Dr. Archana S. Nair

1. College Of Veterinary and Animal
Sciences Mannuthy
Bacteria:- General Characteristics,
Pathogenicity, Virulence Factor And
Its Immune Response
Archana S. Nair
Department Of Veterinary
Public Health

2. Bacterial Introduction
 Bacteria are unicellular
micro-organisms ranging
in length from a few
micrometers to half a
millimeter
 Bacteria can be found in
almost every ecosystem
on Earth

3. • Bacteria are found 2
billion years before
eukaryotes
• Some bacteria are
pathogenic and cause
disease

4. Bacterial History
• Bacteria were first
observed by Anton Van
Leeuwenhoek in 1676
• The term ‘bacteria’,
Greek for ‘small stuff’
was first used in 1838

5. Bacterial history
• Robert Koch and Louis
Pasteur were the first to
discover that bacteria
caused many diseases
(mid 19th century)
• The first antibiotic used
to treat bacterial
disease was made by
Paul Ehrlich in 1910. It
was used to treat
Syphilis

6. The Evolution of Bacteria
 Bacteria are thought (by
some) to be the first
forms of life, about 4
billion years ago
 It is believed that both
the Domain Archaea and
the Domain Eukarya
evolved from bacteria

7. Classification

8. Morphology
• Bacterial cells are
prokaryotic, lacking a
nucleus and complex
organelles
• They have a cell
membrane and a cell
wall made up of
peptidoglycan

10. Teichoic acid in gram positive bacteria

11. Cell wall

12. Gram negative bacteria

13. Cell membrane
• Site of biosynthesis of DNA, cell wall
polymers and membrane lipids. Selective
permeability and transport of solutes into
cells
• Electron transport and oxidative
phosphorylation
• Excretion of hydrolytic
exoenzymes

14. Cytoplasm
Nucleiod
 Chromosomal DNA
 Plasmids
Inclusion bodies
 Storage of excess food and energy
 Metachromatic granules/ Babes ernst granules
 Much granule
Spores
 Resist adverse condition
Ribosomes

15. • Bacteria use flagella or pili for movement and
interaction with the environment
• Pili
Common pili- fimbriae
Sex pili- conjugation

16. Capsule and slime layers
 Attachment
 Protection from phagocytic engulfment
 Resistance to drying
 Depot for waste products
 Reservoir for certain nutrients
 Protection

17. Flagella
 Some bacterial species are mobile and possess
locomotory organelles - flagella. Flagella
consist of a number of proteins including
flagellin

18. Plasmid
• 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

19. Nucleus
• Lacking nuclear membrane, absence of
nucleoli, hence known as nucleic material or
nucleoid, one to several per bacterium

20. Inclusions of Bacteria
• Inclusions are aggregates of various
compounds that are normally involved in
storing energy reserves or building blocks for
the cell

21. Endospores
 Resistant structure
 Heat, irradiation, cold
 Boiling >1 hr still viable
 Takes time and energy to make spores
 Location important in classification
 Central, Subterminal, Terminal
 Bacillus stearothermophilus -spores
 Used for quality control of heat sterilization equipment
 Bacillus anthracis - spores
 Used in biological warfare

22. Morphological classification
• Bacteria can be classified into five major groups on
morphological basis
1. TRUE BACTERIA
• Cocci – These are spherical or oval cells. On the basis of
arrangement of individual organisms they can be
described as
 Monococci (Cocci in singles) – Monococcus spp.
 Diplococci (Cocci in pairs) – Streptococcus pneumoniae
 Staphylococci (Cocci in grape-like clusters) –
Staphylococcus aureus
 Streptococci (Cocci in chains) – Streptococcus pyogenes
 Tetrad (Cocci in group of four) - Micrococcus spp.
 Sarcina (Cocci in group of eight)

23. • Bacilli – These are rod-shaped bacteria. On
the basis of arrangement of organisms, they
can be described as
 Diplobacilli
 Streptobacilli
 Palisades
 Chinese-letter form
 Coccobacilli
 Comma-shaped

25. 2. ACTINOMYCETES (actin- ray, mykes-fungus)
These are rigid organisms like true bacteria
but they resemble fungi in that they exhibit
branching and tend to form filaments

26. 3. Spirochaetes
These are relatively longer, slender, non-
branched microorganisms of spiral shape
having several coils

27. 4. Mycoplasmas
• These bacteria lack in rigid cell wall (cell
wall lacking) and are highly pleomorphic and
of indefinite shape
• They occur in round or oval bodies and in
interlacing filaments

