Introduction:

Bacteria are mainly characterized based on the cell size, shape and structure of cell arrangement, occurrence of special structures and developmental forms, staining reactions and motility and arrangement of flagella. They are as well characterized via the cell wall component, Gram stain reaction, cellular respiration and method of nutrition.

General features of bacteria:

1) They are prokaryotic and are simplest among all the microbial cells.

2) Bacteria are generally single celled organisms.

3) They are measured in the unit known as micrometer.

4) They encompass distinctive cell wall that includes peptidoglycan.

5) Bacteria is deficient in a true nucleus however encompass an area termed as the nucleroid region, that is, DNA is free floating.

6) Their reproduction is through binary fission.

7) They might encompass additional DNA termed as plasmid.

8) They are very diverse and abundant in waters and soils.

Size of Bacterial Cell:

Bacteria are extremely small, 0.5 to 1.0µm in diameter. Due to their small size, they encompass high surface area or volume ratio that outcomes in a high growth and metabolism rate. No circulatory method is required for nutrients taken in as the mass of cell substance to be nourished is much close to the surface. Examinations of the microbial cell need the use of a high power microscope generally of around 1,000 diameters. 

Shape and Arrangement of Bacterial Cell:

The shape of a bacterium is administered through its rigid cell wall which provides it a definite shape.

Characteristic shapes of bacteria are:

1) Cocci: They are oval, round and spherical in diameter characteristic arrangement if multiplying is based on the arrangement of cells, they are termed:

• Diplococci: cocci in pairs, example: meningococci and gonococci.
• Streptococci cocci in the chains.
• Staphylococci: cocci in irregular clusters (similar to a bunch of grapes).
• Tetracocci: cocci in the group of four cells.
• Sarcinae: cocci in the regular clusters.

2) Bacilli (Rod): These are stick such as bacteria having rounded, square, tapered or swollen ends. They compute 1-10µm in length by 0.3-1.0µm in width.

Bacilli are not arranged in prototype as complex as cocci. Most take place singly. The other arrangements are:

• Diplobacilli: Rods in the pairs.
• Streptobacilli: Rods in chains.
• Trichomes: Identical to chains however encompass bigger area of contact among adjacent cells.
• Mass together, example: Mycobacterium leprae.
• Palisade arrangement cells are lined side through side like matchsticks and at angles to one other like Chinese lecters, example: Corynebacterium diptheriae.

3) Vibrios: These are small, slightly curved rods and comma shaped 3 to 4µm in length by 0.5µm in width. Most of them are motile having a single flagellum at one end, example: Vibrio cholerae.

 4) Spirilla: These are helical bacteria, regularly coiled, small, rigid, organisms measuring 3 to 4µm in length. Each and every coil measures around 1µm, example: Spirillum minus.

5) Spirochaetes: They are helical, completely twist, flexible, coiled organisms, can twist and contort their shapes. Spirochaeters are categorized into three major groups: Treponemes, Borreliae and Leptospires.

Bacterial Structures:

Assessment of the bacterial cell will reveal some components and structures. A few of them are external to the cell wall whereas others are internal to the cell wall.

Structure External to the Cell Wall:

1) Flagella (Singular: Flagellum):  These are hair like, helical appendages which protrude via the cell wall, 0.01 to 0.02µm in diameter and simple in structure. Based on their position on the cell, flagella might be polar (that is, at one or both ends of bacterium) or lateral (that is, along the sides of the bacterium).

2) Pili (Singular: Pilus):  They are as well termed as fimbriae. They are hollow, non-helical filamentous appendages which are thinner, shorter and more abundant than flagella: long, thin, straight threads 3 to 25µm in diameter and 12µm in length. They don't function in motility since they are found on the non-motile and motile species. Some of the functions are related by various kinds of pili.

3) Capsules: This is a viscous substance making a covering layer or envelope around the cell wall of several bacteria. Capsules can be classified into three, based on their visualization through light microscope by employing special staining techniques.

4) Sheaths: A few bacterial species form chains or trichomes enclosed through a hollow tube termed as sheaths. These sheaths comprise of a heteropolysaccharides having glucose, glucuronic acid, galactose and fructose.

Some bacteria facilitate modest change of position. Sheaths facilitate individual cells to stay related in the cell colonies.

5) Prosthecae and Stalks:

Prosthecae: They are semi-rigid extensions of the cell-wall and cytoplasmic membrane and encompass a diameter less than that of the cell. Found in several aerobic bacteria from fresh water and marine atmosphere.

Stalks: They are non-living ribbon-like or tubular appendages excreted through some bacterial cells, example: found in Gallionella or Planctomyces.

Structures Internal to the Cell:

1) Cytoplasmic Membrane:

a) This lies instantly under the cell wall.

b) It is around 7.5µm (0.0075µm) thick and comprised primarily of phospholipids (20 - 30 percent) and protein (60 - 70 percent).

c) It serves up as a barrier to most of the water soluble molecules.

d) It includes a variety of enzymes comprised in the respiration and metabolism and in synthesis of capsular and cell wall component.

e) Proteins are as well synthesized in the cytoplasm.

2) Protoplast:

A protoplast is the part of the bacteria, all made up of the cytoplasmic membrane and the cell material surrounded by it.

3) The Cytoplasm:

It is the cell material bounded through the cytoplasmic membrane and it might be splitted into:

a) The cytoplasmic region, granular in appearance and rich in the macromolecular RNA-protein bodies termed as Ribosomes on which proteins are synthesized.

b) The chromatin area is rich in DNA.

c) The fluid part having dissolved substances.

4) Nuclear Material:

Dissimilar to eukaryotic cells bacterial cells don't encompass a distinct membrane enclosed nucleus however they encompass an area close to the centre of the cell which is regarded as the nuclear structure, the DNA of the cell is confined to this region. The DNA is circular and bears the genes of the cell.

5) Spores and Cysts:

Certain bacteria generate spores either in the cells (that is, endospores) or external to the cell (that is, exospores). The spore is metabolically dormant form which under suitable condition can germinate to make a vegetative cell. Endospores are very resistant to staining, desiccation, disinfecting chemicals, radiation and heat.

Cysts are as well dormant, thick walled desiccation resistant forms which can germinate as well under favorable conditioning.

Nutrition in bacteria:

The nutrition necessities of bacteria differ broadly.

1) Based on their source of energy, they are categorized as:

a) Phototrophs: These are the bacteria which utilize light energy as their energy sources.

b) Chemotrophs: They get their energy through oxidizing inorganic or organic - chemical compounds.

2) Based on the source of carbon that is the main source of nutrient for all cells bacteria can be categorized as:

a) Heterotrophs: These are the bacteria which derive carbon from preformed organic nutrients like carbohydrate or sugar.

b) Autotrophs: They derive carbon from the inorganic sources like carbon-dioxide.

Cellular Respiration in bacteria:

Based on whether they require oxygen to survive or not, bacteria might be:

a) Aerobic or strict aerobes: These need oxygen, example: Bacillus cereus.

b) Anaerobic bacteria or strict anaerobes: They can't tolerate oxygen, example: Clostridium spp.

c) Facultative anaerobes: These are usually aerobes however encompass the capacity to grow in the absence of oxygen, example: Staphylococcus spp.

Reproduction in bacteria:

Bacteria reproduce mostly through asexual method which most of the time is transverse binary fission. This is a method or procedure in which a bacterial cell splits to give two daughter cells after developing a transverse septum (that is, cross wall).

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