Microlab quiz

• Medium useful for distinguishing between Enterococcus faecalis (also known as Group D streptococcus) and Staphylococcus epidermidis.
• Streaking the surface of a slant:
• Enterococcus will turn the medium black (positive)
• S. epidermidis will grow poorly; medium will not change color (light tan / negative). --> may change to purple after 48 hours (false positive).

• Identifies bacteria that produce the enzyme catalase and are able to degrade hydrogen peroxide.
• Used to distinguish between Staphylococcus (+) and Streptococcus (-).
• Positive test = immediate, vigorous bubbling
• Negative test = no bubbling, or slight bubbling

• Identifies bacteria that produces the extracellular enzyme coagulase by its ability to clot plasma.
• Used to distinguish between Staphylococcus aureus (+) and other staphylococci (-)

• Identifies bacteria that produce the enzyme cytochrome oxidase and are able to oxidize a dye to a colored product.
• Oxidase positive = purple color develops by 10 seconds
• Slow oxidase positive = purple color develops after ~1 minute (false positive; should be considered negative)
• Oxidase negative = no color develops

• Bacitracin (sensitivity) test
• Used to differentiate group A streptococci from other beta-hemolytic non-group A streptococci.
• Sensitive: any zone of inhibition
• Resistant: growth up to the disc

• Optochin (sensitivity) test
• Used to differentiate Streptococcus pneumoniae from other alpha-hemolytic streptococci.

Test for presence of antigen

Test for presence of specific antibody

• Has type A antigen
• Makes anti-B antibody
• Will agglutinate with anti-A antiserum

• Has type B antigen
• Makes anti-A antibody
• Will agglutinate with anti-B antiserum

• Has neither A nor B antigens
• Makes both anti-A and anti-B antibodies
• Will not agglutinate with anti-A nor anti-B antiserum

• Has both A and B antigens
• Makes neither anti-A nor anti-B antibodies
• Will agglutinate with both anti-A and anti-B antiserum

Stains blue/purple

Stains pink/red

Smear prepared from broth, isolated colonies or clinical specimen. Slide is heat fixed.

Flooded with basic dye (crystal violet) and rinsed with water. All cells appear blue.

Mordant (substance that reacts with dye making it more difficult to decolorize; usually iodine) is put on the smear.

Smear is decolorized by dropping 95% alcohol across surface of slide. G+ cells will be blue. G- cells are colorless.

Counter stain (usually red dye safranin) is added. G- cells are stained and appear red. G+ cells are unaffected and appear blue/purple.

• Medium selective for Gram - bacteria and differentiates them for lactose fermentation.
• Lactose fermentation + = pink
• Lactose fermentation - = yellow

Partial hemolysis; dark/greenish

Complete hemolysis; transparent

No hemolysis

• Triple Sugar Iron
• Tests several characteristics:
• -Fermentation of glucose, lactose and/or sucrose
• -Production of gas
• -Production of hydrogen sulfide (H2S)
• Slant/Butt (K = alkaline; A = acid)
• K/A = only glucose fermenter
• A/A = lactose and/or sucrose fermenter
• K/K = no carbohydrate fermentation
• Blackening in butt = hydrogen sulfide produced

• Motility-Indole-Ornithine
• Identify members of the Enterobacteriaceae on the basis of motility, indole and ornithine decarboxylase

• Positive: organisms mibrate from lab line to medium (appear cloudy)
• Negative: growth along stab line only

• Positive: red color
• Negative: no red develops

• Positive: turbid purple to faded out yellowish gray at bottom of tube
• Negative: bright, clear yellow at the bottom of the tube