2. Steps in pathogenesis - overview

Question 1

What are the 7 stages of pathogenesis?

Answer 1

1. Entry into the host
2. Attain a niche
3. Avoid constitutive or non specific host defences
4. Circumvent specific induced host responses
5. Grow and persist within the host
6. Cause damage to the host
7. Exit from the host

Question 2

What are the five possible entry routes a micro-organism may use to gain entry?

Answer 2

1. Skin
2. Eyes
3. Gastrointestinal tract
4. Respiratory tract
5. Urinogenital tract

Question 3

Which of the aforementioned entry routes are the most often infected?

Answer 3

The gastrointestinal and respiratory tract are most often infected, followed by the other mucosal surfaces and eyes. The skin is the least often infected.

Question 4

How are the alveoli protected from micro-organisms?

Answer 4

No cilia or mucus is present here but macrophages line the alveoli to phagocytose invading micro-organisms.

Question 5

How does the upper and lower respiratory tract prevent micro-organism entry into the body?

Answer 5

1. Hairs lining the notstrils trap larger pathogens.
2. Ciliated cells, goblet cells and subepithelial mucus-secreting glands. The latter two secrete mucus to entrap invaders allowing them to be wafted back up the respiratory tract by ciliary action, or in the case of the upper respiratory tract to the back of the throat to be swallowed and killed in the stomach.
3. Antibodies

Question 6

What are the predominant antibodies present in the upper and lower respiratory tract?

Answer 6

IgA - Upper RT

IgG - Lower RT

Question 7

How does mucus membrane help protect infection in the gastrointestinal tract?

Answer 7

Mucus acts as a mechanical barrier to cells which may become infected and contains secretory IgA antibodies that protect the immune individual against infection.

Question 8

How does the normal intestinal microflora serve to protect from gastrointestinal infection?

Answer 8

1. Killing other bacteria by bacteriocins
2. Competition for nutrition and attachment sites
3. The production of bacterial inhibitors e.g acetic and propionic acids

Question 9

What do pathogens attach to in the oral cavity?

Answer 9

1. Mucosal surfaces
2. Teeth
3. Epithelial cells of cheek
4. Tongue

Question 10

What are the natural defence mechanisms of the oral cavity?

Answer 10

Saliva is secreted continually allowing for mechanical removal of micro-organisms and release of antimicrobial agents such as lysozyme and secretary antibodies.

Question 11

Why is it difficult for micro-organisms to establish a niche in the urinary tract?

Answer 11

Due to the continual flushing of urine.

Question 12

Why are urinary infections more common in woman than men?

Answer 12

The female urethra is 5cm long compared to the 20cm of the male urethra. In addition its close proximity to the anus exposes it to more pathogenic micro-organisms.

Question 13

Name a commensal microorgansim which colonises the vagina?

Answer 13

Lactobacillus

Question 14

Mention a protective defence mechanism established to prevent vaginal infections.

Answer 14

Oestrogen allows the secretion of glycogen in the vagina which is metabolised by lactobacilli to produce lactic acid making the vagina slightly acidic and thus prevents its colonisation by harmful microorganisms.

Question 15

What happens after menopause when glycogen is no longer being secreted like it used to be?

Answer 15

The vaginal secretions become alkaline due to the lack of glycogen and microorganisms such as staphylococci and streptococci can establish themselves in the vagina.

Question 16

How is the uterus protected from invasion by microorganisms?

Answer 16

The downward flow of mucus and cilia as well as the production of lysozyme help to protect the uterus from infection.

Question 17

How is the eye protected from pathogens?

Answer 17

The continuous washing action of the secretions, mechanically wiped away by the windscreen washing action of the eyelids. Lysozyme and other antimicrobial agents are also present such as defensins.

Question 18

Once a pathogen has gained entry into the body, where can it spread to from the initial site of entry?

Answer 18

1. Colonises at the site of entry but does not invade host cells.
2. Colonises at the site of entry to invade host cells only - restricted to mucosal surface.
3. Spread to internal organs causing a systemic infection.

Question 19

What is the first line of defense once a microorganism has entered the body?

Answer 19

Macrophages and neutrophils