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Microbiology: Bacteriology: Gram-Negative Identification Flashcards

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  1. Proteus mirabilis is best identified in the clinical lab by which characteristic feature on blood agar?

    Answer: Swarming motility

    Proteus mirabilis exhibits swarming motility on blood agar due to its abundant peritrichous flagella, creating concentric rings across the plate.

  2. Which gram-negative rod produces both indole and H2S, helping distinguish it from other Enterobacteriaceae?

    Answer: Citrobacter freundii

    Citrobacter freundii is indole-variable but H2S-positive, and some strains produce both; it uniquely combines these traits among common Enterobacteriaceae.

  3. Which selective medium is used specifically to isolate Campylobacter species from stool specimens?

    Answer: Campy-BAP (Campylobacter Blood Agar Plate)

    Campylobacter Blood Agar Plate (Campy-BAP) contains antibiotics (cephalothin, vancomycin, trimethoprim, amphotericin B, polymyxin B) to suppress normal flora while allowing Campylobacter to grow.

  4. The O, H, and K antigens of gram-negative bacteria correspond to which structures, respectively?

    Answer: LPS, flagella, capsule

    O antigen is the LPS somatic antigen, H antigen is the flagellar antigen, and K antigen is the capsular polysaccharide antigen.

  5. A gram-negative, comma-shaped rod that is oxidase-positive and produces a urease rapidly (within minutes), isolated from a gastric biopsy, is most likely:

    Answer: Helicobacter pylori

    Helicobacter pylori's rapid urease activity (used in the CLO test and urea breath test) is its hallmark identification feature, along with its corkscrew morphology and gastric habitat.

  6. Which gram-negative organism is encapsulated, non-motile, and a major cause of lobar pneumonia with 'currant jelly' sputum in alcoholics?

    Answer: Klebsiella pneumoniae

    Klebsiella pneumoniae causes a severe, necrotizing lobar pneumonia with bloody, thick 'currant jelly' sputum, classically in alcoholics and diabetics.

  7. Which test is used to detect beta-lactamase production in Haemophilus influenzae and Neisseria gonorrhoeae in the clinical lab?

    Answer: Nitrocefin chromogenic test

    The nitrocefin test uses a chromogenic cephalosporin that turns red when cleaved by beta-lactamase, providing rapid detection of beta-lactamase-producing strains.