Microbiology & Immunology Pracs

Question 1

What kind of environment can a Gram +ve bacterium survive in (due to its cell wall)

Answer 1

It can survive in a dry environment due to its thick peptidoglycan cell wall

Question 2

1. What are the two diseases caused by Varicella Zoster Virus?
2. What are the layman’s terms for these diseases?
3. How does it cause two diseases?

Answer 2

Varicella = Chickenpox (primary infection)

Herpes Zoster = Shingles (reactivation of latent virus)

Question 3

What is the distinctive colour of Psuedomonas Aeruginosa?

Answer 3

Green

Question 4

What is the difference between primary and secondary immunodeficiencies?

Answer 4

Primary immunodeficiencies: the result of inherent congenital defects in the components of the immune system/their products
Secondary immunodeficiencies: the effects of external agents or alterations in other body systems

Question 5

What are some causes of secondary immunodeficiencies?

Answer 5

Extremes of age
Malnutrition
Anatomic barrier dysfunction (including medical devices)
Non infectious diseases e.g. diabetes, tumours
Infections e.g. Malaria, HIV
Cytotoxic drugs/irradiation
Immunosuppressive drugs

Question 6

Explain the technique of flow cytometry

Answer 6

Tag a cell population with a fluorecent dye (react with a fluorescently labelled monoclonal antibody) so it can be easily recognised and analysed

e.g. you can tag leukocytes to see if they are deficient in numbers, or see of certain cell populations are overabundant.

Question 7

A 2 month old baby presented with swelling and discharge around her umbillical cord stump (the stump had not yet separated). On examination she had a fever, Swabs were taken from the serosanguineous discharge around the stump. On further investigation she was found to be infected by S. Aureus.

What is the significance of the discharge being serosanguineous?

Answer 7

Serosanguineous = blood & serum but NO PUS
No pus = No white blood cells
S. Aureus often causes a purulent discharge.

This should raise suspicion of an immunological deficiency

Question 8

Leukocyte Adhesion Deficiency (LAD) is a primary immunodeficiency. Explain its pathogenesis

Answer 8

There is a normal number of T cells & monocytes but the individual lacks the ability to make certain adhesion molecules = Lack of leukocyte integrin CD18.
The tight binding for B2 integrins to ICAM-1 cannot occur therefore there is no migration, no attachment, no diapedesis and therefore the immune cells cannot get to the site of infection.

Question 9

A young child is suspected to have a primary immunodeficiency.
Upon analysis of his antibodies, the following results were found:
IgG - Low
IgM - High
IgA - None detected
CD40L - Low

Explain these results.

Answer 9

The child’s antibodies are not undergoing isotype switching.
CD40L is required to activate B cells for isotype switching, memory B cells, and stimulating macrophages.

(Diagnosis: Hyper IgM Syndrome)

Question 10

When collecting a sample to test for septicaemia, why should three sets of blood cultures be taken?

Answer 10

Bacteraemia is usually intermittent, rather than continuous

Question 11

What does a catalase test, test for?

Answer 11

The bacteria’s ability to break down H2O2 and therefore its ability to survive in phagocytes

The enzyme catalase is present in most aerobic and faculative anaerobic bacteria.

Question 12

What does a coagulase test look at?

Answer 12

The bacteria’s ability to clot fibrinogen to fibrin.

Question 13

If the colonies on MAC were found to be pink, what does this indicate?

Answer 13

They are lactose fermenters

Question 14

TRUE OR FALSE?

Upper respiratory tract infections are predominantly caused by bacteria.

Answer 14

FALSE!! They are predominantly caused by viruses
Common cold: viral
Sore throat: predominantly viral
Sinusitis: predominantly bacterial (often after viral infection)
Otitis media: bacteria

Question 15

What are some syndromes associated with Lower Respiratory tract infections?

Answer 15

Croup (laryngotracheobronchitis) 
Whooping cough 
Bronchiolitis 
Bronchitis - acute, and acute exacerbation of chronic bronchitis 
Pneuomonoa

Question 16

When diagnosing organisms found in the lower respiratory tract, how do we ensure it isn’t just contaminants from the upper respiratory tract in our sample?

Answer 16

Macroscopically it should be thick mucus
Microscopically there should be NO epithelial squamous cells
Microorganisms that have invaded the LRT would be in greater numbers than in the URT (infection is often associated with isolated of particular microorganism in numbers greater than 10 ^ 7 /ml of sputum)
There should be the prescence of inflammatory cells

Question 17

What are the most common causes of pharyngitis and tonsillitis?

Answer 17

Viruses, but also Strep Pyogenes

Question 18

What antibiotic is Strep. Pyogenes ALWAYS susceptible to?

Answer 18

Penicillin

Question 19

What type of bacteria is not susceptible to pencillin?

Answer 19

Beta lactamase producing bacteria

Question 20

How does Klebsiella evade the immune system?

Answer 20

With its capsule - it will look shiny in colonies

Question 21

What causes the majority of bronchiolitis?

How should it be managed?

