Antibiogram Picture Guide for the Medical Microbiology Practitioner - Part 2: Common Carbapenemases In Enterobacterales

Abstract

Gram-negative bacteria, such as Enterobacterales, can associate enzymatic resistance mechanisms, such as beta-lactamase production, causing hard-to-treat infections. Genes encoding these enzymes are frequently found on mobile genetic elements, which eases their spread. Carbapenemase production, but also other enzymatic and non-enzymatic resistance mechanisms, individually or associated, may be responsible for carbapenem resistance, and their correct identification is important for implementing the correct infection control measures and, sometimes, choosing the right treatment. For this article, disk-diffusion antibiograms were performed according to the EUCAST guidelines, using a collection of well-characterized bacterial strains and various antibiotic disks, including certain ''key'' antibiotics. Supplementary, cloxacillin-infused Mueller-Hinton agar plates were used to differentiate AmpC producers from non-producers phenotypically. The phenotypic features that may be observed on the disk-diffusion antibiogram, which can help identify and differentiate the production of the most common carbapenemases, alone or in combination with other mechanisms, were described. For Ambler class A, KPC alone or combined with ESBL was presented; for Ambler class B, NDM, VIM, and IMP enzymes were chosen, with emphasis on NDM and VIM with different associations; and for Ambler class D, OXA-48 and one of the most problematic mutants, OXA-244, were highlighted. The disk-diffusion antibiogram is a cheap and easy-to-use tool with high diagnostic value when interpreted correctly by the microbiology laboratory practitioner. This article represents the second part of a guide that should help evaluate it.