Abstract
Previous observers showed that many strains of Staphylococcus aureus stimulated the growth of Bordetella pertussis but we have found the reverse: the growth of all available strains of B. pertussis on charcoal-agar medium was inhibited by a standard strain of S. aureus; and 17 of 18 strains of S. aureus (as well as several other organisms) inhibited the growth of a standard strain of B. pertussis. All inhibiting colonies had an unusual brown colouration on the charcoal agar used in the investigation. Both the brown colouration and the inhibitory property were caused by acid production, probably from the starch in the medium. We therefore suggest that media containing starch and blood should not be used in studies of bacterial interference.