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Heshu J. Ahmed/Assist. Lecturer_____________________________________________Pathogenesis of bacterial infections – 2nd Stage /2nd Semester
Heshu.jalal@tiu.edu.iq
https://tiu.edu.iq/
2020 - 2021
TIU - Faculty of ScienceMedical Analysis Department
Medical Microbiology
Introduction
A pathogen is a microorganism that is able to cause disease in a
plant, animal or insect.
Pathogenicity is the ability to produce disease in a host
organism.
Microbes express their pathogenicity by means of their
virulence, a term which refers to the degree of pathogenicity of the
microbe.
Determinants of virulence of a pathogen are any of its genetic
or biochemical or structural features that enable it to produce
disease in a host.
The relationship between a host and a pathogen is dynamic, since
each modifies the activities and functions of the other.
The outcome of such a relationship dependson:
the virulence of the pathogen and
the relative degree of resistance or susceptibility of the host,
mainly due to the effectiveness of the host defensemechanisms.
Introduction
Animals and microbes
Normal flora (beneficial or ignored): GI track, skin, upper respiratory track
Virulent bacteria (actively cause disease): pathogenic islands
Opportunistic bacteria (when host with underline problem): Pseudomonas aeruginosa: cystic fibrosis/burn
TB, Kaposi’s sarcoma (herpesvirus): AIDS
Mechanisms of Bacterial Pathogenicity
1. Invasiveness: the ability to invade tissues.
encompasses mechanisms for
colonization (adherence and initial multiplication),
production of extracellular substances which facilitate
invasion (invasins) and
ability to bypass or overcome host defense
mechanisms.
Mechanisms of Bacterial Pathogenicity
2. Toxogenesis: ability to produce toxins.
Bacteria may produce two types of toxins:
i. exotoxins and
ii. endotoxins.
Exotoxins are released from bacterial cells and may act at
tissue sites removed from the site of bacterial growth.
Endotoxins are cell-associated substance. (classic sense,
endotoxin refers to the lipopolysaccharide component of the
outer membrane of Gram-negative bacteria).
Endotoxins may be released from growing bacterial cells
and cells that are lysed as a result of effective host defense
(e.g. lysozyme) or the activities of certain antibiotics (e.g.
penicillins and cephalosporins).
Hence, bacterial toxins, both soluble and cell-associated,
may be transported by blood and lymph and cause cytotoxic
effects at tissue sites
Some bacterial toxins may also act at the site ofcolonization
and play a role in invasion.
Mechanisms of Bacterial Pathogenicity
Tissue destruction: flesh-eatingbacteria:
Clostridium perfrigens
Immunopathogenesis
Excess immune responses
Autoimmunity
Stages of Bacterial pathogenesis
Infection/entry
Virulence factors
Pathogenesis
Escape of immune surveillance
Infection/entry
Ingestion (fecal-oral)
Inhalation (respiratory)
Trauma (e.g burn)
Arthropod bite (zoonoses:
mosquito, flea, tick,
Tsetse fly)
Sexual transmission
Iatrogenic (needle stick,
blood transfusion)
Maternal-neonatal
Bacteria, virus, fungi
Ingestion: Salmonella, Shigella, Vibrio, Clostridium etc..
Inhalation: Mycobacterium, Mycoplasma, Chlamydia etc..
Trauma: Clostridium tetani
Arthropod bite: Yersinia pestis.
Sexual transmission: Neisseria gonorrhoeae, HIV, chlamydia, etc
Needle stick: Staphylococcus, HIV, HBV
Maternal-neonatal: HIV, HBV, Neisseria, etc.
Modes of infectious disease transmission
Contact transmissionDirect contact (person-to-person): syphilis, gonorrhear, herpesIndirect contact (fomites): enterovirus infection, measles
Droplet (less than 1 meter): whooping cough, strep throat
Vehicle transmissionAirborne: influenza, tuberculoses, chickenpoxWater-borne (fecal-oral infection): cholera, diarrhea
Food-borne: hepatitis, food poisoning, typhoid fever
Vector transmissionBiological vectors: malaria, plaque, yellow fever
Mechanical vectors: E. coli diarrhea, salmonellosis
Extracellular versus Intracellular Parasitism
Extracellular parasites
destroyed when phagocytosed.
damaging tissues as they remain outsidecells.
inducing the production of opsonizing antibodies, they
usually produce acute diseases of relatively short duration.
Intracellular parasites
can multiply within phagocytes.
frequently cause chronic disease.
Virulence factors
Factors enhancing the ability of bacteria to cause disease
Example: Pseudomonas aeruginosa
Adhesins: attachment
Alginate production: mucoid layer
Exotoxin A: inhibits host proteinsynthesis
Exoenzyme S: interferes with phagocytic killing
Elastolytic activity: degrades elastin
Phospholipase C: damages tissue
Pyocyanin: damages tissue by ROS
Antibiotic resistance: complicates therapy
Bacterial Toxins
Many different types of toxins
Exotoxins
Endotoxins
Toxins are are not required forgrowth
Genes for toxins are usually on plasmids
EXOTOXINS.
Produced inside some bacteria as part of theirgrowth and metabolism and released into thesurrounding medium
Are proteins, and many are enzymes
Most bacteria that produce exotoxins are gram-positive
The genes for most exotoxins are carried on bacterial plasmids or phages.
Neurotoxin.
Target the nervous system, and can interfere with normal nerve impulse transmission, e.g. C. tetani, C.botulinum.
ENTEROTOXINS. Affect cells lining the gastrointestinal
tract.
E.g. V. cholerae, C. difficile.
Superantigens
Secreted proteins
(exotoxins) that exhibit
highly potent lymphocyte-
transforming (mitogenic)
activity directed towardsT
lymphocytes.
Known and suspected association of superantigens with
animal diseases
Autoimmune diseases
Lyme disease
Multiple sclerosis
Acute diseases
Food poisoning:
Staph infections
Streptococal
2. Endotoxins: heat stable
Bacterial Endotoxins
Endotoxins Toxin is not internalized
Toxin is located on outside of microorganisms (Part of the outer portion of the cell wall ofbacteria)
LPS of gram – bacteria
Lipoteichoic acid or gram + bacteria
Only toxic at high levels
Liposaccharide
Exert their effects when the gram negative bacteria dies and their cell wall undergo lysis, thus liberating the endotoxin(e.g use of antibiotics)
All endotoxins produce the same signs and symptoms
Endotoxins can also induce miscarriage.
Fever, Disseminated intravascular coagulation, Septic shock and death
Thank you