Bacteria have a simple cell structure compared to eukaryotic cells. They lack membrane-bound organelles and have three basic components - surface appendages, surface layers, and intracytoplasmic structures. Surface appendages include flagella for locomotion and fimbriae/pili for adhesion. The cell envelope varies between gram-positive and gram-negative bacteria, but both contain a cytoplasmic membrane, peptidoglycan layer, and nucleic DNA material. The bacterial cytoplasm contains ribosomes, plasmids, and the nucleoid where DNA is located. Bacteria reproduce through binary fission and have various shapes depending on their structure.

1. BACTERIA STRUCTURE
AND FUNCTION

2. Basic Bacterial Cell Structure
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3. INTRODUCTION
• Bacteria form a large group of single-celled, parasitic,
saprophytic and free-living microorganisms.
• They are prokaryotic and reproduce by binary fission
• Varying size from 0.1- 10um in length.
• The extreme small size makes room for high surface
area to volume ratio.
• They have a simple cell structure, contain both DNA
and RNA and lack a nuclear membrane
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4. • Most bacteria are capable of independent metabolic
existence and growth.
Exceptions: Chlamydia and Rickettsia are obligately
intracellular organisms
• They exist with different shapes – spherical(cocci),
rod(bacilli), comma or spiral shapes.
• Various arrangement – chains, pairs, clusters etc
• Their importance in medicine cannot be over-emphasized
as they are responsible for various diseases.
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8. BACTERIA CELL STRUCTURE
3 basic components:
1. Surface appendages
2. Surface layers
3. Intracytoplasmic structures
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9. SURFACE
APPENDAGES
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10. SURFACE APPENDAGES
• 2 basic types of surface appendages:
1.FLAGELLA [organs of locomotion]
- thread-like appendages present on both Gram
positive and Gram negative bacteria
- their presence is useful for identification.
2.FIMBRIAE (PILI)
- found mostly on Gram negative bacteria and a few
Gram positives .e.g. Corynebacterium renale.
Some bacteria have both fimbriae and flagella. e.g.
E.coli which has numerous flagella and 2 types of
pili.
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11. Flagella
• Chemically, flagella are composed of many
subunits of a single protein called flagellin.
• A flagellum consists of 3 parts:
1. The long filament, which lies external to the cell
surface
2. The hook structure at the end of the filament
3. The basal body, to which the hook is anchored and
which imparts motion to the flagellum
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13. • Functions of flagella:
1. Allow organisms to migrate towards favorable
growth environment(nutrients and other
chemoattractants via chemotaxis) and away from
those that might be harmful.
2. They play a role in the pathogenesis of UTIs by
propelling the bacteria up the urethra into the
bladder.
3. Some species of bacteria(e.g. Salmonella sp.)
are identified in the clinical laboratory by the use
of specific antibodies against flagella proteins
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15. • Flagella can be sheared from the cell surface
without affecting the viability of the cell
The cell then becomes temporarily non motile
In time it synthesizes new flagella and regains motility.
• The protein synthesis inhibitor chloramphenicol,
however, blocks regeneration of flagella
• NB: motility is observed in young cultures (18-24
hours or less); bacteria tend to become non motile
in older cultures because of production of acid and
other toxic products
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16. Motility
• Types of bacterial motility
– run or swim - when a bacterium moves in
one direction for a length of time
- tumbles - periodic, abrupt random changes
in direction
– swarming - rapid wavelike growth across a
solid culture medium
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17. Detection of Motility
• Direct- Hanging drop
method
• Indirect
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18. SWARMING
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19. FIMBRIAE (PILI)
• Fimbriae [pili] are slender, hair-like, proteinaceous
filaments on the surface of many Gram negative
bacteria
• Shorter, finer and more rigid than flagella, 4-8nm
long and 1-5nm thick
• Composed of protein subunits referred to as pilins
• Minor proteins termed adhesins are located on
the tips of the pili and are responsible for the
attachment properties.
