C55 bacteriocin produced by ETB-plasmid positive Staphylococcus aureus strains is a key factor for competition with S. aureus strains.

Exfoliative toxin (ET) produced by Staphylococcus aureus is closely associated with the onset of bullous impetigo. To date, three ETs (ETA, ETB and ETD) have been identified. The gene encoding ETB is located in a plasmid designated pETB. Bacteriocin synthesis genes are also located in this plasmid and pETB-positive strains reportedly produce the C55 bacteriocin. In this study, the antibacterial activity against S. aureus strains of the bacteriocin produced by the pETB-positive strain TY4 was investigated. This bacteriocin demonstrated antibacterial activity against all pETB-negative but not pETB-positive strains, including TY4. Additionally, a TY4- strain from which the pETB plasmid had been deleted exhibited susceptibility to the bacteriocin. Further experiments revealed that two immunity factors (orf 46-47 and orf 48) downstream of the bacteriocin synthesis genes in the pETB plasmid are associated with immunity against the bacteriocin produced by TY4. The TY4- with orf46-47 strain exhibited complete resistance to bacteriocin, whereas the TY4- with orf48 strain exhibited partial resistance. Whether bacteriocin affects the proportion of each strain when co-cultured with S. aureus strains was also investigated. When TY4 or TY4- was co-cultured with 209P strain, which is susceptible to the bacteriocin, the proportion of 209P co-cultured with TY4 was significantly less than when 209P was co-cultured with TY4-, whereas the proportion of TY4- with orf46-48 co-cultured with TY4 was greater than with TY4-. These results suggest that the C55 bacteriocin produced by pETB-positive strains affects the proportion of each strain when pETB-positive and -negative strains co-exist.