GENERAL AND FOOD MICROBIOLOGY PART I - L. V. Krasnikova - 2016

5. STUDY OF THE MORPHOLOGY OF MOLD FUNGI

Objective: to examine the Morphology of the most common mold Fungi responsible for food spoilage.

Most Molds are Saprophytes, but pathogenic species also occur among them, causing diseases in humans, animals, and plants. Many mold fungi cause food spoilage by multiplying on or inside food products. In addition, some fungi are capable of synthesizing toxic metabolic products—mycotoxins—during their life cycle. The accumulation of mycotoxins in food products can cause food poisoning or exert hepatotoxic, teratogenic, or carcinogenic effects.

On solid substrates, molds form rounded, fluffy, cobweb-like, or velvety colonies of various colors: black, brown, green, grayish-blue, yellowish, or white. Mold colonies consist of A large number of branching filaments called hyphae, ranging in thickness from 1.0 to 15 µm and up to 100 mm in length. Hyphae branch and intertwine to form the fungal body, or mycelium. Most hyphae develop in the air, forming aerial mycelium; some hyphae grow into the substrate, forming substrate mycelium. The color of mold colonies depends on the pigmentation of the spores; the mycelial hyphae are colorless.

5.1. Classification of Fungi. Characteristics of Selected Representatives

Currently, fungi are classified under the superkingdom Eukaryota and the kingdom Mycota (Fungi), which comprises two divisions: slime molds (Myxomycota) and true fungi (Eumycota). The division Eumycota is subdivided into seven classes based on their Structure and developmental stages: anamorphic (asexual) and teleomorphic (sexual). Four classes belong to lower fungi: Chytridiomycetes, Hyphochytridiomycetes, Oomycetes, Zygomycetes, and three to higher fungi: Ascomycetes, Basidiomycetes, Deuteromycetes.

Lower fungi possess two distinctive features that set them apart from other fungi. First, they have a unicellular mycelium lacking transverse cross-walls, referred to as aseptate (from the Greek septum meaning partition). Consequently, it is unicellular and multinucleate. Second, the spores of lower fungi (sporangiospores) are always formed endogenously (from the Greek endon meaning within) inside specialized Cells called sporangia, located at the tips of spore-bearing hyphae known as sporangiophores.

Higher fungi possess a well-developed multicellular (septate) mycelium, with their hyphae divided by cross-walls. They reproduce both asexually via exospores—conidia—and sexually with The formation of ascospores or basidiospores.

Lower Fungi

1. Class Chytridiomycetes typically do not form a mycelium; instead, their body is a naked protoplast (plasmodium) that becomes enveloped in a Cell wall as it transforms into a sporangium. Motile zoospores form within the sporangium. Many chytridiomycetes are plant parasites. In infected plant cells, they form thick-walled resting cells known as cysts.

The most important representative of the chytridiomycetes is the potato wart pathogen, Synchytrium endobioticum. Bumpy growths, or "galls," form near the eyes of infected tubers. These growths contain a mass of fungal cysts. Once the growths rupture, the cysts release into the soil and overwinter there. In the spring, the cysts germinate and transform into motile zoospores that infect young plants.

2. Class Hyphochytridiomycetes are unicellular fungi that lack a sexual stage of reproduction. Most species in this class are intracellular parasites of green and brown Algae, as well as cultivated plants. In particular, Plasmodiophora brassicae is the CAUSATIVE AGENT OF clubroot in cruciferous crops, while Spongospora subterranea causes powdery scab in potatoes.

3. Class Oomycetes includes both aquatic and terrestrial forms that reproduce asexually via zoospores and sexually through the formation of oospores. Fungi in this class feature a well-developed mycelium. The oomycetes include the late blight pathogen, Phytophthora infestans, which affects potatoes, tomatoes, and eggplants. Lemon-shaped sporangia develop on short, branched sporangiophores and fall to the ground upon maturation. Inside them, zoospores are formed, which subsequently germinate into hyphae.

4. Class Zygomycetes comprises terrestrial fungi that are widespread in nature. These include fungi of the genera Mucor, Rhizopus, and Thamnidium.

Genus Mucor. Mucoralean molds possess a fluffy, cobweb-like mycelium that is initially white and later turns gray. Numerous spore-bearing hyphae, or sporangiophores, rise from the main mycelium, Swelling at their tips to form large sporangia. Externally, the sporangium is covered with fine spikes made of calcium oxalate crystals. Numerous sporangiospores are formed asexually within the sporangia. Mucor molds thrive On the surface of damp grain, malt, ROOT vegetables, and food products.

Mucor mucedo (Fig. 5.1) forms yellowish-gray colonies with unbranched sporangiophores reaching up to 4 cm in length. The sporangia are gray, occasionally brownish-black.

