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

► Persons with professional exposure to livestock frequently carry MRSA CC398. ► MRSA CC398 has currently not spread extensively into the general population. ► CC398 is probably less virulent than other human MRSA strains. ► Whole genome sequence-based provided evidence that CC398 originated in humans. ► Virulence and transmissibility of MRSA CC398 may therefore increase in the future. Over the past 15years 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 10years ago, this clone has shown a remarkable evolution, which is described in this review.