Novel Class A β-Lactamase Sed-1 from Citrobacter sedlakii : Genetic Diversity of β-Lactamases within the Citrobacter Genus

Abstract

Citrobacter sedlakii 2596, a clinical strain resistant to aminopenicillins, carboxypenicillins, and early cephalosporins such as cephalothin, but remaining susceptible to acylureidopenicillins, carbapenems, and later cephalosporins such as cefotaxime, was isolated from the bile of a patient treated with ␤-lactam and quinolone antibiotics. The isolate produced an inducible class A ␤-lactamase of pI 8.6, named Sed-1, which was purified. Characterized by a molecular mass of 30 kDa, Sed-1 preferentially hydrolyzed benzylpenicillin, cephalothin, and cloxacillin. The corresponding gene, bla Sed-1 , was cloned and sequenced. Its deduced amino acid sequence shared more than 60% identity with the chromosome-encoded ␤-lactamases from Citrobacter koseri (formerly C. diversus) (84%), Klebsiella oxytoca (74%), Serratia fonticola (67%), and Proteus vulgaris (63%) and 71% identity with the plasmid-mediated enzyme MEN-1. A gene coding for a LysR transcriptional regulator was found upstream from bla Sed-1 . This regulator, named SedR, displayed 90% identity with the AmpR sequence of the chromosomal ␤-lactamase from C. koseri and 63 and 50% identity with the AmpR sequences of P. vulgaris and Enterobacter cloacae, respectively. By using DNA-DNA hybridization, a bla Sed-1 -like gene was identified in two reference strains, C. sedlakii (CIP-105037) and Citrobacter rodentium (CIP-104675), but not in the 18 strains of C. koseri studied. Two DNA fragments were amplified and sequenced from the reference strains of C. sedlakii CIP-105037 and C. rodentium CIP-104675 using two primers specific for bla Sed-1 . They shared 98 and 80% identity with bla Sed-1 , respectively, confirming the diversity of the chromosomally encoded class A ␤-lactamases found in Citrobacter.

Molecular and Biochemical Characterization of a Novel Class A β-Lactamase (HER-1) from Escherichia hermannii

“…The ␤-lactam susceptibility pattern of E. hermannii was similar to that of K. pneumoniae, K. oxytoca, Citrobacter koseri, and Serratia fonticola (16,19,21). Susceptibility to cefuroxime distinguished E. hermannii from Rahnella aquatilis, Citrobacter sedlakii, and Proteus vulgaris (2,20,21). These results suggested that E. hermannii produces a class 2a ␤-lactamase (5).…”

“…All the strains were susceptible to cephalothin, cefuroxime, cefoxitin (Table 2), and broad-spectrum cephalosporins (MICs, Ͻ0.06 g/ml), aztreonam (MICs, Ͻ0.06 g/ ml), and imipenem (MICs, 0.06 to 0.12 g/ml). The ␤-lactam susceptibility pattern of E. hermannii was similar to that of K. pneumoniae, K. oxytoca, Citrobacter koseri, and Serratia fonticola (16,19,21). Susceptibility to cefuroxime distinguished E. hermannii from Rahnella aquatilis, Citrobacter sedlakii, and Proteus vulgaris (2,20,21).…”

AmpC β-Lactamases

“…Klebsiella pneumoniae, Klebsiella oxytoca, Proteus mirabilis, and Salmonella spp. (31) lack a chromosomal bla AmpC gene, as do Citrobacter amalonaticus (328), Citrobacter farmeri, Citrobacter gillenii (224), Citrobacter koseri (formerly Citrobacter diversus and Levinea malonatica), Citrobacter rodentium, Citrobacter sedlakii (252), Edwardsiella hoshinae, Edwardsiella ictaluri (312), Kluyvera ascorbata (138,305), Kluyvera cryocrescens (72), Plesiomonas shigelloides (9), Proteus penneri (175), Proteus vulgaris (60), Rahnella aquatilis (30,308), Yersinia pestis, and Yersinia pseudotuberculosis (313) as well as, probably, Escherichia hermannii (91), Francisella tularensis (27), Shewanella algae (123), and Stenotrophomonas maltophilia (111). However, since bla AmpC genes occur on transmissible plasmids, the clinical microbiologist needs to consider this resistance mechanism whatever the identification of an organism.…”

β-Lactamases of Kluyvera ascorbata , Probable Progenitors of Some Plasmid-Encoded CTX-M Types

“…1). Indeed, some class A ␤-lactamases closely related to K. ascorbata enzymes are regulated by an ampR gene: cumR (Proteus vulgaris) (5), sedR (C. sedlakii), (19) and fonR (S. fonticola) (C. Humeniuk, unpublished data).…”