Identification of a chromosomal determinant of alpha-toxin production in Staphylococcus aureus

Abstract

Production of alpha-toxin (the Hla+ phenotype, controlled by the Hla gene and scored as alpha-hemolytic activity) is a property of some isolates of Staphylococcus aureus NCTC 8325 and not of others. Genetic transformation between strains differing in the Hla phenotype revealed that the hla+ gene resides in the following sequence: purB110-bla+-hla+-ilv-129-pig-131; previously, the enterotoxin A (entA) gene of strain S-6 was shown to map very close to hla+. The hla+ mutations occurring naturally in strain Ps6 and after various mutagenic treatments in strains 8325 and 233 also mapped between bla+ and ilv-129. Among the isolates of strain 8325, the Hla+ phenotype was always associated with fibrinolytic activity, whereas Hla- isolates were non-fibrinolytic. This relationship was also observed among transformants selected for their Hla+ or Hla- phenotypes. The failure of Hla- strains and mutants to revert to hla+ at detectable frequencies, the instability of the Hla+ phenotype, and the previously observed pattern of recombination of the hla+ and entA+ determinants lend support to the view that hla+ may reside on a transposon; according to this view, Hla- mutants have lost the hla+-bearing transposon. It remains unclear whether hla+ is the structural gene for alpha-toxin.