Introduction of Yeasts and Molds
Table of Contents
Yeasts are small round or oval in shape, eukaryotic, single-celled microbes classified as members of the fungus kingdom. The first yeast emerged hundreds of millions of years ago, and at least 1,500 species are presently recognized. They are estimated to constitute 1% of all described fungal species. e.g. Cryptococcus, Candida, Saccharomyces, etc. Molds are microscopic fungi that grow in the form of multicellular filaments (hyphae). e.g. Aspergillus, Penicillium, Mucor, Dermatophytes,etc.
Differences between Yeasts and Molds
|1.||Definition||Unicellular, budding fungus||Multicellular, threadlike fungus|
|2.||Form||It grows as large single cells.||Mold grows as multiple tubular branches.|
|4.||Size||2-30 µm||2-5 µm|
|5.||Habitat||It is very common and it can be found on fruit and berries, in the stomachs of mammals, and on the skin, among other places.||It is commonly found in damp, dark, or humid areas.|
|6.||Appearance||White and thready. Normally oval in shape.||It has a fuzzy appearance and can be found in several shapes.|
|7.||Hyphae||Yeast does not have true hyphae. Instead, it forms pseudo-hyphae since the origin of branches with constrictions and septa.||Mold has microscopic filaments called hyphae and it has true hyphae since no constriction in the origin of branches and with the parallel cell wall.|
|9.||Colony morphology||Colonies are soft, opaque, and cream-colored.||Filamentous type colony having vegetative and aerial hyphae.|
|10.||Color||Generally colorless||Usually colorful and maybe orange, green, black, brown, pink or purple, etc.|
|11.||Incubation Temperature||Incubated at room temperature (25- 30oC)||Routine incubation temperature is usually 25-30o C, although 37o C incubation can be used to differentiate some molds based on temperature tolerance or to determine whether organisms are diphasic since |
dimorphic fungi exist as yeasts in the host tissue and the culture at 37°C and hyphal (mycelium) forms in the soil and in the culture at 22-25°C.
|These organisms generally grow within 24 to 36 hours of incubation (fast growers).||These organisms usually grow more slowly than yeasts after inoculation to fungal culture media. e.g. dermatophytes may take up to 4 weeks (slow growers).|
|13.||Aerobic/Anaerobic||Both conditions||Only in aerobic conditions|
|14.||pH range for growth||Its growth is limited to a pH range of 4.0 to 4.5.||It can grow in a wider range of acidity (pH) levels than yeasts.|
|Identification is based on physiologic tests and a few key morphologic differences.||Most clinical molds can be identified by microscopic examination of the ontogeny and morphology of their asexual spores.|
|16.||Reproduction||Most of them reproduce asexually through mitosis. The most common form is called “budding.” A smaller number of yeasts reproduce by binary fission. The growth process is by budding.||Reproduce through small spores, which can be either sexual or asexual. The growth process is by apical elongation.|
|17.||Asexual Spores||Blastospore||Sporangiospores and Conidia|
|18.||Sexual Spores||Absent||Present (zygospores, ascospores, and basidiospores)|
|19.||Energy Production||Yeasts convert carbohydrates to alcohol and carbon dioxide in anaerobic through fermentation. They also obtain carbon from hexose sugars.||Molds secrete hydrolytic enzymes that degrade biopolymers such as starch, cellulose, and lignin into simpler substances that can be absorbed.|
|20.||Health risks||It can cause infection in individuals who are immunocompromised.||It can cause allergic reactions and respiratory problems.|
|21.||Other risks||Yeasts are comparatively less involved in spoilage.||Molds cause a greater threat in terms of food spoilage and sanitation concerns, particularly in fresh produce.|
|22.||Uses||Yeasts are applied for alcohol (ethanol) production, baking, vitamin supplements, and the study of the cell cycle.||Some molds are utilized in food production, for example, Penicillium is used in the production of cheese, Neurospora in the production of oncom (traditional staple food), which is made from the by-product of tofu. Mold is also a crucial saprophyte.|
|23.||Species||1500 known species||400,000 types of molds|
|24.||Yeast and mold %||1% of all fungi||99|
|25.||Examples||Candida, Cryptococcus neoformans, Saccharomyces cerevisiae, Malassezia, Trichosporon, etc.||Aspergillus, Penicillium, Mucor, Rhizopus, Fusarium, Trichophyton, Microsporum, Epidermophyton, Alternaria, etc.|
|26.||Common Fungal Infection||Oral thrush, Vaginal thrush,|
|Ringworm, Aspergillosis, Histoplasmosis, Blastomycosis, Coccidioidomycosis|
Keynotes on Yeasts and Molds
- Fungi are mainly found in two forms-yeast and Mold.
- Dimorphic fungi exist as yeasts in the host tissue and the culture at 37°C and hyphal (mycelium) forms in the soil and in the culture at 22-25°C. Most of them cause systemic infection. e.g. Blastomyces dermatitidis, coccidioides immitis, Histoplasma capsulatum.
- Fungal spores are of two types and they are sexual and asexual spores.
- Sexual spores are of 4 types and they are oospore, zygospore, ascospore, and basidiospore.
- Asexual spores are produced by mitosis and further of two types and they are vegetative spores and Aerial spores. Vegetative spores are further three types-arthrospores, blastospores, and chlamydospores. Similarly, aerial spores are of four types-conidiospores, microconidia, macroconidia, and sporangiospores.
- Dermatophytes are a group of three fungal genera Trichophyton(responsible for skin, hair, and nail infections), Microsporum(causing hair and skin infection), and Epidermophyton(generally causing nail infection).
- True fungal pathogens are Histoplasma, Blastomyces dermatitidis, Coccidioides immitis, and Paracoccidioides brasiliensis.
- Opportunistic fungal pathogens are Candida, Aspergillus Pneumocystis jirovecii, Mucor, and Penicillium.
- Predisposing factors of fungal infection are diabetes, prolonged treatment with corticosteroids, immunosuppression, broad antibiotic therapy, and injury.
- The clinical classification of fungi is as follows-
- Superficial fungi
- Cutaneous fungi
- Subcutaneous fungi
- Systemic fungi
Footages on Yeasts and Molds
Yeasts of Candida albicans in Gram staining of culture at a high magnification
Yeast cells of Cryptococcus neoformans in Gram staining of culture at a high magnification
Yeast cells in wet mount microscopy at a high magnification
Cryptococcus in saline wet mount microscopy
Mould of Aspergillus fumigatus in LPCB tease mount microscopy
- Parija S.C. (2012). Textbook of Microbiology & Immunology. (2 ed.). India: Elsevier India.
- Sastry A.S. & Bhat S.K. (2016). Essentials of Medical Microbiology. New Delhi: Jaypee Brothers Medical Publishers.