28. 5. Rickettsiae and Chlamydiae
These are very small, obligate parasites, and at
one time were considered closely related to
the viruses. Now, these are regarded as
bacteria

29. Based on Cultural characteristics
• Extra growth factors requirements
• Fastidious – Hemophilus influenzae
• Non-fastidious – Escherichia coli
• Hemolysis on Sheep Blood Agar
• Alpha-hemolysis – Streptococcus pneumoniae
• Beta-hemolysis – Streptococcus pyogenes
• Utilization of carbohydrates
• Oxidative - Micrococcus
• Fermentative – Escherichia coli

30. • Growth rate
• Rapid growers– Vibrio cholerae
• Slow growers – Mycobacterium tuberculosis
• Pigment production
• Pigment producer – Staphylococcus aureus
• Pigment non-producer – Escherichia coli

31. • Based on nutrition
• Autotrophs
• Heterotrophs
• Based on environmental factors
• Temperature
• Oxygen dependence
• pH
• Salt concentration
• Atmospheric pressure

32. Temperature
• Psychrophiles (15-200C) – Pseudomonas
fluorescens
• Mesophiles (20-400C) – Escherichia coli,
Salmonella enterica, Staphylococcus aureus
• Thermophiles (50-600C)- Bacillus
stearothermophilus
• Extremely thermophiles (as high as 2500C)

33. Oxygen dependence
• Aerobe (grow in ambient temperature, which contains 21% O2
and a small amount of CO2, 0.03%)
• Obligate aerobes – Strictly require O2 for their growth
(Pseudomonas aeruginosa)
• Microaerophilic (grow under reduced O2, 5-10% and
increased CO2, 8-10%)- Campylobacter jejuni, Helicobacter
pylori
• Anaerobic bacteria use inorganic substances other than
oxygen as a final electron acceptor
e.g. Pseudomonas and Bacillus

34. pH
• Acidophiles (Lactobacillus acidophilus)
• Alkaliphiles (Vibrio)
• Neutralophiles (pH 6-8)
Majority of the medically important bacteria grow
best at neutral or slightly alkaline reaction (pH
7.2-7.6)

35. Salt concentration
• Halophiles e.g. Dunaliella salina
• Non-halophiles

36. Osmotic pressure
• Bacteria are about 80-90% water; they require
moisture to grow
• Bacteria in hypertonic media causes water loss
by osmosis and cell wall separates from cell
membrane called as plasmolysis
• Bacteria in hypotonic media causes water to
enter inside the cell called as osmotic lysis

37. L-Forms
 L-form bacteria, also known as L-phase bacteria,
L-phase variants, and cell wall-deficient (CWD)
bacteria, are strains of bacteria that lack cell
walls. They were first isolated in 1935 by Emmy
Klieneberger-Nobel, who named them "L-forms"
after the Lister Institute in London
 Two types
1. unstable L-forms
2. stable L-forms

38. Bacterial reproduction
• Bacteria usually
reproduce asexually
using the process of
binary fission

39. Sexual Reproduction
• Some bacteria
reproduce sexually by
exchanging some of
their DNA through a
conjugation tube to
another bacterium
• Usually plasmid DNA,
not genomic DNA

40. Bacterial Growth
• Lag Phase: bacteria adjusting to new
environment and growing slowly
• Log Phase: exponential growth
• Stationary Phase: Bacteria have reached the
carrying capacity of the environment
• Death Phase: logarithmic death of bacteria as
nutrients get used up

42. Bacterial pathogenicity
• Capsule
• Capsule production is one of the major virulence
factors utilised by bacteria to evade clearance from
an infectious site
• The capsule provides bacteria with protection from
the host immune response as well as antibiotics
• Some capsules have immunomodulatory effects
• The capsule protects bacteria from phagocytosis by
not allowing opsonising antibodies to be recognized
by phagocytic host defense cells

43. Bacterial Pathogenicity
• The most notorious species of bacteria that
produce capsules are Streptococcus
pneumoniae (pneumococcus), Neisseria
meningitidis (meningococcus), and
Pseudomonas aeruginosa

44. CELL WALL
• The cell wall of both Gram positive and Gram
negative bacteria contain toxic components
that are potent virulence factors and have
central roles in the pathogenesis of bacterial
septic shock
• Toxic component acts via initiation of
inflammatory response by release of
cytokines and interleukin-1 and activation of
cascad system

