Week 18 - Medical Microbiology

After the urine specimens have cultured onto Blood Agar plates and Cysteine Lactose Electrolyte Deficient agar plates, the plates will be incubated at 35°C overnight. The next morning, the plates are read and results are reported. [1st post recap]

Some of the most common Urinary Tract Infection (UTI) causing agents are:

Escherichia coli
- Flat, size varies

Klebsiella spp.
- Mucoid and spready colonies

Proteus spp.
- Swarms in blood agar plates

Pseudomonas aeruginosa
- Spready, irregular shape
- Grape-like smell

Plates are grouped into further and no further tests carried out.

1. No further identification / biochemical tests carried out:

- For plates that do not have any growth -> Report as No bacteria growth.
- For plates that do not have significant number (less than 50,000 cfu/ml) of bacteria growth:
a. Check whether the plate is a pure or mixed culture.

b. For mixed cultures (2 or more organisms present), Report as No significant bacteria growth.

c. For pure cultures, check the colonial morphology and presume the type of microorganism present. If the microorganism is not one of the common UTI causing agents -> report as no significant bacteria growth.

d. If it is one of the common UTI causing agents, check the FEME results. If the WBC count is less than 5, the patient may not be suffering from UTI. Therefore it is reported as no significant bacteria growth.

e. If the WBC count is more than 5, it may suggest UTI and the results will be reported as less than 50,000. But still, identification test is not done because the viable count is less than 100,000 cfu/ml.

**Viable count of more than 100,000 cfu/ml will then be considered as having UTI**

- For plates with mixed bacteria growth:
f. If the viable count is 50,000 cfu/ml containing 2 or more types of organisms -> Report as Mixed 50,000.

g. If the viable count is 100,000 cfu/ml containing more than 2 types of organisms -> Report as Mixed 100,000. These may suggest a contamination or mid stream urine is not collected. It is unclear as to which organism is responsible for UTI therefore the viable count is reported without any further tests done.


2. Further tests carried out:

- For plates with significant bacteria growth,
a. Containing 100,000 cfu/ml of pure culture -> Report as 100,000.

b. Containing 100,000 cfu/ml of 2 types of organisms present -> Report as Mixed 100,000 with doubtful significant.
[Doubtful significant is included because there are 2 types of organisms present and the organism responsible for UTI is uncertain]

c. Containing 50,000 cfu/ml of pure culture -> Report as 50,000 and doubtful significant.
[In this case, even though the viable count is only 50,000 cfu/ml, it is a pure culture that may or may not suggest having UTI. Therefore further biochemical tests are still carried out and doubtful significant is reported together with the results.]

In summary, only plates with more than 100,000 cfu/ml [both pure and mixed cultures (2 types of microorganisms present)] and also pure cultures with 50,000 cfu/ml will be sent to the Investigation Laboratory to conduct biochemical / identification tests.


LeeJin
TG02