Medical Mycology Lecture 2-3 PDF

Medical MycologyLecture 2+3 WHAT ARE FURNGI? Fungi form a separate group of higher organisms, distinct from both plants and animals, which differ from other groups of organisms in several major respects :- First: fungal cells are encased within a rigid cell wall, mostly composed of chi...

Medical MycologyLecture 2+3 WHAT ARE FURNGI? Fungi form a separate group of higher organisms, distinct from both plants and animals, which differ from other groups of organisms in several major respects :- First: fungal cells are encased within a rigid cell wall, mostly composed of chitin and glucan. These features contrast with animals, which have no cell walls, and plants, which have cellulose as the major cell wall component. Chitin is a long-chain polymer of a N- acetyl glucosamine, a derivative of glucose, and is found in many places throughout the natural world. Glucan molecule is a polysaccharide of D-glucose monomers, linked by glycosidic bonds. Many beta-glucans are medically important. They represent a drug target for antifungal medications.  Second: fungi are heterotrophic. This means that they are lacking in chlorophyll and cannot make their organic food as plants can, through photosynthesis. Fungi live embedded in a food source or medium, and obtain their nourishment by secreting enzymes for external digestion and by absorbing the nutrients that are released from the medium. Third: fungi are simpler in structure than plants or animals. There is no division of cells into organs or tissues. The basic structural unit of fungi is either a chain of tubular, filament-like cells, termed a hypha or hyphae (plural) or an independent single cell. Fourth: fungi reproduce by means of microscopic propagules called spores. Many fungi produce spores that result from an asexual process. Many fungi are also capable of sexual reproduction. Some species are homothallic and able to form sexual structures within individual colonies.  General properties of fungi: 1. They are eukaryotic; cells contain membrane bound cell organelles including nuclei, mitochondria, golgi apparatus, endoplasmic reticulum, lysosomes etc. They also exhibit mitosis. 2.They have ergosterols in their membranes and possesses 80S ribosomes. 3. They have a rigid cell wall and are therefore non-motile, a feature that separates them from animals. All fungi possess cell wall made of chitin. 4. They are chemoheterotrophs (require organic compounds for both carbon and energy sources) and fungi lack chlorophyll and are therefore not autotrophic. 5. Fungi are osmiotrophic; they obtain their nutrients by absorption. 6. They obtain nutrients as saprophytes (live on decaying matter) or as parasites (live on living matter).Some of them are symbiotic (living with other plants ,both the plant and fungus get benefit from each other).  7. All fungi require water and oxygen and there are no obligate anaerobes. 8. Typically reproduce asexually and/or sexually by producing spores. 9. They grow either reproductively by budding or non-reproductively by hyphal tip elongation. 10. Food storage is generally in the form of lipids and glycogen. Many fungal pathogens of humans and animals change their growth form during the process of tissue invasion. These dimorphic pathogens usually change from a multicellular hyphal form in the natural environment to a budding, single celled form in tissue. In most multicellular fungi the vegetative stage consists of a mass of branching hyphae, termed a mycelium. Each individual hypha has a rigid cell wall and increases in length as a result of apical growth. In the more primitive fungi, the hyphae remain aseptate (without cross-walls). In the more advanced groups, however, the hyphae are septate. Beneficial Effects of Fungi: 1. Decomposition :Nutrient and carbon recycling. 2. Biosynthetic factories. The fermentation property is used for the industrial production of alcohols, fats, citric, oxalic and gluconic acids. 3. Important sources of antibiotics, such as Penicillin. 4. Model organisms for biochemical and genetic studies. eg: Neurospora crassa 5. Saccharomyces cerviciaeis extensively used in recombinant DNA technology, which includes the Hepatitis B Vaccine. 6. Some fungi are edible like mushrooms and truffles. 7. Yeasts provide nutritional supplements such as vitamins and cofactors. 8. Some Penicillium species are used to flavor Roquefort and Camembert cheeses. 