Susceptibility of 40 Haemophilus ducreyi strains to 34 antimicrobial products

Abstract

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 gramnegative rods which may be contaminants on isolation media. The minimal inhibitory concentration range for colistin (16 to 128 p.g/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 t.g/ml), thiamphenicol (0.12 to 1 pug/ml), chloramphenicol (0.12 to 0.5 ig/ml), and streptomycin (4 to 32 ,g/ml) and moderately susceptible to kanamycin (2 to 8 ,ug/ml). For the heavy metal compounds, a high susceptibility was seen with copper(II) chloride (2 to 8 p,g/ml, corresponding to a concentration of 0.75 to 3 ,ug of Cu2+ ions per ml), sodium selenite (1 to 4 ,ug/ml, or 0.45 to 1.83 p.g of Se-ions per ml), and phenylmercury acetate (0.12 to 0.5 ,ug/ml). The minimal inhibitory concentrations of quaternary ammonium compounds, acridines, and phenolic derivatives were between 1 and 32 pg/ml, 8 and 32 ,ug/ml, and 8 and 250 ,ug/ml, respectively.The isolation of Haemophilus ducreyi from clinical material is often difficult because of a lack of sensitivity and specificity of the primary isolation medium. Several diagnostic criteria have been proposed in the past, but most of these have a low accuracy or yield low recovery rates (1,2,6,8). As a consequence, the diagnosis of chancroid is often based on the exclusion of other sexually transmitted diseases causing genital ulcers and on the clinical aspects of the lesions (5, 9). The isolation medium developed by Hammond et al. (7) and its subsequent modifications (3, 4, 10) contain vancomycin. While effectively inhibiting the gram-positive flora, this medium does not inhibit gram-negative rods which often contaminate isolation plates, particularly when vaginal or cervical specimens are cultured.In this study, the activity of 34 antimicrobial agents, including 10 antibiotics, 3 quaternary ammonium and 15 heavy metal compounds, 3 acridines, and 3 phenolic derivatives was evaluated against 40 strains of H. ducreyi. Eighteen clinical isolates ofEnterobacteriaceae were tested in the same way to define the relative susceptibilities to the same agent. The aim of the study was to select a compound that would enhance the selectivity of the isolation medium. MATERIALS AND METHODS Strains

Chancroid and Haemophilus ducreyi

“…Thus, this model may be amenable to the study of the pathogenesis of chancroid and the virulence of H. ducreyi. SUSCEPTIBILITY TESTING Most of the published studies have used the agar dilution method (18,21,51,53,61,75,123,151,155,164,165) to determine the antimicrobial susceptibility of H. ducreyi. However, disk diffusion (27,53) and broth microdilution (84) techniques have also been used.…”

“…There is no standard medium for performing antimicrobial susceptibility tests on H. ducreyi. Enriched GC agar (21,61,123), enriched Mueller-Hinton agar (27,51,151,164), Beiling medium with Mueller-Hinton agar instead of Columbia agar (155), and Sheffield medium (75) have all been used. For this reason, it is often difficult to compare the MICs obtained in different studies.…”

Antimicrobial susceptibility of Haemophilus ducreyi

“…The introduction of these improved isolation techniques has also made it possible to correlate clinical and bacteriologic responses following therapy and to perform in vitro antimicrobial susceptibility tests (6,17,62 (29), all MIC determinations recorded to date have employed the agar dilution method (6,8,18,19,23,26,48,56,58,60,62 (13,19,23,41,50,56,60). The inoculum size ranged from 104 CFU (58) to 105 CFU (6,13,21,23,41,50,56,62), 106 CFU (7,18,22,63), and 107 CFU (66).…”

“…Both media are enriched with 0.1% glucose, 0.01% glutamine, 0.025% hemin, and 5% lysed horse blood. The incubation temperatures used by various centers have ranged from 33°C (7,8,19,23,26,66) to 35°C (13,18,20,58,62) and 37°C (55), the percent atmospheric CO2 has ranged from 5% (22,58,62) to 10o (17,19,23,41), and the duration of incubation has ranged from 24 h (6, 55) to 48 h (17,19,23,60,62) and 96 h (22,26).…”

“…Most centers have used Mueller-Hinton agar (6,7,56,62) or gonococcal agar-based medium (13,23,41,50,49,58,60) for MIC determinations. Initially, gonococcal agar base was preferred when sulfonamides were being tested (6,8,23,50), but more recently Mueller-Hinton agar base has been used (18,62).…”