Question bank- bacterial virulence: Q1 Explain briefly the following

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Question bank- 5.bacterial virulence:

Q1 Explain briefly the following :

  1. Bacterial classification according to their virulence .

  1. Virulent(ضارية).

  2. A virulent (غير ضارية).

b. Disease production depends on two factors

  1. Virulence of microorganism(الضراوة).

  2. The resistance of invaded subject (host).

c. Bacterial component of virulent state are:

1.Toxigenicity(القدرة على أنتاج السموم):

of the microorganism means the ability of microbe to produce a toxin.

2. Invasiveness (القدرة على الغزو):

The ability of microorganism to invade and multiply into tissues, aggressive substances which the micro produces enters the body to assist the microbes to overcome the defenses

of the body.

3. Nature and physiology :

The capsulated microorganism is highly virulent than non-capsulated because the capsulate protects the microbes against the body.

4. Route and dose of infection :

The route is assisting the microbes to enter to the body also the dose determines the occurrence of infection.

d. Methods used to Measurement of bacterial virulence:

We measure the virulence by:

  1. Minimum lethal dose (MLD).

  2. Lethal dose 50(LD50).

  1. Minimum lethal dose:

It means the number of microorganism or the low number of micro which

kills the test animals in a specific period.

2. LD50:

Number of micro which kills 50% of test animal.

e. Kocks postulate ( routes):

1. The organism must be isolated from every case of disease

2. The organism must be isolated in pure cultures in vitro.

3. Organism must be producing the disease when injected in to laboratory animals.

4. The organism must be isolated from the laboratory animals .Tubercle bacilli cause T.b. Tubercle bacilli isolate from 100 cases for ex: then injected in to lab animals so T.b lesion occurs, isolated of tubercle bacilli from the infected lab animal.

f. Define the following :

  1. Infection: growth and multiplication of a microbe in or on the body with or without the production of disease.

  2. The capacity of a bacterium to cause disease reflects its relative “Pathogenicity.”

  3. Virulence is the measure of the pathogenicity of a microorganism.

  4. Pathogenesis refers both to the mechanism of infection and to the mechanism by which disease develops.

  5. Pathogenesis of sepsis (septicemia) means bacterial growth and multiplication .

  6. Bacteremia introduce of bacteria into blood circulation.

h. Host Susceptibility factor :

  1. Susceptibility to bacterial infections => Host Defenses vs Bacterial Virulence

  1. Host Defenses:

- Barriers (skin & mucus) – first line

- Innate Immune Responses (complement, macrophages &

cytokines) – the early stage

- Adaptive Immune Responses (Ag-specific B & T cells) – the

later stage.

5. Opportunistic pathogens are typically members of normal flora and cause diseases when they are introduced into unprotected sites, usually occur in people with underlying conditions.

g. The most frequent portals of bacterial entry:

1. Mucus .

2. Skin


Ingestion, inhalation, trauma, needles, catheters, arthropod bite, sexual transmission

  1. Characteristics of Pathogenic Bacteria

  1. Transmissibility

  2. Adherence to host cells

3. Invasion of host cells and tissue

  1. Evasion (التهرب)of the host immune system

  2. Toxigenicity

  3. Resistance to antibiotic.

m. Adhesion

Adherence of bacterium to epithelial or endothelial cells allow them to colonize the tissue.

Common adhesins: pili (fimbriae), slime, lipoteichoic acid, surface proteins or lectins.

n. Biofilm

Biofilm, formed on a surface by the bacteria that are bound together within a sticky web of polysaccharide, is a special bacterial adaptation that facilitates colonization on the surgical appliances (e.g., artificial valves or indwelling catheters) and dental plaque. It can protect the bacteria from host defenses and antibiotics.

o. Lipid A of lipopolysaccharide is responsible for endotoxin activity

Q2 Enuemerate the following :

  1. Pathological Mechanisms of Bacterial Infections

  1. Bacteria-mediated Pathogenesis

  2. Host-mediated Pathogenesis

  3. Bacterial virulence factors

=> bacterial factors causing diseases

  1. Bacterial Virulence Mechanisms:

  1. Microbial defenses against host immunologic clearance.

Encapsulation (Inhibition of phagocytosis and serum bactericidal effect)