Fig. 5.1. Mucor mucedo

Genus Rhizopus. Fungi of the genus Rhizopus form a felt-like, grayish-brown mycelium. They differ from mucoralean molds by having arc-shaped curved shoots known as stolons. The stolons attach to the substrate via fine hyphal appendages called rhizoids, above which clusters (tufts) of 3 to 5 (sometimes up to 10) sporangiophores are located. The sporangia are spherical, initially colorless, and turn black upon maturation. Rhizopus nigricans (Fig. 5.2) infects berries and root vegetables, causing soft rot.

Fig. 5.2. Rhizopus nigricans

Genus Thamnidium. Fungi of the genus Thamnidium feature a grayish-white mycelium with long, upright sporangiophores bearing a large sporangium at the tip. Lateral branches develop along the sporangiophores, producing smaller sporangiola containing a limited number of sporangiospores.

Thamnidium elegans (Fig. 5.3) grows well at low temperatures, causing spoilage in meat and meat products during cold storage.

Fig. 5.3. Thamnidium elegans

Higher Fungi

5. Class Ascomycetes (ascomycetes, from the Greek ask - sac). Fungi of this class possess a well-developed multicellular mycelium. During the sexual process, ascomycetes form an ascus containing ascospores. Many ascomycetes can also reproduce asexually via exospores, known as conidia. This form of sporulation is referred to as imperfect. The most common representatives of ascomycetes are fungi of the genera Pénicillium and Aspergillus.

Genus Pénicillium. This group of multicellular fungi is widely distributed in nature due to its high resistance to environmental conditions. Fungi of the genus Penicillium form low, velvety colonies on the substrate surface, colored grey, bluish, or green with lighter margins. The conidiophores of penicills are multicellular and branched. At the tips of the branches lie elongated cells called sterigmata, bearing chains of conidia; the tips of the fruiting hyphae thus resemble a small brush (from the Latin pénicillium - paint brush). Conidia are smooth or spiny, rounded, and pigmented in various colors.

Pénicillium glaucum - green penicillum (Fig. 5.4) forms grayish-green colonies with straight, brush-like branched conidiophores. The chains of conidia are mostly tightly pressed against one another. This species frequently develops on The surface of bread, butter, cheese, confectionery and pasta products, on chilled meat carcasses, and other food products.

Fig. 5.4. Pénicillium glaucum

Fungi of the species Pénicillium roqueforti and Pénicillium camamberti are utilized in The production of mold cheeses, such as Roquefort, Brie, Camembert, Dorblu, etc. They impart a characteristic specific flavor and mushroom aroma to these cheeses.

The species P. chrysogenum and P. notatum are well-known producers of the antibiotic penicillin.

Genus Aspergillus. The conidiophores of aspergilli are non-septate, and their thickened apices terminate in sterigmata from which chains of conidia are abjointed. Upon maturation, the conidia of aspergilli acquire various colors, which is sometimes reflected in the species name of the fungus and, alongside other characteristics, determines their taxonomic identity.

Aspergillus niger (black aspergillus - Fig. 5.5) initially forms a snow-white mycelium, upon which conidiophores with brownish-black heads appear as they mature, giving the fungal colony a black coloration.

Fig. 5.5. Aspergillus niger

Aspergillus glaucum (green aspergillus) forms velvety, yellowish-green colonies with short conidiophores.

Among the black (A. niger, A. awamori), brown (A. terreus), and yellow-green (A. orysae, A. flavus) aspergilli, active producers of various Enzymes, organic acids, and Antibiotics have been isolated. Some aspergilli cause Diseases of the respiratory tract, Skin, and oral mucosa. A. flavus produces toxic substances known as aflatoxins.

Genus Claviceps. The primary representative of this genus is Claviceps purpurea, a phytopathogenic fungus and the causative agent of ergot—a disease affecting predominantly rye, and less frequently wheat and barley during the flowering period (Fig. 5.6). Dark purple sclerotia, resembling small horns, are clearly visible in the inflorescences of ergot-infected plants. During harvesting, the sclerotia shatter, fall to the soil, and overwinter there. In spring, the sclerotia swell and germinate, forming fruiting bodies shaped like heads on thread-like stalks. Several reddish stroma develop from each sclerotium. Inside the HEAD of the stroma are cavities where the sexual process takes place, including the Formation of the ascogonium and antheridium, as well as plasmogamy and karyogamy. The process culminates in the formation of asci containing eight thread-like ascospores. The matured ascospores are discharged onto the flowers of cereal grasses during their mass flowering period, where they germinate into a sporulating mycelium. Several days after infection, the conidial stage of the fungus develops. "Honeydew," in which the conidia are immersed, plays a major role in the dissemination of the fungal conidia by insects. In the infected Ovary by the time rye matures, the mycelium becomes dense, and instead of grains, horns (sclerotia) are formed. Under high humidity, sclerotia form as early as one week after the appearance of the honeydew; in dry weather, after two weeks.

