8 Significant Properties of Bacteria

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1. Shape (forms) of Bacteria:

On the basis of shape, bacteria are of following types- cocci (spherical), bacilli (rod-shaped), vibrios (comma-shaped), spirilla (rigid spiral form), spirochete (flexuous spiral form), mycelia (branched filamentous), pleomorphic (variable shape and size) and stalked (Fig. 11.1).

2. Size of bacteria:

The size of bacteria varies from 0.2-1.5/µm in width and 1-10 µm in length. The smallest known bacterium is Dialister pneumonsintes (0.15- 0.3/µm). The largest known bacterium is Thiomargarita ramibiensis, a marine bacterium with a length of 750/µm. Epulopschium fishelsoni a huge bacterium discovered in the intestine of brown surgeon fish is 600 µm long and 80 µmwide. The micrometer (µm), earlier known as micron ( µ), is the unit of measurement of prokaryotic cell. 1 (µm= 0.0001mm or about 1/25,000 of an inch, lnm (nanometer) or 1 m/x (millimicron) = 0.0001 µm (Fig. 11.2).

3. Structure of bacterial cell:

Bacteria are small unicellular, microorganisms. Bacterial cell have a prokaryotic organization and the details of which can be studied only under electron microscope. A typical bacterial cell consists of a complex cell envelope enclosing the protoplasm. The cell envelope acts as a single protective unit and composed of three basic layers, i.e. the outermost glycocalyx followed by cell wall and innermost cell membrane or plasma membrane.

The protoplasm comprises into nucleoid, cytoplasm and cytoplasmic inclusions such as 70S ribosomes, polyribosomes, mesosomes, granules and vacuoles. (For detail refer prokaryotic cell). The surfaces of some bacteria bear surface appendages such as flagella for locomotion in aquatic medium, pili for conjugation and fimbriae for adhesion (For more details, please refer ultra-structure of Prokaryotic cell).

Gram Staining Technique:

Danish Physician Christian Gram in 1884 divided Bacteria in to two groups based upon Gram Staining method.

1. Gram Positive Bacteria (G⁺)

2. Gram Negative Bacteria (G^–)

4. Flagellation:

The number and distribution of flagella on bacterial surface is called flagellation. On this basis bacteria are of following types (Fig. 11.4):

(a) Atrichous – Flagella absent, e.g., Lactobacillus, Pasteurella.

(b) Monotrichous – Single flagellum present at one end, e.g., Vibrio cholera.

(d) Cephalotrichous – A tuft of flagella present at one end, e.g., Pseudomonas.

(e) Lophotrichous – Two tufts of flagella, one at each end, e.g., Spirillum volutans.

(f) Peritrichous – Flagella all over the surface, e.g., E. coil, Clostridium tetani.

5. Staining of bacteria:

Bacteria are usually transparent like any other cells. So stains or dyes are generally used for microscopic studies which in part different colours to the various cell constituents. The Gram stain and the acid fast stain are the two most widely used stains in bacteriology. According to their wall structure and stainability with the Gram stain (Crystal violet + iodine solution), bacteria are distinguished into 2 types:

(i) Gram positive (Gram +) bacteria

(ii) Gram negative (Gram -) bacteria.

The Gram staining technique was devised by a Danish physician Christian Gram (1884) to stain the peptidoglycan wall. Gram positive bacteria retain the Gram stain and have uniformly thick peptidoglycan wall (10-80nm) with less lipid content and more acidic protoplasm. The Gram negative bacteria don’t retain (loss) the Gram stain, have double layered cell wall (7.5-12 nm), with thin inner peptidogly can wall and high lipid content. The Gram-negative group is larger and more diverse than Gram + negative group.

The Gram-staining technique essentially involves following steps:

Step 1:

Fix bacteria on a microscopic slide.

Step 2:

Stain bacterial smear with a basic dye such as crystal violet or gentian violet for 30 Sec.

Step 3:

Wash the slide and apply a dilute iodine solution. All bacteria appear blue.

Step 4:

Wash the slide with an organic solvent such as alcohol or acetone. Only Gram- positive bacteria retain blue colour and visualized.

Step 5:

Counter stain with safranine. Then only, the Gram- negative bacteria can be visualized.

6. Bacterial growth:

When a bacterium introduced or inoculated into a culture medium, it grows to increase its number until the nutrients are exhausted or toxic metabolites accumulated to inhibitory level. From the beginning of inoculation to the end, if bacterial counts are made at intervals and plotted against time, a sigmoid growth curve appears (Fig. 11.5).

The bacterial growth curve comprises following phases:

(a) Lag phase:

It is the time taken for initial adjustment in the middle.

(b) Exponential or Log phase:

Phase of active growth.

(c) Deceleration phase:

Phase of gradual slowdown in growth.

(d) Stationary phase:

Bacteria don’t grow and stop dividing.

(e) Death or decline phase:

Bacterial population and biomass decrease due to death of ceils Bacteria can grow at temperature between -5 to 80° C, but best growth occurs at 35° C. They can be preserved in liquid nitrogen at -196′ C.

7. Respiration in bacteria:

Bacteria resemble other living organisms in their ability to carry on respiration. Respiration involves a series of reactions by which organic food is oxidized to liberate chemical energy in form of ATP. Bacteria require chemical energy for growth reproduction and other cellular processes.

On the basis of oxygen requirement, respiration maybe aerobic or anaerobic. In aerobic respiration free oxygen participate, so that food (substrate) is completely oxidized to liberate large amount of energy, CO₂ and H₂O. When the oxidation, is carried out in absence of free oxygen the process is called anaerobic respiration or fermentation, in which the substrate is incompletely oxidized to liberate little energy, alcohol or organic acids.

According to their mode of respiration, bacteria may be of following types:

(i) Strict or obligate aerobes:

These bacteria require free oxygen otherwise get killed, e.g. Bacillus subtilis.

(ii) Strict or obligate anaerobes:

These bacteria can’t grow in oxygen and rapidly killed even traces of oxygen present, e.g. Clostridium botulinum.

(iii) Facultative aerobes:

These bacteria are generally anaerobic but can live in presence of oxygen, e.g. Chlorobium limicola.

(iv) Facultative anaerobes:

Normally these bacteria respire aerobically but can switch over to anaerobic mode of respiration if oxygen becomes deficient, e.g. Clostridium tetani, Staphylococcus.

(v) Aeroduricanaerobes:

These are facultative anaerobes continue to live anaerobically even in presence of oxygen, e.g. Lactic acid bacteria, a sub-group of Staphylococcus.

8. Nutrition in bacteria:

On the basis of mode of nutrition bacteria are classified into two categories i.e. autotrophic and heterotrophic bacteria. The vast majority of bacteria are heterotrophs, while some of the bacteria are autotrophs.