Week 12

Hi, this week I'll be sharing on microbiology.

In the microbiology lab, the most commonly received types of specimens would be urine, stool and swabs from any part of the body for culture. Culture basically refers to specimens being plated on specific agar/s to observe for any predominant growth of microorganisms. If culture results confirmed that the patient is infected, CDS (calibrated dichotomous sensitivity) test would be done to determine the antibiotic sensitivity of the microorganism in order for doctors to prescribe the correct type of antibiotic to the patient.

CDS (another term for antibiotic sensitivity) Test

Stool specimens are one of the most frequently received specimens in the lab. Stool cultures would be able to show the presence of certain types of abnormal microorganisms such as Samonella, Shigella, Campylobacter and Vibrio species present in the normal human body. For oncology patients, stool screening is required to detect any multi antibiotic resistant microorganisms such as ESBL producers, VRE or any unusual overgrowth of pathogenic flora such as P. aeruginosa or Candida.

Because of the fact that human stool is not sterile, there is a high possibility that there would be a mixture of gram positive or negative, aerobic or anaerobic bacteria present. Therefore, differential media (eg. XLD, TCBS and Campylobacter agar) need to be used to differentiate the normal from pathogenic flora present in the stool.


Besides stool cultures, high vagina swabs (HVS) are also very commonly received in the lab. Because I work in a gynae hospital, most of the HVS received are to detect for Group B Streptococcus in pregnant women. Although group B Streptococcus can be considered as a type of normal flora in the female genital tract, the presence of this organism in pregnant women is clinically significant.

Swabs used for Culture
Retrieved from http://home.caregroup.org/departments/pathology/lab_manual/PLM_containers.htm

Group B Streptococcus can cause a relatively rare but serious infection in newborns. Approximately 10-30% of pregnant women carry this type of bacteria in the vagina or rectal area, and may pass in to their babies during labour. Some of the consequences include sepsis, pneumonia or meningitis in newborns. Some of these babies, particularly those with meningitis, will have long-term health problems such as hearing or vision loss, cerebral palsy, or developmental disabilities and about 5% would not be able to survive. Therefore, to reduce the risk of the infection in newborns, prenatal testing and treatment is often recommended.

Processing:
1. Swab would be plated on blood agar and put into TGN enrichment broth.
2. Plates and broths are then incubated for 37°C for 24 hours.
3. After incubation, sub-culturing is done by plating the TGN enrichment broth on blood plates.
4. Original blood plates incubated for 24 hours will be checked for organisms such as strep B, Candida and Listeria.
5. Subcultures will be read the following day for strep B.

For all types of suspected microorganism growths on cultures, biochemical tests must be carried out to confirm their identity. Different types of mediums are used to identify the characteristics of the microorganisms based on their biochemical reactions. Below are some examples of mediums that are commonly used (brief explanation):

Kliger Iron Agar (KIA) – Detects the ability of the microorganism to ferment glucose by observing the reaction produced be the organism in the agar. KIA also contains sodium thiosulfate to test for hydrogen sulfide. Iron salts present in the media react with the hydrogen thiosulfide to produce an insoluble black precipitate.

Citrate Test (Simmon Citrate Agar) – Determines the ability of the microorganism to use sodium citrate as the sole source of carbon for metabolism and growth.

Urease Test - Urease is an enzyme that breaks the carbon-nitrogen bond of amides to form carbon dioxide, ammonia, and water.

OF (Oxidation Fermentation) Basal Media – Used to determine oxidative and fermentative metabolism of carbohydrates by gram-negative bacteria on the basis of acid reaction in either the open or closed system. Closed system refers to the surface of the agar being covered with oil.

Motility Indole Lysine Medium – Used to demonstrate motility, indole production, lysine decarboxylase and deaminase activity and hydrogen sulfide production in microorganisms.



Ka Hang
TG02