Bacterial Structure
CYTOPLASMIC MEMBRANE
Cell
membrane
Cell Membrane (Plasma Membrane)
 Double layered membrane.
 2 structural component
– double layer of phospholipids
– proteins
• Fluid Mosaic Model
Cell Membrane (Plasma Membrane)
Functions:
 Selective permeability to different molecules; Selective
barrier (selectively permeable).
Simple diffusion: passage of nutrients from area of high
concentration to low concentration.
 Facilitated diffusion: Solutes combine with a transporter
protein in the cell membrane to enter the cell.
 Active transport of ions like Na, K and nutrients.(
Excretion of hydrolytic enzymes (exoenzymes such as
Amylases, Lipases,peptidases )to degrade
macromolecules into smaller subunits.
 Excretion of pathogenicity proteins like exotoxins.
 Site of respiration; E.T.S. is located here (=mitochondria in
eukaryotes).
 Enzymes for Cell wall synthesis.
 Plays a role in cell division.
 Endocytosis:
o Phagocytosis: engulfing solid particles
o Pinocytosis: engulfing fluids
Phagocytosis
Pinocytosis
Cytoplasm
• Contains ribosomes, nutrient granules, metabolites and
plasmids and nucleoid region composed of DNA.
• Ribosomes: numerous, 15-20nm in diameter with 70S;
distributed throughout the cytoplasm.
Intracytoplasmic structures
Nucleoid
 It has no nuclear membrane.
 It has no nucleolus.
 Contains the bacterial chromosome.
 The chromosome is single circular double stranded DNA
molecule.
Nucleus
• Lacking nuclear
membrane, absence
of nucleoli, hence
known as nucleic
material or nucleoid,
one per bacterium.
Ribosomes
 Clusters of ribosomes are called polysomes.
 Bacterial ribosomes are 70 S and formed of two subunits 30
S and 50 S subunits.
 Consist of (40% protein and 60% ribosomal RNA).
Ribosomes are the sites of protein synthesis in the bacteria.
Ribosomes - protein synthesis
• Prokaryotic Ribosome• Eukaryotic Ribosomes
• 70 S
– 50 S
– 30 S
• 80 S
– 60 S
– 40 S
• Some antibiotics are aimed at the 70 S
ribosomes of bacterial cells
• Streptomycin, Neomycin, Erythromycin and
Tetracycline work by inhibiting protein
synthesis by disrupting the 70 S ribosome
Plasmid
Inclusions of
Bacteria
Plasmids are small circular/extrachromosomal
double-stranded DNA molecules. They are capable
of self-replication and contain genes that confer
some properties such as antibiotic resistance,
virulence factors. Plasmids are not essential for
cellular survival.
Inclusions are aggregates of various 
compounds that are normally involved in
storing energy reserves or building blocks for
the cell. Inclusions accumulate when a cell is
grown in the presence of excess nutrients and
they are often observed under laboratory
conditions.
Plasmids
• Carry 5 to 100 genes
• Carry genes for following functions:
– antibiotic resistance
• penicillase
– production of toxins
• E. coli 0157:H7
• -resistance for ultra violet and heavy metals found in
antiseptics.
• Pilli which mediate the adherence of bacteria to
epithelial cells
• Production of bacteriocins which are toxins or enzymes
killing other bacteria
• Several antibiotics produced by actinomycetes.
Plasmids
 Extra-chromosomal circular double stranded DNA molecules.
 Capable of autonomous replication.
 Transmissible to other bacterial cells.
 Not necessary for bacterial life.
 However, they carry genetic information like: antibiotic
resistance and toxin production.
Transposons
 Fragments of DNA that can move inside the bacterial cell
from one location on the chromosome to another location and
between the chromosome and plasmid.
 So, they are called jumping genes.
Mesosomes
 Invagination of the cytoplasmic membrane.
 Functions:
 Site of respiration of the bacterial cell.
 Play a role in cell division:
o The site of binding of the 2 sister chromosomes.
o The origin of the transverse septum that divides the cell into 2
daughter cells.
Extracytoplasmic structures
Flagellum
 It is the organ of motility.
 Long thin filaments which rotate by basal body attached to
the cell wall and cell membrane.
 Composed of a single protein called flagellin.
 Flagella carry flagellar or H antigen.
 Not stained by Gram stain.
 Seen only by the electron microscope.
Distribution and number of flagella may be:
A. Monotrichous
B. lophotrichous
C. Amphitrichous
D. Peritrichous
Monotrichous and peritrichous
flagella are the most frequent
in pathogenic bacteria.
Motility
Almost all Spiral bacteria are motile
About 1/2 of Bacilli are motile
Almost all Cocci are non-motile
Some bacterial
species are mobile and possess
locomotory organelles - flagella. Flagella consist of a
number of proteins including flagellin
The diameter of a flagellum is thin, 20 nm,
and long with some having a length 10
times the diameter of cell. Due to their small
diameter, flagella cannot be seen in the light
microscope unless a special stain is applied.
Bacteria can have one or more flagella
arranged in clumps or spread all over the
cell.



Identification
of Bacteria
Pathogenesis
Motility of
bacteria
Monotrichate/Amphitrichate/Lophotrichate/Peritrichate
Flagella
GOOD LUCK