Lab 3 Stool Cultures Flashcards
Which organisms cause GI infections?
Most common causative bacteria for GI infections (pathogens):
1. Salmonella spp.,
2. Shigella spp.,
3. E. coli O157,
4. Campylobacter spp.,
5. Yersinia spp.,
6. Aeromonas spp. (controversial, some labs do not culture for it)
–> are the ones that we mostly look for in routine cultures.
- Vibrio spp. are pathogens too but because they are not frequent in North America, they are investigated after a specific request.
Which is the typical “normal flora” for the GI tract?
GI normal flora includes:
1. Majority of anaerobic bacteria
2. Common facultative anaerobes:
- E. coli and
- other Enterobacterales (sometimes referred as “coliforms”),
- Enterococcus and - Streptococcus.
What amount should be collected for a stool culture to be performed? Container?
Approximately 1 to 2 grams of feces should be collected in a CLEAN, wide-mouthed container with a tight lid.
How should rectal swabs be transported?
Rectal swabs should be placed in a transport system containing modified Stuart’s medium, preferably with charcoal (charcoal neutralizes the toxic effects of fatty acids produced by some bacteria – Shigella are particularly susceptible).
What can be done if you need to delay planting a stool specimen?
If there is a delay in planting, specimens should be refrigerated at 4º to 6º C. Some labs recommend a modified Carey- Blair or other transport medium, which contains buffers; however, toxin testing cannot be done on these if needed.
How soon should an unpreserved stool specimen be processed?
Ideally, unpreserved stools should be processed 30 min after collection (but <2 hrs is ok), or go into a transport medium (Shigella is mostly compromised).
Note: Added <2 hrs from lecture notes.
What general contaminates (not other bacteria that is) are not acceptable in specimens?
Specimens contaminated with urine, barium or toilet paper are unacceptable.
Can the lab accept stool from patients that have been in the hospital for over 3 days?
No
What preservatives or transport mediums are not acceptable for stool specimens for culture?
Note acceptable are:
1. Specimens for bacterial investigation collected in a preservative for parasites (SAF)
2. Stool specimens in a transport medium with yellow phenol red indicator, indicating failure of the buffering system to maintain a neutral pH, and thus death of some organisms, especially Shigella
Are dry rectal swabs acceptable?
No
What do you do with a non-acceptable stool specimen?
IF UNACCEPTABLE SPECIMENS ARE RECEIVED,
1. A REJECTED SPECIMEN REPORT SHOULD BE SENT TO THE PHYSICIAN, CLINIC OR WARD.
2. HOLD THE SPECIMEN AT 4º to 6º C WHILE WAITING TO DISCARD IT IN CASE IT IS IRREPLACEBLE AND CRITICAL FOR THE PATIENT.
What is the traditional media for stool cultures?
BA
MAC
Sorbitol-MAC (SMAC)
XLD (could be Hektoen)
Campylobacter agar
Selenite Broth
When is TCBS agar used?
TCBS agar is used if the Dr. is looking for Vibrio spp. and it should be requested on the requisition.
What descriptions for appearance are used for stools?
Liquid, solid, watery, bloody, ‘rice-water.
Do you flame in-between quadrants after planting the stool sample?
Yes, when streaking for isolation.
What are the incubation requirements for most plates, campy plates?
- Incubate plates and enrichment broth at 35C for 18 to 24 hours.
- Incubate campy plates in a jar with a campy pack for microaerophilic incubation at 42C.
When and to what do you sub from the selenite broth?
After 18 hours the selenite broths need to be subbed to an XLD agar with a loopful.
