Acquisition of Resistance to Beta-Lactams by Enterobacter cloacae, Citrobacter freundii, and Serratia marcescens.

Mutants derived from Enterobacter cloacae ATCC 23355 and Serratia marcescens ATCC 8100 by treatment with ceftizoxime were found to be resistant to various newer beta-lactam antibiotics, except for imipenem. E.cloacae ATCC 23355M as a mutant showed no changes in beta-lactamase production and activity during serial passages of the, strain, indicating that its acquisition of resistance was due to chromosomal mutation.In contrast, there was no change in beta-lactamase production by S.marcescens ATCC 8100M in comparison with S.marcescens ATCC 8100 as a parent strain.Mutants of both S. marcescens ATCC 8100 and E.cloacae ATCC 23355 showed changes in their major outer membrane proteins. These findings suggested that the acquisition of resistance to the newer beta-lactam antibiotics by S.marcescens and E.cloacae was dependent on changes of their major outer membrane proteins or changes of beta-lactamase production.