microlab exam 2 real

Catalase test

Detects enzyme catalase that breaks down hydrogen peroxide into water and oxygen

Catalase positive

Staphylococcus (bubbles)

Catalase negative

Streptococcus (no bubbles)

Positive catalase result

Bubbling after adding H₂O₂

Negative catalase result

No bubbling

Mannitol Salt Agar (MSA)

Selective for Staphylococcus (high salt), differential for mannitol fermentation

MSA selective agent

7.5% NaCl (inhibits non-halophiles)

MSA differential agent

Mannitol and phenol red indicator

S. aureus on MSA

Yellow (ferments mannitol → acid)

S. epidermidis on MSA

Pink/red (no mannitol fermentation)

Phenol red indicator

Yellow in acid, red at neutral

MRSA

Antibiotic-resistant Staphylococcus aureus

Staph Latex Test

Detects Staph aureus using antibodies to Protein A and clumping factor

Positive Staph latex test

Visible clumping

Negative Staph latex test

No clumping

Blood Agar

Enriched and differential media showing hemolysis

Hemolysin

Enzyme that lyses red blood cells

Alpha hemolysis

Partial RBC lysis → green zone (S. pneumoniae)

Beta hemolysis

Complete RBC lysis → clear zone (S. pyogenes)

Gamma hemolysis

No lysis → no color change (E. faecalis)

Strep throat gold standard

Throat culture

Strep Latex Test

Detects carbohydrate antigen on Streptococcus using latex antibodies

Positive strep latex

Clumping

Nosocomial infection

Hospital-acquired infection

Most common nosocomial infections

UTIs, surgical site infections, bloodstream infections

Ventilator-associated pneumonia

Pneumonia from mechanical ventilation

MacConkey Agar

Selective for gram-negative, differential for lactose fermentation

MacConkey selective agents

Bile salts and crystal violet (inhibit gram-positive)

Lactose fermenters

Pink/red colonies (acid produced)

Non-lactose fermenters

Colorless colonies

pH indicator

Neutral red (turns pink with acid)

Oxidation-Fermentation (O-F) Test

Determines oxidative vs fermentative carbohydrate metabolism

Fermentative metabolism

Anaerobic, acid production (yellow)

Oxidative metabolism

Aerobic, CO₂ production (slight yellow or green)

Indicator

Bromthymol blue (yellow in acid, blue in base)

Mineral oil purpose

Creates anaerobic conditions for fermentation test

Oxidase Test

Detects cytochrome c oxidase enzyme

Oxidase positive

Pseudomonas aeruginosa (purple color)

Oxidase negative

E. coli (no color change)

Selective media

Allows growth of some bacteria, inhibits others

Differential media

Shows differences in bacteria based on metabolism

Enriched media

Contains extra nutrients for fastidious bacteria

Halophile

Organism that grows in high salt environments

Facultative anaerobe

Can grow with or without oxygen