Fig. 5.6. An ergot-infected ear of grain and a germinated sclerotium

Ergot infection reduces grain yields. Furthermore, the inclusion of sclerotia in grain can cause severe poisoning (toxicoxis) in humans and animals. A large number of ergot toxins and their derivatives possessing high biological activity have been described. These toxins are predominantly Alkaloids derived from ergotic and lysergic acids. Consuming bread made from flour containing ergot sclerotia induces weakness, dizziness, and convulsions (a condition known as "ergotism"). The admixture of ergot sclerotia in grain must not exceed 0.05%.

6. Class Basidiomycetes (basidiomycetes). This is a vast group of highly developed multicellular fungi numbering over 30,000 species. A distinctive feature of fungi in this class is the presence of a specialized organ, the basidium, upon which basidiospores develop As a result of the sexual process. Basidiomycetes include:

✵ cap fungi (tubular, lamellate, hydnoid), which possess an annual fruiting body consisting of a cap attached to a stem;

✵ bracket fungi (polypores), which grow on tree trunks and destroy wood;

✵ dry rot fungi, which multiply on wooden Structural elements of buildings. As these fungi develop, the wood is progressively destroyed and rotted;

✵ parasitic fungi affecting field, vegetable, and garden crops. Their most important representatives are smut fungi (order Ustilaginales) and rust fungi (order Uredinales).

Smut fungi predominantly infect cereal crops, causing a disease known as smut.

Bunt, loose smut, and blister smut are distinguished. Currently, hard (wet) smut of wheat, barley, and rye is widespread, caused by species such as Tilletia caries and Tilletia hordei. Smut spores germinate on the sprouted seed even before seedling emergence. The mycelium penetrates the grain germ and grows alongside the plant. By the time of flowering, the fungal mycelium enters the ovary, and during the milky ripeness phase, the inner part of the grain becomes filled with smut spores, or teleutospores. In the ear, instead of a grain, rounded pouches filled with a mass of black chlamydospores are formed. The infected ear has a darkish tint and resembles a charred splinter—hence the name "bunt" (Fig. 5.7).

Fig. 5.7. Loose smut of cereal crops

During harvesting and threshing, these pouches rupture, and the spores land on the surface of healthy grain. The grain acquires the odor of herring brine due to the presence of trimethylamine in the spores. Bunt spores are hygroscopic and smear easily when damp. Flour made from grain infected with hard smut acquires a dark hue, an unpleasant odor and taste, and can cause irritation of the Salivary Glands and intestinal dysfunction.

7. Class Deuteromycetes (Fungi Imperfecti). This class includes multicellular fungi that reproduce asexually via exospores—conidia or arthrospores (oidia); a sexual process is absent or has not been discovered in them. Sometimes conidial sporulation is also absent, and only sclerotia are formed.

Deuteromycetes are widespread in nature; many of them multiply on cultivated plants and food products, causing their spoilage.

Genus Cladosporium — bunch mold, forms velvety olive-green colonies on the surface of the substrate. The mycelium is multicellular and sparsely branched. Cladosporium produces blastospores that develop at the ends of the conidiogenic cell as swellings and then separate from it by a septum. The reverse side of the colonies adhering to the substrate is black in color. Cladosporium herbarum multiplies during cold storage on the surface of meat, butter, and cheese (Fig. 5.8).

Fig. 5.8. Cladosporium herbarum: a - conidia; b - conidiophores

Genus Endomyces forms a white velvety mycelium whose hyphae break down into individual cells called oidia, also known as arthrospores (Fig. 5.9). A representative of this genus is the dairy mold Endomyces lactis (synonym Geotrichum candidum), which frequently develops on the surface of Fermented milk products, cheeses, and butter. The mycelium of dairy mold never darkens and does not produce colored fructifications. Dairy mold possesses active proteases and lipases; therefore, its proliferation in dairy products causes them to turn rancid and develop an unpleasant specific odor.

Fig. 5.9. Endomyces lactis (synonym Geotrichum candidum)

Genus Fusarium forms white or bright pink-colored colonies. Fusarium has Two Types of conidia: macroconidia—sickle-shaped ones that develop on short conidiophores, and microconidia—smaller, oval, and colorless ones (Fig. 5.10). Macroconidia have a characteristic sickle shape with one or more transverse septa. Microconidia are small, oval, colorless, unicellular, or possess one to two septa. Among the fungi of this genus, There are many parasitic species that cause diseases in fruits and vegetables known as "fusarioses." The most widespread species is Fusarium avenaceum. Some species accumulate mycotoxins.

