Fungi

The study of fungi is referred to as mycology, and mycology is one of the oldest disciplines in microbiology. Fungi are amongst the most generally familiar organisms that are studied in microbiology. Everyone will recognize the furry growths that appear on stale bread and rotting fruits, and since time immemorial, fungi have been exploited for the production of leavened bread and alcoholic beverages. The role of fungi in the decay of vegetable and animal matter has also long been recognized. Fungi have been so intimately linked with the decomposition of organic matter that they have become synonymous with mouldiness, putrefaction and decay.

Fungi range dramatically in size from relatively large and compact structures such as puff-balls, mushrooms, toadstools and bracket fungi that can be seen attached to decaying trees, through the diverse network of filaments in soil, frequently associated with plant roots, down to the microscopic unicellular yeasts. However, all fungi are eukaryotic organisms. They possess a membrane-bound nucleus and nucleoli, cellular respiration occurs in mitochondria present in the cytoplasm, and fungal cells have an elaborate arrangement of internal membrane systems.

Moulds and their structures

The fungi display an astonishing variety of size and shape, but can be broadly divided into two groups, the moulds and the yeasts. The moulds are also referred to as filamentous or mycelial fungi, and they are composed of a network of filaments called hyphae (singular: hypha) that are interwoven into a structure called a mycelium (plural: mycelia). Huphe is the Greek word for a web, and mycelium is derived from mukes meaning mushroom. Moulds reproduce asexually or sexually, and the mycelium and fruiting bodies of a mould are collectively referred to as the fungal thallus, this being the Greek for green shoot. The spores of fungi are of primary importance in the identification of fungi. Mycelial tissue is also sometimes referred to as an anastomosis because it comprises a web of cross-connecting hyphae. The development of mycelial structures is by growth from the tips of the hyphae, with branching of the filaments occurring intermittently. The cytoplasm of young hyphae fills the filament, but further back from the growing tip, the cytoplasm becomes increasingly vacuolated, and the oldest hyphae are empty structures that may become cut off from the rest of the mycelium.

In the majority of moulds, hyphae are divided into sections by the regular occurrence of cross-walls or septa (singular: septum). These structures add rigidity to the filament, and help to control the flow of nutrients through the mycelial network. Septa vary in complexity; simple septa have a single central pore, but some septa seen in higher fungi have a dolipore structure, in which a narrow central pore is flanked by a cap-like, perforated membranes (parenthesomes) made up of amorphous material. Individual compartments in the septate hypha may contain a single nucleus, and these are said to be uninucleate or they may contain several nuclei, and are thus described as multinucleate. In more primitive fungi such as the Phycomycetes, there are no septa to divide the hyphae into sections, and the aseptate hypha is described as coenocytic (Greek: kinos, common; kutos, a vessel). Septa do occasionally develop in Phycomycetes, but their function is to separate the reproductive structures from the vegetative body of the fungus, or to cut off the old sections of the thallus. Unlike the septa of other fungi, the structures elaborated by the Phycomycetes are solid plates, and they do not have a central pore.

In certain higher fungi, adjacent hyphae can fuse vegetatively to give a three-dimensional network structure. It is from such structures that reproductive fruiting bodies are formed. Hyphae can also aggregate to give rise to other specialized structures that exhibit a high degree of internal organization as a result of coordinated growth. Rhizomorphs, literally resembling root structures, are rope-like strands that have a highly differentiated structure. These structures appear to develop in response to stress, and in nature they develop in relatively dry environments such as are found in sandy soils. Sclerotia (singular: sclerotium) are hardened structures that enable certain moulds to survive in a dormant state. In culture, sclerotia are pigmented, and are sufficiently large to be seen by the naked eye. They are generally rounded, but may display an irregular shape. The cells of the outer wall of a sclerotium possess thick walls, and thus the structure has a thick, protective coat. This encloses a central cortex of hyphae that contains the food reserves necessary for dormancy. Nutrients are typically stored either as oil droplets or as glycogen. The most familiar structures generated by fungi, however, are the mushrooms and toadstools. These are highly complex reproductive structures that demonstrate an astonishing level of internal differentiation and organization. A stalk or stipe supports the cap or pileus under which the gills develop. It is from these gills that spores are released. The development of gills is a highly coordinated process that responds to environmental stimuli. In order for spores to be released efficiently, gills must develop vertically. This is achieved by geotropism, as shown by the observation that if the developing structure is tilted, then the gills will still form to lie vertically.

