An overview on Pathogenic mechanisms of microbes of medical importance

1. Pathogenic mechanisms of
microbes of medical importance
Presenter: Dr JOYCE F. MWATONOKA
Mmed PCH, UDOM
December 2018

2. OUTLINE
• Definition of terms
• Introduction
• Mechanisms of Microbial Pathogenicity
-Adherence
-Invasion
-Evasion of host defenses
-Antigenic variations
-Toxigenesis
• References

3. Definition of terms
• Adherence (adhesion, attachment): The
process by which bacteria stick to the surfaces
of host cells
• Infection: Invasion and multiplication of an
infectious agent within the body
• Invasion: The process whereby bacteria,
animal parasites, fungi, and viruses enter host
cells or tissues and spread in the body

4. Cont…
• Nonpathogen: A microorganism that does not
cause disease; may be part of the normal
microbiota
• Pathogen: A microorganism capable of
causing disease
• Pathogenicity: The ability of an infectious
agent to cause disease

5. Cont…
• Toxigenicity: The ability of a microorganism to
produce a toxin that contributes to the
development of disease
• Virulence: The quantitative ability of an agent
to cause disease. Virulent agents cause
disease when introduced into the host in small
numbers. Virulence involves adherence,
persistence, invasion, and toxigenicity

6. Cont…
• Pathogenicity = ability to cause disease
Virulence = degree of pathogenicity
Lethal dose 50 (LD 50)
• Number of microorganism or its toxin which
when administered will kill 50% of a given
animal population (hosts)
• It is a measure of virulence
• The higher the lethal dose the lower the
virulence, and vice versa

7. Introduction
• There are several biochemical, structural, and
genetic factors that play important roles in
bacterial pathogenesis
• Characteristics of bacteria that are pathogens
include transmissibility, adherence to host
cells, persistence, invasion of host cells and
tissues, toxigenicity, and the ability to evade
or survive the host's immune system

8. Cont…
• Resistance to antimicrobials and disinfectants
can also contribute to virulence, or an
organism's capacity to cause disease
• Many infections caused by bacteria that are
commonly considered to be pathogens are
inapparent or asymptomatic
• Disease occurs if the bacteria or immunologic
reactions to their presence cause sufficient
harm to the person

9. The Underlying Mechanisms of
Bacterial Pathogenicity
Two broad qualities of pathogenic bacteria underlie
the means by which they cause disease:
1. The ability to invade tissues: Invasiveness, which
encompasses mechanisms for colonization
(adherence and initial multiplication), ability to
bypass or overcome host defense mechanisms,
and the production of extracellular substances
which facilitate invasion
2. The ability to produce toxins: Toxigenesis

10. Bacterial Adherence
• The interactions between bacteria and tissue
cell surfaces in the adhesion process are
complex
• Depends on several factors like;
- surface hydrophobicity and net surface
charge
- binding molecules on bacteria
(ligands/adhesins), and host cell receptor
interactions

11. Cont…
• The bacterial adhesin is typically a
macromolecular component of the bacterial cell
surface which interacts with the host cell
receptor, eg;pili
• The receptors; are usually specific carbohydrate
or peptide residues on the eukaryotic cell surface
• Adherence, is followed by development of
microcolonies and subsequent steps in the
pathogenesis of infection

12. Cont…
• The mechanisms for adherence may involve
two steps:
1. Nonspecific adherence: reversible attachment
of the bacterium to the eukaryotic surface
(sometimes called "docking")
2. Specific adherence: permanent attachment of
the microorganism to the surface (sometimes
called "anchoring").

13. Some Specific Bacterial Adhesins and
their Receptors
• The adhesins of E. coli are their common pili
or fimbriae
• Pseudomonas, Vibrio and Neisseria possess a
fimbrial protein subunit
• Staphylococcus aureus binds to the amino
terminus of fibronectin by means of a
fibronectin-binding protein which occurs on
the bacterial surface

14. Invasion
• The invasion of a host by a pathogen may be
aided by the production of invasins
• Invasins are proteins (enzymes) that act
locally to damage host cells and/or have the
immediate effect of facilitating the growth and
spread of the pathogen
• The damage to the host as a result of this
invasive activity may become part of the
pathology of an infectious disease

