Effects of amoxicillin, gentamicin, and moxifloxacin on the hemolytic activity of Staphylococcus aureus in vitro and in vivo

In Staphylococcus aureus infection hemolysis caused by the extracellular pr otein alpha -toxin encoded by hla is thought to contribute significantly to its multifactorial virulence. In vitro, subinhibitory concentrations of be ta -lactam antibiotics and fluoroquinolones increase the levels of hla and alpha -toxin expression, whereas aminoglycosides decrease the levels of hla and alpha -toxin expression. In the present study me investigated the effe cts of subinhibitory concentrations of amoxicillin, gentamicin, and moxiflo xacin on hla and alpha -toxin expression and total hemolysis of S. aureus s train 8325-4, a high-level alpha -toxin producer, and its alpha -toxin-nega tive mutant, DU 1090, in vitro and in a rat model of chronic S. aureus infe ction. The levels of expression of hla and alpha -toxin and total hemolysis did not differ significantly when amoxicillin, gentamicin, or moxifloxacin was added to cultures of S. aureus strain 8325-4. In vivo, strain 8325-4 i nduced a significantly increased level of hemolysis in infected pouches com pared to that in uninfected control pouches, but the hemolysis was reduced to control levels by treatment with doses of amoxicillin, gentamicin, or mo xifloxacin that reduced bacterial numbers by 2 orders of magnitude. Additio nally, the effects of subinhibitory concentrations of the three antibiotics on total hemolysis of four methicillin-resistant S. aureus and three methi cillin-sensitive S. aureus (MSSA) clinical isolates were assessed in vitro. A significant increase in total hemolysis was observed for only one MSSA s train when it was treated with amoxicillin but not when it was treated with moxifloxacin or gentamicin. When purified alpha -toxin was incubated with purified human neutrophil elastase, alpha -toxin was cleaved nearly complet ely. The results suggest that the penicillin-induced increases in S. aureus alpha -toxin expression are strain dependent, that reduction of bacterial numbers in vivo counteracts this phenomenon effectively, and finally, that in localized S. aureus infections alpha -toxin activity is controlled by ne utrophil elastase.