Metodo de evaluación

may generate H₂S from the S-containing amino acids in the medium, which in turn shall interact with FeSO₄ to produce a distinct black precipitate of FeS (ferrous sulphide).

5.2.3.3. Selective Media for Staphylococci

The presence of the Staphylococci organisms in various specimens viz., pharmaceutical prod-

ucts, food items, and pathological specimens, may ultimately cause food poisoning as well as serious

systemic infections.

A few typical examples of selective media for various organisms are as follows : (a) Enterobacteria — a surface active agent serves as the main-selector.

(b) Staphylococci — NaCl and LiCl serve as the main selectors. Besides, Staphylococci in general are found to be sufficiently tolerant of NaCl concentrations upto an extent of 7.5%.

5.3.

ACTINOMYCETES

Actinomycetes refers to any bacterium of the order Actinomycetales, which essentially includes

the families : Mycobacteriaceae, Actinomycetaceae, Actinoplanaceae, Dermatophilaceae,

Micromonosporaceae, Nocardiaceae, and Streptomycetaceae.

In fact, Actinomyces represents a genus of bacteria belonging to the family Actinomycetaceae which contain Gram-positive staining filaments. In general, these organisms cause various diseases both in humans and animals.

Another school of thought describes actinomycetes as the filamentous microorganisms. It has been duly observed that superficially their morphology very much looks alike that of the filamentous

fungi. Nevertheless, the filaments of actinomycetes invariably comprise of the prokaryotic cells hav-

ing diameters relatively much smaller in size in comparison to those of the molds. However, there exist certain typical actinomycetes which resemble the molds by making use of externally carried asexual spores for accomplishing the desired reproduction.

Interestingly, actinomycetes are very common inhabitants of soil, whereas filamentous habit of

growth has definitely the added advantages. In this manner, the organism can conveniently bridge the water-free gaps existing between the soil particles to allow them to migrate to a new nutritional site.

It is pertinent to state here that this ensuing particular morphology very much provides the organisms an appreciably higher surface-area-to-volume ratio, thereby the nutritional efficiency gets improved significantly in the highly competitive soil environment.

Importantly, the best-known genus of actinomycetes is Streptomyces, which is one of the bacteria most abundantly isolated from soil.

However, the reproductive asexual spores of Streptomyces, termed as conidiospores, are invari- ably formed at the ends of aerial filaments. If each conidiospore gets attached to an appropriate substrate, it is capable of germinating into an altogether new colony.

Characteristic Features of Streptomyces : The various characteristic features of Streptomyces

are as follows :

(1) These organisms are strict aerobes.

(2) They invariably give rise to extracellular enzymes which essentially enable them to use proteins, polysaccharides viz., starch or cellulose ; and many other organic compounds usually found in soil.

(3) It gives rise to the formation of a gaseous compound known as geosmin, that imparts to the ‘fresh soil’ its typical musty odour.

(4) The species of Streptomyces are of immense value because they cat- egorically produce a host of commercial antibiotics, such as :

Streptomyces nodosus : Amphotericin B Streptomyces venezuelae : Chloramphenicol

Streptomyces aureofaciens : Chlorotetracycline and tetracycline. Streptomyces erythraeus : Erythromycin

Strepromyces fradiae : Neomycin Streptomyces noursei : Nystatin Streptomyces griseus : Streptomycin

5.4.

FUNGI

The kingdom of organisms that essentially includes yeast, molds, and mushrooms, is termed as

fungi.

It has been duly observed and amply demonstrated that fungi invariably grow as single cells, as in yeast, or as multicellular filamentous colonies, as in molds and mushrooms. Interestingly, fungi do not contain chlorophyll (i.e., the nature’s organic green matter), hence they are saprophytic (i.e., they obtian food from dead organic matter) or parasitic (i.e., they obtain nourishment from the living organ- isms), and above all the body’s normal flora categorically contains several fungi. However, most fungi are not pathogenic in nature.

Importantly, the fungi that essentially cause disease belong to a specific group known as fungi

imperfecti. In immunocompetent humans these fungi usually cause minor infections of the hair, nails, mucous membranes, or skin. It is, however, pertinent to mention here that in a person having a com- promised immune system due to AIDS or immunosuppressive drug therapy, fungi critically serve

as a source of the viable opportunistic infections that may even cause death ultimately.

