Infections

Question 1

Mention 5 characteristics of bacteria pathogens?

Answer 1

1. Transmissibility
2. Adherence to host cell
3. Persistence
4. Toxigenicity
5. Invasion of host cells and tissue
6. The ability to evade or survive the host’s immune system

Question 2

Define adherence.

Answer 2

The process by which bacteria attach to the surface of host cells. It may also be referred to as adhesion or attachment.

Question 3

Define the term “carrier”.

Answer 3

A person or animal with an asymptomatic infection that can be transmitted to another susceptible person or animal.

Question 4

Define the term “infection”.

Answer 4

It is the invasion and multiplication of an infectious agent within the body.

Multiplication of bacteria that are part of normal flora in the GIT, skin etc., is generally not considered an infection.

Question 5

Define the term “invasion”.

Answer 5

The process whereby bacteria, parasites, fungi, and viruses enter host cells or tissues and spread in the body.

Question 6

What is microbiota?

Answer 6

Microbial flora harbored by normal, healthy individuals.

Question 7

What is an opportunistic pathogen?

Answer 7

An agent capable of causing disease only when the host’s immunity is compromised.

Question 8

Define pathogenicity.

Answer 8

Pathogenicity pertains to the ability of a infectious agent to cause disease.

Question 9

What are super antigens?

Answer 9

These are protein toxins that activate immune response by binding to MHC molecules and T-cell receptors and stimulate T cells to produce copious amounts of cytokines.

Question 10

What is toxigenicity?

Answer 10

The ability of a microorganism to produce a toxin that contributes to the development of disease.

Question 11

What is virulence?

Answer 11

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.

Question 12

Highlight the Koch postulates.

Answer 12

1. The microorganism should be found in all cases of the disease in question, and its distribution in the body should be in accordance with the lesions observed.
2. The microorganism should be grown in pure culture in vitro (or outside the body of the host) for several generations.
3. When such a pure culture is inoculated into susceptible animal species, the typical disease must result.
4. The microorganism must again be isolated from the lesions of such experimentally produced disease

Question 13

What is the adjunct to Koch postulates?

Answer 13

Development of a rise in specific antibody during recovery from disease is an important adjunct to Koch’s postulates.

Question 14

Mention 2 exceptions to the Koch postulates.

Answer 14

• Treponema pallidum (syphilis) and Mycobacterium leprae (leprosy) cannot be grown in vitro; however, there are animal models of infection with these agents.
• There is no animal model of Neisseria gonorrhoeae (gonorrhea) infection even though the bacteria can readily be cultured in vitro

Question 15

State the molecular Koch postulates.

Answer 15

Molecular cloning has allowed investigators to isolate and modify specific virulence genes and study them with models of infection.
1. The Phenotype or property under investigation should be significantly associated with pathogenic strains of a species and not with nonpathogenic strains.
2. Specific inactivation of the gene or genes associated with the suspected virulence trait should lead to a measurable decrease in pathogenicity or virulence.
3. Reversion or replacement of the mutated gene with the wild-type gene should lead to restoration of pathogenicity or virulence.

Question 16

What are the molecular guidelines for establishing microbial disease causation?

Answer 16

1. The nucleic acid sequences of a putative pathogen should be present in most cases of an infectious disease and preferentially in anatomic sites where pathology is evident.
2. The nucleic acid sequence of a putative pathogen should be absent from most healthy control.
   If the sequence is detected in healthy control, it should be present with a lower prevalence as compared with patients with disease and in lower copy numbers.
3. The copy number of a pathogen-associated nucleic acid sequence should decrease or become undetectable with resolution of the disease and should increase with relapse or recurrence of disease.
4. The presence of a pathogen-associated nucleic acid sequence in healthy subjects should help predict the subsequent development of disease.
5. The nature of the pathogen inferred from analysis of its nucleic acid sequence should be consistent with the known biologic characteristics of closely related organisms and the nature of the disease.

Question 17

What are the methods of transmission of infection?

Answer 17

1. Mild asymptomatic infections: by producing asymptomatic infection rather than death of host, pathogens enhance their chance of transmission from one host to another.
2. Animals to man: some bacteria that commonly cause disease in humans exist primarily in animals and incidentally infect humans. For example, Salmonella typically infects animals and are transmitted in food products to humans.
3. Transmission from one person to another on hands: many opportunistic pathogens that cause nosocomial infections are transmitted from one patient to another on the hands of hospital personnel
4. The clinical manifestations of diseases: (eg, diarrhea, cough, genital discharge) produced by microorganisms often promote transmission of the agents e.g., i. Vibrio cholerae can cause voluminous diarrhea, which may contaminate salt and fresh water; drinking water or seafood.
   ii. M. tuberculosis (tuberculosis) naturally infects only humans; it produces respiratory disease with cough and production of aerosols, resulting in transmission of the bacteria
5. Inadvertent transmission due to error in organism’s life cycle: inadvertent mistake in the normal life cycle of the organism not adapted to humans may produce a severe disease e.g.,
   i. Y. pestis (plague) in rodents inadvertent transmission by the fleas to humans;
   b). Bacillus anthracis (anthrax) transmitted to humans by products such as raw hair from infected animals.
   c). The Clostridium species transmitted to humans by ingestion (e.g. Clostridium botulinum [botulism]) or when wounds are contaminated by soil (e.g., Clostridium tetani [tetanus]).

B. anthracis and the Clostridium species produce spores to protect the their nucleic acid from harsh environmental factors.

Question 18

What are the most common portals of entry for infections?

Answer 18

i. respiratory (upper and lower airways),
ii. gastrointestinal , genital, and urinary tracts.
iii. abnormal areas of mucous membranes and skin (e.g., cuts, burns, and other injuries)

Question 19

Describe with examples the normal infectious process of bacteria.

Answer 19

To cause disease, most bacteria must first
- adhere to host cells
- establish a primary site of infection
- multiply and spread through tissue or lymphatic system to the bloodstream

Example:
V. cholera is ingested and chemotactically attracted to the gut epithelium , with motility of the bacterium by a single polar flagellum, where it penetrates the mucous layer of the intestinal surface.
Adherence to the epithelial cell surface is mediated by pili and other adhesins.
Production of cholera toxin results in flow of chloride and water into the lumen of the gut, causing diarrhea and electrolyte imbalance

Question 20

What are the two classes of toxins produced by bacteria?

Answer 20

i. Endotoxin, which is present in the outer membrane of gram -ve rods.
ii. Toxins which are secreted, such as enterotoxins and exotoxins

Question 21

Differentiate between endotoxins and exotoxins.

Answer 21

1. Endotoxins are an integral part of the cell wall of gram-negative bacteria, which may be released on death, while exotoxins are excreted by living cells.
2. Endotoxins are found only in gram-negative bacteria, while exotoxins are produced by both gram-positive and gram-negative bacteria
3. Endotoxins are lipopolysaccharide complexes, while exotoxins are high molecular weight polypeptides
4. Endotoxins are relatively thermostable, while exotoxins are thermolabile.
5. Endotoxins are weakly immunogenic, while exotoxins are highly immunogenic.
6. Endotoxins produce fever in the host, while exotoxins do not.
7. Endotoxins are moderately toxic, while exotoxins are highly toxic.