Yeasts and their structures

In yeasts, the fungal thallus is generally a single cell. Yeasts are predominantly unicellular fungi that are round, oval or elongated in shape. They vary from 2 to about 10 micrometers in size. A limited number of yeasts elaborate extra cellular capsules. An example is Cryptococcus neoformans, a human pathogen that causes a chronic form of meningitis increasingly seen in patients with acquired immune deficiency syndrome (AIDS). In this instance, the capsule mucopolysaccharide helps the yeast to evade the body's defense mechanisms, and thus allows it to cause disease.

Yeasts generally reproduce by the asexual process of budding. The parental cell develops a protruberance that swells and enlarges into a blastospore that eventually separates from its parent (Greek: blastos, sprout). However, in fission yeasts, such as Schizosaccharomyces pombe, a parental cell divides into two progeny in a manner somewhat similar to the transverse binary fission seen in bacterial reproduction. Yeasts rarely form true multicellular structures. Some yeasts form chains of elongated cells that are called pseudomycelia (singular: pseudomycelium) or pseudohyphae (singular: pseudohypha). Pseudomycelia are elongated yeast cells that arise from buds adhering together in branching chains. The individual cells within a pseudomycelium are independent of one another and, unlike the units within the septate hyphae of moulds, they are not connected by pores. Yeast cells with a typical unicellular morphology may cluster terminally or along the side of a pseudomycelium. These are called secondary blastospores. Some yeasts can produce septate, true mycelia under certain growth conditions.

Dimorphic fungi

Although it is convenient to divide fungi into two groups, moulds and yeasts, there are fungi that are capable of adapting their structures in response to changes in their environment. They may grow in either a mycelial or a yeast form, depending on the prevalent growth conditions. These are referred to as dimorphic fungi. The mycelia formed by dimorphic fungi are true mycelia, unlike the pseudomycelia produced by some yeasts. Many of the fungi that cause diseases in humans and animals are dimorphic, for example Candida albicans, the fungus that causes thrush. This is an infection that affects mucous membranes such as are found in the mouth or the genital tract. The visible symptoms of oral thrush are white plaques in the mouth, and vaginal thrush appears as an itchy white vaginal discharge.

The fungal cell wall

Fungi have been described as plant-like, because they are generally non-motile, and because their cells are bounded by a well-defined, multi-layered cell wall. However, the cell wall structure of plants and fungi differs considerably, with plant cell walls being made of celluloses and hemi-celluloses, and the fungal cell walls composed mainly of other polysaccharides including chitin, a polymer of N-acetylglucosamine. As well as being one of the principal components of fungal cell walls, chitin is a structural polymer that is found in the exoskeleton of the arthropod invertebrates.

The major component of the cell walls of both moulds and yeasts is polysaccharide, with up to 80% of the cell wall material comprising crystalline micro fibrils in an amorphous matrix material. Of the remaining 20% of cell wall components, protein and lipid are present in approximately equal proportions. The cell wall polysaccharide component depends upon the type of fungus. In moulds, chitin is the principal fibrillar component and polymers of glucose known as glucans form the amorphous matrix material. In contrast to moulds, mannans, polymers of mannose, are the predominant structural components of yeast cell walls where they are also found with glucans. In baker's yeast, Saccharomyces cerevisiae, the cell wall contains less than 1 % chitin, and this polymer is principally associated with bud scars, in which it forms a plug of material. In a special group of fungi (Oomycetes) within the Phycomycetes, cellulose forms the dominant structural component of the cell wall material.

The cell wall of fungi has five layers. It is exemplified by the architecture of the cell wall of the mature hyphae of the mould Neurospora crassa. The plasmalemma forms the foundation of the cell wall. Above this lies a layer of chitin micro fibrils in an amorphous matrix of proteins, mannans and glucans that is about 20 nanometers thick. Beyond this lies a discrete protein layer of about 10 nanometers in thickness. This supports a glycoprotein network embedded in protein. This layer is about 50 nanometers thick. The outer layer of the cell wall is the thickest, and is made up of amorphous glucans. This layer is about 90 nanometers thick in Neurospora crassa.

Reproduction in fungi

Reproduction in fungi may be asexual or sexual and, in both cases, spores are the structures that are responsible for dispersing progeny to colonize new locations. Some spores are designed to withstand adverse growth conditions or to provide for a period of dormancy. The mycelia of moulds may also become fragmented, and the resulting fragments may each subsequently develop into an individual thallus by the process of vegetative reproduction. The term vegetative reproduction is used to refer to asexual reproduction where special reproductive structures other than spores are not formed. The vegetative or asexual state of a fungus is known as the anamorph and the sexual state as the teleomorph.