Figure 5.6, illustrates the magnified diagramatic representations of yeast, rhizopus, aspergillus,

ringworm, and cryptococcus.

1* %*

%*

Yeast (× 750) Rhizopus (× 40) Aspergillus (× 40)

Ringworm (× 750) Cryptococcus (× 500)

Fig. 5.6. Magnified Diagramatic Representations of Yeast, Rhizopus, Aspergillus, Ringworm, and Cryptococcus.

Another school of thought defines fungi as — ‘those microorganisms that are invariably

nucleated, spore-bearing and do not possess chlorophyll, generally reproduce both asexually and sexually, and have somatic structural features that are essentially surrounded by cell walls com- prising of polysaccharides, cellulose and/or chitin, mannan, and glucan.

In fact, fungi are considered to be mostly saprophytic, making use of dead organic matter as a

source of energy, vital natural organic decomposers, and destroyers of food stuffs. While a major

segment of species happen to be facultative parasites that specifically able to feed upon either live or dead organic matter, and a relatively minor quantum of species may only survive on the living

protoplasms. These fungi are designated as obligate parasites thereby overwhelmingly causing dis-

ease of man, animals, and plants. They also prove to be of reasonably great economic and medical importance.

Industrial Research — Certain fungi are intimately associated with the manufacture of bread, beer, and wines (fermentative procedures) ; production of edible varieties of cheese, vitamins, and

organic acids (viz., lactic acid, citric acid, acetic acid etc.) ; and several ‘antibiotics’.

Biological Research — Geneticists and Biochemists exploit the fungi profusely by virtue of

5.4.1. Reproduction (Cultivation) of Fungi

A large number of fungi invariably get reproduced both asexually and sexually. Nevertheless, the ensuing morphology, and the cycle of these reproductive structures is employed extensively in carrying out their elaborated and logical classification.

5.4.1.1. Asexual Reproduction

The most common procedure of asexual reproduction is usually accomplished by the help of

spores. In common practice most of them are found to be colourless (hyaline), while a few of them are

duly pigmented as green, yellow, red, orange, black or brown. In fact, their size may invariably range from small to large and their shape from globose via oval, oblong, needle-shaped to helical. Virtually, the ensuing infintie variation in adequate spore appearance and their arrangement prove to be of im- mense utility for proper identification. Asexual reproduction may be borne particularly in a sac-like

structure termed as sporangium ; and the spores being referred to as sporangiospores being called as conidia as depicted in Fig. 5.7.

Conidiophore Conidia 1° Sterigma 2° Sterigma Septate Hypha Conidiophore Metulae Sterigmata Conidia Aspergillus sp. Penicillium sp.

Fig. 5.7. Asexual Reproductive Structures (spores) of Aspergillus and Penicillium [Adapted From : Hugo and Russell : Pharmaceutical Microbiology, 3rd edn, 1984]

Salient Features : The salient features related to asexual reproduction are as follows :

(1) The simplest form of available fungal spore is known as the zoospore, which possess no rigid cell wall, and is duly propelled by flagella.

(3) Flagellum is made up of 11 parallel fibrils, of which 9 forming a cylinder and 2 placed

centrally.

(4) Base of flagellum enters the cell and gets attached to the nucleus by a structure termed as

rhizoplast.

(5) Flagellum structure (9 + 2 fibrils) is usually found to be fairly consistent with that shown for other flagellated organisms.

(6) Sporangium designates the asexual reproductive structure pertaining to these aquatic

fungi. In its early stages it is found to be loaded with nuclei and protoplasm.

(7) Cleavage takes place subsequently whereby the numerous sections invariably get developed into the corresponding uninucleate zoospores.

(8) Finally, following a motile phase, the resulting zoospore encysts, losing its flagellum, and rests quietly just prior to germination.

5.4.1.2. Sexual Reproduction

Importantly, the sexual reproduction is characterized by the strategical union of two compat-

ible nuclei ; and the entire phenomenon may be distinctly divided into three phases, namely :

Phase I : The union of the gametangia (i.e., sex-organs) brings the nuclei into close proximity

within the same protoplast. It is also referred to as plasmogamy.