Microbiology Laboratory Tests and Differential Media

Vocabulary and definitions for standardized microbiology laboratory biochemical tests and differential growth media based on class lecture notes.

Lysine/Ornithine Decarboxylase Broths (LD/OD)

Differential media used to detect glucose fermentation and the production of decarboxylase enzymes.

Bromocresol purple

The pH indicator used in Lysine/Ornithine Decarboxylase Broths; it turns yellow in acidic conditions and purple in alkaline conditions.

Cadaverine

The alkaline product yielded by the decarboxylation of lysine.

Putrescine

The alkaline product yielded by the decarboxylation of ornithine.

Sterile mineral oil

A layer placed over inoculated LD/OD tubes to provide an anaerobic condition required for the reaction.

Urease

An enzyme produced by an organism that breaks down urea into ammonia and $CO_2$, resulting in an alkaline pH and a hot pink color in Urea Broth.

Triple Sugar Iron Slant (TSI)

A differential medium containing lactose, sucrose, and glucose that identifies carbohydrate fermentation and $H_2S$ production.

TSI Slant Reaction

Represents lactose and/or sucrose fermentation; Yellow (A) indicates acid/fermentation, and Pink (K) indicates alkaline/no fermentation.

TSI Butt Reaction

Represents glucose fermentation; Yellow (A) indicates acid/fermentation.

SIM tubes

Media used to test for Sulfide production (S), Indole production (I), and Motility (M).

Kovac's reagent

The reagent added to SIM tubes (8-10 drops) to detect indole; a red layer indicates a positive result.

Tryptophanase

The enzyme required to break down tryptophan into indole as a by-product.

MR (Methyl Red) test

A test that detects organisms capable of performing mixed acid fermentation to overcome a phosphate buffer, turning the broth red.

VP (Voges-Proskauer) test

A test that detects glucose fermenters that convert acids to acetoin and $2,3\text{ butanediol}$; a red layer indicates a positive conversion.

MacConkey agar (MAC)

A selective and differential medium used to isolate Gram-negative organisms and differentiate lactose fermenters.

Neutral red

The pH indicator used in MacConkey agar; lactose fermenting colonies appear hot pink.

Durham tube

A small inverted tube placed in Phenol Red Broths to detect gas production ($G+$).

Citrase

An enzyme that allows an organism to use sodium citrate as a carbon source and ammonium phosphate as a nitrogen source, turning Citrate slants blue.

Bromthymol blue

The pH indicator used in Citrate slants; turns blue in alkaline conditions ($+$, citrase present).

Mannitol Salt Agar (MSA)

A selective and differential medium containing 7.5\text{% }NaCl for isolating Staphylococcus species and mannitol for differentiating fermenters.

Bile Esculin Azide Agar (BEA)

Selective and differential medium where esculin breakdown results in blackening; inhibits most Gram-positives except $S. bovis$ and $E. faecalis$.

6.5\text{% }NaCl broth

A selective medium used to differentiate salt-tolerant Enterococcus faecalis from Streptococcus species based on turbidity.

Catalase test

An enzyme test where the application of hydrogen peroxide ($H_2O_2$) results in vigorous bubbling if the organism is positive.

Alpha-hemolysis

Incomplete breakdown of hemoglobin on Blood agar, resulting in a greening of the medium around growth.

Beta-hemolysis

Complete breakdown of hemoglobin and red blood cells resulting in a clearing of the Blood agar medium.

DNase

An enzyme that breaks down the DNA-methyl green complex in DNA agar, resulting in a halo or clearing around the growth.