Diagram of Bacteria (With Forms) | Microbiology

The below mentioned article provides a diagram of bacteria along with additional information as follows:- 1. Occurrence and Distribution of Bacteria 2. Size of Bacteria  3. Forms 4. Staining Bacteria (Gram Reaction).

Occurrence and Distribution of Bacteria:

The bacteria constitute a highly specialised group of one-celled plants. There are about 2,000 known species. They have a closer association with man because of their universal distribution.

They are among the most numerous of all living things, being present in almost every conceivable environment. They occur in abundance in both fresh and salt water. The air we breathe is full of them.

They are present in the food we eat and the objects we touch. They flourish in our mouth and intestines. They live in the bodies of other organisms and their dead remains. Some live in hot springs and others in ice.

They are found in all soil and above the earth. In fact they live in all conditions not fatal to living matter.

The features which contribute to their universal distribution are:

(a) Extremely simple structure.

(b) Small size and consequent large surface-to-volume ratio. In order to maintain their small size and this favourable ratio, cell division must occur rapidly.

(c) Resistance of vegetative cells to adverse environmental factors.

(d) Formation of highly resistant endospores.

(e) Diversity of their modes of nutrition.

Plant Characteristics:

The bacteria are microorganisms that possess rigid cell walls and when motile have flagella. Mostly they are unicellular organisms lacking true nucleus and typical plastids.

The plant characteristics of the bacteria are:

1. Presence of a definite and rigid cell wall which in a few species contains cellulose.

2. The tendency of some to grow in filaments.

3. The ability of autotrophic bacteria to synthesize organic food from inorganic materials such as carbon dioxide and water.

4. The absorption of food in soluble form through the cell wall.

5. Structure of the bacterium cell and reproductive methods essentially similar to some of the thallophytes.

6. Ability to synthesize all amino acids from inorganic nitrogen.

7. Existence of transitional forms between bacteria and fungi.

Size of Bacteria:

Bacteria are exeedingly small unicells. In fact they are the smallest of all known cellular organisms and are visible only with the aid of the microscope. Some of them are so small that they are hardly visible under the high power of ordinary light microscope.

Their size is usually expressed in microns, a smallest unit of the metric system. A micron (µ) is equivalent to 0.0001 mm. or about 1/25,000 of an inch. An average spherical bacterium cell is 1/2 micron (µ) in diameter.

In comparison, a grain of salt is about 500 microns across. Of course the size of bacteria varies considerably ranging from about 0.2µ (micron) to 2 micron (µ) in width and less than 2-10 microns in length.

The coccus forms vary from 0.5 to 2.5 microns in diameter. The variations in the Bacillus forms are still greater from 0.3 to 15 µ in length and from 0.2 to 2.0µ in width. Escherichia coli is 0.4 to 0.7 µ in diameter and 1.0 to 3.0 µ long.

The average bacillus size is 3 microns long. A single drop of liquid can contain about 50 million bacteria. A few Bacillus and Spirilla forms are much larger.

Forms of Bacteria:

The shape of these tiny organisms is their only readily visible characteristic. Thus it is used as a basis for describing them. The bacteria vary in their shape which is usually constant for each species.

The three fundamental forms among the bacteria are rods, spheres and helices.

True bacteria belong to one of these three types:

1. Rod-shaped Bacteria:

The majority of bacterial cells look like hyphens (-) or tiny rods (A) under the microscope. The rods may be cylindrical and straight or slightly curved or wavy with rounded or blunt ends.

These are the bacilli (sing, bacillus). Bacillus is a Latin word meaning “Little stick or rod”. The bacillary forms are by far the commonest and range around 1.5µ in diameter and 10µ in length. Some bear flagella and others not.

The bacilli occur singly (A), in groups of two (diplohacillus, B), or in a chain (C) forming a filament (streptobacillus) or occasionally in parallel or palisade arrangement (D).

B. suhtilis, B. anthracis and Lactobacillus are the common examples of rod-shaped bacteria. In some bacilli the rods are so short as to be indistinguishable from spherical forms.

