Geotrichum candidum is a plant pathogenic fungus that causes sour rot of citrus fruits, tomatoes, carrot and some vegetables. The fungus can also cause a disease of the lung or other organs in humans known as geotrichosis. This species also used widely in the production of certain dairy products including rind cheeses such as Camembert, Saint-Nectaire, Reblochon and others. The fungus can also be also found in a Scandinavian yogurt-like product known as viili where it is responsible for the product's velvety texture.
The genus Geotrichum was described by Johann Heinrich Friedrich Link in 1809 to accommodate the species G. candidum found on decaying leaves. Since then, over 130 taxa have been described in the genus, and hundreds of synonyms have been generated. For example G. candidum was misclassified as the Oidium lactis in much early literature. Species of Geotrichum resemble the genera Trichosporon and Protendomycopsis; however, Geotrichum is of ascomycetous affiliation whereas the latter are members of the Basidiomycota. Species of Geotrichum are occasionally mistaken for fast growing members of the genus Dipodascus, which are characterized by irregularly branched, 10-14 μm wide hyphae and the production of single-spored asci. However, unlike Geotrichum, members of the genus Dipodascus lack dichotomous branching of the peripheral hyphae and their growth rates are generally less than 3 mm per day.
Galactomyces candidus, formerly thought to be a distinct taxon, was found to be the sexual state of G. candidum using sequence-based methods Geotrichum candidum in the broad sense comprises 3 clades, corresponding to the species G. candidum, G. clavatum and G. fici, all of which are thought to have pathogenic potential. Species of Geotrichum can be differentiated by sequence analysis of the nuclear ribosomal large subunit (18S ribosomal RNA) or the internal transcribed spacer region of the nuclear ribosomal RNA gene.
Geotrichum candidum colonies are thin, spreading, soft, creamy and white in the anamorph state. The fungus G. candidum is characterized by hyphae that appear creeping, mostly submerged and septate. The hyphae colour appears to be hyaline or lightly pigmented. When the hyphae becomes airborne it changes shape from arthrocondia to cylindrical or barrel-shaped or ellipsoidal. Chlamydospores are subglobose, solitary, borne on undifferentiated hyphae. Blastoconidia sometimes develop on hyphae laterally. Conidia appear arthrosporous, terminal or intercalary, aerial on an agar surface. The conidia size ranges from 4.8–12.5 μm x 2.4–2.5 μm.
Geotrichum candidum is thought to be homothallic but most isolates are self-sterile. Sexual reproduction was first observed in strains isolated from soils in Puerto Rico. The fungus produces globose asci that contain a single, thick walled, uninucleated, globose to oval ascospore measuring 6–7 μm by 7–10 μm. The ascospores have a smooth inner wall and a furrowed outer wall. The septa are perforated by microspores, arranged in a ring structure. The colonies appear to be growing faster in the sexual stage than the asexual stage. Colonies grow at a rate of 5–7 mm daily at 24 °C (75 °F).
Geotrichum candidum is fast growing colony that can grow 5–6 cm diameter at 5 days on Sabouraud-glucose agar, wort agar and synthetic media. Microscopically, the growth is characterized by the production of dichotomously branched hyphae that resemble tuning forks along the colony margin. The condia chains become aerial, erect or decumbent and measures 6–12(–20) x 3–6(–9) μm. The fungus can grow on a variety of citrus fruits and cause rotting. It tends to cause rotting in fruits that are stored at 0–5 °C (32–41 °F). The conidia are colourless and have a slimy texture. Geotrichum candidum is also found occasionally in the human gut, feces, sputum and on skin. The fungus grows in soil, water, sewage, various plant substrates, baker’s dough, husks of fermentation, bread, milk and milk products The optimal temperature for growth is 25 °C (77 °F) with a pH range of 5.0–5.5. The temperature range changes depending on the surface that the fungus grows on. For example in plants the optimum temperature ranges from 25–27 °C (77–81 °F). In animals the optimum temperature ranges from 30–31 °C (86–88 °F). The maximum temperature for growth is 35–38 °C (95–100 °F). Fungal growth can be supported by D-glucose, D-mannose, D-xylose, L-sorbose, D-fructose, D-galactose, sucrose, D-mannitol, D-sorbital, ethanol and glycerol. Sporulation often requires balance of carbon and nitrogen.
Geotrichum candidum is extremely common in soil and has been isolated from substrates in Canada, United States, Britain, Germany, Austria, India, South Africa, Japan, Brazil and Peru. It is also found as a causal agent in sour rot in citrus fruits— a soft rot associated with the emission of a fruity odour. The fungus is also known as a post-harvest spoilage agent of muskmelon, squash and cucumber. It plays a role in tomato fruit rot when it is stored at 0–5 °C (32–41 °F). It is a naturally occurring colonist of certain dairy products particularly cheeses and is sometime used to inoculate wash-rind and bloomy rind cheeses.
Human colonization and disease
Geotrichum candidum is also a frequent member of the human microbiome, notably associated with skin, sputum and feces where it occurs in 25-30% of specimens. The fungus can cause an infection known as geotrichosis, affecting the oral, bronchial, skin and bronchopulmonary epithelia. The inoculum may arise from endogenous or exogenous sources.
In 1847 Bennett described Geotrichum candidum causing a superinfection in the tuberculous cavity. Bennett was able to differentiate infection by Geotrichum candidum from candidiasis, and diagnose the first case of geotrichosis. Other early medical case reports in 1916 and 1928 also described lung infections. Most cases affect the bronchopulmonary tree, although other sites can be involved, such as oral mucosa and vagina. Skin and gut infections are also known. Reported cases of geotrichosis have been characterized with symptoms of chronic or acute bronchitis. Exogenous geotrichosis may arise from contact with contaminated soil, fruits or dairy products.
