Susceptibility of 40 Haemophilus ducreyi strains to 34 antimicrobial products

A study was performed to examine compounds that might improve the selectivity of the primary isolation medium for Haemophilus ducreyi. The susceptibility of 40 H. ducreyi strains to 34 antimicrobial agents, including 10 antibiotics, 3 quaternary ammonium compounds, 3 phenolic derivatives, 3 acridines, and 15 heavy metal compounds, was investigated by using an agar plate dilution technique. Results were compared with the susceptibilities of other gram-negative rods which may be contaminants on isolation media. The minimal inhibitory concentration range for colistin (16 to 128 micrograms/ml) indicated that this antibiotic might be of use as a selective agent. H. ducreyi was susceptible to spectinomycin (minimal inhibitory concentration range, 16 to 64 micrograms/ml), thiamphenicol (0.12 to 1 microgram/ml), chloramphenicol (0.12 to 0.5 micrograms/ml), and streptomycin (4 to 32 micrograms/ml) and moderately susceptible to kanamycin (2 to 8 micrograms/ml). For the heavy metal compounds, a high susceptibility was seen with copper(II) chloride (2 to 8 micrograms/ml, corresponding to a concentration of 0.75 to 3 micrograms of Cu2+ ions per ml), sodium selenite (1 to 4 micrograms/ml, or 0.45 to 1.83 micrograms of Se- ions per ml), and phenylmercury acetate (0.12 to 0.5 micrograms/ml). The minimal inhibitory concentrations of quaternary ammonium compounds, acridines, and phenolic derivatives were between 1 and 32 micrograms/ml, 8 and 32 micrograms/ml, and 8 and 250 micrograms/ml, respectively.