Analysis of TEM-type Genes Derived from Cefotaxime-resistant Serratia marcescens

We investigated the TEM-type extended spectrum beta-lactamases (ESBLs) in cefotaxime-resistant S. marcescens strains by means of a 24-month retrospective study between October 1996 and September 1998 at Showa University Hospital. Among the 579 S. marcescens strains isolated during this period, 51 were resistant to cefotaxime. We tested the expression of ESBLs in the cefotaxime-resistant strains using the disk susceptibility test with cefotaxime and cefotaxime / clavulanic acid. We then amplified the betalactamase gene in each strain by the polymerase chain reaction (PCR) .The inhibition diameters observed with cefotaxime and cefotaxime plus clavulanic acid were similar in all of the resistant strains, suggesting that the cefotaxime-resistant S. marcescens strains do not express ESBLs. Sequence analysis of the PCR-amplified beta-lactamase gene from clinical isolates of the cefotaxime-resistant strains identified it as TEM-lb. We detected the TEM-type beta-lactamase genes, TEM-1a and TEM-1b, in four cefotaximesensitive strains. In the TEM-1a-like gene of a cefotaxime-sensitive strain, we found one base substitution at nucleotide position 753 (C to A) causing an amino acid substitution from alanine 184 to valine. No ESBL TEM enzymes were found among these strains, but we isolated a novel TEM-type enzyme.The cefotaxime-resistant S. marcescens isolates, like many gram-negative enterobacteria, had the TEM-1b gene of beta-lactamase. Resistance to cefotaxime seemed to have been acquired through other ESBLs, cephalosporinases or carbapenemases, and not through TEM-type ESBLs.