Before I begin, I apologize for this late post and to make up for this, I will try to share my encounters weekly. Today, I shall start of with my very first week (22nd June - 27th June) in the lab. I title it, The Urine Bench.
The Urine Bench
Urine culture is a very common laboratory test performed to diagnose urinary tract infections (UTIs) which are frequently due to bacterial infections. Escherichia coli, Klebsiella, Enterobacter, Proteus and Pseudomonas sp. are the common culprits of UTIs.
Urine culture requires midstream urine as the specimen because the first bit of urine that passes out may be contaminated from the skin. This is to eliminate possible normal flora growth which can interfere in our identification of pathogenic microorganisms.
In the lab, I was tasked to perform urine cultures for the whole week. Specimens arrived in batches, either in the form of urine, fluid from the kidneys, or dipslides which has already been dipped in urine.
Urine specimen is used to perform aerobic cultures on a split blood agar plate and cysteine lactose-electrolyte-deficient (CLED) media. Direct plating is performed using a disposable 1 micron wired loop and the plates are later sent for incubation at 35°C overnight.
Percutaneous nephrostomy (PCN) is performed on fluid from kidneys. Procedures are similar to aerobic cultures of urine. The only difference is direct plating require the use of both 1 and 10 micron wired loop on 2 separate split blood/CLED agar plates.
A self drawn picture illustrating how streaking is done. Urine is streaked onto blood agar first, then onto the CLED agar. This is to allow trace back if there is any errors or contaminants encountered.
When dipslides are received, they are sent for incubation at 35°C overnight.
Picture of dipslides from http://www.solarbiologicals.com/pages/medical/images/cled-mac-2.JPG
One side is CLED media, the other can be MacConkey (MAC) agar or blood agar.
Blood agar is an enriched differential media that enhances the growth of fastidious microorganism.
CLED media, due to its electrolyte deficient property, prevents Proteus sp. from swarming. It also contains lactose which differentiates lactose fermenters (yellow colonies) from non-lactose fermenters (blue colonies).
MAC agar is a selective differential media that inhibit the growth of most Gram-positive bacteria to stain Gram-negative bacteria for lactose fermentation.
Dipslides are unpopular compared to plating on agar plates as their area is small, making it difficult to make observations. However, one advantage of dipslides is that it can be retained for 2 days before sending it for incubation, as compared to urine where it has to arrive fresh (not more than 24h if kept in refrigerator).
Well, that is all you will need to know for The Urine Bench! Any questions please do not feel hesitant to ask. Thank you.
Check back for the next post in the next couple of days.