Chapter Two: Morphological Features PDF

MORPHOLOGICAL FEATURES There are three basic morphologies:  YEAST: It is a unicellular organism that reproduces by budding (blastoconidia formation) or by binary fission. The continuation of the budding process can produce a chain of elongated yeast cells called pseudohyphae. Yeasts a...

MORPHOLOGICAL FEATURES There are three basic morphologies:  YEAST: It is a unicellular organism that reproduces by budding (blastoconidia formation) or by binary fission. The continuation of the budding process can produce a chain of elongated yeast cells called pseudohyphae. Yeasts are larger than bacteria and are commonly spherical to egg shaped.  It is usually a facultative anaerobe and prefers warmer temperatures.  It grows at 370c and forms a creamy opaque or pasty colony on culture media. It grows rapidly within 24-48hours. MOULD: It has branching tubular structures called hyphae. It grows at 250c. It forms a mass of hyphae called mycelia on culture media.  The colonies formed by moulds are fluffy, cottony, woolly or powdery.  It grows slowly, requiring about 6 weeks growing.  The pattern of branching and the width of the mycelium are aids to the morphological identification.  If the mycelia do not have septa, they are called coenocytic (non-septate).  The terms “hypha”, “mycelium” and “mold” are frequently used interchangeably. DIMORPHIC FUNGI: These exist as yeasts or spherules in tissues and exudates at 35-370c. Conversion to the yeast form appears to be essential for pathogenicity. The different forms are identified by morphological or biochemical characteristics and the appearance of their fruiting bodies.  HYPHAE  The hyphae may be coenocytic, meaning that they may be aseptate and multinucleated. They may be also septate.  They grow at their tips by apical extension.  They are divided into two main types: Vegetative and aerial hyphae.  The vegetative hyphae are submerged and are responsible for nutrition while the aerial hyphae project above the surface and often produce specialized structures called conidia for asexual reproduction. FUNGAL REPRODUCTION Fungi reproduce by budding, hyphal extension or spore formation. FUNGAL SPORES  They are resistant to heat, cold, acids, bases and other chemicals. They can function as allergens.  They are produced by sexual and asexual means. There are two types of asexual spores:  Sporangiospores- These are spores produced within a containing structure called sporangium. Examples are Rhizopus and Mucor.  Conidia- These are spores borne naked, for example, Aspergillus, Penicillium and Dermatophytes. The asexual spores may be large (macroconidia, chlamydospores) or small (microconidia, blastospores, arthroconidia). They may be single or in groups but are produced by specialized hyphae called conidiophores.  There are three main types of conidia: 1. Arthroconidia 2. Chlamydospores 3. Blastoconidia  ARTHROCONIDIA: This is the simplest type. It is formed directly from the hyphae by fragmentation through points of septation. When it is matured, it may appear as square, rectangular or barrel- shaped thick- walled cells.  CHLAMYDIOSPORES: These are round, thick-walled resistant spores which are formed from rounding up and enlargement of the terminal cells of the hyphae. They are the resistant spores produced during resting period.  BLASTOCONIDIA: Most often found in yeasts. These cells are produced by budding. They may be single and round or multiple. They may also form pseudohyphae from elongation of the blastoconidia.  SEXUAL SPORES These spores are produced as a result of fusion of two gametes (two nuclei). There are four main types 1. Zygospores 2. Ascospores 3. Oospores 4. Basidiospores ZYGOSPORES This sexual spore occurs when morphologically identical cells fuse. ASCOSPORES This is produced within a bag-like structure called ascus, which is contained in a larger structure called a cleistothecium. OOSPORES This sexual spore occurs when morphologically different cells fuse. BASIDOPORES This is produced by extrusion of club-like structures. SIZE OF SPORES MACRO CONIDIA: They are large, usually multi septate, club or spindle shaped. They may be smooth or rough walled. They are usually borne on a short to long conidiophores.  MICRO CONIDIA: They are small, unicellular, round, elliptical or pyriform in shape. They may be borne directly on the side of the hyphal strand or at the end of long or short conidiophores. They are seen on many fungi but are useful in the differentiation of Dermatophytes. CHARACTERISTICS OF MYCOTIC INFECTIONS 1. Fungi are ubiquitous in nature and most people are exposed to them. 2. The establishment of an infection usually depends on the size of the inoculum and the resistance of the host. 3. The severity of the disease seems to depend mostly on the immunologic status of the host. Therefore, the demonstration of fungi, for an example, in the blood drawn from an intravenous catheter, can correspond to colonization of the catheter, to transient fungaemia or to a true infection. The physician must decide which one of these, is the clinical status of the patient. This decision should be based on clinical parameters, such as the general status of the patient, laboratory results etc. The decision is not trivial, since the treatment of systemic fungal infections requires the aggressive use of drugs with considerable toxicity. 4. Most pathologic fungi do not produce toxins. 5. They show physiologic modifications during a parasitic infection. 6. They are thermo tolerant. 7. They are able to with stand many host defenses and can resist the effects of the active oxygen radicals (respiratory burst), released during phagocytosis. 