MYCOLOGY - CHAPTER   THREE  

YEASTS  

There are many species of the genus Candida that cause disease. The infections caused by all species of Candida are called candidiasis. Candida albicans (figure 2 and 3) is an endogenous organism. It can be found in 40-80% of normal human beings. It is present in the mouth (figure 4), gut, and vagina. It may be present as a commensal or a pathogenic organism. Infections with Candida usually occur when a patient has some alteration in cellular immunity, normal flora or normal physiology. Patients with decreased cellular immunity have decreased resistance to fungal infections. Prolonged antibiotic or steroid therapy destroys the balance of normal flora in the intestine allowing the endogenous Candida to overcome the host. Invasive procedures, such as cardiac surgery and indwelling catheters, produce alterations in host physiology and some of these patients develop Candida infections. Although it most frequently infects the skin and mucosae, Candida can cause pneumonia, septicemia or endocarditis in the immuno-compromised patient. The establishment of infection with Candida species appears to be a property of the host - not the organism. The more debilitated the host, the more invasive the disease. The clinical material to be sent to the lab depends on the presentation of the disease: blood cultures, vaginal discharge, urine, feces, nail clippings or material from cutaneous or mucocutaneous lesions. Candida is a polymorphic yeast, i.e., yeast cells, hyphae and pseudohyphae are produced. It has been shown that Candida needs a transcription repressor to maintain the yeast form. This ability to assume various forms may be related to the pathogenicity of this organism. The yeast form is 10-12 microns in diameter, gram positive, and it grows overnight on most bacterial and fungal media. It also produces germ tubes (figure 9 and 10), and pseudohyphae (figure 6 and 7) may be formed from budding yeast cells that remain attached to each other. Spores may be formed on the pseudomycelium. These are called chlamydospores and they can be used to identify different species of Candida. Some mycologists think that the pseudomycelial form represents a more invasive form of the organism. The species are identified by biochemical reactions. The organism occurs world-wide. The drugs of choice for systemic infection are itraconazole and fluconazole. If an artificial heart valve or in-dwelling catheter becomes infected, it must be replaced. Drug therapy alone will not suppress the organism if the foreign body remains in the host. This resistance is due to biofilms which we will discuss later.

Figure 12
Encapsulated pathogenic yeast fungus (Cryptococcus neoformans). A yeast-like fungus that reprouces by budding. A acidic mucopolysaccharide capsule completely encloses the fungus. It can cause the disease called cryptococcosis; especially in immune deficient humans, such as in patients with HIV / AIDS. The infection may cause meningitis in the lungs, skin or other body regions. The most common clinical form is meningoencephalitis. It is caused by inhaling the fungus found in soil that has been contaminated by pigeon droppings.
 

  Figure 13.
Cryptococcosis of lung in patient with AIDS. Histopathology of lung shows widened alveolar septum containing a few inflammatory cells and numerous yeasts of Cryptococcus neoformans   CDC/Dr. Edwin P. Ewing, Jr. epe1@cdc.gov 

Figure 14.
Cryptococcosis of lung in patient with AIDS. Mucicarmine stain. Histopathology of lung shows widened alveolar septum containing a few inflammatory cells and numerous yeasts of Cryptococcus neoformans. The inner layer of the yeast capsule stains red. CDC/Dr. Edwin P. Ewing, Jr. epe1@cdc.gov 

Cryptococcosis of lung in patient with AIDS. Methenamine silver stain. Histopathology of lung shows numerous extracellular yeasts of Cryptococcus neoformans within analveolar space. Yeasts show narrow-base budding and characteristic variation in size. CDC/Dr. Edwin P. Ewing, Jr. epe1@cdc.gov

Cryptococcosis manifests itself most commonly as meningitis but in recent years many cases of pulmonary disease have been recognized. C. neoformans is a very distinctive yeast. The cells which are spherical and 3-7 microns in diameter (figure 12), produce buds which characteristically are narrow-based and the organism is surrounded by a polysaccharide capsule (figure 14). There is evidence that the capsule may suppress T-cell function and can be considered a virulence factor. C. neoformans also produces an enzyme called phenoloxidase which appears to be another virulence factor. The ecological niche of C. neoformans is pigeon and chicken droppings. However, although this organism can be easily recovered from pigeon droppings, a direct epidemiological link has yet to be established between exposure to pigeon droppings and a specific human infection. Infection and disease production is probably a property of the host--not the organism. The source of human infection is not clear. This organism is ubiquitous, especially in areas like abandoned buildings contaminated with pigeon droppings. The portal of entry is the respiratory system. Evidence is developing which indicates that the initial exposure may be many years prior to the manifestation of disease. The organism can be sequestered for this time. Infection may be subacute or chronic. The highly fatal meningoencephalitis caused by C. neoformans has a prolonged evolution of several months. The patients symptoms may begin with vision problems and headache, which then progress to delirium, nuchal rigidity leading to coma and death unless the physician is thinking about cryptococcus and does a spinal tap for diagnosis and institutes aggressive therapy. The CSF is examined for its characteristic chemistry (elevated protein and decreased glucose), cells (usually monocytes), and evidence of the organism. The latter is measured by the visual demonstration of the organism (India Ink preparation) or by a serologic assay for the antigen of C. neoformans. The India Ink test, which demonstrates the capsule of this yeast, is supplemented by the latex agglutination test for antigen which is more sensitive and more specific. The Latex Agglutination test measures antigen, NOT antibody. A decreasing titer indicates a good prognosis, while an increasing titer has a poor prognosis. When you consider Cryptococcosis, think of Capsules and CNS disease. In addition to causing meningitis, C. neoformans may also infect lungs (figure 15) and skin. The disease in the lungs and skin is characterized by the formation of a granulomatous reaction with giant cells. As with other fungal diseases, there has been an increase in the recognition of pulmonary infection. The yeast may also form a mass in the mediastinum called a cryptococcoma. The geographical distribution of this organism is world-wide. The clinical material sent to the lab is CSF, biopsy material, and urine (for some unexplained reason the organism can be isolated from the urine in both the CNS and systemic infections). This organism will grow overnight on bacterial or fungal media at 37 C. but growth is a little slower at room temperature. In culture the organism grows as creamy, white, mucoid (because of the capsule) colonies. Growth in culture is usually visible in 24 to 48 hours. As the culture ages, it turns brown due to a melanin produced by the phenoloxidase. The organism is a round, single cell, yeast surrounded by a capsule. Identification is based on physiological reactions. Pathologists use a mucicarmine stain, which stains the capsule, to identify the organism in tissue sections. There is usually little or no inflammatory response. The Direct Fluorescent Antibody test identifies the organism in culture or tissue section specifically, by causing the yeast cell wall to stain green. To test the patient's serum there are 3 serologic tests: The Indirect Fluorescent Antibody test, the Tube Agglutination test for antibody, and the Latex Agglutination test for antigen. The latex agglutination test can be used as a prognostic test. As the patient improves, the serum antigen titer will also decrease. The drugs of choice to treat cryptococcus infection are amphotericin B and 5-Fluorocytosine (5-FC). 5-FC is an oral drug. If it is given as the only treatment, there are relapses so most physicians use both drugs simultaneously. Actually, these two drugs are synergistic, and thus, their association is advantageous.