DNA-mediated transformation in mutants of phage group 2 Staphylococcus aureus

Abstract

A large pool of antibiotic resistant and auxotrophic mutants was isolated from the Staphylococcus aureus phage group 2 strains UT0002-19 and UT0017 by (1) antibiotic gradient plates, (2) trimethoprim selection, and (3) nitrosoguanidine mutagenesis, which sometimes was coupled by enrichment with either penicillin or methicillin. Strain UT0002-19 has a chromosomal determinant for exfoliative toxin (ET), which causes "scalded skin syndrome" in man. A few mutants were isolated from the phage group 1 strain UT0080, which also produces ET. Two transformation regimens, called the broth and plate methods, were devised for the phage group 2 strains. They employed 80α as helper phage, and recipient cells were incubated with transforming DNA in the presence of Ca2+. Strain UT0080 was transformed using phage 55 as helper. Maximum competence of the phage group 2 strains occurred during early logarithmic growth in trypticase soy broth, but cells grown overnight on heart infusion agar were also competent. Transformation frequencies of all markers ranged from 10−6 to 10−8. For phage 80α, a multiplicity of infection of 4 was optimal in transforming a mutant of strain UT0002-19. Transformation of gly, lin, met, ole, rif, and ser markers in S. aureus is reported for the first time. Ery and ole markers in all three strains exhibited cross-resistance. Mapping studies, similar to those performed by DNA-mediated transformation in the phage group 3 strain 8325, can now be commenced for phage group 2 strains of S. aureus in order to elucidate the molecular genetics of this medically important bacterium.