Notes on Microorganisms | Biology

In this article we have compiled various notes on microorganisms. After reading this article we will have a basic idea about:- 1. Meaning of Microorganisms 2. Origin of Microorganisms 3. Distribution 4. Nature 5. Nutrition 6. Classification 7. Reproduction 8. Importance of the Study.

Contents:

1. Notes on the Meaning of Microorganisms
2. Notes on the Origin of Microorganisms
3. Notes on the Distribution of Microorganisms
4. Notes on the Nature of Microorganisms
5. Notes on the Nutrition of Microorganisms
6. Notes on the Classification of Microorganisms
7. Notes on the Reproduction of Microorganisms
8. Notes on the Importance of the Study of Microorganisms

Note # 1. Meaning of Microorganisms:

Microorganisms are microscopic forms of life, include bacteria, fungi, algae, protozoa, and the infectious agents at the borderline of life that are called viruses which are not cellular organisms. They are also known as microbes. The term microbe is taken from the French and means a microscopic organism or microorganism, being usually applied to the pathogenic forms.

Again the term germ, in popular usage, refers to any microorganism but especially to one of the pathogenic or disease-producing bacteria. Hence the terms microbe and germ are probably synonymous with bacterium (pi. bacteria). A microorganism that harms its host is also called a pathogen. Microorganisms differ widely in form and life cycle.

Some are single-celled, but others are multicellular. Some of them have no well-organized nucleus, but others do. Some of the microorganisms are predominantly plant-like, others are animal-like, again others share characteristics common to both plants and animals. They grow rapidly and reproduce at an unusually high rate, within 24 hours some of them com­plete almost 100 generations.

Microorganisms have pattern of metabolic processes like that of higher plants and animals. Regardless of the complexity of structure of a microorganism, the cell is the basic structural unit of life. All living cells are funda­mentally similar.

Microorganisms may or may not contain chlorophyll and can use inorganic or organic carbon as the source of carbon. Some have the unique ability of utilizing either radiant or chemical energy and may be autophytes or heterophytes.

Again some are able to utilize atmospheric nitrogen for the synthesis of proteins and other complex organic nitrogenous compounds. Some microorganisms synthesize all their vitamins, while others need to be furnished with vitamins.

Microorganisms play an important and often dominant role in all fields of human endeavour like industry, agriculture, problems connected with food, shelter and clothing, and in the conser­vation of human health and combating diseases.

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Note # 2. Origin of Microorganisms:

Microorganisms originate from the earth’s surface and are dispersed in nature through various agencies like air, water, rain, insects and various other media. They occur nearly everywhere in nature where they find food, moisture, and a temperature suitable for their growth and multiplication.

Some are found in the bottom of the ocean and others miles away on the mountain heights, thereby can stand wide range of temperature conditions. Microorganisms occur in water (fresh and marine), stagnant and/or free flowing.

Since they thrive under conditions suitable for human survival, it is inevitable that we live among a multitude of microorganisms. They infest the air we breathe and the food we eat.

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Note # 3. Distribution of Microorganisms:

Microorganisms inhabit on our body surfaces, in our alimentary tract, in our mouth, nose and all other body orifices. Fortunately most microorganisms are harmless to us; and we have means of resisting invasion by those that are potentially harmful.

The microbial population in our environment is both large and complex. For example, a single sneeze may disperse approximately from 10,000 to 100,000 bacteria. One gram of faces may contain millions of bacteria. Our environment—air, soil, water—likewise consists of a menagerie of bacteria and other microbes.

Soil micro­organisms include many species of bacteria, fungi, algae, protozoa and viruses. A study of the microorganisms in these habitats requires specialized knowledge of the specific microbes present.

The bacterial population of the soil exceeds the population of all other groups of microorganisms in both number and variety. Several billions of bacteria per gram of soil have been reported by direct count method.

All microorganisms of the aquatic environment have patterns of seasonal growth which again are controlled by the nature of water— fresh or marine and stagnant or free flowing. As such it is extremely difficult to enlist the nature of aquatic microorganisms.

Still a very broad idea is given in the following manner:

i. Fresh water planktonic microorganisms include primarily the producer orga­nisms (photosynthetic algae) —green and blue-green algae, diatoms, pigmented bacteria, etc.

