Trichophyton tonsurans
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''Trichophyton tonsurans'' is a
fungus A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from t ...
in the family
Arthrodermataceae The Arthrodermataceae are a family of fungi containing nine dermatophyte genera — '' Epidermophyton'', '' Microsporum'', ''Nannizzia'', ''Trichophyton ''Trichophyton'' is a genus of fungi, which includes the parasitic varieties that cause ...
that causes
ringworm Dermatophytosis, also known as ringworm, is a fungal infection of the skin. Typically it results in a red, itchy, scaly, circular rash. Hair loss may occur in the area affected. Symptoms begin four to fourteen days after exposure. Multiple ar ...
infection of the scalp. It was first recognized by
David Gruby David Gruby (20 August 1810 – 14 November 1898) was a Hungarian physician born in the village of Kis-Kér (now Bačko Dobro Polje, Serbia) to a Jewish farmer.Geoffrey Clough Ainsworth, ''Introduction to the History of Medical and Veterinary My ...
in 1844. Isolates are characterized as the "–" or negative mating type of the '' Arthroderma vanbreuseghemii'' complex. This species is thought to be conspecific with '' T. equinum'', although the latter represents the "+" mating strain of the same biological species Despite their biological conspecificity, clones of the two mating types appear to have undergone evolutionary divergence with isolates of the ''T. tonsurans''-type consistently associated with Tinea capitis (particularly in children) whereas the ''T. equinum''-type, as its name implies, is associated with horses as a regular host. Phylogenetic relationships were established in isolates from Northern
Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
, through fingerprinting polymorphic RAPD and M13 markers. There seems to be lower genomic variability in the ''T. tonsurans'' species due to allopatric divergence. Any phenotypic density is likely due to environmental factors, not genetic characteristics of the fungus.


Colony morphology

''Trichophyton tonsurans'' may be identified through analysis of its fast-growing colonies. Colonies tend to be flat, powdery, and yellow with a reddish undercolour. It develops into a folded colony, and may vary in colour from off-white to grey, with dark pigments that may diffuse into the medium. The younger colonies fluoresce green on Sabouraud's agar, and are also flat, but are mahogany red or lemon-yellow coloured. On this agar the fungus grows peripherally and develops into a flatter, creamy disk with raised edges. ''Trichophyton tonsurans'' also produces many inflated, pear-shaped microconidia, borne on matchstick-like stalks. It also forms fewer macroconidia that are 4-6 cells long, with thick cell walls. A different simple method for identification at early stages is through the detection of
chlamydospore A chlamydospore is the thick-walled large resting spore of several kinds of fungi, including Ascomycota such as '' Candida'', Basidiomycota such as '' Panus'', and various Mortierellales species. It is the life-stage which survives in unfavourable ...
-like structures (CLS), which are found on the reverse side of the culture under light microscopy. Chlamydospores are asexual spores that are created through hyphal modification, often with thick cell walls arising from the deposition of
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, t ...
materials along the original cell wall. After inoculation on culture media (such as commonly used Mycosel agar),
mycelia Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates ...
extend into the media and create the CLS. Normally, strains will produce CLS regardless of the media it is grown on. CLS growth is essentially unaffected by antibiotic treatment with chloramphenicol, as well as by cycloheximide. After 5 days of incubation, CLS production will be evident, suggesting the fungus is highly likely to be ''T. tonsurans''. Responses of ''T. tonsurans'' to different growth conditions and nutrient media is useful in aiding identification. For example, growth is enhanced in the presence of
thiamine Thiamine, also known as thiamin and vitamin B1, is a vitamin, an essential micronutrient, that cannot be made in the body. It is found in food and commercially synthesized to be a dietary supplement or medication. Phosphorylated forms of thi ...
, and this exogenous requirement for thiamine distinguishes ''T. tonsurans'' from similar species. Since thiamine stimulates growth, ''T. tonsurans'' also displays this behaviour on
vitamin A vitamin is an organic molecule (or a set of molecules closely related chemically, i.e. vitamers) that is an essential micronutrient that an organism needs in small quantities for the proper functioning of its metabolism. Essential nutrie ...
-free, thiamine-supplemented casamino acids agar but the growth is more sparse, and subsurface growth is absent. BCP-milk solids glucose agar can also be used as an indicator of this fungus. This medium turns from pale blue to purple in colour in 7–14 days of growth at . The colour change is due to an alkaline shift arising from the release of ammonium during protein hydrolysis.


Epidemiology

''Trichophyton tonsurans'' causes
tinea capitis Tinea capitis (also known as "herpes tonsurans", "ringworm of the hair", "ringworm of the scalp", "scalp ringworm", and "tinea tonsurans") is a cutaneous fungal infection (dermatophytosis) of the scalp. The disease is primarily caused by dermato ...
infection globally, but it is especially endemic in
Latin America Latin America or * french: Amérique Latine, link=no * ht, Amerik Latin, link=no * pt, América Latina, link=no, name=a, sometimes referred to as LatAm is a large cultural region in the Americas where Romance languages — languages derived f ...
(especially northern
Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
), Mexico, and
Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
. Infections due to this species have become increasingly common in the United States and Canada since the 1980s as a consequence of changing patterns in global travel and immigration, and it is responsible for a majority of pediatric tinea capitis infections in the US. The modes of dispersal are unclear, though it is associated with homes, schools and other institutions, and barbershops. Transmission can occur through direct transfer, or through the use of shared resources and facilities such as pillows, couches, rugs, and pets, which should be thoroughly examined because they can be carriers of ''T. tonsurans''. Children are most susceptible to Tinea capitis whereas adult infections more often manifest as
tinea corporis Tinea corporis is a fungal infection of the body, similar to other forms of tinea. Specifically, it is a type of dermatophytosis (or ringworm) that appears on the arms and legs, especially on glabrous skin; however, it may occur on any superficial ...
. This species is a major cause of family and institutional outbreaks because of its persistent nature in indoor environments, and its ability to be transmitted through asymptomatic carriers.


