Microbiological Investigations

Question 1

What is the most routine microbiological investigation?

Answer 1

Cultures

Question 2

Name some examples of cultures that are taken

Answer 2

Blood cultures
Sputum cultures
Fluid cultures e.g. CSF, Ascites, Urine, Pus (from abscess)
Tissue cultures e.g. Bone

Question 3

What are cultures used for?

Answer 3

Identifying the pathogen

Antibiotic susceptibility and resistance

Question 4

What pathogens are identified from cultures?

Answer 4

Bacteria and Fungi

Question 5

How is Antibiotic Susceptibility tested for in culturing?

Answer 5

Antibiotic is put into a vial and placed in the agar plate. Circular ring (disc) that forms around the vial indicates sensitivity to the antibiotic.
The bigger the disc, the greater the antibiotic sensitivity.

Question 6

What is a major issue with routine swabs for culture, that you need to consider?

Answer 6

Swabs, especially of superficial lesions, may show commensal bacteria, rather than the causative infective pathogen.

Question 7

When should Urine Cultures be done?

Answer 7

Only when their relevance is indicated i.e. Patients are showing clinical signs and symptoms of UTI

Question 8

What is Asymptomatic Bacteriuria?

Answer 8

As people age, they develop bacteria in their urine and are symptomatic despite the bacterial presence.
This means that you will always find bacteria in an elderly patient’s urine

Question 9

Do you give antibiotics in Asymptomatic Bacteriuria?

Answer 9

NO.

Only give antibiotics if the patient exhibits signs and symptoms of infection!

Question 10

What is Serology used to detect?

Answer 10

Mainly Viruses.

Can also be used for Spirochaeta (Lyme Disease), Syphilis and Protozoa
NOT used for standard bacterial infections

Question 11

What does Serology involve?

Answer 11

Antibody and Antigen Testing to confirm the presence of infection and what the causative pathogen is.

For EBV, CMV, Syphilis: Antibodies
For Hep B: Antigens

Question 12

What does it mean if you detect IgM in Serology?

Answer 12

Indicates recent exposure to the Pathogen (a few weeks ago)

Question 13

What does it mean if you detect IgG in Serology?

Answer 13

Indicates that the patient was exposed to the pathogen at SOME POINT in their life but not recently.

Question 14

What disadvantage do you need to consider with Serology?

Answer 14

Cross-reaction may result in False Positives

Question 15

What is the main microbiological investigations for viruses?

Answer 15

PCR

However, PCR can also be used to detect Strep. Pneumoniae and Neisseria Meningitis bacteria

Question 16

How does PCR work?

Answer 16

Makes copies of the viral DNA and amplifies it, targeting specific but commonly occurring genes in the organism.
The Pre-Set Primer targets a particular gene within the organism which prompts DNA copying.

Question 17

What is a theoretical risk of PCR?

Answer 17

If the specific gene is absent in the viral strain, it may result in a false negative.

Question 18

What bodily substances are used for PCR?

Answer 18

Swabs of lesions
Stool
Blood
Other fluids e.g. CSF

Question 19

What is an important characteristic of PCR?

Answer 19

It is Sensitive, as it amplifies the DNA material

Question 20

What are 2 important questions you need to ask yourself when doing PCR?

Answer 20

1. Is the pathogen present or absent?

2. If present, how much of the pathogen is present?

Question 21

Why is the amount of pathogen present important in PCR?

Answer 21

It is important in the contest of the blood-borne viruses e.g. Hep A, B and C viruses.

For Hep A and B, the amount of pathogen correlates to how well controlled the infection is - persistently negative PCRs indicate infection control.
For Hep C, persistently negative PCRs indicate no pathogen indicates cure.

Question 22

What bacterial infection is PCR now a potentially revolutionary investigation for?

Answer 22

TB - Mycobacterium Tuberculosis

This is because as TB is slow growing and a positive culture may take up to 8 weeks to determine, PCR can yield a positive result within 15 minutes due to its sensitivity.

Question 23

What does Whole Genome Sequencing allow you to do?

Answer 23

Monitor outbreaks
Determine the entire DNA sequence of the organism
Analyse gene mutations associated with antibiotic resistance

Question 24

What is MALDI-TOF?

Answer 24

A newer investigation used to identify organisms with a higher sensitivity
However, it does NOT provide antibiotic susceptibility