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.

FIG. 1

Three-dimensional extract test patterns for five isolates. (A) Enhanced growth of the surface organism, E. coli ATCC 25922, is seen near agar slits (arrows) that contain extracts of E. coli (M563) and K. pneumoniae (M484) test isolates, both of which are AmpC producers. The remaining slit contained an extract of a non-AmpC-producing E. coli isolate (M1601). The extract of AmpC-producing E. coli isolate M477 inhibited the growth of one surface organism, E. coli ATCC 25922 (B) (arrow), but did not interfere with the growth of the second surface organism, E. coli ATCC 11775 (C) (arrow). (D) Swarming growth (dark arrow) of unlysed cells in an extract of AmpC-producing P. mirabilis isolate M910 interfered with detection of growth of surface organism (white arrow) when Mueller-Hinton agar was used. (E) On MacConkey agar, growth of P. mirabilis was inhibited, and enhanced growth of the surface organism was easily seen (arrow).