Starch aggregation as a presumptive test for Haemophilus ducreyi

Chancroid and Haemophilus ducreyi

Haemophilus ducreyi is the causative agent of chancroid, one of the genital ulcerative diseases. H. ducreyi is the major cause of genital ulcer disease in Africa and Southeast Asia and is of increasing concern in the United States. Definitive diagnosis of chancroid requires the isolation and identification of H. ducreyi, but isolation of this organism is difficult and the available medium is not optimal for all strains. Fluorescent antibody and serologic tests are of limited value. In general, our knowledge of this organism is rather limited, and indeed, recent studies have questioned the placement of H. ducreyi in the genus Haemophilus. H. ducreyi has relatively few biochemical activities, and epidemiologic studies are limited because there are limited phenotypic markers available for strain typing. Specific virulence factors of H. ducreyi have yet to be identified. Antimicrobial resistance in H. ducreyi is of special concern, as this organism has acquired both gram-negative and gram-positive resistance determinants. In addition, some of these determinants can be mobilized and transferred to other Haemophilus species or to Neisseria gonorrhoeae.

Sheffield medium for cultivation of Haemophilus ducreyi

Our interest in the role of Haemophilus ducreyi in genital ulceration led us to examine the various media commonly used for the cultivation of the organism. We describe an improved medium for the routine isolation of H ducreyi. In comparative studies using 50 test strains originally isolated in the United Kingdom, Canada, the United States of America, and Kenya, the new medium proved superior to three standard media in requiring a shorter incubation period to first visible growth, giving larger colonies in the same period, and making possible a starch aggregation test which we have found helpful in the presumptive identification of H ducreyi from clinical material.

Characteristics of Haemophilus ducreyi in culture

Growth on different media and the influence of culture conditions were studied on 19 recently isolated strains of Haemophilus ducreyi, none of which had more than four passages on artificial media. The results were compared with 10 laboratory strains, which had an unknown number of passages in vitro. For all strains, growth was best on 30% rabbit blood agar and on Bieling agar. The laboratory strains showed a tendency to grow better on chocolate agar than did the fresh isolates. Of 19 fresh clinical isolates, 12 were CO2 dependent, and 2 needed extra moisture for growth. From the 10 laboratory strains, only one needed CO2 and none needed extra moisture. All 29 strains grew under anaerobic conditions. Of the 19 fresh clinical isolates, 12 grew at 22 degrees C, but only 2 of the 10 laboratory strains grew at this temperature. The laboratory strains grew better than the fresh isolates at 37 degrees C, and the optimal pH for all strains was pH 6.5 to 7.0. All strains showed starch aggregation.

Monoclonal antibodies reactive with all strains of Haemophilus ducreyi

We have isolated plasma cell hybridomas which secrete monoclonal antibodies directed against Haemophilus ducreyi. Two of these monoclonal antibodies recognize all strains of H. ducreyi tested to date and are capable of detecting the presence of H. ducreyi in skin lesions produced by this pathogen in experimental animals. These monoclonal antibodies which react with apparently all strains of H. ducreyi have the potential to be developed into a rapid immunodiagnostic test for chancroid.