Variation in the agr-dependent expression of alpha-toxin and protein A among clinical isolates of Staphylococcus aureus from patients with septicaemia

FEMS Microbiol Lett. 1997 Jul 1;152(1):155-61. doi: 10.1111/j.1574-6968.1997.tb10422.x.


In Staphylococcus aureus synthesis of many virulence factors is regulated by the agr locus. The regulatory molecule RNAIII, induced by agr, activates transcription of the alpha-toxin gene, hla, while it acts as a repressor of the protein A gene, spa. Forty clinical strains of S. aureus from human blood cultures were analysed for alpha-toxin and protein A production. An inverse correlation between alpha-toxin and protein A production was found in most strains. The levels of alpha-toxin and protein A production varied significantly among strains, which indicates various levels of the regulator, RNAIII. This was confirmed by selecting strains producing different amounts of alpha-toxin, showing that the variations in toxin production are due to the variations of RNAIII transcript. However, in one of the selected strains which produced high levels of alpha-toxin, OR153, although RNAIII is also strongly expressed, the specific hla mRNA was unexpectedly low. One partial explanation for the high alpha-toxin production by this clinical isolate might be its lack of extracellular proteases.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacteremia / microbiology*
  • Bacterial Proteins / genetics*
  • Bacterial Toxins / biosynthesis*
  • Hemolysin Proteins / biosynthesis*
  • Humans
  • RNA, Antisense / physiology
  • RNA, Bacterial / physiology
  • Staphylococcal Protein A / biosynthesis*
  • Staphylococcus aureus / metabolism*
  • Trans-Activators*
  • Transcription Factors / genetics*


  • Agr protein, Staphylococcus aureus
  • Bacterial Proteins
  • Bacterial Toxins
  • Hemolysin Proteins
  • RNA, Antisense
  • RNA, Bacterial
  • RNAIII, Staphylococcus aureus
  • Staphylococcal Protein A
  • Trans-Activators
  • Transcription Factors
  • staphylococcal alpha-toxin