Bacterial Structure

Shape & size of Bacterial Structure

Bacterial Structure : Bacteria are classified by shape into three basic groups:

• Cocci,
• Bacilli, and
• Spirochetes
Bacteria range in size from about 0.2 to 5 μm.

Shape & size of bacterial Structure
Bacteria – Size, Shape and Arrangement

Structure of Bacteria/Bacterial Structure

Bacterial Structure
Bacterial Structure Gram Positive and Negative

Bacterial Structure

Essential components in the Bacterial Structure.

•Cell wall

  • Peptidoglycan
  • Outer membrane of gram-negative bacteria
  • Surface fibers of gram-positive bacteria

• Plasma membrane
• Ribosome
• Nucleoid
• Mesosome
• Periplasm

Nonessential components in the Bacterial Structure.

• Capsule
• Pilus or Fimbria
• Flagellum
• Spore
• Plasmid
• Granule
• Glycocalyx

Bacterial cell wall : Bacterial Structure

Cell wall composed of

  • Peptidoglycan
  • Outer membrane of gram-negative bacteria
  • Surface fibers of gram-positive bacteria

Cell Walls of Acid-Fast Bacteria

• Mycobacteria (e.g., Mycobacterium tuberculosis ) have an unusual cell wall, resulting in their inability to be Gram stained. These bacteria are said to be acid-fast because they resist decolorization with acid–alcohol after being stained with carbolfuchsin.
• This property is related to the high concentration of lipids, called mycolic acids, in the cell wall of mycobacteria.

Function of Cell Wall in Bacterial Structure

•Determines the shape of the cell.
•Gives rigidity to the cell
•Gives osmotic protection to the cell
•Plays an essential role in cell division
•Antigenic so, used in laboratory for bacterial identification
•Some antibiotics act on cell wall

Differences between Cell Walls of Gram-Positive and Gram-Negative

Differences between Cell Walls of Gram-Positive and Gram-Negative Bacteria
Differences between Cell Walls of Gram-Positive and
Gram-Negative Bacteria 
Major Difference Between Gram-Positive and Gram-Negative Bacteria
Major Difference Between Gram-Positive and Gram-Negative Bacteria


• Peptidoglycan is a complex, interwoven network that surrounds the entire cell and is composed of a single covalently linked macromolecule.
• It is found only in bacterial cell walls.


Structure of peptidoglycan layer : Bacterial Structure

Structure of peptidoglycan layer Bacterial Structure
Structure of peptidoglycan layer

Lipopolysaccharide• The carbohydrate backbone, which is composed of alternating N -acetylmuramic acid and N –acetyl glucosamine molecules.
• Attached to each of the muramic acid molecules is a tetrapeptide consisting of both d and l-amino acids.
• The other important component in this network is the peptide cross-link between the two tetrapeptides.

Importance of peptidoglycan

• It provides rigid support for the cell,
• Important in maintaining the characteristic shape of the cell,
• Gives osmotic protection
• Peptidoglycan is present in bacteria but not in human cells  so, it is a good target for antibacterial drugs, such as penicillins, cephalosporins, and vancomycin,

L – form Bacterial structure

• Cell wall deficient state of bacteria is called L-form.These cells are able to grow and divide.
• Protoplast: Cell wall deficient state of gram positive bacteria is called protoplast.
• Spheroplast: Cell wall deficient state of gram negative bacteria is called spheroplast.
• When bacteria treated with lysozyme in a media having right osmotic pressure which balance the internal osmotic pressure of the cell, free spherical bodies are liberated called L form.
• Lysozyme, an enzyme present in human tears, mucus, and saliva, can cleave the peptidoglycan backbone by breaking its glycosyl bond.

Outer membrane

• The gram-negative bacteria have a complex outer layer consisting of lipopolysaccharide, lipoprotein, and phospholipid.


• The lipopolysaccharide (LPS) of the outer membrane of the cell wall of gram-negative bacteria is endotoxin.
• It is called endotoxin because it is an integral part of the cell wall, in contrast to exotoxins, which are actively secreted from the bacteria.

Lipopolysaccharide  bacterial structure

The LPS is composed of three distinct units:

(1) Lipid A, which is responsible for the toxic effects.
(2) A core polysaccharide of five sugars linked to lipid A.
(3) An outer polysaccharide. This outer polymer is the important somatic, or O, antigen of several gram negative bacteria that is used to identify certain organisms in the clinical laboratory.