8. Mycotic diseases are generally not communicable from person to person. CLINICAL MANIFESTATIONS There are three main groups of clinical syndrome associated with any fungal infection. 1. It may be Mycotoxicosis 2. Hypersensitivity reaction/disease or 3. Colonization with eventual disease formation. MYCOTOXICOSIS This is due to an ingestion of fungal toxins. Most of these infections are accidental. Examples; 1. Ergot alkaloids of Claviceps purpurea which cause tissue inflammation, necrosis and gangrene. 2. Aflatoxin of Aspergillus flavus which causes liver damage and may be carcinogenic. HYPERSENSITIVITY REACTIONS/DISEASES This results from the fungal spores in the air. The fungal spore count is one of the indices that are measured to determine air pollution. It can trigger off asthmatic attack, rhinitis, pneumonitis and alveolitis. INFECTIONS SECONDARY TO COLONIZATION These infections are classified based on the part of the body that is affected. They are: 1. Superficial mycosis 2. Cutaneous mycosis 3. Subcutaneous mycosis 4. Systemic mycosis 5. Opportunistic mycosis.This is caused by organisms of low pathogenicity that produce disease only under conditions of lowered immunity of the host.  CHAPTER FIVE SUPERFICIAL MYCOSES These groups of infections affect the outermost layers of the skin and hair without actually eliciting host /pathogen hypersensitivity reactions in most cases. Sometimes, the infection can be so minute that the patient is insensitive of the problem. Most times, treatment is sort by the patient due to cosmetic reasons. There are two types: 1. Tinea (Pityriasis and Nigra) and 2. Piedra (White and Black). While the Tinea affects the skin, the piedra affects the hairs. WHITE PIEDRA This infection is caused by Trichosporon beigelii. The pathology is the appearance of a soft loosely attached white or light brown nodule on the axillary, pubic, beard and moustache hairs. The scalp is not always involved. Culture: A cream –colored yeast-like colony with radial furrows and irregular folds are seen in culture. Microscopy: Septate hyaline hyphae is seen which may fragment into oval/rectangular arthrospores and sometimes blastospores. Treatment: Remove the affected hairs and apply a topical antifungal at the affected sites. 3% sulphur ointment can also be used. BLACK PIEDRA This is due to Piedraia hortae. This leads to a firmly attached, hard, black nodule around the outside of scalp hairs. The infection can also affect other parts of the body. Culture: Small brown to black colonies with elevated centre that is dark with a flat periphery. Sometimes, there will be a red pigmentation on the media. Microscopy: This shows dark, thick-walled, septate hyphae with swollen irregular cells (Chlamydoconidia). Treatment: Remove the affected hairs and apply a topical antifungal at the affected sites. 3% sulphur ointment can also be used. 5.1 TINEA NIGRA PALMARIS This is a chronic infection of the stratum corneum (horny layer of the skin) due to Exophiala werneckii. Tinea nigra appears light brown to black non-scaly patches mostly on the palm of hands and fingers. Sometimes, it affects the face and planter aspect of the feet. It is a dimorphic fungus. Culture: Appears as a black yeast colony that may later develop to short olive-gray mycelium on skin scrapings taken from the periphery of the lesion. Microscopy: Using 10% KOH, the organism appears as a brown, pigmented budding yeast cells with branched septate hyphae. The brown colour excludes the diagnosis of dermatophytosis, Candidiasis and pityriasis vesicolor. Therapy :Daily application of Whitfield’s ointment or 3% sulphur or 1 % salicylic acid or any azole drug will clear the infection. Re-infection may occur. TINEA VERSICOLOR (PITYRIASIS VERSICOLOR):This a chronic and non- irritating infection of the stratum corneum caused by Malassezia fur fur (Pityrosporum orbiculare).The infection is characterized by superficial white brown or fawn colored lesions. The lesions are usually non- inflammatory with thin scales and sharply marginated. Discrete, serpentine, hyper pigmented or depigmented maculae occur on the skin of the chest, upper back, arms or abdomen. Invasion of the cornified skin and the host responses are both minimal. It is mostly seen in the tropics and the affected patients seek medical care due to cosmetic reasons. The infection is mostly seen on the trunk and shoulder and can be transmitted from person to person. Culture: Not usually necessary since diagnosis is mostly clinical. Microscopy: On a 10-20% KOH digested skin scrapping; there will be short, unbranched hyphae or spherical cells which are diagnostic. Using wood’s lamp, a golden yellow fluorescence that extends beyond the periphery of the lesions is diagnostic. Therapy: The use of 2% selenium sulphide applied daily over the body for 15mins per day for a few days will clear the infection. In the absence of the sulphide, selsun, a shampoo can be put in the water for bathing. It may recur after treatment.  