Multiple effects of promethazine in Staphylococcus aureus.

The antibiotic resistance of 6 Staphylococcus aureus strains was eliminated with a frequency from 1.2 to 10% in the presence of subinhibitory concentrations of promethazine. The pigment production of the cells was also eliminated by the promethazine to an extent of 0 to 5%. The cell size was increased and the protein A production was markedly decreased in S. aureus cells cultured in the presence of promethazine. Complex formation between protein A and promethazine was detected by differential spectrophotometry. The biological activity of staphylococcus protein A was abolished by promethazine in the passive haemagglutination of rabbit antiserum treated sheep red blood cells. Evidence has been found that plasmid-encoded functions of S. aureus cells can be altered in the presence of promethazine, and the chromosomally controlled synthesis of protein A, one of the weakest virulence factor of S. aureus is also lowered by promethazine.