strains of Serratia marcescens were isolated either in vivo or in vitro. Multiple mechanisms of resistance were found, and these mechanisms involved: a decrease in permeability in each case, associated with modification of the protein composition of the outer membrane in two cases; a variable increase (two-to-six-fold) in the amount of beta-lactamase; and in one case only, some modification of the penicillin binding proteins. Thus, it may be necessary for multiple mechanisms to be present to give resistance to latamoxef in this species.