Periplasmic space

• The space between the outer-membrane layer and the cytoplasmic membrane in gram-negative bacteria is the
periplasmic space.
• It contains the peptidoglycan layer and a gel-like solution of proteins
• Proteins include binding proteins for specific substrates, hydrolytic enzymes and detoxifying enzymes (eg,  lactamase that degrade penicillins and other -lactam drugs).

Teichoic Acid

• Teichoic acids are fibers located in the outer layer of the gram-positive cell wall and extend from it.
• They are composed of polymers of either glycerol phosphate or ribitol phosphate
• Gram-negative bacteria do not have teichoic acids.
• Induce septic shock caused by certain gram positive bacteria;
• Mediate the attachment to mucosal cells
• Major surface antigen of gram positive bacteria

Cytoplasmic membrane

• The cytoplasmic membrane lies just inside the peptidoglycan layer of the cell wall , which is composed of a phospholipid bilayer & proteins.
• The membranes of prokaryotes are distinguished from those of eukaryotic cells by the absence of sterols, The only prokaryotes that have sterols in their membranes are members of the genus Mycoplasma.


(1) Selective permeability & active transport of molecules into the cell,
(2) Energy generation by electron transport & oxidative phosphorylation,
(3) Synthesis of precursors of the cell wall,
(4) Secretion of enzymes and toxins,
(5) Bear receptors & other proteins of chemotactic & other sensory transduction systems.


• Invagination of plasma membrane
• Participates in cell division and secretion


• The capsule is a gelatinous layer covering the entire bacterium.
• It is composed of polysaccharide, except in the anthrax bacillus, which has a capsule of polymerized d – glutamic acid.
• Determine the serologic type (serotype) within a species.

Capsulated bacteria

• Streptococcus pneumonia
• Neisseria meningitides
• Haemophilus influenza
• Klebsiella
• Bacillus anthrasis

Capsule detection

• Capsules are clearly visible in the light microscope when negative stains or special capsule stains are employed; they also can be studied with the electron microscope
• Using antiserum against the capsular polysaccharide. In the presence of the homologous antibody, the capsule will swell greatly. This swelling phenomenon, is called the quellung reaction.

Importance of capsule in Bacterial Structure

• It is a determinant of virulence of many bacteria since it prevent phagocytosis.
• Promote adherence of the bacteria to the host tissue surfaces.
• Specific identification of an organism can be made by using antiserum against the capsular polysaccharide.
• Capsular polysaccharides are used as the antigens in certain vaccines because they are capable of eliciting protective antibodies.
• Protect against the lytic action of complement and bacteriophage invasion.
• Protecting bacterial cells against desiccation.
• Act as a barrier to toxic hydrophobic molecules, such as detergents,
• Act as diffusion barriers against some antibiotics, thus protect bacteria from antibiotics.

Glycocalyx (Slime Layer)

• The glycocalyx is a polysaccharide coating that is secreted by many bacteria.
• It covers surfaces like a film and allows the bacteria to adhere firmly to various structures (e.g., skin, heart
valves, prosthetic joints, and catheters).


• Flagella are long whip like, semi-rigid, helical, hollow tubular structures composed of the protein flagellin that move the bacteria toward nutrients and other attractants.
• Flagella are attached to the cell wall, cell membrane, or both by a basal body, which is a complex molecular machine that rotates the flagellum.
• Many rods have flagella, but most cocci do not and are therefore non motile.
• Cells may have one or many flagella.
• Flagella are highly antigenic.

There are 4 patterns of flagella distribution and these patterns are useful in identifying bacterial structure.

1. Monotrichous : have single polar flagellum (e.g. Vibrio cholerae).
2. Lophotrichous: (lopho means tuft) have a cluster of flagella at one or both ends (e.g. Helicobacter pylori, Spirillum spp).
3. Amphitrichous: single flagellum on both sides ( e.g. Pseudomonas, Alkaligenes faecalis).
4. Peritrichous: (peri means around) numerous flagella are spread evenly all over the whole surface of bacteria (e.g. Escherichia coli).

Flagellated Bacterial Structures
Flagellated Bacteria

• Flagella are invisible in ordinary light microscope preparations, but may be shown by the use of special staining methods, and in special circumstances by darkground illumination.
• Because of the difficulties of these methods, the presence of flagella is commonly inferred from the observation of motility.

