Microbial Infections

Question 1

What is a zoonotic disease and what is an example?

Answer 1

A disease that does not normally circulate within humans, but instead, within an animal reservoir, and afterwards transmitted to humans e.g. Ebola - Hammerhead fruit bat, transmitted by contaminated bush meat

Question 2

How does the Ebola virus infect humans?

Answer 2

Requires transported Niemann-Pick C1

Question 3

Why is the animal acting as the animal reservoir not affected?

Answer 3

They have a mutation in that transported (Npc1) hence they get a low grade infection and are carriers of the Ebola virus

Question 4

What are the 5 main types of infectious agents that cause disease in humans?

Answer 4

1. Viruses
2. Bacteria
3. Fungi
4. Protozoa
5. Helminths

Question 5

Features of viruses:

Answer 5

Obligate pathogens - not cells in their own right, only survive inside host cells
Show host specificity - can infect many life forms (including bacteria), but many can only infect particular animals or tissues
Can contain DNA or RNA genetic material
RNA material requires reverse transcriptase to make viral DNA
Uses the host cell’s machinery to synthesise eukaryotic RNA from the viral DNA, and then undergo protein synthesis
Released by budding out of the cell (e.g. come out of the cell wrapped in eukaryotic cell membrane), or cytolysis (bursting out of the host cell - causes severe host cell damage)
Many routes of infection for viruses - oral-faecal, airborne, insect vectors, blood borne

Question 6

Why can’t the RNA from viruses directly undergo protein synthesis in humans?

Answer 6

Viral RNA is not compatible with the host cell’s machinery, reverse transcriptase is required to synthesis viral DNA, which can be used to synthesise eukaryotic RNA

Question 7

Which disease has been successfully eradicated?

Answer 7

Smallpox

Question 8

What virus causes smallpox and who was the last known case?

Answer 8

Variola Virus

Ali Maow Maalin (Somalia - 1977)

Question 9

Why was it easier to eradicate this disease?

Answer 9

Easy to diagnose - obvious lesions on the body
Efficient vaccine - scarification (scratched onto surface of the skin)
Even with early sign of infection, can still be vaccinated and cured

Question 10

What were the iron lung chambers used to treat and why?

Answer 10

Polio virus

Negative pressure chambers to help patients breathe - musculature in the thoracic cavity becomes disabled

Question 11

Why is the polio vaccine easy to distribute?

Answer 11

Oral vaccine - no needles involved therefore can give on a large scale

Question 12

Features of bacteria:

Answer 12

Prokaryotes - single celled
No internal membranes (except photosynthetic bacteria - but they are not infectious)
Haploid
Unique peptidoglycan cell wall - determines shape
Binary fission - elongates to 2x their size, then splits / divides

Question 13

Why is having only one copy of chromosomes (haploid) an important characteristic?

Answer 13

Important due to the expression of mutations - any mutation in the genome is immediately expressed and so contributes to the rapid speed of antibiotic resistance / outbreaks
No recessive genotype

Question 14

Why is the unique peptidoglycan cell wall important?

Answer 14

Target for chemotherapy - e.g. many antibiotics target the synthesis of the cell wall to prevent cell division (and replication)

Question 15

What are the different shapes bacteria can be?

Answer 15

Rod, coccus (spheres), spirochaete (spirals)

Question 16

How do bacteria move?

Answer 16

Using a flagellum - often via chemotaxis (e.g. towards food spurces or away from toxins)

Question 17

What pathogens use the faecal-oral route and an example (+ its features)?

Answer 17

Gut pathogens e.g. shigella
Impacts epithelial cells of the GI tract
Uses host cell components to move within the cytoplasm of the cell (e.g. nucleation of actin within the cells, pushing bacteria between cells)
Spreads e.g. during floods, issues with getting rid of faecal waste, contaminated water supplies
Symptoms = bloody diarrhoea (due to damaged cells and blood released into the contents of the GI tract)

Question 18

An example of an airborne pathogen (community acquired) and its features?

Answer 18

Neisseria meningitidis (NM)
Vaccination an issue / v. time consuming as it has many serogroups
Many individuals have it e.g. in the nasa pharynx, but it causes no harm (dormant)
Something triggers it to become invasive - into the bloodstream
Medical emergency when it becomes invasive - rapid progression
Initially, non-blanching rash = bleeding into the skin
Further spread - septicimia (other bacteria can enter and circulate the blood)
Further spread - blockage of blood vessels causes loss of digits / limbs
Crossing the blood brain barrier = meningitis (infection of the protective membranes that surround the brain and spinal cord)

Question 19

What are nosocomial infections and an example (+ its features)?

