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Stool Culture, E. coli O157:H7

Detect E. coli O157:H7 from stool specimen or rectal swab and perform sensitivity test

Detect E. coli O157:H7 from stool specimen or rectal swab and perform sensitivity test. The Latex test will demonstrate by slide agglutination, E. coli strains possessing the somatic O157 antigen and Flagellar H7 antigen.
Types of specimen
Stool or rectal swab or stool (fresh random) in fecal transport system

Criteria of specimen rejection

Formed stool, specimen contaminated with urine, residual soap, or disinfectants. Specimens received in grossly leaking transport containers; diapers; dry specimens; specimens submitted in fixative or additives
Time relapse before processing the sample
Stool samples should be examined and cultured as soon as possible after collection.
Storage: Refrigerated (2-8 °C)
Media: Sorbitol MacConkey Agar (SMAC)

Culturing procedure

A loopful of stool is streaked on Sorbitol MacConkey. Incubate at 37 °C. Under aerobic conditions. Examine plates for non-sorbitol fermenting colonies (NSF). NSF colonies may be taken from SMAC plates or alternatively NSF isolates may be inoculated onto non-selective agar media for testing. It is necessary to test up to 10 separate NSF colonies to ensure a high probability of detection from mixed cultures.


1) Bring the latex reagents to room temperature. Make sure the latex suspensions are mixed by vigorous shaking. Expel any latex from the dropper pipette for complete mixing.
2) Dispense 1 drop of the Test latex onto a circle of the black slide. Place it close to the edge of the circle.
3) Add some loopfuls or a Pasteur pipette drop of saline to the circle. Ensure that the latex and saline do not mix at this stage.
4) Using a loop, pick off a portion of the colony to be tested and carefully emulsify in the saline drop.
5) Mix the Test latex and suspension together and spread to cover most of the reaction area using the loop. Flame the loop.
6) Rock the slide in a circular motion, observing for agglutination. Do not rock the card for more than 1 minute and do not use a magnifying glass.
7) If no agglutination occurs, then proceed to test other NSF colonies if these are present.
8) If agglutination with the test reagent does occur, then it is necessary to test a further portion of the colony with the control reagent to ensure that the isolate is not an auto agglutinating strain.
9) When finished, dispose of the reaction slide into disinfectant.


a) Positive result - Agglutination of the Test latex occurs within 1 minute. No agglutination of the Control latex. Perform biochemical tests to confirm that the organism is an E. coli strain.
b) Negative result - no agglutination of the Test latex.
c) Non-interpretable result - clumping of the Control latex.

Interfering factors

Patient on antibiotic therapy.
Improper sample collection.
Result reporting
A positive report will be issued only in case E. coli O157:H7 were isolated, otherwise, a negative report will be issued.
Turnaround time
Negative results are sent out 72 hours after receipt of the specimen. Results of positive cultures can be expected in 4-5days.

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