A gene identical to plasmid-borne bla _CTX-M-3 is present in the chromosome of one Kluyvera ascorbata strain. It is associated with a structure including an inverted repeat right and an open reading frame 477-like gene probably involved in the mobilization of bla _CTX-M-3 . Two other K. ascorbata strains rendered the previously described bla _KLUA-9 gene.

Resistance to extended-spectrum cephalosporins is often associated with plasmid encoded extended spectrum β-lactamases (ESBL). In order to evaluate the prevalence and diversity of ESBLs in enterobacteria in our city, a 1-month-period survey was carried out from April to May 2000. Extended-spectrum-cephalosporin-resistant strains, isolated from inpatient clinical specimens other than stools, were collected among 17 participating hospitals. From a total of 427 enterobacterial strains that were collected during this period, 39 were extended-spectrum cephalosporin resistant. The National Committee for Clinical Laboratory Standards' Screening and Confirmatory Tests for ESBL production were performed using cefotaxime and ceftazidime; cefepime and cefepime-clavulanic acid-containing disks were included. β-Lactamases were characterized by isoelectric focusing and PCR amplification using specific primers. Three different ESBLs were detected: SHV-related (4 isolates), PER-2-type (9 isolates), and CTX-M-2-related (26 isolates). Sequencing of the corresponding genes confirmed CTX-M-2 in 19 of 21 and CTX-M-31 (an allelic variant) in the remaining 2 of 21. CTX-M-2 (or its variant) was detected in all Escherichia coli , Enterobacter aerogenes , Serratia marcescens , Proteus mirabilis , and Providencia stuartii strains, while PER-2 was detected in Enterobacter cloacae , E. aerogenes , and Klebsiella pneumoniae ; SHV-related ESBL were found only in K. pneumoniae . These results clearly show that CTX-M-2 is the most prevalent ESBL produced by enterobacterial species isolated from public hospitals in Buenos Aires.

KPC-2-producing Klebsiella pneumoniae isolates mainly correspond to clonal complex 258 (CC258); however, we describe KPC-2-producing K. pneumoniae isolates belonging to invasive sequence type 23 (ST23). KPC-2 has scarcely been reported to occur in ST23, and this report describes the first isolation of this pathogen in the Americas. Acquisition of resistant markers in virulent clones could mark an evolutionary step toward the establishment of these clones as major nosocomial pathogens.

The genetic organization of the region coding for CTX-M-2 in Salmonella enterica serovar Infantis was determined by PCR mapping. This gene seems to have been mobilized from the Kluyvera ascorbata chromosome to a complex sulI -type integron, similar to In6 and In7.