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1. Bacterial morphology
Dr. Himanshu Khatri
Email: himanshubkhatri@yahoo.co.in

2. Infectious agents
• The agents causing disease in humans can fall into one of the
following groups
1. prions,
2. viruses,
3. bacteria,
4. fungi,
5. protozoa,
6. helminths,
7. and arthropods.

3. Classification
• Bacteria are prokaryotic cells whereas fungi, protozoa, helminths and
arthropods belong to the superkingdom of Eukaryota.
• Fungi fall under the kingdom of fungi, protozoa fall under the
kingdom of ‘protists’ (a protist is any unicellular eukaryote that is not
an animal, a plant or a fungus), whereas helminths and arthropods
belong to the animal kingdom.
• Protozoa, helminths and arthropods are grouped as ‘parasites’.
• Prions and viruses do not fall into any of the above categories, as they
are not considered as cells.

4. Characteristics of bacteria
• The characteristic feature is the absence of nuclear membrane like the
eukaryotes. Hence the region of cell containing the DNA is called nucleoid,
and not nucleus.
• Another characteristic feature of prokaryotes is that, unlike eukaryotes
which divide by mitosis, they divide by ‘binary fission’.
• They lack the membrane bound organelles of the eukaryotes like the Golgi
apparatus, endoplasmic reticulum, mitochondria, and lysozomes.
• The size of ribosome is 70S, in contrast to the eukaryotes, who have 80S
ribosome.
• Finally, the cell wall of prokaryotes contain peptidoglycan, which is absent
if eukaryotes. An exception are mycoplasmas, which do not have a cell wall
of peptidoglycan, but incorporate cholesterol from their host into their cell
membranes.

5. • Size of bacteria
• Shape of bacteria
• Arrangement of bacteria
• Bacterial anatomy

6. • Size of bacteria (usually ranges from 0.2 µm to 5 µm)
• Shape of bacteria
• Arrangement of bacteria

8. Shape of bacteria
• They can be spherical or oval. In that case, they are called cocci.
• They can be rod shaped, in which case they are called bacilli.
• They can be comma shaped like Vibrio species.
• They can have spiral forms. These spiral forms can be rigid or flexible.
• Sometimes, they are branching filaments, and these filaments can
break into smaller fragments.
• Some bacteria do not have a stable morphology, and can take
pleomorphic (many-shaped) forms like spherical, or oval, or short rod-
like.

9. Arrangement of bacteria
• Some cocci are predominantly arranged in pairs. They are then called
diplococci (diplo= in pairs).
• They can be arranged in chains. They are then called streptococci (if they
are cocci) or streptobacilli (if they are bacilli). Strepto- in Greek means a
twisted chain.
• They can be arranged in groups of four (tetrad) or eight (sarcina).
• They can also be arranged in grape-like cluster. Then they are called
staphylococci. Staphyle- in Greek means a bunch of grapes.
• Some Gram-positive bacteria like corynebacteria are arranged in a
cuneiform (system of writing by ancient Sumerians of Mesopotamia) or
Chinese letter pattern.

10. Bacterial anatomy
• Cell envelope
• Ribosomes
• Intracytoplasmic inclusions
• Nucleus
• Slime layer and capsule
• Flagella
• Fimbriae
• Pili
• Spore

11. Cell envelope
• Consists of:
1. Cell wall
2. Plasma or cytoplasmic membrane
3. Outer membrane (only in Gram negative bacteria)

12. Cell wall
• Gives the cell shape
• Confers rigidity to the cell
• Composed of peptidoglycan (also called ‘murein’) in bacteria

15. Peptidoglycan (or murein)
• Peptidoglycan is made up of linear chains made up of N acetyl
glucosamine(NAG) and N acetyl muramic acid (NAM) alternatively.
• The NAM in the chain is linked to an oligopeptide side chain
• This side chain usually contains 4-5 aminoacids
• These oligopeptide side chains of adjacent NAM-NAG linear chains are
crosslinked with each other with the help of an enzyme DD-transpeptidase.
• DD-transpeptidases are also called as ‘penicillin binding proteins’ (PBPs)
• These linear NAM-NAG chains, together with crosslinked oligopeptide side
chains form the meshwork of peptidoglycan
• Gram positive bacteria have a much thicker peptidoglycan layer as
compared to Gram negative bacteria