Motility test can done by:

• Hanging drop preparation
• Semi solid agar media (MIU media)

Functions of flagella in Bacterial Structure

• Flagella are organ of locomotion. Flagella may play a role in pathogenesis by propelling the bacteria.
• Some species of bacteria (e.g., Salmonella species) are identified in the clinical laboratory by the use of specific antibodies against flagellar proteins.

Pili or fimbriae

Pili or fimbriae
Pili fimbriae

• Pili are short hair like filaments that extend from the cell surface and function as attachment organs that promote specific cell-to-cell contact.
• They are shorter, thinner and straighter than flagella and are composed of subunits of pilin.
• They are found mainly on gram-negative organisms.

Types of pili in Bacterial Structure

• Two types of pili:
(1)Ordinary / common pili
(2)Sex pili

• Ordinary pili are far more numerous and are much shorter than sex pili

Name of piliated bacteria

• Neisseria gonorrhoeae
• Escherichia coli
• Salmonella
• Shigella
• Crynebacterium diphtheriae

Functions of pili

(1) They mediate the attachment of bacteria to specific receptors on the human cell surface, which is a necessary step in the initiation of infection for some organisms. e.g. N. gonorrhoeae that do not form pili are non-pathogenic.
(2) A specialized kind of pilus, the sex pilus, forms the attachment between the male (donor) and the female (recipient) bacteria during conjugation.

Differences between pili & flagellaDifferences between pili & flagella


• Plasmids are extra chromosomal, double-stranded, circular DNA molecules that are capable of replicating independently of the bacterial chromosome.
• Plasmids occur both in both gram-positive and gramnegative bacteria,

Importance of plasmid in Bacterial Structure

Plasmids carry the genes for the following functions and structures:

• Antibiotic resistance,
• Resistance to heavy metals, the active component of some antiseptics and silver
• Resistance to ultraviolet light, which is mediated by DNA repair enzymes.
• Pili (fimbriae)
• Exotoxins, including several enterotoxins.
• Bacteriocins are toxic proteins produced by certain bacteria that are lethal for other bacteria, e.g. colicins made by Escherichia coli and pyocins made by Pseudomonas aeruginosa.
• Several antibiotics produced by Streptomyces..

Bacterial spore

• Spore is a resting bacterial cell, highly resistant to desiccation, heat & chemical agents formed in response to adverse condition.
• Spore formation (sporulation) occurs when nutrients, such as sources of carbon and nitrogen, are depleted.
• Spore forming bacteria- Bacillus & Clostridium
• Sporulation: it is the process of formation of spore from vegetative cell in adverse condition is called sporulation.
• Germination: it is the process of formation of vegetative cell from spore when favourable environment return is called germination.


Spore formation begins with the invagination of the parent cell membrane, producing a double membrane that encapsulates and isolates a copy of the bacterial DNA in what will become the core of the spore. The mature spore retains the complete machinery for protein synthesis. When the endospore is completed, the parent cell lyses, releasing the spore.


• It contains a complete copy of the chromosome, minimum concentrations of essential proteins and ribosomes, very little water and a high concentration of calcium bound to dipicolinic acid.
• The spore has an inner membrane, two peptidoglycan layers, and an outer keratin-like protein coat.

Bacterial spore bacterial Structure
Bacterial spore

Spore germination:

• Upon exposure to water and the appropriate nutrients, specific enzymes degrade the coat. Then destruction of the cortex by lytic enzymes, followed by uptake of water & nutrients, and release of dipicolinate from the cell, and germination into a potentially pathogenic bacterial cell occurs.

The marked resistance of spores has been attributed to several factors in which they differ from vegetative cells:

• The impermeability of their cortex and outer coat,
• Their high content of calcium and dipicolinic acid,
• Their low content of water, and
• Their very low metabolic and enzymic activity.
• The appearance of the mature spores varies according to the species, being spherical, ovoid or elongated, occupying a terminal, subterminal or central position, and being narrower than the cell, or broader and bulging it.
• The location of the spore within a cell is a characteristic of the bacteria and can assist in identification of the bacterium.


• The cytoplasm contains several different types of granules that serve as storage areas for nutrients and stain characteristically with certain dyes.
• For example, volutin is a reserve of high energy stored in the form of polymerized metaphosphate. It appears as a “metachromatic” granule since it stains red with methylene blue dye instead of blue.
• Metachromatic granules are a characteristic feature of Corynebacterium diphtheriae, the cause of diphtheria.