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1. Large, gray-white colonies with a double zone of beta-hemolysis on anaerobic blood agar. Gram stain reveals large, boxcar-shaped Gram-positive rods.
Listeria monocytogenes
Bacillus cereus
Clostridium difficile
Clostridium perfringens
2. Small, creamy, beta-hemolytic colonies grow on blood agar. The organism is catalase positive, coagulase positive, and Gram stain reveals Gram-positive cocci in clusters.
Enterococcus faecalis
Staphylococcus epidermidis
Micrococcus luteus
Staphylococcus aureus
3. A colony shows swarming motility on blood agar. The organism is oxidase negative and urease positive. It produces hydrogen sulfide on TSI agar.
Escherichia coli
Salmonella enteritidis
Proteus vulgaris
Proteus mirabilis
4. Lactose-fermenting pink colonies grow on MacConkey agar. The organism is indole positive, oxidase negative, and Gram-negative rods are seen on staining.
Citrobacter freundii
Klebsiella pneumoniae
Escherichia coli
Shigella spp.
5. Tiny gray colonies grow on chocolate agar but not on blood agar. Gram-negative coccobacilli are seen on Gram stain. The isolate requires both X and V factors.
Neisseria gonorrhoeae
Haemophilus influenzae
Neisseria meningitidis
Moraxella catarrhalis
6. Alpha-hemolytic colonies appear on blood agar. The isolate is catalase negative, optochin sensitive, and bile soluble. Gram-positive lancet-shaped diplococci are seen.
Streptococcus pneumoniae
Streptococcus pyogenes
Enterococcus gallolyticus
Viridans group streptococci
7. Beta-hemolytic colonies appear on blood agar. Catalase negative, bacitracin sensitive, Gram-positive cocci in chains.
Staphylococcus saprophyticus
Streptococcus pyogenes
Streptococcus agalactiae
Enterococcus faecium
8. Colonies have a mucoid appearance on MacConkey agar. The organism is urease positive, indole negative, oxidase negative.
Klebsiella oxytoca
Shigella sonnei
Proteus vulgaris
Klebsiella pneumoniae
9. A sputum culture yields colonies with a “breadcrumb-like” appearance. The organism is weakly acid-fast and filamentous on Gram stain.
Mycobacterium tuberculosis
Actinomyces israelii
Nocardia spp.
Corynebacterium diphtheriae
10. A patient with HIV presents with meningitis. India ink staining of CSF reveals encapsulated yeast. Colonies grow on Sabouraud Dextrose Agar (SDA) and are urease positive.