Microbiology

Question 1

Commensal/ normal/ resident flora

Answer 1

Colonising but not capable of causing disease yet. E.g. lactobacilli

Question 2

Opportunistic pathogen

Answer 2

Capable of causing disease only in certain circumstances. E.g. aeruginosa

Question 3

Full pathogen

Answer 3

Will possess virulence factors. E.g. Group A streptococcus

Question 4

What is a virulence factor?

Answer 4

Factors that are produced by a microorganism and evoke disease.

Question 5

Cocci

Answer 5

Spherical bacteria

Question 6

Rods/ bacilli

Answer 6

Cyclindrical shaped bacteria

Question 7

Types of gram bacteria

Answer 7

Gram +ve - Purple
Gram -ve - Pink

Question 8

Variations of bacteria

Answer 8

Diplococci- cocci in pairs
Cocci in chains (streptococci)
Cocci in clusters (staphylococci)
Rod (clostridium perfringens)
Curved gram negative rods (Vibrio cholerae)
Spirochetes (BLT)

Question 9

Acid fast staining

Answer 9

To visualise mycobacteria as it has high proportion of lipids in the cell wall and therefore resist staining.
Auramine is used first as its more sensitive and then Ziehl Neelsen to confirm.

Question 10

Types of cultures

Answer 10

Non-selective - Most bacteria will grow.
Selective - Isolate specific bacteria and may contain antibiotics to restrict growth of other bacteria.
Differential- Distinguish one group from another by colours.
Enriched- Supplemented with nutrients to support growth of bacteria with specific requirements.

Question 11

PCR

Answer 11

Detection of pathogens from culture negative samples e.g. pus
• Detection of pathogens from sterile fluids e.g. CSF
• Detection of slow growing or fastidious bacteria e.g.
Mycobacterium tuberculosis
• Screening assays e.g. Neisseria gonnorhoeae and Chlamydia trachomatis
• Detection of antibiotic resistance e.g. MRSA, MDR-TB
• Quantification of pathogen load e.g. HIV viral load
• Sequencing- resistance and epidemiology e.g. SARS-CoV-2

Question 12

Advantages of PCR

Answer 12

Faster detection of pathogens than traditional techniques. Allows appropriate, timely antimicrobial therapy and infection control interventions
- Increased sensitivity over culture and microscopy based techniques
- Useful in culture negative samples
- Can be automated
- Increased cost compared to other methods (£10-120+) per test.

Question 13

Disadvantages of PCR

Answer 13

-Expensive
-Require expertise
-Labour intensive
-Possibility of contamination
-Require verification/validation
-Require quality control
-Require extraction of nucleic acid

Question 14

Serology

Answer 14

– Detect an antibody response in symptomatic patients e.g. IgM, IgG
– Determine if vaccination has been successful e.g. MMR IgG
– Directly look for antigen produced by pathogens e.g. HIV, HBV

Question 15

Types of Antibodies/Immunoglobulins

Answer 15

• IgG
– Secondary immunity
– Pass the placenta
• IgM
– First antibody produced
– Predominantly found in serum
• IgA
– Secretory found in tears, saliva, sweat etc.
• IgE
– Parasitic diseases and allergy
• IgD
– Function not well understood

Question 16

Antibody affinity

Answer 16

The strength of the interaction between a single antigenic determinant and a single antigen binding site

Question 17

Avidity

Answer 17

A measure of the overall binding strength

Question 18

Why is IgG avidity important

Answer 18

IgG increase with time after infection
• Used to approximately time when an infection occurred
• Useful when timing some infections that can cause congenital infection e.g. CMV and toxoplasma

Question 19

Bacterial vs Viral infections in blood results

Answer 19

Can be difficult.
• Bacterial infections usually cause a higher CRP than viral infections.
• Both can cause fever.
• Raised WBC count in both bacterial/viral infections.
– Raised neutrophil count in bacterial infections.
– Raised lymphocyte count in viral infections. Some viral infections cause neutropaenia e.g. HIV and EBV.
• Culture, microscopy and serology used to diagnose bacteria infection.
• PCR and serology used to diagnose viral infection.