45. Toxins
• Delivered to eukaryotic cells by
(1) Secretion into the surrounding area
(2) Direct injection into the host cell cytoplasm via
type III secretion systems
• Bacterial exotoxins are
(1) A-B toxins (2) proteolytic toxins,
(3) pore forming toxins (4) other toxins
(Wilson et al;2002)

46. Toxins
• A-B toxins producing bacteria are P. aeruginosa, E
coli, Vibrio cholerae, Corynebacterium
diphtheriae and Bordetella pertussis
• A subunit which possesses the enzymatic activity
and the B subunit which is responsible for binding
and delivery of the toxin into the host cell
(Wilson et al;2002)

47. Toxins
• Proteolytic toxins produced from Clostridium
botulinum, Clostridium tetani and P aeruginosa
• Membrane-disrupting toxins are arginine (R) and
threonine (T)
• It causes cell lysis
• Many Gram positive bacteria contain a sulfhydryl
activated cytolysin
• E.g. listeriolysin O that is necessary for the escape
of Listeria monocytogenes from the phagosome
(Wilson et al; 2002)

48. Bacterial pathogenicity
• Adhesions
• Factors that bind to molecules on various host
tissue cells and render the microbe resistant to
these mechanical washing forces
• Initiate its specific biochemical reaction causes
disease including proliferation, toxin secretion,
host cell invasion, and activation of host cell
cascades
• Microbial adherence factors are called adhesins
(Wilson et al; 2002)

49. • Two types :-1) polypeptide
2) polysaccharide
• Polypeptide :- a) fimbrial
• b) afimbrial
• Gram negative bacterial pathogens has
fimbriae for adherence
• E.g. E coli, V cholerae, P aeruginosa,
and Neisseria species
(Wilson et al; 2002)

50. • Gram positive (Staphylococcus spp,
Streptococcus spp) and mycobacterial
pathogens express afimbrial adhesions
• Invasion
• Pathogens gain deeper access into the host to
perpetuate the infection cycle called as
invasion
(Wilson et al; 2002)

51. • Two types:- Extracellular and Intracellular
• Extracellular invasion
• Occurs when a microbe breaks down the
barriers of a tissue to disseminate in the host
while remaining outside of host cells
• E.g. b-haemolytic Streptococcus and S aureus
(Wilson et al; 2002)

52. • Intracellular invasion
• Occurs when a microbe actually penetrates
the cells of a host tissue and survives within
this environment
• E.g. all gram positive and negative bacteria
• Target cells are both phagocytic and non
phagocytic
(Wilson et al; 2002)

53. Intracellular Lifestyles
• Bacterial pathogens have evolved to survive and
replicate within host cells after invasion
• Cell has killing mechanism i.e by:-
1. Lowering the pH of bacteria
2. Production of oxidative intermediates
3. Activation of degradative proteases
• Intercellular niche for bacteria
1. Within acidic environment
(Wilson et al; 2002)

54. 2. Inside a vacuole e.g. Coxiella burnetti
3. In host cell cytosole
• e.g. Shigella and Listeria utilise a pathway of cell-
to-cell spread in which infection is spread from
one cell to adjacent cell
• Bacteria residing in macrophages and
neutrophils may use these cells as vehicles to
spread systemically via the blood or lymphatic
circulatory systems
• E.g. Salmonella typhi, Yersinia spp, and Brucella
(Wilson et al; 2002)

55. Virulence factors
• The ability of an agent of infection to produce
disease is called as virulence
• The virulence of a microorganism is a
measure of the severity of the disease it
causes
(Adams et al; 2014)

56. Virulence factors

57. Virulence factors help
bacteria
• invade the host
• cause disease
• evade host defenses
Include:
1. Attachment (via
adhesins)
2. Colonization
3. Invasiveness
4. Toxins & Enzymes
5. Inhibition of
Phagocytosis
(Adams et al; 2014)

58. Attachment
They allow bacteria to bind to host cells:
• fimbriae
• some bacterial cell walls
• capsules
• These adhesins bind to specific epithelium
receptors or they are able to maintain even closer
contact
• Bordetella bronchiseptica adhesins include;
fimbriae, filamentous haemagglutininadhesin
(FHA) and pertactin
(Adams et al; 2014)

59. Colonization
• The Ability to Adhere to Host Cells and Resist
Physical Removal or the establishment of the
pathogen at the appropriate portal of entry
• Pathogens usually colonize host tissues that
are in contact with the external environment
• E.g. Helicobacter species counter the low pH
of the stomach by producing urease
(Adams et al; 2014)