Nutrition of fungi

Fungi are often erroneously said to resemble plants. However, plants can elaborate complex organic compounds from simple inorganic molecules such as water and carbon dioxide by the process of photosynthesis. Fungi, in contrast, all require a supply of preformed organic compounds for their energy production and growth. They are thus described as heterotrophic organisms (Greek: heteros, other; trophikos, nourishment), hence heterotrophs are nourished from elsewhere, rather than being able to feed themselves as the autotrophic plants do. Most fungi are found in dark, moist habitats, but are universally present where organic matter is found.

Fungi may be saprophytic or parasitic. Saprophytic organisms are defined as those that live on decaying organic matter, and the term is derived from two Greek words, sapros and phuton, meaning decayed plant. Parasites (Greek: para, other or beyond; sitos, food) derive their nutrients from living plants or animals, and generally cause disease in their hosts. The majority of fungi are found as saprophytes in the soil, living on decaying plant material, where they play a vital role in the recycling of organic matter. Fungi feed by secreting hydrolytic enzymes into their local environment. These enzymes digest the various polymers to produce soluble products that the fungus then absorbs.

In artificial culture, many fungi can grow on a mineral salts medium containing a source of nitrogen salts, providing that they have glucose present as a carbon source. They can thus manufacture all the complex organic molecules that they require for growth from the metabolism of glucose. Other fungi require an exogenous supply of vitamins or other growth factors that they cannot manufacture themselves in order to grow in artificial culture. Added growth factors are of particular importance if vegetative cells in culture are to initiate sporulation. Fungi have a specific requirement for trace elements including calcium, magnesium, iron, zinc, copper and manganese. No fungus has been found that can fix atmospheric nitrogen.

Glycogen is the principal storage polymer used by fungi, but oil droplets are also used for nutrient storage. Like most eukaryotes, fungi are obligate aerobes, and they obtain energy from the aerobic respiration of glucose. However, a minority of yeast species are facultative anaerobes, and if held under anaerobic conditions, they can obtain energy by fermentation. This process is not as efficient as respiration. In culture, mutants of the baker's yeast Saccharomyces cerevisiae that lack mitochondria occur at a frequency of about 1 %. These cells are incapable of respiration, because of their inability to assemble mitochondria. Consequently, when they grow on a glucose-based solid medium, they can obtain energy only by fermentation. Such mutants give rise to very small colonies when compared to the wild-type cells that do possess mitochondria, and are thus described as petite mutants.

Classification and identification of fungi

Classification and identification of organisms are two independent but interrelated processes. Classification is the division of organisms into hierarchical groups. These are based upon the degree of relatedness of organisms. Identification is the process whereby an isolate is assigned to a group within a classification scheme.

To illustrate the difficulties of classifying microorganisms, the parasite Pneumocystis carinii has, for many years, been regarded as a protozoan. It causes pneumonia in patients who are immunocompromised, and this is the most common life-threatening disease in patients with AIDS. It is a unicellular flagellate organism that lacks a cell wall. However, recent molecular genetic studies have shown that Pneumocystis carinii is more closely related to the fungi than to other protozoa. This is based upon the structure of its ribosomal RNA molecules, a feature that is commonly exploited in molecular taxonomy.

Classification of fungi

Mycelial fungi, or moulds, are classified according to both their macro- and micro-morphology. Yeasts are structurally more simple and therefore display a limited range of morphologies. This creates difficulties in the classification of yeasts, and this group is subdivided partly on the basis of their reactions in biochemical tests.

There is no universally accepted classification scheme for fungi. In some classification schemes the fungal divisions are referred to as phyla. Also, some have realigned the fungi grouped in the Phycomycetes. Members of the Myxomycota and Mastigomycotina have been regrouped into the kingdom Protoctista, which also includes protozoa and nucleated algae, and are not considered by some to be true fungi. Fungi in the Zygomycotina are retained as a group in the fungal kingdom and referred to as the Zygomycota.

The Myxomycotina or slime moulds are characterized by an amoeboid vegetative stage. However, under appropriate conditions the amoeboid cells congregate and differentiate to form reproductive structures that resemble other fungi. For this reason, members of the group including Dictostelium discoideum and Physarum polycephalum are intensively studied by developmental biologists. Slime moulds are common free-living organisms found in habitats such as leaf litter and soils, but some species are parasitic. The parasites are frequently found in association with higher plants, algae including marine algae, and other fungi. The symptomless parasite Polymyxa graminis is frequently found associated with the roots of cereal crops, and can act as the vector of virus diseases.