Whereas, marine planktonic ones are: diatoms, blue-green algae, dinoflagellates, chlamydomonads; species of the genera Pseudomonas, Vibrio, Flavobac- terium, and Achromobacter which give protection against lethal portion of solar radiation; protozoa—species of Foraminifera and Radiolaria, and many flagellated and ciliated species.

The characteristic colour of the Red Sea is associated with heavy blooms of a blue-green alga, Oscillatoria erythraea. The chytrid Rhizophidium, small animals and viruses parasitize on planktonic algae causing profound effect on the algal population.

ii. In the benthos epilithic forms include many diatoms (e.g., species of the genera Synedra, Meridion, Licmorphora); species of Chamaesiphon, Rivularia, Chaetophora, Cladophora, Sphacelaria and many microscopic Rhodophytes. Some produce mucilage in which filaments are embedded whilst others precipitate Calcium carbonate around themselves and build up nodular growths.

Again some small algal cells are attached amongst the coating of bacteria on both fresh water and marine sand grains— epiplasmmic flora—the majority belong to the Bacillariophyta, though some of Cyanophyta and Chlorophyta. Besides this, small Crustacea bear epizooic algal populations.

iii. Both pigmented and nonpigmented bacteria including species of Pseudomonas, Chromobacterium, Achromobacter, Flavobacterium and Micrococcus are very common in less polluted water. Bacteria washed into water are mainly species of Bacillus (aerobic) and Clostridium (anaerobic).

In water polluted with animal or human excreta some of the common bacteria are: Escherichia coli, Streptococcus faecalis, Proteus vulgaris, Clostridium perfringens, Salmonella typhi and Vibrio cholerae. Where oxygen is available, a wide range of aquatic fungi is available of which chytrids are most important.

Some more common ones are the species of Mycosphaerella, Ceriosporopsis, Saprolegnia, Monoblepharis. Where oxygen is scarce, species of Clathrosphaerina and Helicodendron may be present. Aquatic protozoa are common in both fresh are salt waters. Planktonic protozoa (ciliates, flagellates, and the Heliozoa) and grazing species of other animals fluctuate with the nature of phytoplankon.

Pollution of Natural Waters:

The increasing use of fertilizers and disposal of wastes in streams are resulting in rapidly increasing in nutrient content in many natural water bodies. Enormous algal crops result, followed by their decay giving massive pollution of water. Similar problems of excessive algal growth arise in industrial plants, e.g., in power station cooling towers, and ponds.

The degree of pollution of both fresh and marine waters can be determined by studying the algal growth. Benthic algae are very sensitive indicators. In the extremely polluted zones only algae such as: Oscillatoria chlorina, and Spirulina jenneri occur and in the slightly less polluted zones the diatoms Nitzschia palea and Gamphonema parvulum appear.

The recent growth in number of nuclear power stations, from which large amount of excess heat has to be dissipated, is giving cause for concern over ‘heat pollution’ of rivers and estuaries. It causes direct danger to fish and microorganisms, thereby may disrupt the food chain.

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Note # 5. Nutrition of Microorganisms:

Microorganisms are autotrophs and heterotrophs; mesophiles, thermnphiles, and psychrophiles; aerobes and anaerobes; cellulose digesters and sulfur oxidizers; nitrogen fixers and protein digesters; and other kinds of bacteria in soil which are yet to be discovered.

Large number of actinomycetes, as many as millions per gram, is present in dry warm soils. The most predominant genera of this group are Nocardia, Streptomyces, and Micromonospora which are responsible for the characteristic musty or earthy odour of a freshly ploughed field. They are capable of degrading many complex organic substances and consequently play an important role in building soil fertility.

Fungi are most abundant near the soil surface where an aerobic condition is likely to prevail. They exist in both the mycelial and spore stage. It is difficult to estimate their numbers. Fungi are active in decomposing the major constituents of plant tissues, namely, cellulose, lignin, and pectin. The physical structure of soil is improved by the accumulation of fungal mycelium within it.