60. Virulence Factors that Promote
Bacterial Colonization
1. Using Pili (fimbriae) to Adhere to Host Cells
2. Using Adhesins to Adhere to Host Cells
3. Using Biofilms to Adhere to Host Cells
(Adams et al; 2014)

61. Invasiveness
• The ability of a pathogen to invade tissues
• Invasiveness
(1) Mechanisms for colonization (adherence and
initial multiplication),
(2) Production of extracellular substances
("invasins"), that promote the immediate
invasion of tissues
(3) Ability to bypass or overcome host defense
mechanisms which facilitate the actual invasive
process
(Adams et al; 2014)

62. Invasiveness
• E.g. The intermalin
surface proteins found
on Listeria
monocytogenes helps
to invade mammalian
cells via
transmembrane
proteins
(Adams et al; 2014)

63. Toxins &Enzyme
• They are products of a
pathogen that
destroy/damage/inactivate
one or more vital
component of the host .
• Classes of toxins
Neurotoxins
Enterotoxins
Cytotoxins
Toxins
(Adams et al; 2014)

65. Toxins
• The ability to produce toxins is known as
toxogenesis
• Two main forms of toxin
 Lipopolysaccharides (primarily associated
with the outer cell membrane structure of
Gram-negative bacteria) and
proteins(exotoxins)
 Cell associated toxins (such as the
lipopolysaccharides which are bound to the
outer membrane) (endotoxins)

66. Enzymes
• Excretion of certain pathogens to assist them
in establishing infection and producing a
disease
• There are virulence determinant enzymes that
dissolve the glue between cells, thus allowing
the bacteria to spread rapidly through the
tissue
• E.g. Hyaluronidase and Coagulase

67. Virulence by spore forming bacteria
• Spore-forming bacteria cause some of the most
significant diseases of both humans and animals
• e.g. Tetanus, Botulism, Gas gangrene, Anthrax,
and many different enteric or gastroenteritis
syndromes
• Virulence of this bacteria are due to production
of potent protein toxins, including tetanus and
botulinum toxins, anthrax toxin, and alphatoxin,
epsilon-toxin (ETX), and enterotoxin (CPE) from
Clostridium perfringens
(Adams et al; 2014)

68. • The genes for many of these toxins and
capsule production are located on plasmid
• E.g. Tetanus toxin plasmid, the Conjugative
toxin plasmids of C. perfringens, and the pXO1
and pXO2 virulence plasmids from B. anthracis

69. Regulation of virulence factor
• The regulation and timing of expression of
virulence factors is very important for most
pathogenic bacteria
• Requires rapid adaptation to the new
environment to allow the pathogen to
colonize, survive, and grow within the host
• Major regulatory control mechanism by:-
1. Sigma factor
2. Two component system

70. Sigma factor
• Sigma factors are protein subunits of bacterial
RNA polymerases and control the initiation of
transcription at the promoter sequence
• Regulates prokaryotic gene expression
• Helps to control initiation specificity at
different promoter site
• Regulate the expression of genes in response
to stationary phase, nutrient deprivation, and
oxidative and osmotic stress

71. • RpoS (s38) sigma factor important for
virulence in a number of bacterial
pathogens,including Salmonella typhimurium,
E coli, P aeruginosa
• RpoE (s24) a sigma factor which responds to
periplasmic stress
• RpoH (s32) a heat shock sigma factor which is
important in the regulation of virulence in
Vibrio cholerae

72. Two component systems
(1) A Sensor protein that is embedded in the
bacterial membrane which “senses” different
physiological conditions of the bacterial cell
(2) A Response regulator which usually binds to the
promoter region of a gene to activate or repress
transcription
• Helps in regulation of iron, phosphate, nitrogen,
carbon, capsule production, and flagellar activity

73. EVOLUTION OF BACTERIAL
PATHOGENS
• Genetic makeup of bacterial genome is rapidly
changing by “horizontal gene transfer”
• Horizontal gene transfer refers to the
incorporation of genetic elements transferred
from a donor organism directly into the
genome of the recipient organism and forms
pathogenicity islands
• E.g. E.coli, Salmonella, Vibrio spp. , Shigella
spp. , Yersinia spp. , Listeria spp. , S aureus

74. • Addition to pathogenicity islands, plasmids
and bacteriophages can also be transferred
horizontally

