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2 more weeks and we have to bid goodbye to our respective lab. But, whatever it is, we still have to get back to school and graduate. OKAY, I am still going to discuss more about what I have learnt in my Microbiology laboratory, and here goes...
Do you know that every food handler (hawker stall helper, canteen attendant, Prima-deli) as long as they handle food that me, you and everyone consume have to undergo this food handler screening, where enteric pathogens are screened from their urine and stool samples. In my lab, it is known as Thyphoid culture
Purpose
It is important to undergo this test as the enteric pathogens can be transmitted through means of water and food easily. It is also useful in monitoring the food-handlers hygiene status so that suitable measures can be carried out to prevent and stop any outbreak of enteric pathogens.
Enteric pathogens?
For this screening, enteric pathogens that are screened will be Salmonella sp., Shigella sp., Vibrio sp. that are related to foodborne illness. They can usually be found in patients' stool and urine sample.
Salmonella sp. can be found on meats, infected eggs and milk when not handled properly or cooked totally.
Shigella sp. can be usually found in water and it is also spreaded through fecal-oral route (hand-to mouth) and this usually gives an idea of the hygienic status of the patient or the environment that he/she is in
Vibrio sp. can be found in salt water and usually carried by seafood such as crab, prawns and mussels. Infection could occur is these seafoods are not properly handled and consumed.
How to carry out the test?
Patients' stool and urine samples are send in in sterile and clean bottles in order to have uncontaminated culture results. There are a few types of medium used to culture the samples, they are:
Selenite Broth - Enrichment medium where the stool and urine samples are inoculated into and incubated overnight for the growth of Salmonella sp. It contains peptone, lactose, sodium selenite that enables enrichment of the organism.
XLD ( Xylose Lysine Deoxychalate) Agar - for the selective growth and isolation of Salmonella sp. and Shigella sp. after the incubation in Selenite broth overnight. This agar contains yeast extract, L-Lysine, Xylose, Lactose, Sucrose, Sodium deoxychalate, Sodium Chloride, Sodium thiosulfate, Ferric ammonium citrate, Phenol red that make up this selective medium for the growth.
The phenol red indicator in the medium indicates its neutral pH and any pH changes will change its colour. Salmonella known to ferment the sugar xylose into acid will alter the indicator colour to yellow as there is a drop of pH forming yellow colonies. Shigella will not ferment sugar and therefore remain red forming red colonies. However, red colonies can be formed when all the xylose is metabolised and lysine is decaroxylised that will increase the pH value and forming the red colonies. Some characteristics to differentiate Salmonella and Shigella is the break down of thiosulfate by Salmonella sp. to produce hydrogen sulfide (H2S) which is the black centers of the colonies.
Therefore, after one night of incubation in Selenite broth, the broth will be plated onto the XLD agar and incubated overnight again for the growth of the organism. Then it will be read on the 3rd day after the sample was send in and then reported for the organisms identified through the macroscopic characteristics (black center colonies etc) and biochemical test (Indole, Catalase etc)
Any questions?
Comment (:
LESLIE
7 comments
BLACKBOARD
Group 2- tg01group2
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Group 6- fluidcollectors
Group 7- codec-5
Group 8- madtechs
Group 9- de-incredibles
Group 10- bloody-immortals
Ms Chew