Penicillium: Description, Structure and Reproduction

In this article we will discuss about:- 1. Description of Penicillium 2. Vegetative Structure of Penicillium 3. Reproduction.

Description of Penicillium:

Penicillium is a saprophytic fungus, com­monly known as blue or green mold. According to Raper and Thom (1949), the genus includes 1 36 species, distributed throughout the world. They are present in soil, in air, on decaying fruits, vegetables, meat, etc.

The “wonder drug” penicillin was first dis­covered by Sir Alexander Fleming at Sant Mary’s Hospital, London, in 1929; during his work with a bacterium Staphylococcus aureus responsible for boil, carbuncle, sepsis in wounds and burns etc., get contaminated with mold spore (Penicillium notatum) which after proper growth causes death of 5. aureus showing lytic zone around itself.

He isolated and called this anti­microbial compound as Penicillin. Later Raper and Alexander (1945) selected a strain of P. crysogenum, more efficient than P. notatum, in the production of penicillin. The importance of Penicillium is mentioned in the Table 4.6.

Vegetative Structure of Penicillium:

The vegetative body is mycelial (Fig. 4.42A, B). The mycelium is profusely branched with septate hyphae, composed of thin-walled cells containing one to many nuclei (Fig. 4.42C). Each septum has a central pore, through which cyto­plasmic continuity is maintained.

Some mycelia grow deeper into the substratum to absorb food material and others remain on the substrate and grow a mycelial felt. The reserve food is present in the form of oil globules.

Reproduction in Penicillium:

Penicillium reproduces by vegetative, asexual and sexual means.

1. Vegetative reproduction:

It takes place by accidental breaking of vegetative mycelium into two or more fragments. Each fragment then grows individually like the mother mycelium.

2. Asexual Reproduction:

Asexual repro­duction takes place by unicellular, uninucleate, nonmotile spores, the conidia; formed on conidiophore (Fig. 4.43).

The conidiophore develops as an erect branch from any cell of the vegetative mycelium. The conidiophore may be unbranched (P. spinulosum, P. thomii) or becomes variously branched (P. expansum). The branch of the conidiophore (Fig. 4.44B) is known as ramus (plural rami) which further becomes branched known as metulae. A number of flask-shaped phialid or sterigmata develops at the tip of each metulae.

Each sterigmata develops at its tip a number of conidia arranged basipetally (younger one near the mother and older one away from it). In species (P. spinulosum) with unbranched coni­diophore, the sterigmata develops at the tip of conidiophore. Rarely (P. claviforme) many conidiophores become aggregated to form a club- shaped fructification called coremium, which develops conidia known as coremiospores.

During the development of conidium, the tip of the sterigma swells up and its nucleus divides mitotically into two nuclei, of which one migrates into the swollen tip and by partition wall the swollen region cuts off from the mother and forms the uninucleate conidium.

The tip of the sterigma swells up again and following the same procedure second conidium is formed, which pushes the first one towards the outer side. This process repeats several times and thus a chain of conidia is formed.

The conidia (Fig. 4.44C) are oval, elliptical or globose in structure having smooth, rough, echinulate outer surface and of various colou­rations like green, yellow, blue etc.

After maturation, the conidia get detached from the mother and are dispersed by wind. On suitable substratum, they germinate (Fig. 4.44D) by developing germ tube. The nucleus undergoes repeated mitotic division and all nuclei enter into the germ tube. The septa formation continues with the elongation of germ tube and finally a new septate branched mycelium develops.

3. Sexual Reproduction:

Sexual reproduction has been studied only in few species (Fig. 4.44). It shows great variation from isogamy (P. bacillosporum), oogamy (P. vermiculatum) to somatogamy (P. brefeldianum). Most of the species are homothallic, except a few like P. luteum are heterothallic. Ascocarps are rarely formed. Based on the ascocarps, different genera can be assigned as Europenicillium, Talaromyces and Carpenteles.

The genus Talaromyces consists of 15 species. All the species of Talaro­myces studied are homothallic. The account of sexual reproduction deals with Talaromyces vermiculatus (= Penicillium vermiculatum) was described by Dangeard (1907). This spedes shows oogamous type of sexual reproduction. The female and male sex organs are ascogonium and antheridium’, respectively.

The ascogonium develops from any cell of the vegetative filament as an erect uninucleate and unicellular body (Fig. 4.44E). The nucleus then undergoes repeated mitotic divisions and produces 32 or 64 nuclei (Fig. 4.44F).

The antheridium develops simultaneously with the ascogonium from any neighbouring hypha (Fig. 4.44F). It is also an uninucleate and unicellu­lar branch which coils around the ascogonium. The apical region of antheridial branch cuts off by septum and forms a short, somewhat inflated uni­cellular and uninucleate antheridium (Fig. 4.44G).

After maturation of both ascogonium and antheridium, the tip of the antheridium bends and touches the ascogonial wall. The common wall at the point of contact dissoves and the two cytoplasm then intermixed.

The nucleus of the .antheridium does not migrate (Fig. 4.44H) into the ascogonium (Dangeard, 1907). Later, the pairing of nuclei into the ascogonium takes place by the ascogonial nuclei only. The ascogonium then divides by partition wall into many binucle­ate cells, arrange uniseriately (Fig. 4.44H).

Some of the binucleate cells of the ascogo­nium projects out by the formation of multicellu­lar ascogenous branched hyphae, whose cells are also dikaryotic (Fig. 4.441). The apical cells of the dikaryotic mycelia swell up and function as an ascus mother cells (Fig. 4.44J).

Both the nuclei of ascus mother cell undergo karyogamy and form diploid (2n) nucleus (Fig. 4.44K). The nucleus then undergoes first meiosis, then mito­sis, results in the formation of 8 nuclei; those after accumulating some cytoplasm form 8 ascospores (Fig. 4.44L).

With the development of ascogonium and antheridium, many sterile hyphae gradually entangle with them and finally after the forma­tion of ascospores, the total structure becomes a round fruit body i.e., cleistothecium (Fig. 4.44M). The asci arrange irregularly inside the cleistothecium. The ascospores may be globose, elliptical or lenticular in shape with smooth, echinucleate, pitted (Fig. 4.44N) or branched outer wall like a pully-wheel in lateral view.

The ascospores are released by the dis­solution of ascus and cleistothecium wall. The ascospore germinates on a suitable substratum by developing germ tube (Fig. 4.440) and ulti­mately into a mycelium like the mother.