75. Antibiotic resistance
• What are antibiotics?
• Powerful medicines that treat
bacterial infections
• They work by either killing bacteria or
preventing growth and reproduction of
bacteria
• Widespread use of antibiotics led to the
emergence of antibiotic resistant

76. • Both Gram negative and Gram positive
bacteria have acquired resistance to
antimicrobial drugs
• E.g. Shigella, Salmonella, E coli, and
Enterococcus faecium
• Methicillin resistant S aureus, causes
nosocomial infections
• Vanomycine resistance
Enterococcus

77. How the resistance is produced?
• Three common type of antimicrobial
resistance :-
1. Modification of target site
2. Altering uptake of antibiotics
3. Inactivation of antibiotic
Its occurs by two genetic process
1. Spontaneous mutation
2. By acquication

78. Horizontal gene transfer
• Genetic elements are transferred from one
organism to another, intraspecies and
interspecies
• It is transferred as mobile element i.e.
Transposons or by naked uptake of DNA
through transformation or by sexual
transformation by conjugation or by
incorporation of DNA into a phage genome

79. Antibiotic resistance
• E.g. multidrug resistance of S typhimurium
DT104, a food borne pathogen is due to the
integration of a transposon
• A mutation which alters the binding site of a
drug would decrease antibiotic sensitivity and
thus increase drug resistance
• E.g. multi drug resistant M tuberculosis

80. Interaction of Pathogens with the
Innate Immune System
• Micro-organisms are exposed non-specific
barriers to infection after introduction of the
microbe into the host
• Include
1. Epithelial cells of the skin
2. Antimicrobial substances in secretions
3. Complement proteins in the blood
4. Leucocytes in the blood and tissues

81. Immune response
• The host defense mechanisms are mediated
by the immune system
• Immunity refers to the relative state of
resistance of the host to infectious disease
• The immune system is composed of two
major subdivisions
1. Innate or nonspecific immune system
2. Adaptive or specific immune system

82. • Innate immune system is a primary defense
mechanism while the adaptive immune
system acts as a second line of defense
• Both aspects of the immune system have
cellular and humoral components which carry
out their protective functions
• Cells or components of the innate immune
system influence the adaptive immune system
and vice versa

83. Immunity
• Adaptive immune system requires some time
to react to an invading organism, whereas the
innate immune system acts rapidly
• The adaptive immune system is antigen
specific. The innate system is not antigen
specific
• Adaptive immune system exhibits an
immunological memory but innate immunity
does not

84. Cellular defense
• A variety of tissue cells are involved in innate
and adaptive immunity, hence the term
cellular defense
• It has Neutrophils and Macrophages for
phagocytosis
• Basophils and Mast cells for inflammation
• B cells and T cells which account for antibody
mediated immunity and cell mediated
immunity

85. Innate immune response
• They play major role by
complement activation
by alternative pathway
specially for gram –ve
bacteria
• By phagocytosis
• Release of cytokinins
• Activation of NK cells

86. Adaptive Immunity
• By production of antibodies
• Neutralization of bacterial
toxins
• Mucosa protection
(IgA) and activation of the
complement by the
classical pathway
• Activating phagocytosis by
the stimulation of the Fc
receptor

87. • Specific Immune Responses:- depends on type
of pathogens
• Extracellular bacteria and toxins
• Intracellular bacteria
• Encapsulated bacteria

88. References
• Textbook of Microbiology by Ananthanarayan and Paniker
• Wilson, J.W., Schurr, M.J., LeBlanc, C.L., Ramamurthy, R., Buchanan, K.L.
and Nickerson, C.A., 2002. Mechanisms of bacterial
pathogenicity.Postgraduate medical journal, 78(918), pp.216-224.
• Rahme, L.G., Stevens, E.J., Wolfort, S.F., Shao, J., Tompkins, R.G. and
Ausubel, F.M., 1995. Common virulence factors for bacterial pathogenicity
in plants and animals. Science, 268(5219), pp.1899-1902.
• Adams, V., Li, J., Wisniewski, J.A., Uzal, F.A., Moore, R.J., McClane, B.A. and
Rood, J.I., 2014. Virulence plasmids of spore-forming
bacteria.Microbiology spectrum, 2(6).
• Textbook of Microbiologt by Sharma S. N. and Adalka
• Janeway Jr, C.A. and Medzhitov, R., 2002. Innate immune
recognition.Annual review of immunology, 20(1), pp.197-216.