The Mastigomycotina are zoospore-forming fungi. They may form branched chains of cells that attach to their substrate by a root-like structure called a rhizoid. Many are soil saprophytes where they are found as important decomposers. Alternatively, they are found in freshwater habitats, and may be associated with water that is polluted with sewage. Some species are found as parasites of plants or algae, and a few are parasites of insects or fish. The downy mildews are obligate parasites unable to grow in standard laboratory cultures. The group includes important plant pathogens, such as Phytophthora infestans. This is the cause of potato blight.

The Zygomycotina are common soil saprophytes and several species are associated with animal dung. Species of the genus Entomophthora are parasites of aphids and houseflies. The Zygomycotina also include a very important group of fungi that can form symbiotic associations with higher plants that are known as mycorrhizas. These structures involve the intimate association of a fungus and the root system of its associated plant. For example orchids have mycorrhizal associations in their roots. The fungus derives its organic nutrients from the plant and thus, in return, is provided with mineral nutrients that the mycorrhiza extracts from the surrounding soil. Fungi that form mycorrhizal associations may be impossible to grow in artificial culture.

The Ascomycotina include yeasts such as those of the genus Saccharomyces. These include Saccharomyces cerevisiae. This yeast forms the basis of the baking and brewing industries, and is of immense economic importance. Yeasts are frequently found associated with fruits, but can also be found in freshwater and marine environments. The mycelial Ascomycotina are common soil saprophytes, or are associated with animal dung. Fungi of the genus Tuber form mycorrhizal associations with the roots of trees. Their fruiting bodies are harvested as truffles that are highly prized culinary delicacies. In France, pigs are specially trained to hunt out truffles by the smell that they emit. Not all of the Ascomycotina are benign. Dutch elm disease, responsible for the decimation of elm trees in England is caused by Ceratocystis ulmi, and the mildews of roses are caused by other ascomycete fungi. The dermatophyte fungi that cause diseases such as ringworm and athlete's foot are classified as members of the Ascomycotina.

The Basidiomycotina includes many fungi that live in association with plants. Some cause disease, but most are saprophytes that grow in leaf litter, composts, soil or dung. Fungi such as those of the genus Agaricus form fairy rings. Many of these fungi form mycorrhizas with trees. Merulius lacrymans is the cause of dry-rot in timber. Basidiomycotina of the class Teliomycetes include fungi that are responsible for plant rusts or smuts, and these are also very important economically, since they frequently affect cereal crops. Those of the order Gasteromycetes, as the name implies, include the edible fungi.

The Deuteromycotina or Fungi Imperfecti, of necessity, include a wide variety of saprophytic and parasitic fungi. Many, like those of the genera Aspergillus, Cladosporium and Penicillium are important food spoilage fungi. Aspergillus flavus and related species of fungus are responsible for aflatoxin production. The presence of aflatoxin in foodstuffs is of great concern since aflatoxins are among the most powerful carcinogens so far discovered. Aspergillus fumigatus is responsible for the human disease aspergillosis, some forms of which cause serious and often fatal infection in immunocompromised individuals such as transplant patients. However, Aspergillus niger is of economic benefit, since it is used in the industrial production of citric acid. Similarly, members of the genus Penicillium are important in the production of antibiotics. Penicillium chrysogenum is used in the industrial production of the antibacterial penicillin family of antibiotics, and Penicillium griseofulvum is used to produce the antifungal agent griseofulvin.

Identification of moulds

The identification of a mycelial fungus involves the study of both macro- and micro-morphology. These will vary depending upon the growth medium and temperature used to cultivate the mould, hence it is often necessary to grow a fungus on a variety of media in order to complete its identification. Careful attention should be paid to the color and texture of the colony. These features may be different in different areas of the colony, and the color of a colony may be different on the surface and underneath the colony. The presence and nature of special structures including fruiting bodies, sclerotia etc. should be noted.

Prior to disturbing the colonial growth to make microscopic preparations the entire culture may be examined using a low-power objective, so that fruiting bodies can be examined in situ. Lactophenol or lactophenol cotton blue mounts should then be made from the colony. A different mount should be made from each area of the colony that shows a different macro-morphology. Several mounts from each different area of the colony may be necessary to see all of the structural features associated with a particular fungus. Accurate observation is essential for the successful identification of a fungus growing in artificial culture. If a fungus has delicate sporing structures it is often necessary to use special culture techniques such as slide- or cellophane cultures that allow the micro-morphology of the fungus to be examined undisturbed.

