Strains of Enterobacteriaceae producing an extended spectrum beta-lactamase have become a concern in medical bacteriology as regards both antimicrobial treatment and infection control in hospitals. Extended-spectrum beta-lactamase (ESBL) detection tests should accurately discriminate between bacteria producing these enzymes and those with other mechanisms of resistance to beta-lactams, e.g., broad-spectrum beta-lactamases, inhibitor-resistant beta-lactamases and cephalosporinase overproduction. Several phenotypic detection tests, based on the synergy between a third-generation cephalosporin and clavulanate, have been designed: the double-disk synergy test (DDST), ESBL Etests, and the combination disk method. These tests often need to be refined in order for them to detect an ESBL in some bacterial strains, such as those that also overproduce a cephalosporinase. The sensitivity of the DDST can be improved by reducing the distance between the disks of cephalosporins and clavulanate. The use of cefepime, a fourth-generation cephalosporin that is less rapidly inactivated by cephalosporinase than by ESBL, improves the detection of synergy with clavulanate when there is simultaneous stable hyperproduction of a cephalosporinase; alternatively, the cephalosporinase can be inactivated by performing phenotypic tests on a cloxacillin-containing agar. Some beta-lactamases can hydrolyse both third-generation cephalosporins and carbapenems, such as the metallo-beta-lactamases, which are not inhibited by clavulanate, but rather by EDTA. The production of an ESBL masked by a metallo-beta-lactamase can be detected by means of double inhibition by EDTA and clavulanate. Since extended-spectrum Ambler class D oxacillinases are weakly inhibited by clavulanate and not inhibited by EDTA, their detection is difficult in the routine laboratory.

i Escherichia coli is the species most frequently associated with clinical infections by extended-spectrum-␤-lactamase (ESBL)-producing isolates, with the CTX-M ESBL enzymes being predominant and found in genetically diverse E. coli isolates. The main objective of this study was to compare, on the basis of a case-control design, the phylogenetic diversity of 152 CTX-M-producing and 152 non-ESBL-producing clinical E. coli isolates. Multilocus sequence typing revealed that even though CTX-M enzymes were largely disseminated across the diversity of E. coli isolates, phylogenetic group B2 showed a particularly heterogeneous situation. First, clone ST131 of group B2 was strongly associated with CTX-M production (55 [79%] of 70 isolates), with CTX-M-15 being predominant. Second, the remaining members of group B2 were significantly less frequently associated with CTX-M production (9 [12%] of 75) than E. coli phylogenetic groups A, B1, and D (88 [55%] of 159). CTX-M-producing ST131 E. coli isolates were significantly more frequent in patients hospitalized in geriatric wards or long-term care facilities. Besides, the non-ESBL ST131 isolates significantly more frequently showed resistance to penicillins than the non-ESBL, non-ST131 isolates did. In conclusion, the present study emphasizes the particular antimicrobial resistance and epidemiologic characteristics of clone ST131 within group B2, which could result from the higher antibiotic exposure of this clone, as it is the predominant clone of group B2 carried in the human gut. E scherichia coli is the Enterobacteriaceae species that causes the largest number of infections, but this species is also a widespread gut commensal of humans and animals. Strains of E. coli are therefore frequently exposed to antimicrobial agents, which has resulted in the emergence and rapid diffusion of antibioticresistant E. coli clinical isolates. Extended-spectrum ␤-lactamases (ESBLs), which confer resistance to extended-spectrum cephalosporins, were, until the year 2000, produced essentially by Klebsiella pneumoniae and Enterobacter sp. isolates responsible for nosocomial infections. However, E. coli has become dominant among the ESBL-producing enterobacterial species. This is especially worrisome as the human gut carriage of E. coli favors the dissemination of ESBLs in the community (31). Interestingly, the switch of dominant species among the ESBL-producing enterobacterial isolates occurred concomitantly with the emergence of novel ESBL enzymes that belong to the CTX-M family. The CTX-M enzymes have superseded the TEM and SHV ESBLs (2) and are found in genetically diverse E. coli isolates. Among CTX-M enzymes, CTX-M-15 predominates and has been associated with the dissemination of a particular E. coli clone of sequence type 131 (ST131) (23,27).The species E. coli is genetically diverse, and strains have been classified into a number of phylogenetic groups, the most frequent ones being A, B1, B2, and D (12, 14, 29, 32). The ST131 clone belongs to phylogenetic group B2 and is associated wi...

fWe report here the complete nucleotide sequence of two IncR replicons encoding multidrug resistance determinants, including ␤-lactam (bla DHA-1 , bla SHV-12 ), aminoglycoside (aphA1, strA, strB), and fluoroquinolone (qnrB4, aac6=-1b-cr) resistance genes. The plasmids have backbones that are similar to each other, including the replication and stability systems, and contain a wide variety of transposable elements carrying known antibiotic resistance genes. This study confirms the increasing clinical importance of IncR replicons as resistance gene carriers.