Looking for clues

Derek Sullivan and David Coleman on why understanding C.dubliniensis is so important

The oral cavity is home to a myriad of micro organisms. This is not surprising as it is the portal of entry for food, drink and air that are often contaminated with bacteria, fungi and viruses and it offers a warm and moist environment that is ideal for microbial colonisation and growth.

Shortly after birth, the oral cavity becomes colonised with micro organisms that establish themselves as the normal commensal flora, also known as the normal oral flora. These micro organisms are present throughout life, and play an important role in protecting oral epithelial surfaces and tissues by excluding more pathogenic micro organisms.

For the most part these microbes exist in harmony with their human host. However, if factors develop that disturb the balance of the normal flora, commensal bacteria and other micro organisms can overgrow and cause a wide range of diseases, including caries and periodontitis.

While the role of bacteria in oral infections is well established, the role of fungal species in these infections is far less well known and their pathogenesis is poorly understood. The most common manifestation of oral fungal infection is orophrayngeal candidosis (OPC), more commonly known as oral thrush.

The aetiological agents of this infection are yeast species belonging to the fungal genus Candida.

These single-celled eukaryotic organisms (which are related to the yeasts used in brewing and baking) are members of the normal oral flora in most individuals and cause little or no problems for healthy individuals because they are held in check by the host’s immune system. If, however, the immune system becomes compromised (e.g. by HIV infection, topical steroid use, head and neck irradiation, endocrine disorders, etc) or dentures and other oral prostheses if not cleaned adequately, the host-pathogen balance tips in favour of Candida, resulting in overgrowth of the fungus and subsequent infection.

The most common manifestation of OPC is pseudomembranous candidosis, characterised by white curd-like lesions on the palate and tongue that sometimes resemble the pattern of feathers of the songbird thrush (hence the common name for the infection).

Other forms of Candida infection include, erythematous and hypertrophic candidosis and angular cheilitis. These infections can be treated topically using nystatin or amphotericin, but recurrent or recalcitrant infections are often treated with systemic azole drugs, such as fluconazole and itraconazole, although resistance to these drugs can be a problem following repeated use.

The genus Candida is composed of almost 200 species, only a handful of which have been associated with oral infection. Candida albicans is by far the most common cause of OPC.

Other related species, such as Candida glabrata and Candida parapsilosis, can also be associated with disease. In 1995, we identified a new Candida species in Irish HIV-infected and AIDS patients with severe OPC, especially in patients with recurrent disease.

The novel organism was named Candida dubliniensis (after the city and University of Dublin) and has been subsequently identified in HIV-infected patients and other groups, such as cancer patients, throughout the world. Since 1995 we have been investigating the epidemiology, drug resistance and pathogenesis of C. dubliniensis.

Although it has been isolated from a wide range of body sites in patients around the world, C. dubliniensis is rarely found in healthy people and primarily associated with oral disease in HIV-infected individuals.

Population analysis using sophisticated genetic fingerprinting and DNA analysis tools revealed that C. dubliniensis is composed of three distinct groups and that isolates of the species tend to be closely related or clonal in nature, exhibiting far less diversity than the closely related species, C. albicans.

The first non-human C. dubliniensis isolates were recovered recently on the surface of ticks associated with the droppings of guillemots living on the Saltee Islands off the South East coast of Ireland. More recently, we have also found this species in herring gull nests in Dublin. The avian isolates were found to comprise a distinct group except for one isolate, which was indistinguishable from human isolates, suggesting that transmission of this fungus can occur between humans and birds, possibly as a result of urban sea birds scavenging in human garbage.

The vast majority of C. dubliniensis isolates are susceptible to all of the commonly used antifungal drugs. However, they do have the capacity to become resistant to azole drugs which might explain why it was most often seen in cases of recurrent OPC in HIV-infected and AIDS patients. Since oral candidosis is now less common in HIV-infected patients due to the introduction of highly active anti-retroviral therapy in the late 1990s, the level of C. dubliniensis oral infection has decreased dramatically, and antifungal drug resistance has not emerged as a significant clinical problem.

Detailed genetic analysis has revealed that the most closely related species to C. dubliniensis is C. albicans. This is intriguing, because while C. albicans is by far the most pathogenic yeast species, epidemiological and infection model data indicate that C. dubliniensis is far less pathogenic. Our current research is directed towards comparing the two species with a view to furthering our understanding of candidal pathogenesis with the ultimate aim of identifying targets for the development of novel antifungal drugs.

One difference between the two species lies in their differing ability to undergo morphogeneisis, whereby the two species have the ability to switch between yeast and filamentous (e.g. hyphal) growth forms. This has long been known to be an important virulence trait of C. albicans, however, while C. dubliniensis has the capacity to produce hyphae, it does so far less efficiently than C. albicans. To investigate the molecular basis of the different virulence potential of the two species we are comparing their genomes to identify what factors are expressed by C. albicans that gives it the pathogenic edge over such a close relative.

The Wellcome Trust Sanger Institute in Cambridge has just deciphered the C. dubliniensis genome sequence and, as expected, the genomes of the two species are highly similar. However, C. dubliniensis lacks a number of genes that encode putative C. albicans virulence factors (e.g. surface adhesion proteins and proteinases) as well as a number of genes whose products have no known function. It is hoped that by identifying the function of these proteins we will further our understanding of how C. albicans and C. dubliniensis cause oral infections and potentially identify new targets for the development of novel classes of antifungal agents.

By Derek J. Sullivan, B.A.(Mod.), Ph.D., F.T.C.D. associate professor in microbiology, and David C. Coleman, B.A.(Mod.), Ph.D., F.R.C.Path., F.T.C.D. professor of oral and applied microbiology at the Microbiology Research Unit, Division of Biosciences, Dublin Dental School and Hospital.