It may be difficult to differentiate fruiting structures such as pycnidia, perithecia, cleistothecia and sclerotia from each other. Some sclerotia are easily distinguished by their irregular shape. Others are not so easy to distinguish. The best way to identify these structures is to squash them to see what comes out, but sometimes sclerotia are difficult or impossible to squash. Sclerotia yield numerous oil droplets. Large numbers of conidia are released from squashed pycnidia, and asci and ascospores are released from cleistothecia and perithecia. Ascospores and asci tend to ooze out of their fruiting structures.

Identification of yeasts

Yeasts are relatively simple structures and, paradoxically, this makes their identification more difficult than that of other fungi. Identification of yeasts is founded upon their limited morphological differences and on their biochemical properties. Morphological features used in the identification of yeasts include such things as cell size, shape and the presence or absence of a capsule. Some yeasts possess the ability to produce pseudohyphae, whilst a minority can produce a true septate mycelium identical with that found in moulds. Some yeasts are capable of sexual reproduction to produce ascospores. A small number of yeasts reproduce sexually to produce basidiospores. The capsulated yeast Cryptococcus neoformans that causes meningitis, particularly in patients with AIDS, is a yeast that belongs to the Basdiomycotina. Candida albicans is an important commensal and opportunist pathogen of humans. It can be identified from clinical specimens by its ability to produce a mycelial germ-tube when incubated for a period of 1-2 hours in serum at 37 °C. The biochemical tests involve observing the ability of the isolate to assimilate various compounds, mainly sugars and nitrogen sources, in addition to the ability to ferment different sugars. The identification of yeasts has been standardized and there are commercially available strips that can be inoculated to test for carbon and nitrogen assimilation. Depending upon the pattern of substrate usage an identification profile can be generated, and the identity of the yeast obtained either by reference to a manual or by using a computer database.

