Cladosporium herbarum

Cladosporium herbarum is the type species of the genus Cladosporium.[5][7] Its spores are highly prevalent in the air; the genus Cladosporium is the dominant genus of spores found in the air, with the C. herbarum species contributing the highest percentage to this group.[2] These airborne spores are more common during the summer and fall seasons;[2] in the summer there can be as many as 15,000 spores per m³ air of C. herbarum.[8] Cladosporium herbarum is also found all over the world on dead organic material, in the soil, and sometimes appears as a plant parasite.[9][2] It is also a common fungus found in indoor environments.[8][10]

In young cultures, newly-formed spores show growth by budding into a large, multi-branched mycelium.[1] It has an optimal growth temperature between 18 °C (64 °F) and 28 °C (82 °F), with a maximum growth temperature between 28 °C (82 °F) and 32 °C (90 °F).[2][11][10] It has been shown to grow at temperatures as low as −6 °C (21 °F),[2][10] and −10 °C (14 °F);[11] allowing it to grow actively (albeit slowly) on frozen materials.[5] Cladosporium herbarum can also grow in dry environments (xerophile).[5][12] C. herbarum grows optimally at pH 6 but tolerates environments with a pH as low as 4.4.[2] Conidium production is higher in wet than dry conditions.[2] The production of spores is inhibited by rubratoxin B and aflatoxin.[2][13] Its growth is inhibited in atmospheres of reduced oxygen[11] and increased carbon dioxide.[2] Mycosphaerella tassiana is the sexual reproductive stage (teleomorph) of C. herbarum.[5][2]

Spores produced by C. herbarum can be lemon-shaped or cone-shaped, often composed of 2-4 cells (3-23 mm).[5][14][15] One-celled conidia (5.5-13 x 3.8-6 µm) also exist.[2][11] In culture, newly-formed spores appear mostly 1-celled, later forming tree-like colonies with multiple branches.[1] The stalks which produce C. herbarum spores are 250 µm long and 3-6 µm wide.[2][15] They appear swollen at the tip and in between cells, and have irregularly bent branches.[2][15][11][10] Under microscopy, these stalks appear pale or dark brown in color and have smooth or rough walls.[15][11][10] When submerged in fluid, they become brittle and break up completely into spores and rod-like fragments.[1] Colonies grown on Czapek yeast extract agar (CYA) and malt extract agar (MEA) appear velvety or fluffy, with smooth or slightly wrinkled walls, and are green or brown in color.[15][2] The reverse side of the colony appears a darker green or grey/black.[15][2][10][1] Colonies can appear both sparse or densely grown.[11] At 5 °C (41 °F), colonies are 1-2 mm in diameter.[11]

This species is distributed worldwide, inhabiting polar, temperate, mediterranean, subtropical, tropical, forest, grassland, and arable soil regions.[2] It is found frequently in wood exposed to soil, deep soil depths, and the highly nutritious soil directly surrounding the roots of plants (rhizosphere).[2] It is one of the early colonizers on dying and dead plants (saprobe), especially the leaves and stems of both aquatic and desert plant species (for example ferns and mosses).[2][15] It is the most common fungal species found on living leaves (given suitable conditions) and dead plant material in very moist environments.[2][4] It also has an adaptation to high salt concentrations[2] (has been found in high salinity sediments) and extremely dry areas (xerophilic).[2]

Cladosporium herbarum causes spoilage of fresh fruits and vegetables including yams, peaches, nectarines, apricots, plums, cherries, tomatoes, and melons.[11] Its common occurrence on fresh apples can lead to contamination of apple juice and fruit based products.[11] It has also been found in eggs, hazelnuts, cereals, chickpeas, soybeans, and frozen fruit pastries.[11] Cladosporium herbarum can also spoil cheese and pasteurized soft cheese, causing problems during its manufacturing.[13] Being able to survive at temperatures below freezing, C. herbarum can cause “black spot” spoilage of meat in cold storage (between −6 °C (21 °F) and 0 °C (32 °F)).[4][13] It has been isolated from fresh, frozen, and processed meats.[11]

Cladosporium herbarum has been isolated from caterpillars, nests, feathers, pellets of free-living birds, nests of gerbils, bee honeycombs, internal organs of frogs, and earthworms.[2] It has also been found indoors on walls, wallpaper, textiles, rubber strips of window-frames, and bathrooms.[10] Increased release of C. herbarum spores is correlated with increasing temperatures, daylight, and declining humidity.[2][4][10] This species appears more frequently during the summer than the winter with peak concentrations of airborne spores found during the afternoon of a 24 hour cycle.[4][10] The spores are easily carried through the air and can be transported long distances including over oceans.[2]

This fungus is non-pathogenic, but its ability to freely produce spores that are easily dispersed in air currents adds to its effect as a fungal airway allergen; it is one of the main fungal causes of asthma and hay fever in the Western Hemisphere.[5][6] More than 60 antigens derived from C. herbarum have been detected, and 36 of these have immunoglobulin E (IgE) binding properties.[16][5] Most of these antigens are proteins found inside cells,[6][5] and eight of these antigens are members of the World Health Organization's official allergen list.[5] There is variation in allergen content between different strains of C. herbarum .[10]

Toxic effects of C. herbarum on warm-blood animals have been reported when they were fed with heavily infected wheat.[2] It can produce a toxin causing mucosal damage in horses, and mycelium extracts are shown to have low-level toxicity in chicken embryos.[5]

C. herbarum is also fungal plant pathogen. Its hosts include Bryum, Buxbaumia, Gyroweissia, Tortula, and Dicranella.[17] It can cause Cladosporium ear rot on corn.

Most often, C. herbarum conidia have 1 nucleus, but some can have 2 nuclei.[2] During mitosis, 5 to 8 dot-like chromosomes have been observed.[2] Based on DNA analysis, they have a guanine-cytosine content (GC-content) of 55%.[2] Using molecular diagnostics, C. herbarum internal transcribed spacer (ITS) sequences have been found to be identical to those of Cyphellophora laciniata.[15]