23 AmpC beta-lactamases: epidemiology, infection control and treatment strands of DNA apart from the chromosomal DNA of the bacterium. They can be transferred from bacterium to bacterium by horizontal transfer. Several types of plasmidencoded ampC (pampC) genes have been detected in Enterobacterales species often referred to as “group I Enterobacterales” (including E. coli, K. pneumonia, P. mirabilis, Salmonella enteritidis), with blaCMY-2 being the most common pampC resistance gene in the Netherlands (E. Ascelijn Reuland et al. 2015). Other less commonly isolated pampC genes are other variants of blaCMY, as well as variants that differ more from blaCMY, e.g. blaDHA, blaACC, blaACT, blaMIR, blaMOX and blaFOX (Philippon, Arlet, and Jacoby 2002) (see Table 1.). All of these different pampC genes are originally derived from chromosomal ampC genes. For example, the blaCMY originates from the C. freundii chromosome, but several mutations have resulted in a great diversity of variants of the blaCMY gene (Jacoby 2009). Depending on the variant of AmpC beta-lactamase the hydrolysing capacity varies (Philippon, Arlet, and Jacoby 2002). This leads to different phenotypes per variant of ampC gene (see Table 1). The blaCMY-2 gene most common in the Netherlands generally leads to increased minimum inhibitory concentrations (MICs) for ceftriaxone, ceftazidime and cefoxitin (Philippon, Arlet, and Jacoby 2002; Coolen et al. 2019). However, in isolates with blaDHA this hydrolysing activity is less prominent, so that the effect on ceftriaxone or ceftazidime MICs may be less pronounced (Coolen et al. 2019). Moreover, the blaDHA gene is inducible when exposed to different antibiotics, e.g., imipenem. The hydrolysing effect can increase under the influence of these antibiotics (Jacoby 2009). Another example is the blaACC gene, which has the specific effect that it cannot hydrolyse cephamycins (e.g., cefoxitin) (Jacoby 2009; Philippon, Arlet, and Jacoby 2002). This is unique, as the hydrolysis of this antibiotic group is considered typical for AmpC beta-lactamases and the increased cephamycin MICs are used in many diagnostic algorithms (J.A.J.W Kluytmans et al. 2021; Martinez and Simonsen 2017). It seems that certain pampC genes are more common in certain species of the group 1 Enterobacterales. For example, blaCMY-2 is more commonly detected in E. coli and Salmonella spp and blaDHA in Klebsiella spp (Rodríguez-Guerrero et al. 2022). Furthermore, which variant is most prevalent can differ regionally. For example, in North-western Europe, the blaCMY-2 gene is most commonly detected in human and veterinary samples, while the blaDHA gene is more often found in equivalent samples in East Asia (e.g., South Korea and Japan) (Rodríguez-Guerrero et al. 2022). 2
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