The effect of lysogeny on the genomic organization of Staphylococcus aureus

Abstract

The genome of Staphylococcus aureus strain S6C was shown to contain a prophage inserted within the β-toxin (BT)-encoding structural gene ( Mb). The phage att site was identical to that reported for the BT-converting phages φ 13 and φ 42. The prophage carried the genes encoding staphylokinase ( sak) and enterotoxin A (sea), which suggests that it is similar to φ 42. However, it was not induced in the presence of mitomycin C (MC) and appears to be defective. Mapping studies revealed that the genomes of the BT-converting phages present in strains S6C and PS42D (a φ 42 lysogen) encode at least one SmaI restriction site. Moreover, the PS42D chromosome contained a second prophage that also had at least one Smal site, carried both sak and sea, and hybridized with DNA probes that also hybridize with the BT-converting phages. The second phage in strain PS42D was mapped to a Smal fragment corresponding to fragment A of the S. aureus strain 8325 genomic map. Although the BT-converting phage present in strain S6C could not be induced, a phage was induced from strain S6C using MC. Southern blots suggest that it is similar to φ 11; however, the restriction patterns of DNA from the induced phage and φ 11 were clearly distinct. We have designated the inducible phage present in strain S6C as φ15, to denote that distinction. Relatively weak hybridization signals were also observed when φ15 DNA was used to probe genomic DNA from S. aureus strains lysogenized with the BT-converting phages, φ 13, φ42 and 42E. Taken together, our results demonstrate that all of the Smal-defined restriction-fragment length polymorphisms observed among the genomes of the strains examined can be accounted for by the presence of prophage DNA within the staphylococcal genome.