3.2.1 β-lactam resistant S. aureus

Resistant strains of S. aureus pose a big threat to both human and animal health worldwide, with MRSA S. aureus resistant to β-lactam antimicrobials of particular importance.

MRSA is easily transmitted between patients and is a very important HCAI pathogen globally.

MRSA is also found increasingly in livestock (Livestock-associated MRSA, LA-MRSA) and companion animals and transmits readily between humans and other species (Butaye et al., 2016; Chen and Wu, 2020).

Figure 6 A boy feeds a Vietnamese Pot-bellied pig on a farm in Thailand. © Anant Kasetsinsombut

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Resistant strains of S. aureus pose a big threat to both human and animal health worldwide, with MRSA S. aureus resistant to β-lactam antimicrobials of particular importance. MRSA is easily transmitted between hospital patients and is also found increasingly in livestock, so care must be paid to how these are reared.

Figure 6 A boy feeds a Vietnamese Pot-bellied pig on a farm in Thailand. © Anant Kasetsinsombut

Resistance occurs by two main mechanisms. The most common form is due to the presence of the mecA gene on a mobile genetic element carried by all MRSA strains. MecA encodes an altered penicillin-binding protein (PBP) which confers resistance to all penicillin and inhibitor combinations and almost all cephalosporins, other than novel ‘anti-MRSA’ ones such as Ceftobiprole. There is another similar gene, mecC.

This type of resistance is different from staphylococcal β-lactamases. These enzymes only cause resistance to penicillins (pencillin, ampicillin and amoxicillin) and the organism remains susceptible to isoxazolyl-penicillins (flucloxacillin and cloxacillin), and first generation cephalosporins. As such, these basic β-lactamase producers do not pose a public health problem at the moment as they remain relatively easy to treat.