The population of algae in soil is generally smaller than that of either bacteria or fungi. The major types present are the green algae and diatoms. Their photosynthetic nature accounts for their predominance on the surface and just below the surface layer of soil. On barren and eroded lands algae may initiate the accumulation of organic matter because of their ability to carry out photosynthesis and other metabolic acti­vities.

The blue-green algae are known to grow on the surfaces of freshly exposed rocks where the accumulation of their cells results in simultaneous deposition of organic matter. This establishes a nutrient base that will support bacterial species. The growth and activities of the initial algae and bacteria pave the way for the growth of other bacteria and fungi.

The mineral nutrients of the rock are slowly dissolved by acids resulting from microbial metabolism. This process continues with a gradual accumu­lation of organic matter and dissolved minerals until a condition results that supports growth of lichens, then mosses, then higher plants. The blue-green algae play a key role in the transformation of rock to soil, a first step in rock-plant succession.

Most soil protozoa are flagellates or arnoebas. Their number per gram of soil ranges from a few hundred to several hundred thousand in moist soils rich in organic matter. Their mode of nutrition involves ingestion of bacteria with some selectivity. Hence protozoa is one of the factors in maintaining equilibrium of microorganisms in soil.

Plant and animal viruses, as well as bacterial viruses (bacteriophages) gain entry into soil microbial population through plant and animal wastes.

In general, some microorganisms survive utilizing solar energy, others use various chemical substances as their fuel, again others have special mechanism of livelihood. As such, microorganisms may behave either as autophytes, as saprophytes, as parasites, or as symbionts.

Microorganisms bring about in nature many changes some desirable (beneficial) and others undesirable (harmful) to us. The diversity of their activities ranges from the enhancement of soil fertility by decomposing dead organic matter, transforma­tion and deposition of minerals in the soil, formation of coal, production of various useful substances, causing diseases of humans and animals and plants to reducing soil fertility and various other activities.

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Note # 6. Classification of Microorganisms:

The subject of microbial classification has a long history. One of the first systems was proposed in 1773, and many more appeared since then. Until the eighteenth century, the living organisms were classified into two Kingdoms: plant and animal. Since there are organisms which are typically neither plants nor animals, naturally they do not fall into either plant or the animal Kingdom.

Hence it was proposed that new kingdoms be established to include these organisms. In 1866 E. H. Haeckel, a German zoologist suggested that a third kingdom, Protista, be formed to include those unicellular microorganisms that are typically neither plants nor animals.

These organisms, the protists, include bacteria, algae, fungi and protozoa. Since viruses are not cellular organisms, hence they were not included with the protists.

He referred bacteria to as lower protists; and called algae, fungi and protozoa higher protists. But Haeckel’s concept of Kingdom Protista failed to put forward criteria that could be used to distinguish a bacterium from a yeast or certain microscopic algae. Late in the 1940s with the help of powerful magnification provided by electron microscopy it was possible to make more definitive observation of internal cell structure.

It was dis­covered that in some cell, particularly in bacteria, the nuclear substance was not enclosed by a nuclear membrane. Whereas, in other cells, for example typical algae and fungi, the nucleus was enclosed in a membrane. This discovery enabled to sepa­rate one group of protists (bacteria) without membrane bound nucleus from all the others (algae, fungi, protozoa) which possess membrane bound nucleus.

These two cell types have been designated as: procaryotic and eucaryotic and organisms of each cell type are called procaryotes and eucaryotes respectively. Hence bacteria are procaryotic organisms and eucaryotic organisms include algae, fungi and protozoa.

A comprehensive system of classification, designated as five-Kingdom system, was proposed by R. H. Whittaker (1969). This system of classification is based on three levels of cellular organization which evolved to accommodate three principal modes of nutrition: photosynthesis, absorption, and ingestion. The procaryotes are included in the Kingdom Monera, they lack the ingestive mode of nutrition.

Unicellular eucaryotic microorganisms are placed in the Kingdom Protista, all nutritional types are represented here. It includes microalgae whose mode of nutrition is photosynthetic and protozoa with ingestive mode of nutrition.

