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Microbiology: Bacteriology: Gram-Negative Identification Flashcards

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  1. Which gram-negative organism is identified by the 'string of pearls' appearance when cultured on a wet mount and causes melioidosis?

    Answer: Burkholderia pseudomallei

    Burkholderia pseudomallei causes melioidosis, is endemic in Southeast Asia and northern Australia, and is a Tier 1 select agent recognizable by its bipolar staining pattern.

  2. Which virulence factor of enterohemorrhagic E. coli (EHEC, O157:H7) directly causes hemolytic uremic syndrome?

    Answer: Shiga toxin (Stx)

    Shiga toxin (Stx1 and Stx2) produced by EHEC inhibits protein synthesis in endothelial cells, causing thrombotic microangiopathy that leads to HUS.

  3. On TCBS (Thiosulfate-Citrate-Bile-Sucrose) agar, which organism produces yellow colonies indicating sucrose fermentation?

    Answer: Vibrio cholerae

    Vibrio cholerae ferments sucrose and produces yellow colonies on TCBS agar, while Vibrio parahaemolyticus does not ferment sucrose and produces blue-green colonies.

  4. Which gram-negative rod is the causative agent of tularemia and requires biosafety level 3 precautions in the laboratory?

    Answer: Francisella tularensis

    Francisella tularensis causes tularemia ('rabbit fever'), is highly infectious via aerosol, and is classified as a Tier 1 select agent requiring BSL-3 handling.

  5. The IMViC test pattern of '+ + - -' (Indole+, MR+, VP-, Citrate-) is characteristic of which organism?

    Answer: Escherichia coli

    E. coli has an IMViC pattern of + + - - (indole positive, methyl red positive, Voges-Proskauer negative, citrate negative).

  6. Which gram-negative rod produces a characteristic red pigment (prodigiosin) and is associated with opportunistic infections in immunocompromised patients?

    Answer: Serratia marcescens

    Serratia marcescens produces the red pigment prodigiosin at room temperature; it is an opportunistic pathogen causing UTIs, pneumonia, and septicemia in hospital settings.

  7. Bordetella pertussis is best isolated from nasopharyngeal specimens using which selective medium?

    Answer: Bordet-Gengou agar

    Bordet-Gengou agar (potato-glycerol-blood agar) with methicillin added selectively supports Bordetella pertussis growth, producing pearl-like 'mercury drop' colonies.