Antigenic mimicry

Antigenic masking

Antigenic or phase variation

Intracellular multiplication

Escape phagosome

Inhibition of phagolysosome fusion

Resistance to lysosomal enzymes

Production of anti-immunoglobulin protease

Inhibition of chemotaxis

Destruction of phagocytes

  1. Regulation of bacterial virulence factors

  2. Environmental factors often control the expression of the virulence genes.

  3. Common factors:

  4. temperature, iron availability, osmolarity, growth phase, pH, specific ions, specific nutrient factors, bacterial cell-density, interaction with host cells.

m. Bacterial virulence factors


Pili (fimbriae)

Nonfimbrial adhesins

Invasion of host cells

Tissue damage

Growth byproducts

Tissue-degrading enzymes



Exotoxins (cytolytic enzymes and A-B toxins); enterotoxins;


endotoxin and other cell wall components

Antiphagocytic factors

Intracellular survival

Antigenic heterogeneity

Antigenic variation

Phase variation

Iron acquisition


Receptors for

iron-containing molecules

Resistance to antibiotics

n. Regulation of bacterial virulence factors.

Environmental factors often control the expression of the virulence genes.

Common factors:

temperature, iron availability, osmolarity, growth phase, pH, specific ions, specific nutrient factors, bacterial cell-density, interaction with host cells.

Q2 Explain why the fallowing.

  1. A bacterium may cause diseases by?

  1. Destroying tissue (invasiveness)

  2. Producing toxins (toxigenicity)

  3. Stimulating overwhelming host immune responses

  1. Mechanisms for escaping phagocytic clearance and intracellular survival?

  1. Exceptions to Koch’s Postulates

  2. Koch’s principles do not apply to all diseases.

  3. 1. Some microbes cannot be cultured in artificial media.

  4. Treponema pallidum (syphillis)

  5. Mycobacterium leprae (leprosy)

  6. Ricketsias, chlamydias, and viruses only multiply within cells.

  7. 2. One disease may involve several different pathogens.

  8. Diarrhea

  9. Pneumonia

  10. Meningitis

  11. Peritonitis

  12. Nephritis

  13. 3. Some pathogens may cause several different diseases.

  14. Streptococcus pyogenes: Scarlet fever, sore throat, skin infections, bone infections, etc.

  15. Mycobacterium tuberculosis: Causes disease of lungs, skin, bones, and internal


d. Define the following

Symptoms: Subjective feelings not obvious to an observer.

Example: Pain and malaise.

Signs: Objective changes that can be measured.

Example: Fever, redness, swelling, paralysis.

Syndrome: A group of signs and symptoms that are associated with a disease.

Morbidity rate is number of cases of illness divided by population at risk

Mortality rate reflects population that dies from disease

Incidence reflects number of new cases per specific time period

Prevalence reflects total number of existing cases

Diseases that are constantly present are endemic

Unusually large number of cases in a population constitutes an epidemic

When epidemics spread worldwide they are termed pandemic

Outbreak refers to a cluster of cases in a brief time affecting a specific population

Acute Disease: Develops rapidly, but lasts a short time.

Examples: Flu and common cold.

Chronic Disease: Develops more slowly, and reactions are less severe. Tend to recur for long periods or to be continual.

Examples: Tuberculosis, hepatitis B, and infectious mononucleosis.

Subacute Disease: Intermediate between acute and chronic.

Examples: Subacute bacterial endocarditis (streptococci).

Latent Disease: Causative agent remains inactive for a time, but then becomes active and produces disease symptoms.

Examples: Shingles, genital and oral herpes, AIDS.

Example: AIDS (Acquired Immune Deficiency Syndrome) is a syndrome that varies from one individual to another.

e. Classifying Infectious Diseases according to their spreading:

  1. Communicable Diseases:

Spread from one host to another, directly or indirectly.

Example: Tuberculosis, herpes, flu, AIDS, chickenpox, mumps, polio, and hepatitis.

  1. Contagious Diseases:

Spread easily from one person to another.

Example: Chickenpox and measles.

  1. Noncommunicable Diseases:

  2. Not spread from one host to another. Caused by microbes that live outside the body or by opportunistic pathogens that live inside the body.

Example: Tetanus, botulism, and yeast infections.

f. Propability for Exposure to Infectious Agents:

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