Emerging Fungal Pathogens

EMERGING FUNGAL PATHOGENS

In recent years a number of fungal species previously regarded as nonpathogenic have emerged as causes of disease in certain groups of patients (e.g. HIVpositive patients, cancer patients and transplant patients). While a number of these fungal species may have been causing disease previously, their emergence as significant pathogens is a cause of great concern. Undoubtedly the ability to keep immunocompromised patients alive for long periods is providing a set of novel niches for fungi, as is the introduction of novel drugs and therapies. This section focuses on a number of emerging fungal pathogens and demonstrate how the availability of novel niches has facilitated the appearance of this group of fungal pathogens.

Saccharomyces cerevisiae

The yeast S. cerevisiae is widely distributed in nature. It has been used for millennia in the production of bread and alcoholic beverages, and can be consumed directly as a dietary supplement. Traditionally it is better known as brewer ’s or baker’s yeast. Because of the ease with which it can be genetically and biochemically manipulated it is probably the most studied and bestcharacterized organism on the planet. Recently a considerable number of reports have implicated this yeast as a cause of disease in immuno-compromised patients but also in those with no apparent predisposing condition. As a result, S. cerevisiae is no longer regarded as a GRAS (‘generally regarded as safe’) organism but is now classified as a Bio-safety Level 1 pathogen, indicating that it can cause superficial or mild systemic infections in certain instances. This change of status has serious implications for industries using this yeast and indicates that, in cases of impairment of the immune system, what was once regarded as a harmless yeast is capable of causing life-threatening illness.

S. cerevisiae has been implicated in a number of superficial and systemic cases of disease and often in patients who have no apparent predisposing illness. In the majority of cases it has been found in association with other microbes, but in a significant number of instances it was identified as the sole pathogen. It has been identified as a cause of vulvovaginitis in women, being responsible for possibly up to 5% of cases, and is responsible for pneumonia and widespread dissemination in AIDS patients. Other conditions attributed to this yeast include septicaemia, postoperative peritonitis and the induction of fever and coughs in transplant patients. In one instance a bone marrow transplant (BMT) patient taking a self prescribed course of brewer’s yeast tablets developed a fever, cough and chest pains. S. cerevisiae was isolated from the lung and identified as the source of the infection. In relation to BMT patients the contamination of food with S. cerevisiae is now regarded as a potential source of infection.

Isolates of S. cerevisiae that have been associated with disease display a number of attributes that facilitate their persistence and dissemination in the host. Clinical isolates display the ability to grow at 42 °C, which is significantly greater than the upper temperature range for brewing yeast. This ability is regarded as important since febrile patients can attain this temperature and it is advantageous for a pathogenic microorganism to survive at this elevated temperature. Pathogenic isolates are capable of producing a number of extracellular enzymes such as acid proteinases and phospholipases which play a role in modulating the immune system’s response to infection and allow the degradation of cell membranes, respectively. Isolates also demonstrate the ability to grow in a pseudohyphal form which may assist in the penetration of tissue. Brewing strains of S. cerevisiae are known to flocculate at the end of the fermentation and this phenomenon is also seen in pathogenic isolates where it plays a role in obstructing capillaries particularly in the brain with concomitant damage to surrounding tissue. Pathogenic isolates adhere to epithelial tissue via a proteinaceous adhesin which is critical to the survival of the yeast in areas where it could be washed away by the action of swallowing as in the mouth. Potentially the most important attribute of virulent isolates is their ability to alter their phenotype. In clinical isolates of S. cerevisiae this ability contributes to the yeast’s persistence in the body where in complement factor 5 mice isolates were capable of persisting in the brain for up to 7 days without being cleared.

Conventional therapy for superficial or systemic S. cerevisiae infection relies on the use of azoles or polyenes. Interestingly, clinical isolates demonstrate a high level of resistance to fluconazole, which has been regarded as the first choice for the treatment of C. albicansinduced superficial candidosis in HIV positive individuals. While the mechanism that confers resistance to fluconazole in S. cerevisiae is still poorly characterized it is thought to be mediated via a multidrug efflux pump, which would remove the drug from the cell before it can act.

