Superficial mycoses- Tinea nigra, Piedra

 Superficial Mycoses

(1) Pityriasis versicolor

• Common superficial mycoses caused by a lipophilic yeast Malassezia furfur 
• Chronic usually asymptomatic involvement of stratum corneum 
•  Normal flora of skin - so the disease re-occurs.
• common in the tropics with more than 60 % of the population infected-young adults mainly
• Cause opportunistic infection in normal skin
• Multiple patchy well-demarcated lesions (oval shape) hypo or hyper pigmented -either light in color or brown -
• Scattered over the back, neck, chest, shoulders - Seldom the face.
  

Predisposing factors:

• Humidity –Immunosuppression –Poor hygiene – Sweat – Greasy skin – Chronic bacterial infections – Steroids
• Affects the epidermal area with strong affinity to keratin

Diagnosis:

• Skin scraping using scalpel (sharp bladed instrument) followed by 10% KOH preparation (skin scrapings placed into 10% KOH , which will destroy skin cells and only fungal cells are retained)

Sampling  can be done with Wood ́s lamp (producing ultraviolet radiation) -uses ultraviolet (UV) light to look at the skin closely. Normally, the light will look purple or violet and skin will not fluoresce (glow) or show any spots under the Wood’s lamp. Skin will change color there is fungal infection 

• Direct Microscopy – abundant yeast-like cells and short branched hyphal filaments- “Spaghetti and meatballs”

• Culture: Malassezia furfur (Lipophilic) can be grown in a laboratory only if the culture is covered with oil. --Sabouraud Dextrose Agar flooded with Olive oil

    Dixon's agar- A specialized isolation medium containing glycerol-mono-oleate for primary isolation of Malassezia furfur 

  Tinea nigra

Localized chronic superficial fungal infection of stratum corneum of the palms and soles- brown to black macules– found mainly in the tropics

Caused by Cladosporium werneckii/Hortae werneckii/Exophiala werneckii

Laboratory Diagnosis

Direct Microscopy of 10% KOH preparation of Skin scrapings - pigmented brown to dark septate hyphal elements and budding yeast cells

Culture: Primary isolation media is Sabouraud's dextrose agar - colonies are mucoid, yeast-like grey or black

 Piedra

fungus infection of the hair-characterized by the appearance of firm, irregular nodules along the hair shaft- nodules are fungal cells cemented together with the hair

Two varieties-

Black piedra – caused by Piedra hortae - Chronic fungal infection of the hair shaft  mostly affects young adults  epidemics in families-Infected hairs: hard black nodules on the shaft •

Direct Microscopy: – 10% KOH • darkly pigmented nodules in hair shaft –asci detected

Culture: – On SDA, colonies are dark, brown-black -Take about 2-3 weeks to appear

White piedra – caused by Trichosporon beigelii –  common in young adults - Nodules: white, follicles– irregular, soft, white or light brown nodules adhering to the hairs

   Direct Microscopy: – 10% KOH – Arthrospores

2.   Culture: primary isolation media – white or yellowish to deep cream colored – smooth, wrinkled, velvety, dull colonies with a mycelial fringe. Laboratory Diagnosis

Mycorrhiza (fungus root)

·      Symbiotic association between fungi and plant roots-  Rhizosphere interaction

·      The term Mycorrhiza introduced by A B Frank

·      Mutualistic association where both partners depend on each other and have coevolved

Important component of soil life and soil chemistry

Mycorrhizae form a mutualistic relationship with the roots of most plant species (80% of plant families are predominantly mycorrhizal; (with arbuscular mycorrhizas being the predominant form). Mycorrhizal fungi has a unique ecological position of being partly inside and partly outside the host

Mycorrhizal fungi use photosynthetically derived organic C from their host and unlike most fungi, are not saprophytic

Mycorrhizae differ from other rhizosphere interactions due to

·      Greater specifity & organization of plant fungus relationship           

·         Association for prolonged periods

 

Types of mycorrhiza

Mycorrhizas are broadly divided into ectomycorrhizas and endomycorrhizas, though six types are recognized based on the characteristic features.

·         Ectomycorrhizae:  Extracellular - Fungi do not penetrate individual cells within the root, instead penetrate the intercellular spaces of epidermis and of the cortical region of the root, forming a sheath of interconnecting hyphae around the roots called Hartig net and mantle.