Pathophysiology

Once the fungal infection has been contracted, it invades hairs and sporulates in the hair shaft, causing it to burst and curl, creating a black dot on the scalp.
Tinea capitis Tinea capitis (also known as "herpes tonsurans", "ringworm of the hair", "ringworm of the scalp", "scalp ringworm", and "tinea tonsurans") is a cutaneous fungal infection (dermatophytosis) of the scalp. The disease is primarily caused by dermato ...
is the clinical disease, but it may also cause
Tinea corporis Tinea corporis is a fungal infection of the body, similar to other forms of tinea. Specifically, it is a type of dermatophytosis (or ringworm) that appears on the arms and legs, especially on glabrous skin; however, it may occur on any superficial ...
,
onychomycosis Onychomycosis, also known as tinea unguium, is a fungal infection of the nail. Symptoms may include white or yellow nail discoloration, thickening of the nail, and separation of the nail from the nail bed. Toenails or fingernails may be affected, ...
, and
Tinea pedis Dermatophytosis, also known as ringworm, is a fungal infection of the skin. Typically it results in a red, itchy, scaly, circular rash. Hair loss may occur in the area affected. Symptoms begin four to fourteen days after exposure. Multiple a ...
. Cutaneous lesions due to ''T. tonsurans'' do not fluoresce under Wood's Lamp. Although some people may not show the symptoms of carrying ''T. tonsurans'', it has a distinctive manifestation. During pathogenesis, the fungus undergoes
protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
elaboration to hydrolyze structural proteins (such as the
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
found in hair), and isolates show peak values between days 18–22 during the sporulation phase. There are potentially 23 genes that may have mechanistic roles of this skin infection, and 21 show significant differences in infection rates, especially among children. The genes are typically involved in leukocyte activation and migration, and formation and integrity of the
extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...
. In molecular studies of its virulence, common target genes include CarbM14, CER, and Sub2, which encode the proteases
carboxypeptidase A carboxypeptidase ( EC number 3.4.16 - 3.4.18) is a protease enzyme that hydrolyzes (cleaves) a peptide bond at the carboxy-terminal (C-terminal) end of a protein or peptide. This is in contrast to an aminopeptidases, which cleave peptide bonds ...
, ceraminidase, and subtilisin, respectively. Among other virulence-related
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s, ''T. tonsurans'' also produces
urease Ureases (), functionally, belong to the superfamily of amidohydrolases and phosphotriesterases. Ureases are found in numerous bacteria, fungi, algae, plants, and some invertebrates, as well as in soils, as a soil enzyme. They are nickel-contai ...
. This fungus has also been found to produce
melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
, which may be phenotypically demonstrated through ''in vitro'' induction in
caffeic acid Caffeic acid is an organic compound that is classified as a hydroxycinnamic acid. This yellow solid consists of both phenolic and acrylic functional groups. It is found in all plants because it is an intermediate in the biosynthesis of lignin, one ...
media. Melanin acts as an antioxidant molecule, providing protective properties to the fungus from damaging UV rays. Since it is endemic in sunny regions, the melanin production is perhaps crucial for survival. In early stages of infection, the lesion has a clear and raised border, although there is not much hair loss yet. However, as it progresses, infected hairs break off at the scalp surface and the scalp is eventually coated in a scaly layer, with short hair stubs remaining. Twisted hairs may be found in keratotic follicular papules that will be formed. The infection is often called "black dot ringworm" due to the small dark hair stubs that are found on the scalp. Inflammatory reactions are also quite common and can manifest as edema, abscess, or highly inflammatory
kerion Kerion or kerion celsi is an acute inflammatory process which is the result of the host's response to a fungal ringworm infection of the hair follicles of the scalp (occasionally the beard) that can be accompanied by secondary bacterial infection( ...
. Hair regrowth does occur, although some scarring may remain. Males have a tendency to show greater improvement in non-inflammatory presentation as well. Although there are several treatments available, tinea capitis often has no subjective symptoms, so people at risk for infection should still receive fungal examinations regularly.


Treatment

Treatment options include antifungal shampoo, systemic antifungals, or both. Oral therapy is indicated for complicated infections of those that fail to respond to topical treatment. Still, the use of selenium sulphide or
povidone-iodine Povidone-iodine (PVP-I), also known as iodopovidone, is an antiseptic used for skin disinfection before and after surgery. It may be used both to disinfect the hands of healthcare providers and the skin of the person they are caring for. It may ...
shampoos greatly reduce fungal viability and may be helpful in person-to-person transmission. Advancements have been made in detection of ''T. tonsurans'' in patients with Tinea capitis, using TaqMan PCR assay and primers and probes designed to detect this fungus rapidly and specifically, excluding contaminating skin microorganisms.


References

{{Taxonbar, from=Q5928559 Arthrodermataceae Fungi described in 1848