Answer 19

Hospital acquired infections
e.g. Clostridium Difficile
Has a spore forming stage hence may exist as a spore in hospital (standard disinfectants may not work)
e.g. MRSA
Issue as they are resistant to most standard drugs and do spread more easily

Question 20

Why does mycobacterium tuberculosis (TB) have such a high mortality rate?

Answer 20

Chronic infection
Difficult to diagnose - need diagnostic tools that can detect TB at earlier stages)
Treatment drugs work well, but take a long time i.e. 6 months
TB vaccine (AKA BKG vaccine) works better in infants than in adults
TB transmitted through adults - need more effective vaccines in adults to reduce transmission

Question 21

What is myobacterium leprae (leprosy) and its features?

Answer 21

Chronic infection of the skin and nerves
Slow incubation period (i.e. 5 years)
Different forms depending on the patients’ immune response to leprosy
Cell mediated = tuberculoid (skin lesions)
Humoral = lepromatous (numb - cuts / wounds go unnoticed)

Question 22

What is helicobacter pylori?

Answer 22

Strongly linked to peptic ulcers and gastric cancer

Question 23

What is pathogenic E-coli?

Answer 23

Line gut epithelial cells
Zoonotic - often picked up in petting zoos / farms
Mutations cause them to become pathogenic

Question 24

Are mutation rates higher in other micro-organims compared to humans?

Answer 24

No, more or less the same in humans, mice, bacteria etc - 10^-8
(viruses a little higher - 10^-4)

Question 25

So why do mutations arise faster in becteria than in humans?

Answer 25

Generation time - replicate much faster from several minutes to several hours
When mutations arise - replicate faster so more organisms carry that mutation

Question 26

Features of fungi:

Answer 26

Single celled eukaryotes - have nuclei
Cause different types of infections e.g. scalp infection - dandruff, mucosal - mouth infections, systemic - in the body
Budding yeast - attached to epithelial cells
During cell cycle - form hyphae (long filaments to attach to cells)

Question 27

Features of protozoa:

Answer 27

Single celled eukaryotes - unicellular
Intestinal e.g. giardia
Blood borne e.g. malaria
Replicate by binary fission (asexual stage), but also sexual stage
Complex life cycles - encompassing more than 1 host e.g. first inside a different animal (vector), then acquired through ingestion into the human

Question 28

What is malaria caused by and features of malaria?

Answer 28

Plasmodium species
Infection via mosquito vector - distribution of malaria depends on distribution of mosquitoes
e.g. high altitudes = no mosquitoes = no malaria
Infect RBCs - staining blood sample paints infected cells

Question 29

What mutation reduces malaria infection?

Answer 29

Sickle cell anaemia - mutation selected for as it protects against malaria
Carrier = SCA symptoms less severe but also little / no malaria impact

Question 30

What is an example of a blood / tissue parasitic infection and its features?

Answer 30

Leishmania species - several different species causing several different symptoms
Blood and tissue parasite infections
Sandfly vector
Replicating stage found on blood films = diagnostic tool
Cutanous form = skin lesions, Visceral form = massive swelling of internal organs
Zoonotic disease - dogs (sandflies bite dogs and humans)

Question 31

Features of Helminths:

Answer 31

Mmetazoa - multi-cellular eukaryotes
Visible with the naked eye - no microscope required to see them
Life cycle outside human host - many environmental e.g. live in soil / water and later, picked up by humans
e.g. worms - roundworms in guts, tapeworms, flatworms

Question 32

An example of a helminth and its features?

Answer 32

Ascaris - roundworms
Faecal-oral route - e.g. eggs in soil / water picked up through contaminated food/water supply
Occupy GI tract, absorb nutrients from food, leaving the body of the human malnourished
Treatable, no vaccines

Question 33

Another example of helminths and its features?

Answer 33

Schistosomiasis - Flukes (Tapeworms)
Snail is the intermediate vector
Live in hepatic portal vein in adults
Produce eggs with a spine - allows for migration from hepatic portal vein towards the gut, released into the environment
Migration causes inflammation in organs - tremendous swelling due to inflammatory immune response
Hatch in environment - miracidia, which go on to infect snails
Snails release cercaria into the water supplies
Control spread by controlling snails