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20. Pili
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21. Pili cont’d
• Pili are anchored on the outer membrane of the
Gram negative bacteria
• 2types of pili are recognised:
1. Ordinary pili- short, abundant, for adhesion
2. Sex pili – a small number (one to six), very long,
for DNA transfer during conjugation
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F-Pili in Conjugation
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23. • Pili have two important roles;
• 1. they mediate the attachment of bacteria to
specific receptors on the human cell surface.
• 2. specialized sex pilus-------conjugation
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24. SURFACE LAYERS
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25. SURFACE LAYERS
• The principal surface layers are:
- Capsules and Loose Slime
- Cell wall of Gram positive bacteria
- Complex cell envelope of Gram negative bacteria
- Cytoplasmic membranes
- Mesosomal membrane vesicles
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26. CAPSULE
• High molecular weight extracellular condensed
polymers surrounding bacteria (up to 10um thick)
• Synthesized by enzymes located on the surface of
the cells
• For most bacteria, it is made up of polysaccharides
(except B. anthracis- D-glutamic acid)
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27. • LOOSE SLIME- formed when the extracellular polymers
are detached from the cell but the cell is entrapped in it
• GLYCOCALYX- loose meshwork of fibrils extending from
the cell. This form the basis for the formation of biofilm
- Plays a role in adherence of bacteria to
surfaces. This form the basis for the formation of
biofilm
Dental caries – Strept mutans forms biofilm on which
other bacteria adhere to form plaque, the precondition
for the destruction of the enamel and ultimate
formation of caries
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28. Dental plaque

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30. IMPORTANCE OF CAPSULES
• Capsules are known to confer resistance to phagocytosis
• Specific identification of an organism can be made by using
antiserum against the capsular polysaccharide.(quellung
reaction)
• The adherence role of glycocalyx is the basis for the formation
of biofilm
• Capsular polysaccharides are used as the antigens in certain
vaccines.
• The capsules may play a role in adherence of bacteria to
human tissues.
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31. CELL WALL
• Lies between the cytoplasmic membrane and capsule
• Has a high tensile strength due to peptidoglycan (aka
murein or mucopeptide)
• Important for bacterial ID- bacteria are classified as
Gram +ve or Gram –ve
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32. Cell wall
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33. Importance of cell wall:
• Protects protoplasm from external noxious stimuli
• Withstands and maintains the osmotic pressure
gradient between the cell interior and extracellular
environment
• Gives the cell its structural support and maintains its
characteristic shape.
• Facilitates communication with the environment
• Plays a role in cell division
• It is a good target for antibacterial agents
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34. CELL WALL OF GRAM POSITIVE
BACTERIA
Gram +ve bacterial cell wall composed of:
• Peptidoglycan- may be as much as 40-50 layers
• Teichoic acid; peculiar to Gram positive bacteria. They are
composed of polymers of glycerol phosphate or ribitol
phosphate. Two types:-
– Membrane teichoic acid- covalently linked to membrane
glycolipid therefore called lipoteichoic acid.
– Wall teichoic acid- attached covalently to peptidoglycan
• Cell wall associated protein
– In Streptococcus pyogenes LTA is associated with the Mprotein
that protrudes from the cell membrane through the
peptidoglycan layer.
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35. Wall of Gram +ve Bacteria
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36. • PEPTIDOGLYCAN LAYER made up of 3 parts
1. A glycan back bone composed of alternating N-
acetylmuramic acid (NAM) and N-
acetylglucosamine (NAG) linked together by B-
1,4 glycosidic linkages.
