CTX-M-5, a Novel Cefotaxime-Hydrolyzing β-Lactamase from an Outbreak of Salmonella typhimurium in Latvia

Abstract

At a children’s hospital in Riga, Latvia, isolates identified asSalmonella typhimurium were found to be resistant to expanded-spectrum cephalosporins. Two of the resistant strains were analyzed for the mechanism of cephalosporin resistance. Isoelectric focusing revealed a common β-lactamase with a pI of 8.8. In addition, one of the strains produced a pI 7.6 β-lactamase. A transconjugant producing only the pI 7.6 enzyme was susceptible to expanded-spectrum cephalosporins; therefore, this enzyme was most likely SHV-1. Transformants producing only the pI 8.8 β-lactamase were resistant to cefotaxime and aztreonam but were susceptible or intermediate to ceftazidime. A substrate profile determined spectrophotometrically with purified enzyme revealed potent activity against cefotaxime, with a relative k cat value of 95 (benzylpenicillin equal to 100). The enzyme showed lower relativek cat values for ceftazidime (3.3) and aztreonam (9.3). In addition, the enzyme was inhibited by clavulanate, sulbactam and tazobactam, with 50% inhibitory concentrations of 19, 100, and 3.4 nM, respectively. These results indicated the presence of an unusual extended-spectrum β-lactamase. The gene expressing the pI 8.8 β-lactamase was cloned. Nucleotide sequencing revealed a β-lactamase gene that differs from the gene encoding CTX-M-2, which also originated from S. typhimurium, by 11 nucleotides, 4 of which result in amino acid substitutions: Ala27Thr, Val230Gly, Glu254Ala, and Ile278Val. These results indicated the presence of a novel extended-spectrum β-lactamase, designated CTX-M-5, that specifically confers resistance to cefotaxime.

Expanded-spectrum cephalosporin resistance in non-typhoid Salmonella

“…CTX-M-producing salmonella strains also seem to be widely disseminated in the Baltic countries and Russia. Bradford et al [43] reported an epidemic of ESC-resistant, CTX-M-5-producing S. Typhimurium in Latvia during 1996-1999. The plasmid coding for this ESBL also mediated resistance to a variety of other antibiotic classes.…”

CTX-M-37 in Salmonella enterica serotype Isangi from Durban, South Africa

“…Resistance to expanded-spectrum ␤-lactam antibiotics in Enterobacteriaceae is often due to the presence of extendedspectrum ␤-lactamases (ESBLs), which are plasmidmediated bacterial enzymes found in enteric Gram-negative organisms [1]. Although reports of ESBLs associated with Salmonella spp.…”

What’s new in β-lactamases?

“…More recently, isolates expressing these enzymes have been identified in Greece, Spain, and Brazil; however, the isolates from Greece came from immigrants from the outbreak areas in Eastern Europe [25,26]. Interestingly, a number of these enzymes have been found among isolates of Salmonella typhimurium [24,28].…”

“…In addition, the CTX-M-1 and CTX-M-2 enzymes were first detected in single isolates of S. typhimurium [65,66]. In the past few years, large outbreaks of S. typhimurium strains that are resistant to cefotaxime have been reported in eastern Europe [24,28,67]. These strains have all been found to express plasmid-mediated variants of the CTX-M-type ␤-lactamase.…”

“…These enzymes are not closely related to TEM or SHV ␤-lactamases, but instead share a high homology to the class A, chromosomally encoded ␤-lactamase of Klebsiella oxytoca. In addition to the preferential hydrolysis of cefotaxime, another unique feature of these enzymes is that they are inhibited better by tazobactam than the other ␤-lactamase inhibitors, sulbactam and clavulanate [28,29]. There have been outbreaks of strains expressing these enzymes in Eastern Europe, Argentina and Japan.…”