Department of Microbiology

- Bacteriology
- Virology
- Mycology
- Parasitology
- Mycobacterology
- Molecular Microbiology

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FAQs

Clinical Microbiology

Blood cultures

Why isn't Gram stain done on blood cultures as soon as they are received in the microbiology lab?

For organisms to be detected on Gram stain, there must be > 10E5 bacteria per mL of fluid and that would not be compatible with life. Therefore, Gram stain of the blood culture is performed only after the incubator indicator becomes positive for possible growth of organism.

Specimens

Why are swabs not good specimens?

Several reasons, but most importantly: The superficial surfaces and membranes are usually colonized with the local normal flora or “opportunists”. The pathogen(s) invading the tissues may not be detected by swabbing that area, you will sample only the surfaces and membranes that are contaminated with the normal flora or “opportunists”.

Also, by sending a swab means you are likely to miss certain pathogens, especially tuberculosis or fungus infections. These are harder to detect with the small amount of specimen collected by a swab because these organisms are not found in very large numbers like the common pyogenic bacteria (staphylococci, streptococci, etc.) Swabs can make it difficult to detect anaerobic bacteria too.

The swab picks up only a small volume of the specimen, and by inoculating the specimen onto multiple plates, you will run out of specimen before all the cultures have been inoculated.

What are the best specimens?

The best specimens are pieces of tissues or aspirated fluid or pus from the infected site, collected after disinfection of the surface. We don’t need more tissue than a piece the size of a penny and no more than 20 mL of fluid.

The amount of specimen larger than what is collected by a swab ensures that we will have sufficient specimen to inoculate all the necessary media and perform the microscopic examinations.

Does that means, we no longer need the swab?

No, Swabs are still useful in the following situations:

Throat swabs to detect GAS
Vaginal swabs for detecting causes of vaginitis
Vaginal/rectal swabs for detecting GBS
Infection Control surveillance swabs

Why is sending the best quality specimen so important?

Sending the best specimen to microbiology laboratory will enable the Microbiology Laboratory to perform optimal investigations and produce the most reliable results.
This will help the physician to establish the right diagnosis. That will facilitate initiation of appropriate antibiotic therapy as directed by the culture result.
Also, it will help avoiding unnecessary empiric antibiotic therapy, which is overall better for the patient’s care.
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