16. Teichoic acids
• In addition to peptidoglycan, Gram positive bacteria also contain
teichoic acid in the cell walls
• They project on the outer surface of Gram positive bacteria
• They are absent in Gram negative bacteria
• When they are anchored to the peptidoglycan, they are called as ‘wall
teichoic acids’ (WTAs)
• When they are anchored to the cell membrane/protoplasmic
membrane (composed primarily of lipids), they are called ‘lipo-
teichoic acids’ (LTA)

18. Teichoic acid- functions
• They play a role in
1. bacterial cell division and morphology,
2. prevention of autolysis of bacterial cells,
3. protection of bacteria from host defences and antibiotics,
4. adherence of bacterial cells to tissues, and
5. induction of shock in the host in a mechanism similar to endotoxins
of Gram negative bacteria

19. Cell membrane/Protoplasmic membrane
• Present in Gram positive and Gram negative bacteria
• Present inner to the cell wall
• Made up of phospholipids

20. Outer membrane
• Present only in Gram negative bacteria
• Present outer to the cell wall
• Is made up of phospholipids, lipopolysaccharides, and porins

22. Porins
• Porins are transmembrane channels made up of proteins through
which small molecules can pass through the outer membrane

23. Lipopolysaccharide (LPS)
• Present in Gram negative bacteria
• Are responsible for O antigen specificity and endotoxic activity
• Consist of three regions (I, II, and III)
• Region I (outermost)- determines O antigen specificity
• Region II- is core polysaccharide
• Region III (innermost)- contains lipid A, which is responsible for
endotoxic activity

24. Lipid A (endotoxin)
• Potent stimulator of immune system
• It therefore has ‘adjuvant’ properties
• Excessive stimulation of immune system can lead to ‘endotoxic shock’
• Also has ‘pyrogenic’ effects

25. Ribosomes
• Are centers of protein synthesis
• Bacterial ribosomes (70S) are smaller than eukarytic ribosomes (80S),
where S is the ‘Svedberg unit’, which is related to the molecular
weight and shape of the compound
• Prokaryotic ribosomes consist of a larger 50S subunit and a smaller
30S subunit, which is in contrast to the eukaryotic ribosomes which
consist of 60S and 40S subunits
• Are integrated in linear strands of mRNA forming ‘polysomes’

27. Polysomes

28. Intra cytoplasmic inclusions
• Can be of various types including volutin, polysaccharide, lipid and
crystal
• Volutin granules contain inorganic polyphosphate, and are
characteristically present in diphtheria bacilli

29. Nucleus
• Bacterial nucleus do not have a nuclear membrane or nulceolus
• The DNA is not associated with histones to form chromatin
• The DNA is mostly circular, but sometimes linear
• This type of DNA is called ‘nucleoid’ and not nucleus
• Usually only one DNA molecule is present per cell
• Bacteria may also contain extranuclear genetic elements consisting of
DNA, called ‘plasmids’ or ‘episomes’
• Plasmids are not essential for life, but are important in virulence and
antibiotic drug resistance

31. Binary fission
• Bacteria divide by ‘binary fission’
• Binary fission, like mitosis (in eukaryotic cells), results in the
formation of two daughter cells which are similar to mother cell
• However, binary fission is different from mitosis in a number of
aspects
• In binary fission, there is no formation of ‘spindles’ and ‘sister
chromatids’ like mitosis. This makes it a much rapid process
• Therefore, some bacteria have ‘generation time’ (time required for
doubling of population) as low as 20 minutes
• Also, binary fission does not have four distinct phases like mitosis