Fig. 5.10. Fusarium avenaceum: a - macroconidia; b - mycelium

Genus Botrytis. Botrytis develops on the substrate as a creeping grey or brownish-olive coating. Sometimes it forms sclerotia (clusters of mycelium made of densely intertwined hyphae) rich in nutrient reserves, which serve to preserve the fungus under adverse conditions. They are brown or black in color and visible to the naked eye. Botrytis features treelike branched conidiophores, at the ends of which short, densely packed sterigmata with clusters of conidia are formed (Fig. 5.11). Conidia are ellipsoidal, sometimes spherical, and colorless. Botrytis cinerea (from the Greek botrytis - bunch, cinerea - grey) causes diseases in apples, pears, and berries. The surface of infected fruits becomes covered with a fluffy grey coating, and the plant tissue turns brown and softens under the action of the fungus's active pectinases.

Fig. 5.11. Botrytis cinerea: a - conidia; b - conidiophore

Genus Trichothecium has a creeping mycelium and forms a powdery coating on the substrate surface consisting of conidiophores and conidia. Conidiophores are straight, with sparse septa. Conidia are oblong, formed singly at the apex of the conidiophore or forming a head that rapidly disintegrates (Fig. 5.12). Trichothecium roseum forms fluffy apricot-pink colonies.

Fig. 5.12. Trichothecium roseum: a - development of conidia; b - conidiophores; c - mycelium with conidia

Genus Alternaria (Fig. 5.13) forms low web-like colonies, initially olive-green and subsequently brownish-black. Short conidiophores extend from the mycelium, bearing multicellular pear-shaped or pointed conidia. The fungus secretes a pigment that stains the substrate beneath the colony black. Fungi of this genus multiply on butter and cheese, forming dark spots on their surfaces. Various species of this genus cause a plant disease known as alternariosis (black rot).

Fig. 5.13. Alternaria radicina: a - conidia; b - mycelium

Genus Trichoderma (Fig. 5.14). The mold grows in the form of loose colonies with a floccose surface. The mycelium is creeping, colorless, and becomes dark green with age. Conidiophores are branched, elevated above the mycelium. Pointed sterigmata develop at their ends, bearing heads of conidia, 20 pieces in each. The spores are glued together by mucus. Trichoderma lignorum has emerald-green colonies. Conidiophores grow out on the mycelium as lateral branches.

Fig. 5.14. Trichoderma lignorum: a - conidia; b - conidiophore

The genus Phoma (Fig. 5.15) initially forms a white, spiderweb-like mycelium on the substrate surface, which subsequently darkens and develops black pycnidia spots. The spherical pycnidia are embedded in the substrate and feature a small, round opening. The pycnidial wall is dark brown to nearly black. Conidiophores are unbranched, and conidia are oval or egg-shaped. Phoma causes root crop diseases known as phomosis. For instance, Phoma betae causes Heart rot in beets, while Phoma rostrupii causes phoma rot in carrots. Occasionally, Phoma can be found within blocks of butter in the form of dark spots.

Fig. 5.15. Phoma betae

The genus Catenularia (Fig. 5.16) forms small, slow-growing colonies of a chocolate-brown color. The conidia are glossy, yellow-brown, extending from the terminal hyphae of the aerial mycelium in long chains. Catenularia fuliginea frequently proliferates in sweetened condensed milk, forming chocolate-colored clumps or "buttons".

Fig. 5.16. Catenularia fuliginea

The genus Monilia. This represents a transitional form between Yeasts and mycelial fungi. Initially, the colonies resemble those of yeasts; only later do the cells elongate and form a true mycelium, from which aerial hyphae rise. The mycelium is endogenous, emerging onto the surface as dense tufts. Conidia form in chains and subsequently break apart into individual cells. Many species of the genus Monilia can infect pome and stone fruit crops. Monilia fructigena causes fruit rot in apples, pears, quinces, and other fruits. Monilia cinerea develops on cherries and plums, causing brown rot.

Monilia nigra forms black spots on the rind of hard cheeses that penetrate deep into the cheese. Monilia roseum is capable of proliferating on the surface of butter, forming pink spots.

Review Questions

1. What characteristics distinguish lower fungi from higher fungi?

2. List the classes of higher and lower fungi.

3. Name Representatives of the lower fungi.

4. Name representative fungi of the class Ascomycetes.

5. Name representative fungi of the class Basidiomycetes.

6. Name representative fungi of the class Deuteromycetes.

7. What role do molds play in the food industry?