title: 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 beta-lactam drugs and that may thereby create serious therapeutic problems. Although reported with increasing frequency, the true rate of occurrence of AmpC beta-lactamases in Escherichia coli, Klebsiella pneumoniae, and Proteus 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. mirabilis isolates 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.
authors.affiliation: Pathology and Laboratory Medicine Service/113, McGuire Veterans Affairs Medical Center, Richmond, Virginia 23249-0001, USA. Philip.Coudron@med.va.gov
authors.forename: P E
authors.initials: PE
authors.lastname: Coudron
authors.forename: E S
authors.initials: ES
authors.lastname: Moland
authors.forename: K S
authors.initials: KS
authors.lastname: Thomson
mesh_terms: D001426:Bacterial Proteins; D002440:Cefoxitin / Q000494:pharmacology*; D004926:Escherichia coli / Q000187:drug effects / Q000201:enzymology* / Q000302:isolation & purification; D006787:Hospitals, Veterans; D006801:Humans; D007711:Klebsiella pneumoniae / Q000187:drug effects / Q000201:enzymology* / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests; D011513:Proteus mirabilis / Q000187:drug effects / Q000201:enzymology* / Q000302:isolation & purification; D014481:United States; D001618:beta-Lactamases / Q000032:analysis*
chemical_list: D001426:Bacterial Proteins; D002440:Cefoxitin; C048795:AmpC beta-lactamases; D001618:beta-Lactamases