Prevalence of AmpC and other beta-lactamases in enterobacteria at a large urban university hospital in Brazil

Abstract

Production of extended-spectrum beta-lactamases (ESBLs) has been reported in virtually all species of Enterobacteriaceae, which greatly complicates the therapy for infections caused by these organisms. However, the frequency of isolates producing AmpC beta-lactamases, especially plasmid-mediated AmpC (pAmpC), is largely unknown. These beta-lactamases confer resistance to extended-spectrum cephalosporins and aztreonam, a multidrug-resistant (MDR) profile. The aim of the present study was to determine the occurrence of ESBL and pAmpC beta-lactamases in a hospital where MDR enterobacterial isolates recently emerged. A total of 123 consecutive enterobacterial isolates obtained from 112 patients at a university hospital in Rio de Janeiro, Brazil, during March to June 2001 were included in the study. ESBL was detected by the addition of clavulanate to cephalosporin containing disks and by double diffusion. AmpC production was evaluated by a modified tridimensional test and a modified Hodge test. The presence of plasmid-mediated ampC beta-lactamase genes was evaluated by multiplex polymerase chain reaction. Sixty-five (53%) of 123 enterobacterial isolates were MDR obtained from 56 patients. ESBL production was detected in 35 isolates; 5 clonal Escherichia coli isolates exhibited high levels of chromosomal AmpC and ESBL production. However, no isolates contained pAmpC genes. Infection or colonization by MDR enterobacteria was not associated with any predominant resistant clones. A large proportion of hospital infections caused by ESBL-producing enterobacteria identified during the study period were due to sporadic infections rather than undetected outbreaks. This observation emphasizes the need to improve our detection methods for ESBL- and AmpC-producing organisms in hospitals where extended-spectrum cephalosporins are in wide use.

Figure 1

Hodge and tridimensional test patterns. (A) Hodge test (upper arrow): distorted inhibition zone (2mm) of the indicator organism by a positive isolate; (lower arrow) distorted inhibition zone (1mm) of the indicator organism by an indeterminate isolate; right and left: no effect on the zone of the indicator organism by negative isolates. (B) Metal apparatus specially prepared for molding Müeller-Hinton agar plates for AmpC detection. (C) Tridimensional test. Enhanced growth (6 or 7mm) of the surface organism E. coli ATCC 25922 is seen near agar slits (arrows) that contain extracts of E. coli (Fer110, Fer141, and Fer170) AmpC positive isolates; left: enhanced growth (3mm) of surface E. coli ATCC 25922 is seen near agar slits that contain extract of E. coli (Fer026) test isolate with indeterminate result; the extract of non-AmpC-producing E. coli isolate Fer136 inhibited the growth of surface E. coli ATCC 25922. On MacConkey agar, the growth of P. mirabilis isolate Fer094 inhibited the growth of surface E. coli ATCC 25922 (D) (arrow), but did not interfere with the growth of surface E. coli ATCC 35218 (not shown). The extract of E. coli isolate Fer136 inhibited the growth of surface E. coli ATCC 25922 (E) (arrow), but did not interfere with the growth of surface organism E. coli ATCC 35218 (F).

Figure 2

Dendrogram generated from computer-assisted analysis of the ERIC2-PCR fingerprints of 29 E. coli isolates: all 12 MDR and 17 (57%) of the non-MDR isolates. Five clonal E. coli isolates (Fer043, Fer044, Fer110, Fer141 and Fer170) showing similar electrophoretic patterns exhibited high levels of chromosomal AmpC and ESBL production.