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


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:-

  1. protection- protect the cell from drying, desiccation, phagocytosis (virulence), and toxicity
  2. adherence- help in adhering bacteria to surfaces
  3. storage- it is a reservoir for nutrients
  4. 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 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.

monotrichoussingle polarvibrio cholerae
lophotrichoustwo or more in the endPseudomonas fluorescence
amphitrichoussingle or multiple in both endsaquaspirillum serpense
peritrichousall over surfaceEscherichia 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.

CharacterGram positive Gram negative
pili absentpresent
basal body1 ring pair (L and M)2 ring pair (L, P, S, M)
teichoic and
lipoteichoic acid
porins lipoprotein
and lipopolysaccharides
periplasmic spaceabsentpresent
peptidoglycangreater than 50%10-20%
penicillin sensitivityyesno
resistance and inhibition to dyeshighless
dyepurple to crystal violetred to safranin


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