Aflatoxin
Toxic compounds produced by fungi such as Aspergillus flavus and A. parasiticus, which are associated with episodes of animal poisoning. These toxins were first discovered around 1960, when over 100,000 turkeys in England died as a result of consuming moldy peanut meal imported from South America and Africa. The compounds are aromatic chemicals that are only produced by the fungi under the right temperature and moisture conditions. For instance, the optimal temperature for aflatoxin production appears to be about 24-28°C, and fungi that grow at temperatures less than 15°C or at a relative humidity over 75 percent will not produce these poisons. But even under controlled laboratory studies, the production of aflatoxins has proven unpredictable, and it is possible that other factors such as special nutrients also determine their production. They have been found in numerous foods, typically from agricultural sources-for example, in nuts, grains, meats, cheese, and flour-held at ambient temperatures. They appear to act by binding to DNA, and inducing mutations, which in turn are carcinogenic. The liver appears to be especially susceptible to aflatoxins. Circumstantial evidence connects these toxins with human disease, but no firm links have been established.
Cytoplasm
The matrix of any cell regardless of whether it is a prokaryote or eukaryote. The cytoplasm of eukaryotic cells is distinguished by the presence of different membranous structures called organelles, e.g., mitochondria and chloroplasts, which form the site for different metabolic and other activities. Prokaryotic cells lack organelles but contain different kinds of granules for nutrition. Metabolic and digestive activities typically occur in different vacuoles. Ribosomes, albeit of different types, are present in the cytoplasm of both prokaryotes and eukaryotes. The cytoplasm is the site for all cellular activity except DNA replication and RNA transcription. In the nucleus-free prokaryotic cells, these activities also take place in the cytoplasm.
Dermatophytes
Species of fungi belonging to the genera Epidermophyton, Microsporum, and Trichophyton, present ubiquitously in nature, that infect the hair, nails, or skin of a living animal or human host and cause acute inflammatory conditions known as dermatophytosis. Generally known in lay terms as tinea or ringworm due to their clinical appearance, the dermatophytoses are classified according to the body area affected and not on the basis of the infecting species. Thus, tinea pedis refers to a foot infection (athlete's foot), tinea capitis affects the scalp, and tinea unguium refers to an infection of the nail plate. Generally, Microsporum species show a preference for hair and skin, Epidermophyton infects skin and occasionally nails, but not hair, while Trichophyton may infect all three sites with equal facility. More than one species may be isolated from a single lesion. All dermatophytes produce a number of proteolytic enzymes that enable them to degrade keratin, the main protein in surface tissues, and thus colonize the host. Normally, the chemicals produced by the sweat glands serve as natural inhibitors for the organism, but infections take hold under weakened circumstances or in areas where fewer sweat glands exist. Primary symptoms are due to inflammatory reactions, leading to the development of erythematous, scaly, or vesicular lesions at the site of infection.
Eukaryote
Living organisms whose cells have a special, membrane-bound compartment called the nucleus that houses the organism's DNA material (i.e., genes) separately from the rest of the cellular matrix (known as the cytoplasm). The nucleus and cytoplasm of eukaryotic cells may be distinguished under an ordinary light microscope with the use of selective dyes. In addition to the nucleus, eukaryotic cells also possess special membrane-limited structures called organelles (little organs) to perform various functions that the cell needs to carry on living, such as respiration, metabolism, and synthesis. The eukaryotic world encompasses a wide variety of organisms, from unicellular microbes such as yeasts and algae, to multicellular macroscopic beings, including plants, fish, corals, sponges, birds, and humans.
Filovirus
This family of viruses, which consists of agents of certain viral hemorrhagic fevers, are among the newly emerging pathogens to have gained prominence in the past few decades. Well-known entities within this group include the Ebola and Marburg viruses. Structurally, these viruses are identical and appear as long filamentous structures consisting of a helical nucleocapsid within a tightly bound lipid envelope with viral proteins on the surface. The genome is a linear, single-stranded RNA molecule of negative polarity with complimentary ends. Individual filoviruses differ with respect to their antigenic structure. Disease symptoms are typical of hemorrhagic fevers, with acute onset of fevers, muscular weakness and pains, and headaches some 2-21 days after infection, followed by systemic signs such as diarrhea, vomiting, pharyngitis, and a rash. The hemorrhagic manifestations of the disease-namely pinpoint bleeding (petechiae) and bleeding along the gastrointestinal tract-become apparent rather quickly (around the third day of infection). Of all the viruses known to cause hemorrhagic fevers, Ebola and Marburg have the highest fatality rate as well as the severest symptoms. Diagnosis may be achieved by virus isolation or by serological tests, although there are no specific medications or vaccines against these viruses. Treatment is therefore mainly supportive.
Exactly how humans first acquired Ebola virus infections is not clear because the natural reservoirs have not yet been ascertained. The first outbreak of the Marburg virus was traced to the handling of infected tissues from African green monkeys. Infection is spread through close contact, including sexual intercourse, and control methods are aimed at minimizing close contact between infected and infection-free individuals. Careful handling of clinical and other laboratory specimens and proper treatment and disposal of laboratory equipment and materials (particularly in primate labs), are some of the most important precautionary measures against unexpected outbreaks.
Flavivirus
Family of mostly arthropod-borne (tick-borne) RNA viruses associated with such human diseases as yellow fever, dengue, and viral encephalitis, which are among the most important viral diseases in the developing world. Originally classified under the arbo-virus label as the Group B viruses, the flaviviruses were separated into an independent family based on genomic characteristics, antigenicity, and replication strategies. As it stands today, this family contains not only the original arbo-virus members, but also certain non-arthropod-borne members, including the hepatitis C virus and certain veterinary pathogens called pestiviruses. Viruses are spherical, enveloped particles, consisting of an inner core made of a single protein surrounded by a tightly attached lipid envelope derived from the host. The flavivirus genome is a single-stranded, positive-sense RNA molecule of about 11 kilo-bases, capable of infecting cells on its own, i.e., when naked. Replication occurs in the cytoplasm.
Hypha
A single filament of a multicellular fungus. Typical vegetative hyphae are horizontal and may be either septate with separate cells or a continuous multinucleate mass. Aerial hyphae usually represent spore-bearing or reproductive portions of a fungus.
Nucleus
Membrane-bound compartment found exclusively in eukaryotic cells, whose function is to house all the genetic information an organism requires for living. The principal component of the nucleus is the cellular DNA, packaged into one or more chromosomes, depending on the organism. In addition, the nucleus may contain special DNA-binding proteins such as histones and various nucleic-acid-polymerizing enzymes. This compartment is the site for DNA replication as well as for the transcription of genes into RNA during gene expression and protein synthesis. The nuclear membrane is a phospholipid bilayer with two distinct faces-nuclear and cytoplasmic-containing embedded proteins as well as pores that form the route of communication between the nucleus and the rest of the cell (cytoplasm). The nuclei of many cells may be visualized easily under an ordinary light microscope with the use of selective stains.