Evaluation of high-level carbapenem resistance in atypical Serratia marcescens by a comparison with its revertants

A clinical isolate of Serratia marcescens (FHSM4055) was highly resistant to carbapenem (MIC of imipenem, 100 mg/L) and was atypical in that it was negative in the aesculin-hydrolysis test. This parent strain was compared with the revertants S1 and S5, which had lost resistance to imipenem alone and to imipenem and piperacillin respectively. The MICs of imipenem for the revertants were 16- to 32-fold lower than that for the parent strain. Only the crude extract of the parent strain had imipenem-hydrolyzing activity (0.27 unit/mg: panipenem-, meropenem- and biapenem-hydrolyzing activities were 53.4, 22.6 and 26.7% of the imipenem-hydrolyzing activity, respectively). From the isoelectric focusing profiles of beta-lactamases detected with nitrocefin, the parent strain produced beta-lactamases with pls of 9.2, 8.7 and 5.5, whereas the revertants did not produce the pI 8.7 beta-lactamase, which is a metalloenzyme. In the induction test, the pI 8.7 beta-lactamase was constitutive. These results indicated that the constitutive pI 8.7 beta-lactamase contributes to the carbapenem resistance of the parent strain. On the other hand, the MICs of imipenem for the revertants were 8- to 16-fold higher than that for the typical S. marcescens reference strain IFO3736, which was susceptible to various beta-lactam antibiotics other than first generation cephalosporins (according to the disk sensitivity test). This result revealed that the revertants were low-level resistant to carbapenem. The protein corresponding to the 42 kDa porin of strain IFO3736 (the major transport-channel) was absent on the outer membrane protein profiles of the parent strain and the revertants, and [3H]glucose uptake concomitantly decreased. Hence a combination of constitutive pI 8.7 metallo-beta-lactamase and decreased outer membrane permeability are major factors involved in the high-level carbapenem resistance of S. marcescens.