Prevalence of the immune evasion gene cluster in Staphylococcus aureus CC398

The immune evasion gene cluster (IEC) is typical for Staphylococcus aureus isolated from humans but is usually absent in S. aureus isolated from animals. Previous studies have shown that methicillin resistant S. aureus (MRSA) CC398 obviously lost the IEC when evolving as livestock-associated MRSA from a human-adapted, methicillin-susceptible ancestor. This study aimed to look for the presence of IEC in MRSA from pigs and horses as well as from the colonization of humans with occupational animal contact and from infections in humans. For comparison, methicillin susceptible S. aureus (MSSA) isolates from infections in humans were included. We did not detect the IEC among 94 isolates from the nasal colonization of pigs; however, the IEC was found in 6 of 61 isolates from nosocomial infections in horses. MRSA CC398 isolates from the nasal colonization of 138 pig farmers were negative for the IEC. It was detected, however, in 4 of 69 veterinarians treating horses. Among 99 epidemiologically unrelated MRSA isolates attributed to CC398 originating from infections in humans, 19 were positive for the IEC. Only three of these isolates which also contained luk-PV were attributed to the ancestral, human-adapted subpopulation of CC398 by means of PCR for detection of canonical SNPs. A considerable proportion of LA-MRSA CC398 attributed to the animal subpopulation and originating from infections in humans had acquired the IEC; this acquisition is, however, obviously not a prerequisite to the capacity of LA-MRSA CC398 to cause infections in this host. Among 15 MSSA CC398 isolates from infections in humans, 11 contained the IEC, and of these, two were attributed to the animal subpopulation. Six isolates containing both the IEC and luk-PV were attributed to the ancestral, human subpopulation. Re-acquisition of the IEC by LA-MRSA CC398 suggests readaptation to the human host. In epidemiological surveillance, discrimination from the ancestral human subpopulation is important.