Bacteria is a unicellular, microscopic, prokaryotic, heterotrophic organisms that are present in almost all places in this world. These organisms have both RNA and DNA as their genetic material. They can survive high temperatures, Ph, oxygen, acidity, and pressure. These are micron in size and are observable in compound and electron microscopes. The following flowcharts represent the shape and arrangement of bacteria.
Anatomy of bacteria
The following are the components of a bacterial cell:-
- flagella
- pili
- capsule
- plasma membrane
- cytoplasm
- cell wall
- ribosome
- mesosomes, spore and inclusions
Capsule
Capsules are tightly attached amorphous organic exopolymers composed of polysaccharides (2%), water (98%), and polypeptides. If the capsule is loosely associated, it is called a slime layer since it can be easily washed off. The bacteria with capsules produce smooth colonies and others produce rough colonies. These may be thick or thin. The following are the functions of a capsule:-
- protection- protect the cell from drying, desiccation, phagocytosis (virulence), and toxicity
- adherence- help in adhering bacteria to surfaces
- storage- it is a reservoir for nutrients
- diagnosis and serotyping
Plasma Membrane
This is similar to the eukaryotes and can be best explained by the fluid mosaic model consisting of a lipid bilayer with two types of proteins. Peripheral proteins (20-30%) are easily removable and integral proteins (70-80%) cant be easily removed. The plasma membrane helps in the transportation of nutrients, house enzymes, and have selective permeability.
Pili and Fimbriae
Pili is the longer thick microfiber hair on the bacterial surface. This is made of pilin protein present only in gram-negative bacteria, that have 1-10 pilus. Used for sexual conjugation and are formed by plasmid (the additional genetic material in bacteria) genes.
Fimbriae are shorter thin hair on bacterial surfaces. this is made up of fimbrillin protein present in both strains of bacteria, having 200-400 fimbriae. Used for attachment and are formed by chromosomal genes.
Flagella
Flagella are long slender hair structures with constant diameter helping in bacteria motility. These are present in both strains, a few cocci, most bacilli, and all spirilla. They have three different components, namely the basal body, hook, and filament. The filament is a helical structure exposed to the environment made with identical subunits of flagellin. The basal body attaches the filament to the cell membrane and the hook attaches the filament to the cell surface. The following are the different type of flagella.
| Name | Description | Example |
| monotrichous | single polar | vibrio cholerae |
| lophotrichous | two or more in the end | Pseudomonas fluorescence |
| amphitrichous | single or multiple in both ends | aquaspirillum serpense |
| peritrichous | all over surface | Escherichia coli |
Cell Wall
It is a 20-30mm thick rigid structure that gives shape to the cell. They constitute 20-30% of the weight and are not visible in a light microscope even after staining. It is made up of muramic acid, diaminopimelic acid, and teichoic acid, all combining together to form peptidoglycan. They carry bacterial antigens important for virulence and immunity. Several antibiotics interact with the cell wall.
Gram positive and negative strains
These are two different strains of bacteria and there are several differences between them.
| Character | Gram positive | Gram negative |
| pili | absent | present |
| basal body | 1 ring pair (L and M) | 2 ring pair (L, P, S, M) |
| teichoic and lipoteichoic acid | present | absent |
| porins lipoprotein and lipopolysaccharides | absent | present |
| peptidoglycan | thick | thin |
| periplasmic space | absent | present |
| Layers | one | two |
| peptidoglycan | greater than 50% | 10-20% |
| penicillin sensitivity | yes | no |
| resistance and inhibition to dyes | high | less |
| dye | purple to crystal violet | red to safranin |
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