CHAPTER SIX CUTANEOUS MYCOSES These are fungal diseases that are confined to the outer layers of the skin, nail or hair. They only affect the keratinized layers of the body, rarely invading the deeper tissues or viscera. The fungi involved are called Dermatophytes. They are a group of 40 related fungi that belong to three genera: Microsporum, Trichophyton and Epidermophyton. Dermatophytes are probably restricted to the nonviable skin because most are unable to grow at 370c or in the presence of serum. They may cause persistent infections but they are neither debilitating nor life-threatening. In skin, they are diagnosed by the presence of hyaline, septate, branching hyphae or chains of arthroconidia. In culture, the many species are closely related and are often difficult to identify. Dermatophytes are classified as anthropophilic, geophilic and zoophilic , depending on whether their usual habitat is humans, soil or animals. Anthropophilic:- They are usually associated with humans only. Transmission from man to man is by close contact or through contaminated objects. Anthropophilic species, which cause the greatest number of human infections, cause relatively mild and chronic infections in humans. They produce few conidia in culture, and may be difficult to eradicate. Geophilic:- They are usually found in the soil, and are transmitted to man by direct exposure. This is less adapted to human hosts; they produce more acute inflammatory infections that tend to resolve more quickly. This also occurs in zoophilic cases. The most common geophilic species causing human infections is Microsporum gypseum. Zoophilic:- They are usually associated with animals and transmission to man is by close contact with animals (cats, dogs, cows etc) or with contaminated products. The knowledge of the species of Dermatophytes and source of infection are important for proper treatment of the patient and control of the source. Invasion by zoophilic or geophilic organisms may cause inflammatory disease in man. Dermatophytes have a worldwide distribution but some species are limited to a particular geographic location and race. For instance, in South Carolina, USA, Trichophyton tonsurans is more common in blacks while Microsporum canis is found commonly in Caucasians. The key feature is an annular scaling patch with a raised margin showing a degree of inflammation, the centre usually less inflamed than the edges. The precise appearance varies with the affected site, the fungal species involved and the host immune response. Inappropriate application of topical steroids may lead to an infection showing none of the classical signs. Interdigital fungal infection may cause cracks in the skin and lead to bacterial cellulitis. These groups of infections are also called “Tinea” or “ringworm” or “moth like”. They are as follows: TINEA CORPORIS- This is small lesions occurring anywhere on the body. It is a disease of childhood. It is also known as ringworm. It is commonly caused by all species of Dermatophytes. TINEA PEDIS- “athlete’s foot”. Infection of toe webs and soles of feet. This is seen in children and young adults. It is usually due to T. mentagrophytes and T. rubrum. It causes toe-web fissures, maceration, scaling of soles, erythema, vesicles/pustules, and bullae. Bacterial organisms can produce an identical appearance. TINEA UNGUIUM (ONYCHOMYCOSIS)- The infections are on the nails. Sometimes, the infections are lifelong. In the commonest form, the nail is invaded from the nail bed. Thick, discolored (white, yellow, brown, black) , dystrophic nails. It can occur at any age but is commoner with increasing age. A number of other fungi may cause onychomycosis including Scopulariopsis brevicaulis, Acremonium species and Fusarium species. TINEA CAPITIS- This is fungal infection of the hair, mostly seen in children. Those infections in which the arthrospores are found on the hair surface are termed ectothrix infections while those with spores that develop inside the hair are called endothrix infections. Mainly, there is scalp scaling and hair loss. It may resemble dandruff. In ectothrix infections, hair tends to break a few millimeters above the skin, unlike in endothrix infections where the hair breaks at the skin surface. Inflammation is variable and may be severe with pustules and an exudative crest. Untreated scalp ringworm usually remits spontaneously after puberty. TINEA CRURIS- “JOCK ITCH”- Infection of the groin, perineum or perianal area. These are erythematous lesions with central clearing and raised borders in the groin and less commonly, the scrotum. It is usually seen in young men. TINEA BARBAE- Fungal infection of the bearded areas of the face and neck resulting into scaly plaques, pustules and vesicles. Tinea imbracata is a variant caused by T. concentricum, which is characterized by a rash that is made of concentric rings of scales. It is endemic in parts of Southeast Asia, the South Pacific, Central America and South America. TINEA MANNUS- Infection of the fingers. As stated earlier, there are three genera of Dermatophytes: A. TRICHOPHYTON The species are more than 19. They are Trichophyton tonsurans, T. rubrum, T. mentagrophytes, T.equinum, T. verrucosum, etc. It infects skin, hair and nails. It takes about 2-3 weeks to grow in culture. It forms large conidia (macroconidia), which are smooth, thin –walled, septate (0-10 septa), and pencil- shaped. The colonies have loose aerial mycelia, which produce a variety of pigments. The identification requires special biochemical and morphological techniques. Trichophyton rubrum may cause subcutaneous infections in immunocompromised individuals, although, this is very rare. B. MICROSPORUM The species are more than 13. They are Microsporum gypseum, Microsporum gallinae, Microsporum canis, Microsporum nanum, M. audouini etc. It affects skin and hair. It causes infected hairs to fluoresce a bright green color when illuminated with a UV emitting wood’s light. The loose, cottony mycelia produce macroconidia which are thick-walled, spindle shaped, multicellular, and spiny (echinulate). C. EPIDERMOPHYTONFLOCCOSUM This is the only one specie in this genus and it affects skin and nails. It appears as yellow-colored and cottony in culture. It is usually identified by the thick, bifurcated hyphae with multiple, smooth, club shaped macroconidia of 2-4cells.