·         Endomycorrhizae: Fungi penetrate the cell wall and invaginate the cell membrane. Endomycorrhiza includes arbuscular, ericoid, and orchid mycorrhiza.

·         Ectendomycorrhizae: Shares the features of both ecto- and endomycorrhiza.- Hartig net and mantle present but occasional penetration of hyphae into root cells.  Arbutoid mycorrhizas can be classified as ectoendomycorrhizas. 

                Monotropoid mycorrhizas form a special category.

Ectomycorrhiza

·         Ectomycorrhizas, or ECM

·      Predominately seen in  mostly woody plants in cooler climates, like the birch, eucalyptus, oak, pine, and rose families

·         Fungi belonging to the Basidiomycota and some Ascomycota.

·         Strict host specifity is rare - An individual tree may have 15 or more different fungal ECM partners at one time.  Some ECM fungi, such as many Leccinum and Suillus, are symbiotic with only one particular genus of plant, while other fungi, such as the Amanita, form mycorrhizas with many different plants

·         Unlike Endomycorrhizae, ectomycorrhizal fungi do not penetrate their host’s cell walls.

·        ECM fungi secrete signalling molecules that limit the growth of root hairs – ECM colonized roots often appear blunt and covered in fungi    

·         From the root surface, fungi extend hyphae into the soil, which aggregate to form rhizomorphs- dense mats of hyphae often visible to the naked eye

·         Hyphae form characteristic structures like the hyphal sheath, or mantle, covering the root tip and a Hartig net of hyphae surrounding the plant cells within the root cortex.

·         Mantle is a dense, and multi-layered covering of fungal mycelia surrounding the root surface. Thickness of mantle varies from 20-40 mm depending on mycorrhizal fungi, temperature, nutritional factors etc. The hyphae extend up to several centimetres into the surrounding soil. 

    `    In some cases the hyphae may also penetrate the plant cells, in which case the mycorrhiza is called an ectendomycorrhiza.

·         Hartig net is made of highly branched hyphae forming a mycelial network between epidermal and cortical root cells.

·         Outside the root, the fungal mycelium forms an extensive network within the soil and leaf litter- efficient two way transfer of soil nutrients to the plant and carbohydrates to the fungus

·         This hyphal network aids in water and nutrient uptake and help the host plant to survive adverse conditions and in exchange, the fungal symbiont is provided with access to carbohydrates. Nutrients such as Carbon, Nitrogen and Phosporous can move between different plants through the fungal network (sometimes called the wood wide web).

·         Many ECM fungal fruiting bodies are well known including the economically important and edible truffle (Tuber), deadly death caps and destroying angels (Amanita).

 

Endomycorrhiza

An Endomycorrhiza is a type of mycorrhiza in which the fungus penetrates the cortical cells of the roots of a vascular plants including bryophytes, pteridophytes, gymnosperms and angiosperms. Fungi are predominately from Zygomycotina (eg; Glomus, Gigaspora etc).  Endomycorrhizas have been further classified as arbuscular, ericoid and orchid mycorrhizas.

1.      Arbuscular mycorrhiza

Arbuscular mycorrhizas, or AM (formerly known as vesicular-arbuscular mycorrhizas, or VAM), are mycorrhizas whose hyphae enter into the plant cells, between the plant cell wall and invaginations in the plasma membrane, producing unique highly branched structures that are either balloon-like (vesicles) or branching invaginations (arbuscules).  

 The fungal hyphae enter root cells The structure of the arbuscules increases the contact surface area between the hypha and the cell cytoplasm and act as the functional site of nutrient exchange.

Arbuscular mycorrhizas are formed only by fungi in the division Glomeromycota. They are found in 85% of all plant families, and occur in many crop species (eg, wheat).

 They provide the host plant with mineral nutrients (phosphorus and micronutrients) and water, in exchange for photosynthetic products. The AMF mycelium that emerges from the root system can acquire nutrients from soil volumes that are inaccessible to roots.

Furthermore, fungal hyphae are much thinner than roots and are therefore able to penetrate smaller pores. Carbohydrates and mineral nutrients are then exchanged inside the roots across the interface between the plant and the fungus.

The hyphae of arbuscular mycorrhizal fungi produce the glycoprotein glomalin, which may be one of the major stores of carbon in the soil. Thus, AMF can alleviate the limitation in plant growth caused by an inadequate nutrient supply.

