Objective:
Staphylococcus aureus is an important pathogen which also commonly asymptotically colonizes healthy people, while S. epidermidis even though capable of causing infections is considered much less virulent. MRSA pose particular problem, but the extent and frequency of SCCmec transfer among staphylococci, including S. aureus, remain poorly understood. This study aims at better understanding of evolutionary and ecological relationships between the two species and their role in horizontal transfer of SCCmec.
Methods:
400 healthy volunteers have been sampled for the presence of S. aureus and S. epidermidis. Multiple isolates per each colonized individual were collected. All isolates were genotyped by AFLP. All isolates of S. aureus deemed non-identical by AFLP were then genotyped by MLST and spa. Selected isolates of S. epidermidis were also typed by MLST. SCCmec was typed according to published methods.
Results:
32% of the volunteers were colonized with S. aureus, and nearly always with a single strain. S. epidermidis was found on 75% of people and colonization was nearly always with multiple strains revealing a key difference in the biology of S. aureus and S. epidermidis. The interference between the two species appeared to be of only minor importance. Both S. aureus and S. epidermidis showed similar and high diversity. i.e., nearly every carrier had a unique strain.
Only two individuals were colonized with MRSA. On the other hand, approx. 25% of S. epidermidis isolates harbored mecA. Thus, simultaneous colonization with S. aureus and S. epidermidis harboring SCCmec occurred in 5%of cases, while two strains of S. aureus nearly never co-occured. Considerable diversity among SCCmec harbored by S. epidermidis was found with up to three different types present on one colonized host. All SCCmec types known from S. aureus were found.
Conclusions:
S. epidermidis constitutes an important reservoir and recombination arena of SCCmec. Due to proximity and frequent co-colonization with S. aureus this reservoir is particularly accessible. The transfer of SCCmec among different S. aureus strains may be hampered by mono-clonal colonization of the carriers. S. epidermidis may play an important role in on-going evolution of methicillin resistance in the latter.
The question why there is interference among S. aureus and not S. epidermidis and its potential relationship to virulence of the former remains to be answered. We are also investigating the question why S. epidermidis harbors mecA so much more frequently compared to S. aureus.