Effect of Temperature of Incubation on Performance of Media in the Detection of Enteric Pathogens

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The effect of incubation temperatures on the efficacies of both plating media and transport or enrichment broths was determined by the analysis of 391 diarrheal stools for salmonellae and shigellae. Each analysis resulted in 90 observations. Stool specimens were homogenized in saline and used to inoculate eosin methylene blue (EMB), Salmonella-Shigella (SS), and xylose lysine deoxycholate (XLD) agar plates, Amies and Cary-Blair (CB) transport media, and gram-negative (GN) enrichment broth. All media were incubated at 25, 30, and 35 C for 24 and 48 h. In order of efficacy, GN and saline were significantly better than Amies and CB, which were still better than direct streaking for both salmonellae and shigellae. Forty-eight hours was a significant improvement over 24 h only at 25 C on direct streaking for both pathogens. Salmonella detection was also improved at 30 over 25 C on direct streaking. In direct plating, XLD was better than both SS and EMB for both pathogens. After broths, for salmonellae, XLD > SS > EMB, and for shigellae, XLD > EMB > SS, with all differences significant. SS agar was significantly improved for detection of shigellae with 48-h broth inocula versus 24-h broth inocula. The differences thus observed at the various temperatures tested proved to be less important than the media used. The efficient media, GN broth, saline-stool, and XLD were shown to be affected very little by either temperature or time variance of the magnitude tested.

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