Medical Microbiology

Hi, this is Muna. For the past 4 weeks, including this week, Shameema, Jennifer and I have been posted to the Central Processing Area(CPA) in the Department of Diagnostic Bacteriology. Each week, we rotate to a different testing bench. We receive patient samples from the wards and clinics for testing.

I shall start by posting what I did last week - Urine Culture.

The purpose of urine culture is to isolate and identify potential pathogenic organisms from urine. This helps in the diagnosis of renal or urinary tract infections (UTI). Since urine is mostly sterile, any presence of microorganisms is usually clinically significant, unless it is contaminated by normal flora.

The samples we receive are usually mid-stream urine (MSU) for aerobic culture and dipslides. Mid-stream urine, as its name implies is collected in the middle part of the urination flow. The initial part is not collected as it usually contains the flushed out normal flora and cellular debris from the urethra and urinary tract. This may cause a false positive reading.

The samples are inoculated onto a split plate containing CLED agar and blood agar (BAP). CLED stands for Cystine Lactose Electrolyte Deficient agar, which can differentiate between lactose fermenters and lactose non-fermenters. Lactose fermenters produce a lower pH in the media, thus causing the indicator (bromothymol blue) to change from the original greenish yellow to yellow. It also enhances the growth of dwarf-colony coliforms.

Blood agar plates (BAP) supports growth of fastidious bacteria. It is also able to detect the pattern of bacterial haemolytic activity, which can determine the pathogenicity.

CLED and BAP split plate.


The streaking of plates for urine culture is unique as it does not follow the ocnventional way of streaking plates. I was taught that this method is used so that the pattern of growth can be obseved and the calculation of colony forming units per ml(cfu/ml) of urine can be counted easily. Also, instead of a 10 ul inoculating loop, a 1ul loop is used since bacterial presence in urine should be minimal or none in normal persons.

Urine culture streaking technique.

Normally, the BAP is streaked first, followed by the CLED plate so that any residual inhibitory substances picked up from the cLED plate in streaking does not affect the growth on the BAP.


NOTE: I shall continue where I left off soon. Need to clarify some doubts before continuing.