The multicellular and multinucleate eucaryotic organisms are found in the Kingdoms Plantae (multicellular green plants and higher algae), Animalia (multicellular animals), and Fungi (higher fungi). The diversified modes of nutrition lead to a more diversified cellular organization of the five kingdoms: Monera (bacteria and cyanobacteria), Protista (microalgae and protozoa) and Fungi (yeasts and molds).

The scheme of classification is pre­sented below:

A. Unicellular organisms

B. Procaryotes…. Monera

BB. Eucaryotes…. Protista

AA. Multicellular organisms

B. Higher algae and green plants.. Plantae

BB. Animals…… Animalia

BBB. Higher fungi …. Fungi

Bergey’s Manual of Determinative Bacteriology in 8th edition (1974) has recogni­zed the Kingdom Monera, but has called it Kingdom Procaryotae because of the procaryotic nature of the cells. This Kindgom is divided into two divisions: the blue- green algae or cyanobacteria (some microbiologists consider the blue-green algae as bacteria) and bacteria (procaryotic organisms other than blue-green algae).

In 1984 the scope of Bergey’s Manual was greatly broadened by bringing together various other information concerning bacterial classification and identification.

It was published under a new name—Bergey’s Manual of Systematic Bacteriology in which all bacteria were placed in the Kingdom procaryotae which has been divided into 4 divisions as follows:

Division 1. Gracilicutes

Procaryotes with a complex cell wall structure -Gram-negative bacteria

Division 2. Firmicutes

Procaryotes with a cell wall structure characteristic of Gram-positive bacteria.

Division 3. Tenericutes

Procaryotes that lack a cell wall.

Division 4. Mendosicutes

Procaryotes with characteristics of an earlier phylogenetic origin than above Divisions.

The study of cell organization by the electron microscope and by advanced bio­chemical techniques has revealed fundamental resemblances and differences between various microorganisms and offers a sound basis for their division into major groups (i.e. procaryotes and eucaryotes, and viruses) and for their farther subdivision.

L. E. Hawker and A. H. Linton (1978) included under the Kingdom Protista the so-called microorganisms (both procaryotic and eucaryotic) having relatively lowly differentiated body and subdivided it in the following manner.

A. Organization subcellular Viruses

AA. Thallus unicellulr, multicellular, or plasmodial

B. Nucleoplasm not bounded by a membrane Prokaryota

C. Chlorophyll absent, or if present of type different from that of plants

CC. Chlorophyll present, together with charac­teristic blue-green pigment, pigments not located in discrete plastids

BB. Cells or plasmoclia containing one or more discrete membrane-bounded nuclei

D. Cell(s) of vegetative thallus (with a few exceptions possessing a cell wall(s)

E. Chlorophyll present and located in discrete chloroplasts

EE. Chlorophyll absent

DD. Gell(s) of vegetative thallus lacking true cell walls

F. Thallus unicellular, remaining so Protozoa

FF. Thallus unicellular at first, becoming a plasmodium or pseudoplasmodium and even­tually forming a fructification Slime molds

In Bergey’s Manual of Systematic Bacteriology (1984) the Cyanophyta (Blue- green algae) have been placed in volume 3—Bacteria with unusual properties in the Section: Oxygenic Phototrophic Bacteria. They were designated as the Cyanobacteria as they contain chlorophyll, can use light as an energy source, and evolve O₂ in a manner similar to that of green plants.

This arrangement has not been accepted by the algologists. According to them, the Cyanophyta (Blue-green algae) have closer affinities with algae than bacteria and as such they should be placed under the group Algae and not under Bacteria.

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Note # 7. Reproduction of Microorganisms:

Microorganisms reproduce by the simple method of binary fission. Besides this, there are other methods by which they reproduce.

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Note # 8. Importance of the Study of Microorganisms:

Microorganisms though microscopic in size are most interesting life forms which attracted attention of workers through ages. They possess immense potentialities which are continuously being explored for the welfare of human society. Not only that they are most fascinating group of organisms, but also have a very simple life form, easy to handle, and are pliable.

With increase in know ledge of the microbial world, the task of befriending the microbial regime—particularly bacterial has now become an urgent agendum in the struggle for existence of the human beings together with the necessary organisms. Following are some of the essential aspects of human society that have opened up based on the study of microorganisms and have emphasized the importance of their study.