Inhibition of motility of Pseudomonas aeruginosa and Proteus mirabilis by subinhibitory concentrations of azithromycin.

Exposure to subinhibitory concentrations (4-8 micrograms/ml) of azithromycin resulted in loss of motility in Proteus mirabilis strains and a significant reduction of motility in Pseudomonas aeruginosa strains. Examination revealed that the loss of motility was due to a total absence of flagella in Proteus mirabilis while the poor motility observed in Pseudomonas aeruginosa was due to absence of flagella in the majority of the population. Since motility may be considered a pathogenicity trait in the two species, these results confirm the unusual ability of azithromycin to reduce the expression of virulence factors in gram-negative pathogens.