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Stool Culture | Salmonella and Shigella

Stool or rectal swab or stool (fresh random) in fecal transport system

Aim of the test
Detect bacterial pathogenic organisms in the stool; diagnose typhoid fever, enteric fever, bacillary dysentery, Salmonella infection
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
Gastrointestinal tract



  • Helicobacter pylori
  • Salmonella spp
  • E. coli O157:H7
  • Staphylococcus aureus
  • Campylobacter spp.
  • Vibrio cholerae
  • Yersinia enterocolitica
  • Clostridium difficile
  • Shigella spp.

Commensal flora
  • Enterobacteriaceae other than the common pathogens
  • Bacteroides spp
  • Streptococcus spp
  • Lactobacilli
  • Pseudomonas spp.
  • Coagulase negative staphylococci
  • Bacteroides
  • Clostridium
  • Peptostreptococcus

Specimen collection

A single stool specimen cannot be used to rule out bacteria as a cause of diarrhea. More than two specimens should only be submitted from patients for whom there is a high degree of suspicion.

The stool should be collected on collected in sterile bedpan. A sample is transferred with the sticks to the container. The specimen should contain at least 5 g of faeces and, if present, those parts that contain blood and/or mucus should be selected. The specimen should not be contaminated with urine. Close the lid.

Rectal swab: Pass swab beyond anal sphincter, carefully rotate, and withdraw. Swabbing of lesions of rectal wall or sigmoid colon during proctoscopy or sigmoidoscopy is preferred.
Duodenal or sigmoid aspirate: Specimen should be collected by a physician trained in this procedure

Quantity of specimen

The specimen should contain at least 5 g of faeces
Time relapse before processing the sample
Stool samples should be examined and cultured as soon as possible after collection. As the stool specimen cools, the drop in pH will inhibit the growth of most Shigella spp. and some Salmonella spp.
Storage: Refrigerated (2-8 °C)

Media

  • SSA or XLD
  • Selenite-F broth or tetrathionate

Reagents
API 20 E Kit
Salmonella and Shigella antiserum (polyvalent and monovalent)

Culturing procedure

1 gram of stool is transferred to a tube of Selenite-F broth and a loop is streaked on XLD or SSA. Incubate at 37 °C. After an overnight incuba incubation do subculture from Selenite-F broth onto a fresh plate of XLD or SSA.
Interfering factors
  • Patient on antibiotic therapy.
  • Improper sample collection.

Result reporting: A positive report will be issued only in case salmonella or shigella were isolated, otherwise, a negative report will be issued.

Turnaround time

Negative results are sent out 48 hours after receipt of the specimen. Results of positive cultures can be expected in 3-4days.
In enteric fever caused by Salmonella typhi, S. choleraesuis, or S. enteritidis, blood culture may be positive before stool cultures, and blood cultures are indicated early; urine cultures may also be helpful. Stool samples should be examined and cultured as soon as possible after collection. As the stool specimen cools, the drop in pH will inhibit the growth of most Shigella spp. and some Salmonella spp.

Diarrhea is common in patients with the acquired immunodeficiency syndrome (AIDS). It is frequently caused by the classic bacterial pathogens as well as unusual opportunistic bacterial pathogens and parasitic infestation. (Giardia, Cryptosporidium, and Entamoeba histolytica frequently reported.)

Cryptosporidium and Pneumocystis can occur with AIDS. Rectal swabs are useful for the diagnosis of Neisseria gonorrhoeae and Chlamydia infections. AIDS patients are also subject to cytomegalovirus, Salmonella, Campylobacter, Shigella, C. difficile, herpes, and Treponema pallidum gastrointestinal tract involvement.



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