In this article we will discuss about:- 1. Occurrence of Penicillium 2. Distinction between Penicillium and Aspergillus 3. Salient Features.
Occurrence of Penicillium:
The penicillia are commonly known as the green or blue molds. However, their colour varies through a wide range. They are cosmopolitan in their distribution. Usually Penicillium grows as a saprophyte on decaying fruits and vegetables.
Decaying oranges, lemons and other fruits of the family Rutaceae usually exhibit a bluish growth on them. This is due to the blue coloured spores of Penicillium. It also grows on the preserved fruits and jellies.
Some species of Penicillium have been reported to grow on the forest floors and a few of them grow on cultivated and manured ground. Penicillium italicum and P. digitatum are well known plant pathogens which attack citrus fruits causing the familiar soft rot disease.
P. funiculosum induces the pine apple (Ananas comosus) fruit disease termed fruitlet core rot. Wild species of Penicillium that grow upon damp wood have been known to digest hemicelluloses. Some species (P. roqueforti and P. camemberti) cause fermentation in cheese and lend it characteristic aroma and consistency.
Raper and Thom (1949) in their monograph of the genus reported 136 species. Some mycologists make 140 species.
Penicillium can be grown artificially in the laboratory. Take any moist citrus fruit or a piece of cheese Cover it with a bowl and place it in a damp, warm place. It will become moldy. Small velvety patches of blue or sky blue or bluish green colour will appear on the substrate.
Distinction between Penicillium and Aspergillus:
The two genera can easily be distinguished from each other by the following facts:-
1. The conidiophore in Aspergillus is unseptate and unbranched. It aries from a specialized, T-shaped thick-walled foot cell. In Penicillium it is long, slender, septate and branched. It arises from any vegetative cell of the mycelium. There are no foot cells.
2. In Aspergillus the conidiophores enlarge into a vesicle at its tip which bears the finger-like sterigmata. The whole structure has the form of a spherical head. The conidiophore in Penicillium ends in a whorl of branches which give it a broom-like appearance.
3. The peridium of cleistothecium of Penicillium is thicker and generally consists of loosely interwoven hyphae than that of Aspergillus which is yellowish in colour and quite soft
Salient Features of Penicillium:
The life cycle comprises three phases, the haplophase, the dikaryophase and the transitory diplophase.
1. It is represented by a well-developed, saprophytic mycelium consisting of colourless, septate, branched hyphae. The cells are uninucleate to bi-or multinucleate and the septa between the cells have each a central pore.
2. The haplomycelium during the growing season multiplies by means of light, dry, non- motile, wind borne, spores called the conidia which are uninucleate, sometimes multinucleate.
3. The conidia arise in chains from the tips of bottle-shaped sterigmata and are thus exogenous in origin. They are arranged in a basigenous manner.
4. The sterigmata are borne in tufts on the ultimate branches (metulae) of the conidiophores.
5. The conidiophores are long, erect, septate special hyphae of the mycelium. They branch in a broom-like manner in the upper portion. This brush-like portion of the conidiophore is called the penicillus.
6. The haplomycelium is homothallic. Both the antheridia and the ascogonia are borne on the same mycelium.
7. The ascogonium in P. vermiculatum at maturity is an erect, tubular, aseptate, multmucleate structure.
8. The antheridium is a short, club-shaped uninucleate cell, surmounting the antheridial branch which coils spirally round the ascogonium.
9. The tip of the antheridium comes in contact with the ascogonium. The double wall at the point of contact dissolves.
10. The male nucleus, however, does not migrate into the ascogonium m Talaromyces vermiculatus (P. vermiculatum).
11. Pairing of female nuclei takes place in the ascogonium. Each pair is called a dikaryon. With the establishment of dikaryons the haplophase ends and the dikaryophase starts.
12. Establishment of dikaryons is followed by the septation of the ascogonium, each segment possessing a dikaryon.
13. From the middle segments arise the ascogenous hyphae which branch. The branches are of different lengths.
14. The terminal, binucleate cell of each ascogenous hypha or its ultimate branches develops into an ascus. The ascus mother cell is binucleate. It is the last structure of the dikaryophase which consists of the ascogonium after pairing of nuclei, the ascogenous hyphae and the ascus mother cells. The dikaryophase is, however, dependent for its nutrition on the haplomycelium.
15. Karyogamy takes place in the ascus mother cell, with this the dikaryophase ends.
16. The young ascus containing a synkaryon or diploid nucleus represents the transitory diplophase.
17. The young ascus enlarges into a sac-like structure. The synkaryon immediately undergoes meiosis followed by mitosis. The 8 haploid nuclei are organised into ascospores which are the pioneer structures of the future haplophase.
18. The mature, sac-like asci, each containing eight ascospores are spherical or pear-shaped in form.
19. The sexual apparatus in the meantime is surrounded by loosely interwoven envelope of sterile hyphae in some species and compact in others. It is called the peridium. The entire ascus fruit consisting of the septate ascogonium, ascogenous hyphae, asci containing ascospores and the surrounding peridium is called the ascocarp.
20. The mature ascocarp of Penicillium is a round, closed structure. It is known as the cleistothecium.
21. At maturity the ascus wall dissolves away releasing the ascospores into the cleistothecium. The wall of the cleistothecium disintegrates to liberate the ascospores.
22. The liberated uninucleate ascospore has sculptured spore wall. It is pulley-like in appearance in side view and round to star-shaped in face view.
23. Each ascospore, on falling, on a suitable substratum germinates to produce the haplomycelium.