Occurrence and Detection of AmpC Beta-Lactamases among Escherichia coli , Klebsiella pneumoniae , and Proteus mirabilis Isolates at a Veterans Medical Center

Abstract

AmpC beta-lactamases are cephalosporinases that confer resistance to a wide variety of β-lactam drugs and that may thereby create serious therapeutic problems. Although reported with increasing frequency, the true rate of occurrence of AmpC beta-lactamases inEscherichia coli, Klebsiella pneumoniae, andProteus mirabilis remains unknown. We tested a total of 1,286 consecutive, nonrepeat isolates of these three species and found that, overall, 45 (3.5%) yielded a cefoxitin zone diameter less than 18 mm (screen positive) and that 16 (1.2%) demonstrated AmpC bands by isoelectric focusing. Based on the species, of 683 E. coli, 371 K. pneumoniae, and 232 P. mirabilisisolates tested, 13 (1.9%), 28 (7.6%), and 4 (1.7%), respectively, demonstrated decreased zone diameters and 11 (1.6%), 4 (1.1%), and 1 (0.4%), respectively, demonstrated AmpC bands. Cefoxitin resistance was transferred for all but 8 (E. coli) of the 16 AmpC producers. We also describe a three-dimensional extract test, which was used to detect phenotypically isolates that harbor AmpC beta-lactamase. Of the 45 cefoxitin-resistant isolates, the three-dimensional extract test accurately identified all 16 AmpC producers and 28 of 29 (97%) isolates as non-AmpC producers. Interestingly, most (86%) isolates in the latter group were K. pneumoniae isolates. These data confirm that, at our institution, E. coli, K. pneumoniae, and P. mirabilis harbor plasmid-mediated AmpC enzymes.

Multidrug-resistantPseudomonas aeruginosastrains harbouring R-plasmids and AmpC β-lactamases isolated from hospitalised burn patients in a tertiary care hospital of North India

“…3). The three-dimensional extract test has been reported to be a fairly sensitive and reliable method of detection of isolates that harbour AmpC enzymes [12]. We could not compare our results due to lack of studies performed in India regarding the occurrence and detection of AmpC L-lactamases among P. aeruginosa isolates exclusively in hospitalised burn patients.…”

“…Three types of results were recorded: AmpC producers, showing a clear distortion of the zone of inhibition of cefoxitin, AmpC non-producers with no distortion and isolates showing minimal distortion, which were taken as indeterminate. All the isolates found harbouring AmpC enzymes were further con¢rmed by the use of cloxacillin discs that showed decreased distortion or complete inhibition of growth [4,12].…”

“…The three-dimensional extract test of Coudron et al [12] as modi¢ed by Manchanda et al [4] was used for phenotypic screening for the presence of AmpC L-lactamases in all 30 isolates of P. aeruginosa. Cefoxitin (30 Wg) discs were used for the detection of AmpC L-lactamases.…”

Environmental selection of antibiotic resistance genes

“…For example, chromosomal AmpC beta‐lactamases are now present in plasmids (Coudron et al ., 2000) that disseminate among bacterial populations contributing to the acquisition of an antibiotic resistance phenotype by previously susceptible bacteria. What is the origin of some other plasmid‐encoded antibiotic resistance determinants?…”

Molecular detection of TEM broad spectrum -lactamase in clinical isolates of Escherichia coli

“…Recently, co-production of ESBLs and AmpC βlactamases in some pathogens, particularly E. coli, is increasing and therefore provides these strains with a mechanism for a broader spectrum of resistance. In light of these findings, diagnostic laboratories should adopt conclusive methods to detect the bacteria expressing these enzymes, since they play a significant role in the control of antibiotic resistant pathogens (Coudron et al, 2000;Rupp and Fey, 2003). The number of β-lactamase enzymes is rapidly increasing worldwide due to the continuous mutation of their coding sequences.…”