8 Screening for resistance mechanisms

EUCAST (2017) states that, ‘The need for identification of resistance mechanism and the level of identification needed for public health or infection control purposes may vary both geographically and temporally depending on the prevalence and heterogeneity of different resistance mechanisms’. The resistance mechanisms however do not necessarily confer clinical resistance, and instead, screening of resistance mechanisms may be important for infection control and public health.

Detection of the mechanism in extended-spectrum β-lactamases and carbapenemases in Gram-negative bacilli does not in itself lead to classification as clinically resistant. This is because carbapenemase enzymes may be expressed at differing levels and may have different biochemical properties against β-lactams.

However, many carbapenemase-producing strains, such as Pseudomonas aeruginosa and Klebsiella pneumoniae, may be resistant to a wide range of antimicrobial agents.

AmpC-type cephalosporinases are Ambler class C β-lactamases. They hydrolyse penicillins, cephalosporins and monobactams, which have become important in humans but where originally detected in farm animals. Whilst many Enterobacteriaceae and other Gram-negative bacilli produce AmpCs at a low level, hyperproduction due to genetic changes has conferred resistance to cephalosporins and to penicillin-β-lactamase inhibitor.