Bacteria: Structure and Role (With Diagram)

In this article we will discuss about:- 1. Structure of a Bacterial Cell 2. Beneficial Activities of Bacteria 3. Role in Industry.

1. Structure of a Bacterial Cell:

It is a prokaryotic cell in which genetic material is not organised in the form of nucleus but instead lies freely in a naked super-coiled state in the cytoplasm whence it is known as prochromosome or nucleiod. Prokaryotic (= eukaryotic) cells are known for their rapid multiplication.

The average size is 2.0-2.6 µm long and 1.1-1.5 µm wide. The small­est bacterial cells are 100-200 nm (0.1-0.2 µm). The small­est bacterium is Dialister pneumosintes. It has a size of 0.15 µm. The largest bacte­rial cells belong to spirochaetes and blue green algae. Here the cell may reach a size of 500 µm. Epulopscium fishelsoni, a bacterium found in the in­testine of Brown Surgeon Fish is 600µm long and 80 µm wide.

Marine bacterium Thiomargarita ramibiensis is 750 µm long. Therefore, some bacterial cells are quite large as compared to eukaryotic cells (5-100 µm). In shape, bacterial cells are of many types (Fig. 2.7). Mycelial form is found in actinomycetes.

1. Coccus (Gk. kokkos- berry):

Coccus bacteria are spherical or ovoid in outline.

Depending upon their grouping they are called:

(i) Monococcus (occurring singly),

(ii) Diplo-coccus (in twos),

(iii) Tetracoccus (in tetrads),

(iv) Streptococcus (in chains),

(v) Staphy­lococcus (irregular grape-like clusters) and

(vi) Sarcina (3-dimensional geometrical forms).

2. Bacillus (L. bacillus- small rod):

The bacterium is straight and cylindrical like a rod with ends being flat, rounded or cigar shaped.

It has three special types:

(i) Diplobacillus (in twos),

(ii) Palisade Bacillus (like a stack) and

(iii) Streptobacillus (in chains).

3. Spirillum (L. spira- coil):

The bacterium is coiled like a cork-screw, e.g., Spirillum, Spirochaete. Aggregation does not occur.

4. Vibrio:

The body of the bacterium is like a comma, curved rod or single turn of the

Spiral e.g. Vibrio cholera. Like spirillum bacteria, the vibrio forms live singly.

5. Stalked:

The bacterium possesses a stalk, e.g., Caulobacter.

6. Budding:

The bacterium is swollen at places, e.g., Rhodomicrobium.

Flagellation (Fig. 2.8):

Depending upon the presence or absence of flagella, bacteria are grouped into flagellate and non-flagellate types.

The various forms of flagel­lation (Fig. 2.9) are as follows:

(a) Atrichous:

Flagella absent.

(b) Monotrichous:

A single flagellum occurs at or near one end of bacterium.

(c) Amphitrichous:

A flagellum at each of the two ends,

(d) Lophotrichous:

A group or tuft of fla­gella is found only at one end.

(e) Cephalotrichous:

A tuft or group of flagella occurs at each of the two ends or poles. Many authors use the term amphitrichous for both single flagellum and tuft of flagella at each end.

(f) Peritrichous:

A number of flagella are distributed all over the surface.

2. Beneficial Activities of Bacteria:

Role in Agriculture:

(i) Nature’s Scavengers:

Along with saprotrophic fungi, saprotrophic bacteria cause decay and decomposition of dead bodies of plants and animals. In the process they cleanse the earth and release raw materials for new generations.

(ii) Sewage Disposal:

Organic content of sewage is broken down by the bacteria.

(iii) Ammonifying Bacteria:

Bacillus vulgaris releases ammonia from amino acids.

(iv) Nitrifying Bacteria:

Nitrosomonas and Nitrosococcus oxidise ammonium salts to nitrites. The nitrites are further changed into nitrates by Nitrobacter and Nitrocystis.

(v) Nitrogen Fixing Bacteria:

A few free living bacteria are able to pick up di-nitrogen from the soil atmosphere and convert it into organic nitrogenous materials like amino acids, e.g., Azotobacter, Beijerinckia, Clostridium pasteurianum. Symbiotic nitrogen fixing bacteria of the genus Rhizobium occur in the root nodules of a number of legumes (Fig. 2.15).

These legumes are used in crop rotation and green manuring. The plants as well as their seeds are rich in proteins. Root nodules containing symbiotic nitrogen bacteria also occur in Casuarina and Alnus. Leaf nodules containing such bacteria are found in Ardisia.

(vi) Manure:

The saprotrophic bacteria convert farm refuse, dung and other organic wastes into manure.

(vii) Gobar Gas Plants:

They employ bacteria for converting animal dung and other organic wastes into manure along with production of fuel gas.

(viii) Sulphur Bacteria:

Beggiatoa and other sulphur bacteria pick up H₂S released during putrefaction of proteins to produce sulphates.

(ix) Ensilage:

Ensilage is preserved cattle feed or fodder. It is formed by packing fresh chopped fodder in silos sprinkled with molasses. Fermentation activity of bacteria produces lactic acid which has a preservative action.

3. Role of Bacteria in Industry:

(x) Dairy Industry:

Lactic acid bacteria (e.g., Streptococcus lactis) convert milk sugar lactose into lactic acid.

C₁₂H₂₂O_n + H₂O – Lactose → Lactic Acid 4 C₃H₆O₃ + Energy

Lactic acid coagulates milk protein casein and converts milk into yoghurt, curd and cheese.

(xi) Lactic Acid:

Lactic acid is commercially got from ammoniated sugar solution through fermentation caused by Lactobacillus delbreuckii. Acid is used in food preservation, tanning and preparation of a number of drugs.

(xii) Vinegar:

Acetic acid bacteria (Acetobacter aceti) oxidise ethyl alcohol into acetic acid. Ethyl alcohol is got from molasses.

(xiii) Butyl Alcohol and Acetone:

Clostridium acetabutylicum is able to produce butyl alcohol, methyl alcohol and acetone from molasses.

(xiv) Retting of Fibres:

Stem and leaf fibres are separated from softer tissues by bacterial action of two types— dew retting (e.g., Pseudomonas fluorescence) and anaerobic retting (e.g., Clostridium or butyric acid bacteria).

(xv) Curing:

Leaves of tea and tobacco are cured off their bitterness with the help of certain bacteria, e.g., Bacillus megatherium. Beans of coffee and cocoa are similarly cured.

(xvi) Cleaning of Hides:

Hides are cleaned of their fat, hair and other attached tissues by bacterial action. Commercial sponges are cleaned similarly.

(xvii) Antibiotics:

A number of antibiotics are obtained from mycelial bacterium Strepto­myces (e.g., Streptomycin, Chloramphenicol, Tetracycline, Oxytetracycline). A number of antifungal medicines are also produced by this genus, e.g., hamycin, trichomycin, primaricin. Bacitracin, subtilin, polymyxin, gramicidin are some other antibiotics obtained from bacteria.

(xviii) Vitamins:

Riboflavin was formally prepared from Clostridium butylicum. Cobal- amine (B₁₂) is obtained from bacteria like Bacillus megatherium. Acetic acid bacteria are used in some steps during the preparation of vitamin C. Escherichia coli present in the human intestine produces large quantities of vitamin K and В complex vitamins.