# Haemophilus ducreyi

Haemophilus ducreyi are fastidious gram-negative coccobacilli bacteria.
This species causes the sexually transmitted disease chancroid, a major cause of genital ulceration in developing countries characterized by painful sores on the genitalia. The first study linking this disease with the agent Hemophilus ducreyi was published in 1889 by Auguste Ducrey. Each year in the United States, there are over 2,000 cases of chancroid. Chancroid starts as an erythematous papular lesion that breaks down into a painful bleeding ulcer with a necrotic base and ragged edge. It has also been found to cause chronic skin ulceration away from the genitalia, infect children and adults, and behave in a manner that mimics yaws.
H. ducreyi can be cultured on chocolate agar and incubated in an environment with elevated humidity and CO$_2$ enrichment at 33° to 35 °C. It is best treated with a macrolide, e.g. azithromycin, and a third-generation cephalosporin, e.g. ceftriaxone.

## Morphology

Haemophilus ducreyi is a Gram-negative coccobacillus, and has a shape between a spherical coccus and a rod-shaped bacterium. This species of bacterium has pili, fine and tangled appendages composed predominantly of protein, that allow bacteria to attach to surfaces, including those of cells.

## Colonies

Colonies of Haemophilus ducreyi are described as yellow-grey, small, and semiopaque as well as nonmucoid. Scanning electron microscopy has been used to observe that the microbe can form a colony of many cells; the cells adhere to each other because of an intercellular matrix. This bond can make it difficult to isolate a single cell of Haemophilus ducreyi, hindering the genetic studies that have been done on the microbe.

## Metabolism

Haemophilus ducreyi has been shown to have high phosphatase activity (acid phosphatase, alkaline phosphatase, and phosphoamidase). There are specific temperature and nutritional necessities for the pathogen to grow, requiring advanced laboratory equipment to study the bacteria. A saturated atmosphere with elevated CO$_2$ levels is considered optimal for most strains, and the most favorable growth has been observed to occur under micro-aerophilic conditions achieved in a sealed anaerobic jar without a catalyst, using two envelopes that generate CO$_2$ and H$_2$, commonly referred to as Campylobacter growth conditions.

## Pathogenesis

Haemophilus ducreyi is a human pathogen; and there are no known animal or environmental reservoirs. H. ducreyi is an opportunistic microorganism that infects its host by way of breaks in the skin or epidermis. Inflammation then takes place as the area of infection is inundated with lymphocytes, macrophages, and granulocytes. This pyogenic inflammation causes regional lymphadenitis in the sexually transmitted disease chancroid.

## Toxins

Haemophilus ducreyi is able to defend itself against the immune response's T cells through two toxins: a hemolysin and the cytolethal distending toxin (CDT). CDT is characterized by its ability to arrest epithelial cells in the G2 phase of the cell cycle and combats T cells by inducing apoptosis. Together, these toxins showcase the adeptness of H. ducreyi in manipulating host cell processes.

## Diagnosis

Although antigen detection, serology, and genetic amplification methods are sometimes used to diagnose infections with H. ducreyi and the genetic tests have greater sensitivity, they are not widely available, so cultures are currently considered the "gold standard" test, which has about 80% sensitivity under optimal combination of media.

## Treatment

Single-dose antibiotic treatments using macrolides, third-generation cephalosporins, or fluoroquinolone continue to be effective in treating chancroid. The first line treatments recommended by the U.S. Centers for Disease Control and Prevention are one of four options: azithromycin one gram orally in a single dose, ceftriaxone 250 mg intramuscularly in a single dose, ciprofloxacin 500 mg orally two times a day for three days, or erythromycin base 500 mg orally three times a day for seven days. Some antibodies were specific to all strains, while others targeted only certain groups of strains H. ducreyi, indicating that the outer membrane proteins of H. ducreyi can vary in their immune recognition. Infected individuals are still susceptible to reinfection due to the absence of developed protective immunity.
A rise in antimicrobial resistance among H. ducreyi strains result in a shift away from benzylpenicillin as the preferred treatment.