 In addition to an improved nutritional supply, AM interactions provide other benefits to plants, such as improved drought and salinity tolerance and disease resistance.

AM fungi are known to reduce heavy metal toxicity in the host plants and to tolerate high metal concentrations in the soil.

AM fungi can also have a direct effect on the ecosystem, as they improve the soil structure and aggregation and productivity AM fungi contribute to reducing emissions of N₂O, which is an important greenhouse gas, thus suggesting that they could play a role in controlling climate change.

It is believed that the development of the arbuscular mycorrhizal symbiosis played a crucial role in the initial colonization of land by plants and in the evolution of the vascular plants.

AMF or AM fungi represent a key link between plants and soil mineral nutrients, and are of great interest as natural fertilizers.

2.      Ericoid mycorrhiza

Ericoid mycorrhizas have a simple intraradical (grow in cells) phase, consisting of dense coils of hyphae in the outermost layer of root cells. Plants are woody shrubs or small trees of the Ericacea subfamily Ericoidae, found in open or acid peaty soil. They have usually fine roots on which fungus attached. Fungi is usually Ascomycetes (Eg. Clavaria, Pezizella spp). Ericoid mycorrhizas have been shown to have saprotrophic capabilities, which would enable plants to receive nutrients from by the decomposing actions of their ericoid partners. 

3.      4.  Orchid mycorrhiza

All orchids, in nature germinate only when infected with endomycorrrhizal fungi which subsequently colonize the host plants. fungi. The fungi are mostly from Rhizoctonia genus of Basidiomycetes and Ascomycetes. Their hyphae penetrate into the root cells and form typical coils are similar to ericoid mycorrhizas. Their carbon nutrition is exclusively to supporting the host plant as the young orchid seedling is non-photosynthetic and depends on the fungus partner. The fungus utilises complex carbon sources in the soil, and make carbohydrates available to the young orchid. All orchids are achlorophyllous in the early seedling stages, but usually chlorophyllous as adults, so in this case the seedling stage orchid can be interpreted as parasitising the fungus.

Ectendomycorrhizae-   Arbutoid mycorrhiza

This type of mycorrhiza involves plants of the Ericaceae subfamily Arbutoideae. It is different from ericoid mycorrhiza and resembles ectomycorrhiza, both functionally and in terms of the fungi involved. The difference to ectomycorrhiza is that some hyphae actually penetrate into the root cells, making this type of mycorrhiza a form of transition between ecto- and endo mycorrhizae and hence classified as ectendomycorrhiza. Mycosymbionts are of Basidiomycetes. The host plants are mostly woody shrubs and trees. The roots are covered with mycorrhiza with a sheath and a Hartig net, but the fungus penetrates cortical cells to form extensive coils of hyphae.

Monotropoid mycorrhiza

This type of mycorrhiza occurs in the subfamily Monotropoideae of the Ericaceae. These plants are achlorophyllous and heterotrophic or mixotrophic, hence derive their carbon from the fungus partner. This is thus a non-mutualistic, parasitic type of mycorrhizal symbiosis. Roots form ball throughout which fungal mycelium grows enclosing mycorrhizal roots of surrounding green plants. Root ball is the survival organ in winter for Monotropa and when the favourable conditions come, they form flowering shoots. A Hartig net and sheath (mantle) is also seen occasionally. The structure and function of monotropoid mycorrhiza change with seasonal development of host plants.

Benefits

·         Mycorrhiza has a broad ecological adaptability and is known to occur in deserts as well as arctic, temperate, tropical and other inhospitable habitats.

·         Healthy physiological interaction between plant and fungus- Nutrient exchange between both partners, Plants occupy habitats they otherwise cannot.

·         Mycorrhiza increases the rate of photosynthesis and hence improves plant growth, productivity and yield.

·         Mycorrhiza facilitates better uptake of nutrients like phosphorus and immobile trace elements like zinc, cobalt, magnesium, iron, copper, molybdenum, etc, leading to better nutrition for plants.

·         Mycorrhiza offers tolerance against a range of soil stresses like heavy metal toxicity, salinity, drought, and high soil temperatures. This enhances the chances of plant survival immensely.

·         Mycorrhiza with hyphae that extend much beyond a few meters away the plant root zone, can acquire nutrients from a much wider soil area and is categorized as a biofertilizer. Mycorrhiza offers up to 50% reduction in chemical fertilizer application.