Nonalbicans Candida species

The rather cumbersome term ‘nonalbicans Candida species’ covers a range of Candida species that have emerged as significant human pathogens in recent years. The principal species are Candida dubliniensis, Candida krusei and Candida glabrata, although it must be emphasized that other Candida species may be problematic in specific situations. All the emerging Candida species share a number of common characteristics i.e. they were either unknown or regarded as inconsequential pathogens until recently and their emergence as significant causes of disease has occurred by exploiting a series of novel niches produced either as a result of therapy or disease.

Candida dubliniensis was first identified in 1995 in samples taken from HIVpositive patients suffering from oropharyngeal candidosis. Upon streaking samples on CHROMagar plates C. albicans appeared green but the newly discovered C. dubliniensis produced colonies displaying a different shade of green. The yeast is similar to C. albicans in many ways but displays different carbohydrate assimilation and DNA restriction patterns. This yeast has been identified in HIVpositive and negative populations from many parts of the world and is now the dominant cause of oral candidosis in the former group. The discovery of C. dubliniensis demonstrates that the provision of a novel niche allows previously unrecognized pathogens to emerge and outcompete the perceived dominant pathogen.

C. krusei was regarded as a harmless, transient commensal, being commonly found in the environment (on fruit) and on the human body, but now it is regarded as a significant cause of disease in HIVpostive patients, diabetics and cancer patients (both solid tumour and leukaemia where it is capable of colonizing the hastrointestinal, respiratory and urinary tracts). In terms of virulence attributes C. krusei demonstrates a reduced ability to adhere to epithelial cells compared to C. albicans although it does display a high cell surface hydrophobicity which allows it stick to and colonize catheters and implants. Its main virulence attribute is the high level of inherent resistance to fluconazole. Upon the introduction of this drug in 1990 to treat oropharyngeal candidosis in AIDS patients and systemic candidosis in transplant patients there was a reduction in the number of cases of disease caused by C. albicans. However the elimination of this yeast may have facilitated the emergence of C. krusei as the dominant fungal pathogen in certain classes of patient.

C. glabrata has emerged in recent years as a serious cause of disease in neutropenic cancer patients and has been responsible for mortality rates of 5–38% in some surveys. It appears to be a particular problem in the late stages of haematological malignancies, where mortality rates of 70–100% have been described. The incidence of C. glabrata infection has increased in recent years; it is now the fourth most commonly isolated Candida species and this increase may be attributable, in part at least, to an alteration in local epidemiology. Since C. glabrata is common amongst leukaemia patients and bone marrow recipients, a range of other risk factors such as prolonged hospitalization, use of cytotoxic drugs and catheterization may also play a role in its prevalence. C. glabrata is recognized as being a yeast of relatively low virulence but its emergence as a serious pathogen has been attributed to it being partially resistant to fluconazole. In addition, other factors that may have contributed to its appearance as a serious pathogen have been identified as azole prophylaxis which eliminates C. albicans but not C. glabrata or C. krusei and local factors such as the use of broadspectrum antibiotics or vascular catheters or the presence of neutropenia as part of the disease state.

Penicillium marneffei

Until recently Penicillium marneffei was regarded as a very rare and inconsequential cause of disease in humans. Now, however, it is the most frequent cause of fungal disease in AIDS patients who reside in, or have visited, South East Asia. The principal areas of infection are Thailand and southern China although cases have been reported from Malaysia, Taiwan, Japan and Hong Kong. While the nature of the infection in humans is well characterized, the natural habitat or reservoir of this fungus is still unknown although it may be soil or decaying vegetation. P. marneffei is an asexual, dimorphic fungus growing as a mycelium at 37 ° C in tissue and as single cells at 28 °C. It reproduces by a process known as fission. In the yeast form of growth it can be difficult to distinguish microscopically from Histoplasma capsulatum and Cryptococcus neoformans, so monoclonal antibody based assays specific for the detection of mannoproteins associated with P. marneffei have been developed. PCR (polymerase chain reaction) fingerprinting is also used to identify P.marneffei and to distinguish between infecting strains.

Infection in humans follows inhalation of fungal material, and in AIDS patients dissemination throughout the body can result. Pulmonary involvement is often seen particularly in AIDS patients but the conditions most associated with infection are fever, weight loss, anaemia, skin lesions, and liver and spleen inflammation. The condition is fatal if untreated with antifungal drugs. While the condition responds well to therapy if it is initiated at an early stage, relapse is common in AIDS patients and continuing antifungal therapy may be required. Because of the immunocompromised nature of AIDS patients other infections such as tuberculosis and pneumonia are often seen along with P. marneffei infection.