9. Ergot produced by Claviceps purpurea contains medically important alkaloids that help in inducing uterine contractions, controlling bleeding and treating migraine. 10. Fungi (Leptolegnia caudate and Aphanomyces laevis) are used to trap mosquito larvae in paddy fields and thus help in malaria control.  Harmful Effects of Fungi: 1. Destruction of food, wood, paper, and cloth. 2. Animal and human diseases, including allergies. 3. Toxins produced by poisonous mushrooms and other filamentous fungi within food (Mycotoxicosis). 4. Plant diseases. 5. Spoilage of agriculture products such as vegetables and cereals in the storage places and silos. 6. Damage the products such as magnetic tapes and disks, glass lenses, marble statues ‫ اﻟﻣرﻣر ﺗﯾل‬and wax.  The differences between bacteria and fungi: 1. Fungi are eukaryotes while bacteria are prokaryotes. 2. Bacteria are single celled whereas most fungi are multicellular except for yeast. 3. The compositions within their cell walls are different. Peptidoglycan (also called murein)in bacteria and Chitin in fungi. 4. Fungi are heterotrophs while Bacteria can be autotrophs or heterotrophs. 5. Bacteria have 3 distinct shapes while fungi have various shapes. 6. Bacteria reproduce asexually via binary fission whereas, fungi are capable of reproducing both sexually or asexually.  Morphology of Fungi Fungi exist in two fundamental forms; the filamentous (hyphal) and single celled budding forms (yeast). But, for the classification sake they are studied as moulds, yeasts, yeast like and dimorphic fungi. Moulds: The thallus of mould is made of hyphae, which are cylindrical tube like structures that elongates by growth at tips. A mass of hyphae is known as mycelium. It is the hypha that is responsible for the filamentous nature of mould. The hyphae may be branched or unbranched. They may be septate or aseptate. Hyphae usually have cross walls that divide them into numerous cells. These cross walls, called septa have small pores through which cytoplasm is continuous throughout the hyphae. Unicellular budding yeast Filamentous fungus Hypha :- Elongation of apical cell produces a tubular, thread like structure called hypha , Hyphae may be septate or non-septate(coenocytic)hyphae. Mycelium :- Tangled mass of hyphae is called mycelium. Fungi producing mycelia are called molds or filamentous fungi.  Mycelium are of three kinds: 1. Vegetative mycelium are those that penetrates the surface of the medium and absorbs nutrients. 2. Aerial mycelium are those that grow above the agar surface 3. Fertile mycelium are aerial hyphae that bear reproductive structures such as conidia or sporangia. Since hypha is the structural unit of mould, the mycelium imparts colour, texture and topography to the colony. Those fungi that possess melanin pigments in their cell wall are called phaeoid or dematiaceous and their colonies are coloured grey, black or olive. Examples are species of Bipolaris, Cladosporium, Exophiala, Fonsecaea, Phialophora and Wangiella. Those hyphae that don't possess any pigment in their cell wall are called hyaline like Acremonium and Fusarium Colony and microscopic morphology of dematiaceous fungus Fonsecaea pedrosoi  White colonies and hyaline hyphae and conidia of Acremonium sp. Hyphae may have some specialized structure or appearance that aid in identification. Some of these are: a) Spiral hyphae: These are spirally coiled hyphae commonly seen in Trichophyton mentagrophytes. b) Pectinate body: These are short, unilateral projections from the hyphae that resemble a broken comb. Commonly seen in Microsporum audouinii.  c) Favic chandelier: These are the group of hyphal tips that collectively resemble a chandelier or the antlers of the deer (antler hyphae). They occur in Trichophyton schoenleinii and Trichophyton violaceum  d) Nodular organ: This is an enlargement in the mycelium that consists of closely twisted hyphae. Often seen in Trichophyton mentagrophytes and Microsporum canis. e) Racquet hyphae: There is regular enlargement of one end of each segment with the opposing end remaining thin. Seen in Epidermophyton floccosum, Trichophyton mentagrophytes.  f) Rhizoides: These are the root like structures seen in portions of vegetative hyphae in some members of zygomycetes.  