- Pulmonary geotrichosis is the most frequent form of geotrichosis. The symptoms appear to be secondary symptoms of tuberculosis. This includes symptoms such as light, thick, grey sputum, which in some cases may be blood-tinged. Patients often have a cough that produces clear or yellow sputum. Another symptom of pulmonary geotrichosis includes fine to medium rales. Patients may develop fever, rapid pulse and leukocytosis. The condition appears chronic with the presence of a little debilitation and fever. There is no chest pain and occasional wheezing can occur.
- Bronchial geotrichosis does not involve the lung instead the disease persists within the bronchial. Geotrichum candidum grows in the lumen of the bronchi. The disease is characterized as an endobronchial infection. Bronchial geotrichosis is similar to the allergic reaction of aspergillosis. Symptoms include prominent chronic cough, gelatinous sputum, lack of fever and medium to coarse rales. Patients with the bronchial condition their pulse and respiration are rarely elevated. Fine mottling may be present in the middle or basilar pulmonary region. Colonization of the bronchi can be associated with Candida albicans and usually occur with patients with chronic obstructive lung disease.
- Oral and vaginal geotrichosis is similar to thrush in its appearances and was often confused with this infection. The difference between oral and vaginal geotrichosis can be determined using microscope analysis. The infected area forms a white plaque and patients usually report burning sensation in the affected areas. The vaginal geotrichosis is more common pregnant women and is often associated with vaginitis.
- Gastrointestinal geotrichosis is enterocolitis associated with glutamic therapy. The symptoms usually stop once the glutamic therapy is discontinued. Establishment of the etiology of the fungi is difficult since G. candidum is found within the gut normal flora. The difference between normal gut flora form and the disease causing form is the production of toxins.
- Cutaneous geotrichois has two different types of variants which include superficial and deep infection. The superficial form the infection occurs on skin folds including submammary, inguinal, perianal and interdigital folds. The deep form develops nodules, tumours and ulcers on legs, face and hands. Geotrichosis can cause a cystic lesion appears as soft tissue on the skin.
The diagnoses of geotrichosis cannot be determined without using culture or microscopic measurements. The laboratory diagnosis of geotrichosis involves collected fungi samples areas of infections without contamination. Scraping of the mouth lesions and the ulcers can provide a sample of G. candidum. Samples can also be collected from pus and mucus can be obtained from the feces. Sputum can be searched for the mucoid-like white flakes for further examination. Culturing the cylindrical barrel-shaped or elliptical fungi in considerable numbers in oral lesions is an indicator that a patient may have geotrichosis. Under the microscope the fungi appears yeast-like and septate branching hyphae that can be broken down into chains or individual arthrospores. Arthrospores appear rectangular with flat or rounded ends. Under the microscope the arthroconidia size range from 6-12μm x 3-6μm. Arthroconidia and coarse true hyphae can be observed can be observed under the microscope. Another identification method for G. candidum is selective isolation method. A selection isolation method based on the fungi tolerance to novobiocin and carbon dioxide can determine if G. candidum is the cause of illness.
X-rays can be used to examine the lung tissue, however it can not be used to positively diagnose geotrichosis. X-rays may show cavitation that is located the walls of the lungs tissues. The lung tissue resemble the early signs of tuberculosis. The results of an x-ray examination of pulmonary geotrichosis presents smooth, dense patchy infiltrations and some cavities. Bronchial geotrichosis shows peribronchial thickening with fine mottling may be present on middle or basilar pulmonary fields. Bronchial geotrichosis usually present itself as non-specific diffuse peribronchical infiltration.
Geotrichosis generally has a good prognosis and patients generally have successful recovery. However there is not a standard treatment for geotrichosis. There are several types of antimicrobial or antifungal compounds that can be used for geotrichosis treatment. One type of treatment of geotrichosis can involve miconazole and ketoconazole, which has shown to improve cutaneous, branchopulmonary, intestinal and joint conditions. Another method of treatment involves symptomatic care, bed rest, iodine therapy, aersol nystatin and amphotericin B. Azole drugs including isoconazole and clotrimazole are used for geotrichosis treatment. Associated treatment for pulmonary geotrichosis includes the use of potassium iodine, sulfonamides or colistin. The associated asthma can be treated with desensitization and prednisolone. Amphotericin B, clotrimazole and S-fluorocytosine have become more susceptible to G. candidum. Antimycotic resistance can appear due to repeated treatment.
Geotrichum candidum can be used commercially to inoculate wash-rinds and bloomy rind cheeses. Cultures can be added to milk, brine or sprayed onto cheese surface. The optimum pH range for growth on cheese ranges from 4.4 to 6.7. The fungus colonizes nearly the entire surface of the cheese during the early stages of ripening. It is found on soft cheeses like Camembert cheese and semi-hard cheese Saint-Nectaire and Reblochon. For the Camembert cheese the fungi grows on the outside of the cheese forming a rind. The fungus is responsible for the uniform, white, velvety coat on Saint-Marcellin cheese. Lipases and proteases from G. candidum release fatty acids and peptides that provide the cheese with distinctive flavors. Geotrichum candidum reduces the bitterness in Camembert cheese through the activity of the aminopeptidases that hydrolyze low molecular weight hydrophobic peptides. Aminopeptidases also contributes an aroma in traditional Norman Camembert. The fungus also neutralize the curd by catabolizing lactic acid produced by bacteria. Geotrichum candidum prepares the cheese for colonization of other acid sensitive bacteria such as Brevibacterium. The fungus has inhibits growth of the bacteria Listeria monocytogenes. Commercial strains of G. candidum are available for cheese ripening.
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