15. Cont…
• Invasins are not clearly distinguished from
some extracellular protein toxins (exotoxins)
which also damage the host
• Invasins usually act at a short range (in the
immediate vicinity of bacterial growth) and
may not actually kill cells in their range of
activity; exotoxins are often cytotoxic and
may act at remote sites (removed from the
site of bacterial growth)

16. Cont…
• Also, exotoxins typically are more specific and
more potent in their activity than invasins
• Even so, some classic exotoxins (e.g.
diphtheria toxin, anthrax toxin) may play some
role in invasion in the early stages of an
infection, and some invasins (e.g.
staphylococcal leukocidin) have a relatively
specific cytopathic effect

17. A Survey of Bacterial Invasins
"Spreading Factors" describes a family of
bacterial enzymes that affect the physical
properties of tissue matrices and intercellular
spaces, thereby promoting the spread of the
pathogen
• Hyaluronidase; is the original spreading factor
It is produced by streptococci, staphylococci,
and clostridia. The enzyme attacks the
interstitial cement (ground substance) of
connective tissue by depolymerizing
hyaluronic acid (polysaccharide)

18. Cont…
• Collagenase is produced by Clostridium
histolyticum and Clostridium perfringens. It
breaks down collagen, the framework of muscles,
which facilitates gas gangrene due to these
organisms
• Neuraminidase is produced by intestinal
pathogens such as Vibrio cholerae and Shigella
dysenteriae. It degrades neuraminic acid (also
called sialic acid), an intercellular cement of the
epithelial cells of the intestinal mucosa

19. Cont…
• Streptokinase and Staphylokinase are
produced by streptococci and staphylococci,
respectively. Kinase enzymes convert inactive
plasminogen to plasmin which digests fibrin
and prevents clotting of the blood. The
relative absence of fibrin in spreading
bacterial lesions allows more rapid diffusion of
the infectious bacteria

20. Some Extracellular Bacterial Proteins That Are
Considered Invasins
INVASIN Bacteria Involved Activity
Hyaluronidase Streptococci, staphylococci
and clostridia
Degrades hyaluronic of connective
tissue
Collagenase Clostridium species Dissolves collagen framework of
muscles
Neuraminidase Vibrio cholerae and
Shigella dysenteriae
Degrades neuraminic acid of
intestinal mucosa
Coagulase Staphylococcus aureus Converts fibrinogen to fibrin which
causes clotting
Kinases Staphylococci and
streptococci
Converts plasminogen to plasmin
which digests fibrin
Leukocidin Staphylococcus aureus Disrupts neutrophil membranes and
causes discharge of lysosomal
granules
Streptolysin Streptococcus pyogenes Repels phagocytes and disrupts
phagocyte membrane and causes
discharge of lysosomal granules

21. Cont…
Invasin Bacteria involved Activity
Hemolysins Streptococci, staphylococci
and clostridia
Phospholipases or
lecithinases that destroy
red blood cells (and other
cells) by lysis
Lecithinases Clostridium perfringens Destroy lecithin in cell
membranes
Phospholipases Clostridium perfringens Destroy phospholipids in
cell membrane
Anthrax EF Bacillus anthracis One component (EF) is an
adenylate cyclase which
causes increased levels of
intracellular cyclic AMP
Pertussis AC Bordetella pertussis One toxin component is an
adenylate cyclase that acts
locally producing an
increase in intracellular
cyclic AMP

22. Evasion Of Host Defenses
• Some pathogenic bacteria are inherently able
to resist the bactericidal components of host
tissues
• Pathogenic mycobacteria have a waxy cell wall
that resists attack or digestion by most tissue
bactericides. And intact lipopolysaccharides
(LPS) of Gram-negative pathogens may protect
the cells from complement-mediated lysis or
the action of lysozyme

23. Overcoming Host Phagocytic Defenses
• Microorganisms invading tissues are first and
foremost exposed to phagocytes
• Most bacteria that are successful as parasites
interfere to some extent with the activities of
phagocytes or in some way avoid their attention
• Microbial strategies to avoid phagocytic killing are
numerous and diverse, but are usually aimed at
blocking one or of more steps in the phagocytic
process

24. Cont…
Phagocytosis:
1. Contact between phagocyte and microbial cell
2. Engulfment
3. Phagosome formation
4. Phagosome-lysosome fusion
5. Killing and digestion

