Diagnostic microbiology laboratories in Australia and the UK commonly incorporate
p-nitrophenylglycerol (
PNPG) into solid susceptibility testing media in order to prevent the swarming of Proteus spp. We have investigated the effects of
PNPG and adjusting the
cation concentrations of the media to physiological levels on the MICs of
aminoglycosides and other
antibiotics for 128 strains of Pseudomonas aeruginosa isolated from the sputa of children with
cystic fibrosis. The addition of
PNPG to the media led to higher MICs of
gentamicin and
tobramycin for up to 23% of the isolates. Depending on the base medium, supplementation with the
cations,
calcium and
magnesium, also increased the MICs of these
aminoglycosides for 12-27% of the strains tested. Both incorporation of
PNPG and
cation adjustment led to higher MICs for 25-53% of isolates, again depending on the base medium. The MICs of
ticarcillin,
ciprofloxacin and
colistin (on Iso-Sensitest
agar) for significant numbers of strains were lower in the presence of
PNPG, while those of
ceftazidime varied from higher to lower, according to the concentration of the
drug and the base medium. In many instances these changes would have altered the way in which the susceptibilities of the organisms would have been reported.
PNPG clearly exerts an important effect when the in-vitro activities of various
antibiotics against P. aeruginosa are determined by the
agar dilution method. Recommendations for the inclusion of
PNPG in susceptibility testing media should therefore be reviewed.