Investigating Infections

Question 1

What do Gram+ve and Gram-ve bacteria stain?

What is done on a Streak plate?

Answer 1

• Gram+ve stain PURPLE, Gram-ve stain RED
• Culture put in one section (Initial Inoculum) of the Agar plate and swept in 4 different directions to separate the different bacteria

Question 2

Enriched, Differential and Selective Media:
What do these use?

What does a MacConkey Agar contain?
→ What does the Bile show?
→ What does the Lactose show? What result is seen?

What is a Blood Agar used for?
→ What type of media is it?
→ What does it become when heated?

What can differentiate bacteria on a Blood agar?
→ What are the 2 types and what will be seen?

In blood cultures, what’s in the Grey and Purple lids?

Answer 2

• Salts, sugars, chemicals, indicators, antibiotics
• Bile salts and Lactose - Differential media - cultured overnight in air
  → If the microbe can withstand its conditions
  → If the microbe can ferment it or not = Turns RED if it can
• Campylobacter jejuni - incubated in an anaerobic jar-microaerophilic
  → Enriched media (contains nutrient-rich blood)
  → Chocolate agar
• Haemolysis
  → • Alpha Haemolysis - Oxidised blood around each colony - GREEN/BROWN tinge
  • Beta Haemolysis - Complete lysis of blood around each colony
• Aerobic blood culture in the Grey lids, Anaerobic blood culture in the Purple lids

Question 3

Antibiotic Resistance Testing and MIC (Minimum Inhibitory Conc.):
What is used to test for this?

What test is used for antibiotic resistance?

What test is used to measure a drug’s MIC against a specific microbe?
→ How is this carried out?

How can the MBC (Minimum Bactericidal conc.) be tested using Tubes?

Answer 3

• PURE colonies
• Disc diffusion test
• E-test
  → A strip is soaked in the drug at differing concentrations along it, then placed on the agar
• After adding the drug, the tubes that still have bacteria growing can be poured onto an agar plate to see how many colonies form
  o The number of colonies shows the drug’s MBC

Question 4

When is a Urine sample taken for a healthy patient? Why’s it done at this time?

When is a Urine sample taken for a UTI patient? Why’s it done at this time?

Answer 4

• As soon as they start peeing to maximise pathogen load

- Midway through peeing (mid-stream) to minimise commensals