25. Cont…

26. Cont…
1. Avoiding Contact with Phagocytes
• Invade or remain confined in regions inaccessible
to phagocytes. Certain internal tissues (e.g. the
lumen of glands) and surface tissues (e.g. the
skin) are not patrolled by phagocytes
• Avoid provoking an overwhelming inflammatory
response. Some pathogens induce minimal or no
inflammation required to focus the phagocytic
defenses

27. Cont…
• Inhibit phagocyte chemotaxis. e.g. Streptococcal
streptolysin (which also kills phagocytes)
suppresses neutrophil chemotaxis, even in very
low concentrations
• Fractions of Mycobacterium tuberculosis are
known to inhibit leukocyte migration
• Hide the antigenic surface of the bacterial cell. Eg;
pathogenic Staphylococcus aureus produces cell-
bound coagulase which clots fibrin on the
bacterial surface

28. Cont…
2. Inhibition of Phagocytic Engulfment
• Some bacteria employ strategies to avoid
engulfment if phagocytes do make contact
with them. They bear on their surfaces
substances that inhibit phagocytic engulfment
• Resistance to phagocytic ingestion is usually
due to a component of the bacterial cell wall,
or fimbriae, or a capsule enclosing the
bacterial wall

29. Cont…
• Eg; Polysaccharide capsules of S. pneumoniae,
Haemophilus influenzae, Treponema pallidum
and Klebsiella pneumoniae
• M protein and fimbriae of Group A
streptococci
• O antigen associated with LPS of E. coli
• Cell-bound or soluble Protein A produced by
Staphylococcus aureus

30. Cont…
3. Survival Inside of Phagocytes
• Some bacteria survive inside of phagocytic
cells, in either neutrophils or macrophages.
Bacteria that can resist killing and survive or
multiply inside of phagocytes are considered
intracellular parasites
I. Inhibition of phagosome-lysosome fusion
II. Survival inside the phagolysosome
III. Escape from the phagosome

31. Antigenic Variation
• Periodically changing antigens in order to avoid a
host immune response
• Some bacteria change from one type of fimbriae
to another, by switching fimbrial tips. This makes
the original AMI response obsolete by using new
fimbriae that do not bind the previous antibodies
• They can also change other surface proteins that
are the targets of antibodies
• Antigenic variation is prevalent among
pathogenic viruses as well

32. Toxigenesis
• Toxins are proteinaceous (exotoxins) or non-
proteinaceous molecules (endotoxins)
produced by bacteria to destroy or damage
the host cell
• Examples of non-proteinaceous toxins are LPS
(endotoxin) for Gram negative organisms and
teichoic acid for Gram positive organisms

33. Exotoxins
• Are polypeptides, excreted by living cell; high
concentrations in liquid medium
• Produced by both gram-positive and gram-
negative bacteria
• Relatively unstable; toxicity often destroyed
rapidly by heating at temperatures above 60°C
• Highly antigenic; stimulate formation of high-
titer antitoxin. Antitoxin neutralizes toxin

34. Cont…
• Highly toxic; fatal to animals in microgram
quantities or less eg: botulinum toxin
• Usually bind to specific receptors on cells
• Usually do not produce fever in the host
• Converted to antigenic, nontoxic toxoids by
formalin, acid, heat, etc. Toxoids are used to
immunize (eg, tetanus toxoid)
• Frequently controlled by extrachromosomal
genes (eg, plasmids)

35. Endotoxins
• Are integral part of the cell wall of bacteria.
Released on bacterial death and in part during
growth
• Lipopolysaccharide complexes. Lipid A portion
probably responsible for toxicity
• Relatively stable; withstand heating at
temperatures above 60°C for hours without
loss of toxicity

36. Cont…
• Weakly immunogenic
• Moderately toxic; fatal for animals in tens to
hundreds of micrograms
• Specific receptors not found on cells
• Usually produce fever in the host by release of
interleukin-1 and other mediators
• Synthesis directed by chromosomal genes

37. Endotoxins Vs Exotoxins

38. References;
• Jawetz, melnick, & adelberg's medical
microbiology, 24e
• Wilson JW, Schurr MJ, LeBlanc CL, et al
Mechanisms of bacterial pathogenicity
Postgraduate Medical Journal 2002;78:216-
224
• Todar's Online Textbook of Bacteriology

39. Thank You For Your Attention!