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Microbiology: Food and Water Testing

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  • Components of Salmonella-Shigella (S-S) agar

    Peptones for growth, lactose as a fermentable sugar, neutral red as pH indicator, sodium thiosulfate and ferric citrate for H2S detection, and brilliant green to inhibit gram-positive bacteria.

  • Purpose of components in S-S agar

    Peptones support growth; lactose and neutral red detect fermentation; sodium thiosulfate and ferric citrate detect H2S production; brilliant green inhibits gram-positive bacteria.

  • Colony colors on S-S agar for Salmonella

    Salmonella colonies are colorless with black centers due to H2S production.

  • Colony colors on S-S agar for Shigella

    Shigella colonies are colorless and do not produce H2S, so no black centers.

  • Colony colors on S-S agar for fecal coliforms

    Fecal coliforms ferment lactose, producing pink to red colonies.

  • Use of phenol red-lactose broth in MPN method

    It detects lactose fermentation by color change from red to yellow, indicating gas production in Durham tubes.

  • Definition of Durham tube

    A small inverted tube inside a broth tube used to capture gas produced by microbial fermentation.

  • Reason for using different volumes of water in MPN tubes

    To increase accuracy and sensitivity by detecting bacteria at varying concentrations.

  • How to determine MPN from test results

    Use the number of positive tubes at each dilution and consult a reference MPN table to estimate bacterial concentration.