Livestock-associated Staphylococcus aureus CC398: animal reservoirs and human infections

Over the past 15 years the epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) has changed significantly. Being initially a nosocomial pathogen, other clones have been detected in the community, leading to infections in relatively young and healthy individuals lacking contact with healthcare. More recently, a specific clone of MRSA CC398 emerged, which has spread extensively in livestock animals and is also found in retail meat. People in contact with food production animals are at high risk of colonization. The ways in which MRSA CC398 can be transmitted to humans are direct contact with animals, environmental contamination, and eating or handling contaminated meat. The role of MRSA CC398 as a food pathogen needs further research. Recently, whole genome sequencing and other genetic analyses have shown that livestock-associated strains are distinct from human-derived strains. However, there is also an exchange of strains between the reservoirs. Livestock-associated and human-associated strains of CC398 share some virulence factors, but there are also distinct virulence factors that appear to be important in host adaptation. Exchange of genes encoding these virulence factors between strains may expand the host range and thereby threaten public health. Since the emergence of MRSA CC398 in humans, approximately 10 years ago, this clone has shown a remarkable evolution, which is described in this review.