2. Spherical Bacteria:

The second largest group of bacteria have spherical or ellipsoidal cells varying from 0.5 to 1.25µ in diameter. These are called cocci (sing, coccus). Almost all coccal forms lack flagella and are non-motile.

The spherical cells may occur singly but often tend to stick together and various cell arrangements occur. Diplococcus sp. tend to occur in groups of two (E) or multiples of two (D. pneuomoniae). Tetrads of cocci (F) are formed by Tetracoccus.

In Streptococcus sp. the cocci occur in chains (S. lactis). The coccal cells form loose masses of variable size and irregular in shape like bunches of grapes (H) in Staphylococcus (S. albus and S. aureus).

In Sarcina sp. the spherical cells divide regularly in three planes producing box-like cubical packets of eight or multiples of eight with the cells arranged in rows (G).

3. Helical Bacteria:

The helical forms are of two types, spirilla and vibrios.

Spirilla (J):

The bacteria having the body twisted spirally like a cork-screw through 1-5 complete turns are the Spirilla (sing. Spirillum). The Spirilla are larger than the cocci and bacilli ranging form 10-50µ in length though only 0.5µ to 3µ in diameter.

They are smaller in number probably with fewer than 100 species. The spirilla forms usually bear two or more flagella at one or both poles. They occur as single cells or in short chains but do not form colonies. Spirillum urdulum and S. volutans are the common examples of this type.

Vibrios (I):

The bacterium cell in the vibrios is short and looks like a curved rod (I) or twine of a spiral. In fact it is less than the full turn of a helix and shaped much like a slightly twisted comma.

The vibrios have flagella at one pole only and seldom exceed 10µ in length and 1.5 to 1.7µ in diameter. Vibrio comma is a typical example of this category.

4. Filament (K):

A fourth type of form may occur in a few species. It is a thread or filament. The number of filamentous forms, as compared with others, is very small. A group of filamentous forms including Beggiatoa and Thiothrix oxidize hydrogen sulphide. Granules of sulphur accumulate within the cells.

5. Pleomorphic Bacteria (L):

Some bacteria are able to change shape and size with changes in the environmental conditions. These changes are usually temporary. In a few species, however, several types of cell forms may occur in the life cycle.

For example, Acetobactor, a group of vinegar bacteria occur as small rods, long rods, ellipsoids or a chain of small rods. Such bacteria are called pleomorphic.

Staining Bacteria (Gram Reaction):

The most important and commonly used of bacterial stains is the Gram stain. It is a differential stain which has a simple staining procedure. It was devised by the Danish bacteriologist.

Gram, in 1884 and thus named, the Gram stain after its discoverer. The procedure involves the staining of a bacterial smear containing a variety of bacteria with crystal violet solution.

The dye is prepared by dissolving 2.0 gm of Crystal violet (80% dye content) in 20 ml of Ethyl alcohal (95%). The dye is applied for 30 seconds to the bacterial smear. It is then gently rinsed off.

The smear is next with an aqueous solution of iodine which is prepared by adding 1.0 gm Iodine and 2 to 5 gm potassium iodine to 300 ml of distilled water.

Before use the solution is allowed to stand for 24 hours for iodine to dissolve. The iodine solution, in turn, is rinsed off the smear after 30 seconds.

Examination of the smear with the microscope reveals that all the bacteria have taken the purple colour. The next step in the staining procedure is to apply and renew 95 percent ethyl alcohol to the smear until all except the thickest layers of the smear have ceased to give off the dye.

Examination of the smear at this stage reveals that many of the bacteria still retain the original purple colour whereas from others it has been washed off. The bacteria that keep the original purple stain are called gram-positive.

Those which yield it to the solvent (alcohol) and become decolourised are called gram-negative. The decolourised bacteria (gram-negative) are nearly invisible.

To study their details the final step in the staining procedure is to apply the counter stain such as eosine or safranine to the smear. It gives a good colour to the gram-negative bacteria and at the same time contrast with the purple of the gram-positive bacteria.

The division of bacteria into gram-positive and gram-negative forms, on the basis of the reaction to gram-stain, is correlated with other characterisic differences between them. All spore- producing bacteria are gram-positive. The polarly flagellated bacteria are gram negative.