Bacterial strains that produce AmpC beta-lactamase enzymes are resistant to the cephamycins (e.g. cefoxitin and cefotetan) but susceptible to the fourth generation cephalosporin, cefepime. The susceptibility of the test isolates to cefoxitin disk (30 µg) should be used as the primary screening test to screen all the test isolates for possible production of AmpC enzymes.
All the test isolates should be subjected to the antimicrobial activity of cefoxitin disk (30 µg) on an aseptically streaked Mueller-Hinton agar plate(s). All the test plates should be incubated at 37oC for 18-24 hrs.
AmpC enzyme production should be suspected in those test isolates that showed reduced susceptibility to cefoxitin disk as per the breakpoints recommended by the Clinical Laboratory Standard Institute (CLSI). Test isolates showing inhibition zone diameter (IZD) ≤ 18 mm should be selected and tested and/or confirmed phenotypically for the production of AmpC beta-lactamase enzyme using any of the phenotypic confirmation test.
Brief note on AmpC enzymes
Chromosomal AmpC enzymes (which can also be called inducible AmpC enzymes) and plasmid-borne AmpC enzymes are the two main types of AmpC beta-lactamases that exist amongst bacteria especially in Gram negative organisms – in which these multidrug resistant enzymes are produced. AmpC enzymes are broad-spectrum beta-lactamase enzymes that are usually encoded on bacterial chromosome, and which are active on cephamycins (e.g. cefoxitin and cefotetan) and oxyimino-β-lactam agents. They can also be plasmid encoded; and AmpC enzymes like other extended or expanded beta-lactamases such as ESBLs and MBLs confer on pathogenic Gram negative bacteria the exceptional ability to be resistant to a wide array of beta-lactam drugs and non-beta-lactams. AmpC beta-lactamases are bacterial enzymes that hydrolyze third-generation extended spectrum cephalosporins and cephamycins (e.g. cefoxitin), thus engendering antimicrobial resistance to these categories of antibiotics. AmpC beta-lactamases are differentiated from extended spectrum beta-lactamases (ESBLs) by the ability of the former (i.e. AmpC enzymes) to hydrolyze cephamycins (e.g. cefoxitin) and their lack of inhibition by clavulanic acid. The expression of AmpC enzyme is typically inducible in several Enterobacteriaceae and other Gram negative bacteria including but not limited to Escherichia coli, Klebsiella species, Enterobacter species and Pseudomonas aeruginosa; and the production of this enzyme facilitates the emergence under antibiotic pressure of highly resistant but stable depressed mutants of the organisms. And these highly resistant but stably depressed mutants of the organisms have the ability to hydrolyze extended spectrum cephalosporins and other beta-lactam agents even though they may still remain susceptible to the carbapenems (e.g. imipenem and meropenem). The genes that codes for the production of AmpC enzymes in bacteria are normally chromosomally-mediated. Plasmid-mediated AmpC enzyme production in bacteria is also possible amongst bacterial organisms through genetic transfer mechanisms such as conjugation and transduction
Ejikeugwu Chika, Esimone Charles, Iroha Ifeanyichukwu, Adikwu Michael (2018). First Detection of FOX-1 AmpC β-lactamase gene expression among Escherichia coli isolated from abattoir samples in Abakaliki, Nigeria. Oman Medical Journal, 33(3):243-249.