·         Mycorrhiza offers higher resistance to various soil and root-borne pathogens, thus becoming a potential disease control agent.

·         Mycorrhiza helps in soil conservation and soil structure stabilization, thus restoring land productivity.

Ultrastructure of bacteria- External structures-glycocalyx & capsule

BACTERIAL CELL STRUCTURE

Procaryotic cells are in general smaller, grow extremely rapidly and lack the complex vesicular transport systems in comparison to eucaryotic cells. A variety of structures is found in procaryotic cells. Not all structures are found in every genus. Furthermore, gram- negative and gram-positive cells differ, particularly with respect to their cell walls. Despite these variations procaryotes are consistent in their fundamental structure and most important components.

Procaryotic cells are bounded by a chemically complex cell wall. Inside this wall, and separated from it by a periplasmic space is the plasma membrane. The genetic material is localized in a discrete region, the nucleoid and is not separated from the surrounding cytoplasm by membranes. Ribosomes and larger masses called inclusion bodies are scattered about in the cytoplasmic matrix. Both gram-positive and gram-negative cells can use flagella for locomotion. In addition, many cells are surrounded by a capsule or slime layer external to the cell wall.

Structure	Functions
Plasma membrane	Selectively permeable barrier, mechanical boundary of cell, nutrient and waste transport, location of many metabolic processes  (respiration, photosynthesis), detection of environmental cues for chemotaxis
Gas vacuole	Buoyancy for floating in aquatic environments
Ribosomes	Protein synthesis
Inclusion bodies	Storage of carbon, phosphate, and other substances
Nucleoid	Localization of genetic material (DNA)
Periplasmic space	Contains hydrolytic enzymes and binding proteins for nutrient processing and uptake
Cell wall	Gives bacteria shape and protection from lysis in dilute solutions
Capsules and slime layers	Resistance to phagocytosis, adherence to surfaces
Fimbriae and pili	Attachment to surfaces, bacterial mating
Flagella	Movement
Endospore	Survival under harsh environmental conditions

 

External cell structures

Glycocalyx

·         All polysaccharide containing substances found external to cell wall

·         Thickest capsule to thinnest slime layer

Capsules

·         Protective structure secreted by the organism; seen outside the cell wall

·         Capsules -clearly visible in the light microscope using negative stains or special capsule stains; also with the electron microscope

·         Well organized & not easily washed off

·         Chemical composition unique to the organism

·    Usually composed of polysaccharides; may be constructed of other materials. Eg., Bacillus anthracis - poly- D-glutamic acid capsule

·  Pathogens usually are capsulated; prevents phagocytosis by host phagocytic cells eg., Streptococcus pneumoniae -Without a capsule, more vulnerable to destruction & less likely to cause disease, whereas the capsulated variant is pathogenic

·         Capsules contain a great deal of water; protect bacteria against desiccation

·         Protection against bacterial viruses and hydrophobic toxic materials such as detergents also

Slime Layers

·         A slime layer is a zone of diffuse, unorganized material that is removed easily.

·         Capsules and slime layers usually are composed of polysaccharides

S-Layers

·         Many gram-positive and gram-negative bacteria have a regularly structured layer called an S-layer on their surface- S layer has a pattern like floor tiles and is composed of protein or glycoprotein

·         S- layers also are very common among Archaea, where they may be the only wall structure outside the plasma membrane.

·    In gram-negative bacteria the S-layer adheres directly to the outer membrane whereas it is associated with the peptidoglycan surface in gram-positive bacteria.

·     It may protect the cell against  pH fluctuations, osmotic stress, enzymes, or the predacious bacterium Bdellovibrio

·         The S-layer also helps maintain the shape and envelope rigidity of at least some bacterial cells.

In general, glycocalyx, capsules, slime layers, s-layers

• helps maintain the shape and envelope rigidity of at least some bacterial cells
• aids bacterial attachment to surfaces of solid objects in aquatic environments or to tissue surfaces in plant and animal hosts
• protect bacteria against desiccation
• protect the cell against pH fluctuations, osmotic stress, enzymes, or the predacious bacterium Bdellovibrio
• protection against bacterial viruses and hydrophobic toxic materials such as detergents
• protect some pathogens against complement attack and phagocytosis, thus contributing to their virulence