3.1.1 β-lactam resistance and ESBLs

The resistance genes for β-lactamases are located either on the bacterial chromosome or on plasmids, leading to both intrinsic and acquired resistance. The genes have mutated and evolved so that each one represents a ‘family’ of related genes. Two separate systems are used to classify β-lactamases: a structural scheme based on primary structure (Ambler, 1980; Hall and Barlow, 2005); and a functional scheme based on chemical reactivity and DNA sequence (Bush and Jacoby, 2005).

ESBL-producers among Enterobacterales are significant as they cannot be treated with the usual ‘workhorse’ antimicrobials ‒ amoxicillin and cephalosporins. They also tend to be MDR as several resistance genes may be carried on the same plasmid. This means that often no oral options are available because the majority of second- and third-line drugs to which the organisms are still susceptible can only be given intravenously. The presence of ESBL is also a driver of carbapenemase resistance as the choice of drug to treat them is usually a carbapenem.

ESBL-producers, mainly E. coli and Klebsiella, are also widely distributed in poultry, including those which have had little direct exposure to antimicrobials themselves. They are readily transmitted to humans via food consumption, for example by handling uncooked meat or consuming vegetables fertilised with animal manure; they can also be transferred in the other direction from humans to livestock (Subramanya et al., 2020; Uttapoln et al., 2019).

Figure 4 Market day, Bahir Dar, Ethiopia

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ESBL-producers, mainly E. coli and Klebsiella, are widely distributed in poultry which is frequently on sale in markets and readily transmitted to humans, for example by handling uncooked meat or consuming vegetables fertilised with animal manure. The bacteria can also be transferred from humans to livestock.

Figure 4 Market day, Bahir Dar, Ethiopia

Table 2 Cephalosporin hydrolysing enzymes important in human and animal health