Microbiological Investigations Flashcards
What is the most routine microbiological investigation?
Cultures
Name some examples of cultures that are taken
Blood cultures
Sputum cultures
Fluid cultures e.g. CSF, Ascites, Urine, Pus (from abscess)
Tissue cultures e.g. Bone
What are cultures used for?
Identifying the pathogen
Antibiotic susceptibility and resistance
What pathogens are identified from cultures?
Bacteria and Fungi
How is Antibiotic Susceptibility tested for in culturing?
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.
What is a major issue with routine swabs for culture, that you need to consider?
Swabs, especially of superficial lesions, may show commensal bacteria, rather than the causative infective pathogen.
When should Urine Cultures be done?
Only when their relevance is indicated i.e. Patients are showing clinical signs and symptoms of UTI
What is Asymptomatic Bacteriuria?
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
Do you give antibiotics in Asymptomatic Bacteriuria?
NO.
Only give antibiotics if the patient exhibits signs and symptoms of infection!
What is Serology used to detect?
Mainly Viruses.
Can also be used for Spirochaeta (Lyme Disease), Syphilis and Protozoa
NOT used for standard bacterial infections
What does Serology involve?
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
What does it mean if you detect IgM in Serology?
Indicates recent exposure to the Pathogen (a few weeks ago)
What does it mean if you detect IgG in Serology?
Indicates that the patient was exposed to the pathogen at SOME POINT in their life but not recently.
What disadvantage do you need to consider with Serology?
Cross-reaction may result in False Positives
What is the main microbiological investigations for viruses?
PCR
However, PCR can also be used to detect Strep. Pneumoniae and Neisseria Meningitis bacteria
How does PCR work?
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.
What is a theoretical risk of PCR?
If the specific gene is absent in the viral strain, it may result in a false negative.
What bodily substances are used for PCR?
Swabs of lesions
Stool
Blood
Other fluids e.g. CSF
What is an important characteristic of PCR?
It is Sensitive, as it amplifies the DNA material
What are 2 important questions you need to ask yourself when doing PCR?
- Is the pathogen present or absent?
2. If present, how much of the pathogen is present?
Why is the amount of pathogen present important in PCR?
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.
What bacterial infection is PCR now a potentially revolutionary investigation for?
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.
What does Whole Genome Sequencing allow you to do?
Monitor outbreaks
Determine the entire DNA sequence of the organism
Analyse gene mutations associated with antibiotic resistance
What is MALDI-TOF?
A newer investigation used to identify organisms with a higher sensitivity
However, it does NOT provide antibiotic susceptibility