Yeasts: Yeasts are unicellular spherical to ellipsoid cells. They reproduce by budding, which result in blastospore (blastoconidia) formation. In some cases, as the cells buds the buds fail to detach and elongate thus forming a chain of elongated hyphae like filament called pseudohyphae. This property is seen in Candida albicans. The same species also have the ability to produce true hypha, which is seen as germ tube. The difference between the two is that there is a constriction in psueudohyphae at the point of budding, while the germ tube has no constriction.  Some yeast such as Cryptococcus and the yeast form of Blastomyces dermatatidis produce polysaccharide capsule. Capsules can be demonstrated by negative staining methods using India ink or Nigrosin. The capsule itself can be stained by Meyer Mucicarmine stain. Some yeasts are pigmented. Rhodotorula sps produces pink colonies due to carotenoid pigments while some yeasts such as Phaeoannellomyces werneckii and Piedraia hortae are dematiaceous, producing brown to olivaceous colonies. True yeasts such as Saccharomyces cerviciae don't produce pseudohyphae. Yeast-like fungi may be basidiomycetes, such as Cryptococcus neoformans or ascomycetes such as Candida albicans.  Classification of fungi: Fungi were initially classified with plants and were a subject of interest for botanists; hence the influence of botany can be seen on their classification. In 1969 R.H Whittaker classified all living organisms into five kingdoms namely Monera, Protista, Fungi, Plantae and Animalia. Traditionally the classification proceeds in this fashion: Kingdom - Subkingdom - Phyla/phylum - Subphyla - Class - Order - Family – Genus-Species.This classification is too complicated to be dealt here. There are alternate and more practical approaches, one based on sexual reproduction and the other based on morphology of the thallus (vegetative structure).Accordingly, the kingdom fungi divided in several classes:  1. Zygomycetes: which produce through production of zygospores 2. Ascomycetes: which produce endogenous spores called ascospores in cells called asci (single:ascus).  3. Basidiomycetes: which produce exogenous spores called basidiospores in cells called basidia. 4. Deuteromycetes (Fungi imperfecti): fungi that are not known to produce any sexual spores (ascospores or basidiospores).  Based on Morphology: 1. Moulds (Molds): Filamentous fungi eg: Aspergillus sps, Trichophyton rubrum  2. Yeasts: Single celled, cells that buds eg: Cryptococcus neoformans, Saccharomyces cerviciae Cryptococcus neoformans Saccharomyces cerviciae  3. Yeast like: Similar to yeasts but produce pseudohyphae eg: Candida albicans 4. Dimorphic: Fungi existing in two different morphological forms at two different environmental conditions. They exist as yeasts in tissue and in vitro at 37C° and as moulds (hyphae) in their natural habitat and in vitro at room temperature. eg: Histoplasma capsulatum, Blastomyces dermatidis, Paracoccidiodes brasiliensis, Coccidioides immitis Some 200 "human pathogens" have been recognized from among an estimated 1.5 million species of fungi.  Based on the site of infection ( Clinical Classification ):- 1- Superficial infection. 2- Cutaneous infection. 3- Subcutaneous infection. 4- Systemic infection. 5-Opportunistic infection.  Reproduction in fungi: Fungi reproduce by asexual, sexual and parasexual means (The parasexual cycle, a process restricted to fungi and single-celled organisms, is a nonsexual mechanism of parasexuality for transferring genetic material without meiosis or the development of sexual structures). Fungi can reproduce asexually by fragmentation, budding, or producing spores, or sexually with homothallic or heterothallic mycelia. Asexual reproduction is the commonest mode in most fungi ,while fungi participating in sexual mode only under certain circumstances. The form of fungus undergoing asexual reproduction is known as anamorph (or imperfect stage) and when the same fungus is undergoing sexual reproduction, the form is said to be teleomorph (or perfect stage). The whole fungus, including both the forms is referred as holomorph. A-Penicillium Anamorph (asexual stage) B-Eupenicillium Teleomorph (sexual stage)  Importance of Spores: A. Biological 1) Allows for dissemination 2) Allows for reproduction 3) Allows the fungus to move to new food source. 4) Allows fungus to survive periods of adversity. 5) Means of introducing new genetic combinations into a population B. Practical 1) Rapid identification (also helps with classification) 2) Source of inocula for human infection 3) Source of inocula for contamination