32. Slime layer and capsule
• Many bacteria secrete a viscid layer around the cells surface
• This layer is usually, but not always, a polysaccharide
• When it is an unorganized structure, it is called a ‘slime layer’
• When it is an organized structure, it is called a ‘capsule’
• Capsules, which are too thin to be seen under a light microscope, are
called ‘microcapsules’
• Capsules are not stained using a Gram stain, where they appear as
unstained halos around a stained bacterium
• Capsules can be demonstrated with negative staining like ‘India ink’
and serologic methods like ‘Quellung reaction’

34. Demonstration of capsules
• In negative staining methods like the India ink, the dye is unable to
penetrate the cell, and the capsule appears as ‘halos’ around the
cells. Further, India ink gives a black background
• In serologic methods like the Quellung reaction, the specific
antibodies react with the capsular antigens, and increase its
refractivity. The capsules, therefore, appear ‘swollen’

36. Role of slime layer/capsules
• They protect the bacteria from host defences like phagocytosis
• They are also protective for bacteria against detergents and
antibiotics
• They help in the adhesion of bacteria to surfaces
• They are antigenic and cause stimulation of immune response
• Capsular material of some bacteria are used as ‘subunit’ vaccines

37. Biofilms
• Aggregate of microorganisms in which cells that are frequently
embedded within a self-produced matrix of extracellular polymeric
substance (EPS) adhere to each other and/or to a surface.
• EPS consists usually of polysaccharides, proteins, and nucleic acids
• Biofilms can be formed on living tissues (e.g. dental plaque) or inert
substances (e.g. urinary catheters, joint prostheses, heart valves etc.)
• Biofilms protect bacteria from host defences like phagocytosis and
agents like detergents and antibiotics

38. Flagella
• Are organs of locomotion for motile bacteria
• Are long and sinuous whip-like structures
• Are antigenic, and possess the ‘H’ antigen, which may be different in
different genera of bacteria
• Antibodies formed against the flagellar antigen (H antigen) are usually
non-protective, but are useful in serodiagnosis

40. Flagellar arrangement
• Monotrichous- bacteria have a single flagellum
• Lophotrichous- bacteria have multiple flagella originating at the same
spot on bacterial surface
• Amphitrichous- bacteria have a single flagellum at each of the
opposite ends. Only one flagellum works at a time, which determines
the direction of motility of bacterium
• Peritrichous- flagella originate throughout the bacterial cell surface
• Some bacteria like the spirochetes have ‘endoflagella’, which do not
protrude outside, but are located in the periplasmic space between
the cell wall and the outer membrane

41. Demonstration of flagella
• Flagella are usually less than 0.02 µm in diameter, and hence beyond
the limit of resolution of light microscope
• They can be visualized with the help special staining methods under
light microscope or without staining under electron microscope
• Their presence can be indirectly determined by assessing the motility
of organisms in semisolid media or by ‘hanging drop’ preparation

42. Fimbriae
• Are thinner and shorter than flagella
• Hence, they are also visible only under electron microscope
• They are organs of adhesion
• Fimbriae are also antigenic, and members of different genera may
possess the same fimbrial antigen

43. Pili
• They are similar in structure to fimbriae
• But they are not organs for adhesion
• They are used for the purpose of bacterial ‘conjugation’
• They are also antigenic

46. Spore
• They are resting stages of bacteria, which are resistant to various
factors like extreme temperature, dessication, chemicals, radiation
etc.
• During unfavourable conditions, vegetative bacteria get transformed
into spores. In this form, they can survive for many years.
• During favourable conditions, these spores germinate again into
vegetative forms
• Each vegetative bacterial cell transforms only into one spore

48. Types of spores
• According to their location in the bacterial cell, they may be central
(equatorial), subterminal, or terminal
• According to their shape, they may be oval or spherical
• The diameter of the spore may be greater than the diameter of the
bacillary body. In that case, spores may distend the bacillary body.
• If the diameter of the spores is smaller than the diameter of the
bacillary body, they do not distend the bacillary body.

49. Thank you