Answer 21

Respiratory Syncitial Virus (RSV)

DON’T USE ANTIBIOTICS - can only provide symptomatic treatment (oxygen, fluid, rest…)

Question 22

In bacterial contamination of food, what are the symptoms normally due to?

Answer 22

Ingesting the toxins produced by the pathogens in the contaminated food.
Common offenders include: Steph aureus, Bacillus cereus and Clostridium botulinum

Question 23

Compare the differences in likely pathogens between developing and developed countries that cause gastroenteritis w. diarrhoea

Answer 23

Developing countries: Most likely to be viruses, less likely to be bacteria, least likely to be protozoa
Developed countries: Most likely to be bacteria, less likely to be viruses, least likely to be eukaroytic parasites

Question 24

A group of friends went to have lunch together. The next day over half of them were unwell. By the following night, they all showed signs of gastroenteritis.

The doctor decided the pathogen was NOT Staph Aureus.
Why?

Answer 24

This case demonstrated a long incubation period.

Staph aureus is associated with a short incubation period (only 4-6 hours) as it produces toxins directly in the food.

Question 25

Explain how the pathogenesis of EHEC can cause Haemolytic Uremic Syndrome (HUS)

Answer 25

EHEC = Enteroheamorrgic E. Coli

• Production of Shiga toxin when ingested
• It is not an invasive organism, it is restricted to the enterocyte surface
• It binds to endothelial cells or organs that express G3B receptors, these are found in kidneys, pancreas and CNS
• After binding, the toxin inhibits protein synthesis and causes an inflammatory response.
• There is fibrin deposition that traps platelets and breaks RBCs (leads to thrombocytopaenoa)
• The endothelium dies and there is damage to kidneys and therefore acute renal failure

Question 26

What kinds of bacteria are associated with a foul odour?

Answer 26

Anaerobic bacteria

Question 27

A patient was found to be infected with Giardia lamblia. How should she be treated?

Answer 27

This is not a self limiting disease like the other gastroenteritis diseases. Needs to be treated with matranodozole - this antibiotic targets the metabolic anaerobic pathway

Question 28

What antibodies should be tested for when looking for coeliac disease?

Answer 28

DGP and Ttg (tisse trans-glutaminase) antibodies

Question 29

Describe the capsid of rotavirus

Answer 29

Triple shell, 3 lots of proteins that make a protective layer

Question 30

How can you prevent rotavirus infections?

Answer 30

Hygiene + vaccines! (given at 2, 4 and 6 months of age)

Question 31

Should you use antidiarroeals in children?

Answer 31

No! Let them flush out the organism!

Question 32

In a liver function test, what does the presence of HBs Ab indicate?
(Hep B surface antibody)

Answer 32

Previous vaccination

Question 33

What is the most common cause of UTIs?

List some other common causes.

Answer 33

Most common: E. Coli

Other common causes: Staphylococcus saprophyticus (10% in sexually active women), Proteus, Kelbsiella, Enterococci, Pseudomonas

*Increasingly common in complicated UTI

Question 34

What factors predispose to UTIs?

Answer 34

• Being female ( 50 x more common in females)
• Anatomical abnormalities
• Neurological abnormalities
• The presence of foreign bodies e.g. urethral catheterisation
• Diabetes

Question 35

What colour should acid-fast bacteria appear in a Ziehl-Neelsen stain?

Answer 35

Red

Question 36

What features of E. coli allow it to cause UTI?

Answer 36

Only uropathogenic E. colis will cause UTIs due to their expression of adhesins (fimbriae) for the UTI
epithelium. These bacteria HAVE to be able to sit on the UTI wall to be able to cause disease. Biofilms and colonies can form on the uroepithelium and some can even invade.
(This invasion is often impeded by interactions with TLR-4 and such defence mechanisms, releasing defensins and preventing invasive dissemination.)
-When the uropathogenic E. coli are at their site of colonisation, they stop expressing flagella so they do not activate TLR-5, but in ureters and parts proximal to the bladder, the cells don’t express TLR-5, so the E. coli can start expressing flagella again and then swim around.
- They also express haemotoxins that can lyse RBCs.

Question 37

A woman in her first trimester of pregnancy was found to have a UTI caused by Staph saprophyticus.
Antimicrobial tests showed the pathogen was susceptible to: Ciprofloxacin, Norfloxacin, Cotrimoxazole and Trimethoprin.

The doctor decided to use Cotrimoxazole.

Why didn’t the doctor prescribe Ciprofloxacin or Trimethoprin in this case?

Answer 37

Ciprofloxacin: is a broad spectrum quinolone that bacteria becomes resistant to very easily. We generally only use this for Pseudomonas

Trimethoprin: causes inhibition of folate, we do NOT want to be using this during preganancy as it will affect the fetus.

Question 38

HBA (Horse Blood Agar) can be used as an indicator of various types of haemolysis.
What are the two types of haemolysis?

Answer 38

1. Clear haemolysis - beta haemolysis
   Seen with cultures of Strep. pyogenes, Clostridium perfingens, Bacillus cereus
2. Greening haemolysis - alpha haemolysis
   Seen by organisms which reduce Hb to a green product, like Strep. pneumoniae, Viridans streptococci