Table 2.

Examples of bacterial genes and their roles in antimicrobial resistance.

Bacterial Resistance Genes	Role in Antimicrobial Resistance
Genes encoding for extended-spectrum beta-lactamase (ESBL-) and AmpC beta-lactamase (AmpC-) in Enterobacteriaceae (for ex. CTX-M type)	Exhibit resistance to a wide range of β-lactam antibiotics, including penicillins and 3rd and 4th generation cephalosporins in Gram negative bacteria. They are one of the fastest emerging resistance problems in both humans, companion and production animals worldwide. Third and 4th generation cephalosporins are now classified as Highest Priority Critically Important Antimicrobials (HPCIAs) by WHO.
Carbapenemase-encoding genes in Enterobacteriaceae	Carbapenems are classified by WHO as HPCIAs. Resistance to carbapenems is emerging in humans and constitute a major public health concern. Although carbapenems are not licensed for use in animals, carbapenem resistance has been isolated in bacteria from companion, food-producing animals and wildlife.
Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) Clonal Complex 3398	Found in the skin and nose of pigs, cattle and horses without causing a disease and they can transmit it to humans. Exposed individuals can become colonised asymptomatically; LA-MRSA can cause skin infections and occasionally invasive disease (septicaemia). Veterinarians and farmers are at high risk of exposure through their occupations.
Plasmid mediated colistin resistance (mcr genes)	Colistin (Polymyxin E) is a last resort antibiotic in humans. This new gene is transmitted horizontally, which can cause a higher risk of spread between animals and humans. Colistin resistance has been identify in livestock (pigs and poultry) and animal-derived food and in humans.
Vancomycin resistance encoding genes in Enterococcus faecium	A major cause of nosocomial infection and is categorized as high priority by the WHO global priority list of antibiotic-resistant bacteria. Vancomycin-resistant Enterococci (VRE) have been isolated from food-producing animals as a result of the previous use of avoparcin as a growth promoter (Note: antibiotic growth promoters have been banned in Europe since January 2006 but are still used in other countries).