Gram Positive and Negative Biochemical Tests

Mannitol Salt Agar (MSA) Selectivity

Selective for halophiles (organisms thriving in high salt concentrations).

MSA Carbohydrate & Indicator

Contains mannitol (carbohydrate) and phenol red (pH indicator).

MSA Positive Result

Turns yellow, indicating mannitol fermentation producing acid.

DNase Enzyme Function

Hydrolyzes polymerized DNA into oligonucleotides or nucleotides.

DNase Positive Result

A clear zone of no coloration around growth on green methyl green agar.

Bacitracin Mechanism

Inhibits bacterial cell wall synthesis.

Novobiocin Mechanism

Inhibits nucleic acid synthesis.

Hemolysins

Exotoxins produced by Gram-positive cocci that lyse red blood cells.

Hemolysis Types

Beta (complete clearing), Alpha (partial/greenish), Gamma (no hemolysis).

Catalase Reaction

Converts hydrogen peroxide into water and oxygen gas (bubbling is positive).

Bile Esculin Agar (BEA) Selectivity

Selective for organisms that can survive in oxbile (e.g., enterococci).

BEA Positive Indicator

Esculetin reacts with iron to produce a dark brown/black precipitate.

Snyder Test Purpose

Measures glucose fermentation to acid to estimate susceptibility to dental caries.

Snyder Test pH Indicator

Bromcresol green.

Turbidity in TSB

Cloudiness in tryptic soy broth indicating microbial growth.

MacConkey Agar Differentiation

Differentiates between organisms that can and cannot ferment lactose.

EMB Agar Selectivity

Selective for Gram-negative organisms.

EMB Lactose Fermentation

Produces a green metallic sheen or purple/black/pink colonies.

Durham Tube

A small inverted tube in phenol red broth to detect gas production.

Methyl Red (MR) Test

Determines if an organism is a mixed acid fermenter (turns red).

Voges-Proskauer (VP) Test

Detects if glucose fermentation products are converted to acetoin.

Simmons Citrate Test Positive

Turns blue, indicating citrate utilization and ammonium hydroxide production.

Deaminase vs. Decarboxylase

Deaminase removes amino groups (-NH₂); decarboxylase removes carboxyl groups (-COOH).

LIA Slant Deamination Location

Observed on the slant (red slant = positive).

LIA Slant Decarboxylation Location

Observed *********** (purple butt = positive).

Positive Indole Test

Formation of a red ring at the top after adding Kovac's reagent.

Positive Urease Test

Turns bright pink due to ammonia production raising the pH.

Phenylalanine Deaminase Reagent

Ferric chloride, which turns green in the presence of phenylpyruvic acid.