2. A set of identical tetrapeptide side chains
attached to N-acetylmuramic acid ( L-Alanine, D-
Isoglutamine, L-Lysine and D-Alanine)
3. A set of identical cross-bridges between
neighbouring tetrapeptide side chains
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37. The Structure of Peptidoglycan
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39. CELL ENVELOPE OF GRAM NEGATIVE BACTERIA
• Highly complex and multilayered of abt 5-10nm thickness
• Composed of:
1. A peptidoglycan layer- relatively thinner
abt 2nm thick (5-10% of dry mass)
2. Outer membrane- bilayered structure
3. Lipopolysaccharides – attached to outer membrane
4. Lipoprotein – cross-link outer membrane & peptidoglycan
5. Periplasmic space – space bw inner and outer membrane
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40. Cell Envelope of Gram –ve Bacteria
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41. OUTER MEMBRANE
• Lipopolysaccharides is attached to it by a weak
cohesive forces
• Lipoprotein and ompA anchor outer membrane to
the peptidoglycan layer
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42. LIPOPOLYSACCHARIDE
• Only one Gram +ve bacteria contains LPS- Listeria
monocytogenes
• LPS (endotoxins) is released when the cells are lysed
• The LPS molecule have 3 covalently linked regions:
(1) Lipid A, (2) Core polysaccharide, (3) O- specific
polysaccharide chain (O-Antigen)
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43. Lipopolysaccharide complex
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45. BACTERIA LACKING CELL WALL
• Bacteria lacking cell walls- 4 groups have been identified:
1. Mycoplasma spp- Mycoplasma pneumonia, Ureaplasma
urealyticum
2. L-forms- discovered in Lister Institute, London. Produced in
lab but could be found in the body of patients being treated
with penicillin
3. Protoplasts- Gram positive bacteria lost cell wall to lysozyme.
4. Spheroplasts- Gram negative bacteria loss cell wall to
lysozymes
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46. Bacterial Identification
India ink technique -
– Bacteria is suspended in diluted India ink
– Bacterial cells appear to lie in a lacunae
Quellung reaction - Homologous antibody is added to a
preparation of capsule.
– microprecipitation at the periphery of the capsule altering its
refractive index rendering the capsule to be visible
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47. Quellung reaction
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48. Gram stain technique
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50. PERIPLASMIC SPACE
• Region between outer and inner membrane
(cytoplasmic membrane)
• Approximately 20-40% of cell volume
• Periplasmic space contains peptidoglycan layer and
gel-like solution of protein including antibiotics
degrading enzymes (e.g B-lactamase and
aminoglycoside phosphorylase)
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51. CYTOPLASMIC MEMBRANE
• Plasma [cytoplasmic ]membrane: composed
primarily of lipids bilayer with interspersing proteins
• Protein to lipid ratio is usually 3:1
• Thickness about 4-5nm
• Lack sterols (except for Mycoplasma )
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52. Functions of the cytoplasmic membrane
• Transport of nutrients into and waste products out of
the cell by providing high SA/volume ratio
• Site of electron transport and oxidative
phosphorylation
• Anchoring site for DNA
• Provides the cell with mechanism for the separation
of sister chromosomes.
• Excretion of hydrolytic exoenzymes for the digestion
of macromolecular organic polymer nutrients
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53. INTRACYTOPLASMIC
STRUCTURES
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54. Cytoplasm
• 80% Water {20% Salts-Proteins)
• DNA is circular, Haploid
• Plasmids; extra circular DNA
–Antibiotic Resistance
• No organelles ( no mitochondria, golgi, etc.)
• Inclusion bodies/granules; e.g. glycogen,
polyphosphates
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55. NUCLEOID
• Contain a single double stranded DNA located in the
cytoplasm
• The DNA can be visualized with the aid of Feulgen
staining, and is seen as a single, continuous, giant
circular molecule with molecular weight of approx.
3 x 109. The unfolded nuclear DNA would be about
1mm long.
• Nucleoid lacks nuclear membrane and mitotic
apparatus
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58. PLASMIDS
• Extrachromosomal genetic apparatus capable of
replication within prokaryotic cell line
• Circular twisted DNA molecule that transfer genetic
information from one cell to another
• Drug resistance plasmids renders bacteria
resistance to antibiotic treatment
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59. Cell division in Prokaryotes
• Prokaryotes use a relatively
simple form of cell division -
binary fission.
• The cell wall and membrane
are in red,
• the bacterial chromosome in
blue,
• the cytoplasm in light green,
• the yellow dot represents a
point of attachment of the
chromosome to the cell
membrane.
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60. RIBOSOMES
• Densely packed ribosomes of 70S in size
• Each 70S ribosome has 30S and 50S subunits
• Ribosomes are involved in the synthesis of various
bacteria proteins
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