Microbiology Media Quiz 2

Sulfur Indole Motility (SIM) (what are the following tests used to determine, how do they work, differential or selective or both, what does a positive or negative test look like)

• a semisolid, differential medium that differentiates different members of Enterobacteriaceae using 3 distinct biochemical characteristics

  • sulfur reduction test - ferrous ammonium sulfate (a metallic indicator) reacts with H2S to form a black precipitate (FeS or ferrous sulfide)

    • positive result = blackened medium (ex: P vulgaris, M morganii)

    • negative result = normal color (ex: E. coli, B subtilis, S marcescens, P aeruginosa, E faecalis)

  • indole production test - if the bacteria contains tryptophanase, it breaks down tryptophan into indole, pyruvic acid, and ammonia. after incubation, Kovac’s reagent is added to react with the indole

    • positive result = pink/red ring at the top (ex: E. coli, P vulgaris, M morganii)

    • negative result = normal color (ex: B subtilis, S marcescens, P aeruginosa, E faecalis)

  • motility test - low agar concentration allows flagellated bacteria to swim away from the stab line

    • positive result = cloudy growth that spreads outward from the stab line (ex: E. coli, P vulgaris, S marcescens, P aeruginosa, M morganii)

    • negative result = growth only stays near the stab line (ex: B subtilis, E faecalis)

Starch Agar (what are the following tests used to determine, how do they work, differential or selective or both, what does a positive or negative test look like)

• differential medium that distinguishes bacteria that can and cannot hydrolyze starch

  • can hydrolyze starch if the enzyme amylase is present

• the bacteria is streaked, incubated, and flooded with purple/black iodine (iodine stains starch, but not glucose)

  • positive result = clear halo forms around the bacterial growth (ex: B subtilis)

  • negative result = no halo present (ex: S marcescens, P aeruginosa, P vulgaris, E coli, E faecalis, M morganii)

MacConkey Agar (what are the following tests used to determine, how do they work, differential or selective or both, what does a positive or negative test look like)

• selective AND differential medium that contains bile salts, crystal violet dye, lactose, and a pH indicator

  • selects FOR Gram-negative bacteria b/c the bile salts + crystal violet dye inhibit the growth of Gram-positive bacteria

  • distinguishes between lactose-fermenting and non-lactose fermenting bacteria b/c the medium contains a pH indicator and lactose that can break down and release acid

• No growth = no colonies (ex: B subtilis, E faecalis)

• Non-lactose fermenter = colorless colonies (ex: S marcescens, P aeruginosa, P vulgaris, M morganii)

• Lactose fermenter = pink/red colonies (ex: E coli)

Eosin Methylene Blue (EMB) Agar (what are the following tests used to determine, how do they work, differential or selective or both, what does a positive or negative test look like)

• slightly selective AND differential medium that contains eosin (dye), methylene blue (dye), and lactose

  • selects FOR Gram-negative enteric bacteria b/c eosin and methylene blue inhibit the growth of Gram-positive bacteria

  • distinguishes between lactose-fermenting and non-lactose fermenting bacteria b/c the medium contains lactose that can break down and release acid, which absorbs the dyes to form colonies

• No growth = no colonies (ex: B subtilis, E faecalis)

• Non-lactose fermenter = colorless to amber colonies (ex: S marcescens, P aeruginosa, P vulgaris, M morganii)

• Lactose fermenter = blue/black colonies (gram negative)

  • strong lactose fermenters (like E. coli) form metallic green colonies

Nutrient Agar (what are the following tests used to determine, how do they work, differential or selective or both, what does a positive or negative test look like)

• generally used to grow and observe bacteria (doesn’t specifically test for anything)

• contains basic nutrients like peptone (amino acids + nitrogen), beef/yeast extract (vitamins + minerals), agar (allows for bacterial growth)

  • positive result = visible colonies

  • negative result = no visible colonies