micro lab quiz all

Bright Field Microscopy

Uses objective and ocular lenses; resolving power decreases and less light is admitted as magnification increases, requiring oil immersion. Forms a dark image against a bright background.

Dark Field Microscopy

Uses a dark stop to alter the light path so only light coming from the specimen enters the objective. Forms a bright image with a dark background; ideal for unstained living organisms, internal structures, and spirochetes.

Simple Staining

Uses a single stain (crystal violet for 20–30 seconds) that binds to (-) charged molecules like nucleic acids and proteins; uses heat fixation to attach/kill cells without distortion. Enhances cell shape, size, and arrangement; 1000 total magnification reveals morphology (e.g., E. coli rods, M. luteus cocci).

Negative Staining

Mixes liquid suspension of cells with acidic stains (Nigrosin, India Ink, Eosin Blue, Congo Red); spreads on slide using a second slide at a 45 angle. Air dry only—do not heat fix. Repulsion between (-) stain and (-) cell surface leaves microbes unstained against a dark deposit background.

Gram Staining

Differential stain; heat-fixed smear is treated with crystal violet, iodine (mordant), ethanol decolorizer, and safranin counterstain. Gram (+) cells have thick peptidoglycan and appear purple; Gram-negative cells have thin peptidoglycan/outer membrane and appear pink.

Aseptic Technique

Transferring microorganisms without contamination. Loops/needles use a Baccinerator, glass pipettes use an autoclave, plastic pipettes go to biohazard/UV. Turbidity in broth indicates bacterial presence. Contamination happens if tubes stay open too long or heating is improper.

Spread Plate Technique

Pipette 0.1ml of sample into sterile saline, transfer to TSA plate, and spread with a hockey stick sterilized in ethanol and flamed.

Streak Plate Technique

Separates a petri dish into 4 quadrants, spreading from Q1 to Q4 and sterilizing the loop between each. Q1 has most growth; Q4 yields isolated pure colonies for transfer.

Blood Agar

Enriched differential medium used to test hemolytic patterns. beta-hemolysis = clear zone (complete breakdown); alpha-hemolysis = greenish halo (partial); gamma-hemolysis = no breakdown.

Mannitol Salt Agar (MSA)

Selective for Gram (+) Staphylococci (inhibits others) and differential for mannitol fermentation. Mannitol fermentation positive turns the medium yellow; negative remains pink.

EMB & MacConkey (MAC)

Selective for Gram (-) bacteria and differential for lactose fermentation. EMB: lactose (+) shows green metallic sheen, (-) is clear. MAC: lactose (+) is pink/red, (-) is cream.

Kirby-Bauer Antibiotic Test

Creates a bacterial lawn (swab rotated 60 after 3 streaks), applies antibiotic discs with sterile forceps, adds (+) control (bleach) and (-) control (disinfectant #20). larger zones of inhibition mean greater sensitivity.

Phenol Red Carbohydrate Broth

Detects fermentation of specific sugars (lactose, sucrose, dextrose) using Durham tubes. Yellow (acidic, pH < 6.8) + optional gas bubble. Red (neutral, pH 6.8–7.4) or purple-pink (basic). Do not shake/tip tubes (false bubbles).

Triple Sugar Iron (TSI) Agar

Tests catabolism of 1% lactose/sucrose, 0.1% glucose, and H2S production. Yellow butt = glucose only; Yellow butt & slant = multiple sugars; Gas lifts agar; Blackening = H2S. Red butt and slant = no sugars fermented.

Starch Hydrolysis

-Amylase breakdown of starch; Gram iodine added after incubation. Clear halo surrounding growth after adding iodine. Blue-to-brown complex right up to the streak line.

SIM Deep (Indole, H2S, Motility)

Tryptophanase (indole), cysteine desulfurase (H2S), and flagellar movement. Indole: Bright red surface line with Kovac's reagent. H2S: Black ferrous sulfide precipitate. Motility: Growth spreads away from stab line. No red line; no black precipitate; growth strictly restricted to stab line.

Methyl-Red (MR) Test

Detects mixed acid fermentation pathways from glucose broth. Red (acidic end products lower pH). Yellow (less acidic / butanediol fermenter).

Voges-Proskauer (VP) Test

Detects 2,3-butanediol / acetoin accumulation using glucose-phosphate broth, 40% KOH (Sol A), and 5% -naphthol (Sol B). Cherry red color within 20 minutes after vigorous vortexing/aeration. Clear / not red.

Citrate Test

Citrate permease; uses sodium citrate, ammonium salt, and bromothymol blue indicator. Blue (citrate oxidized, raises pH, sodium carbonate formed). Green (citrate not utilized).

Casein Hydrolysis

Proteolytic enzymes breaking down milk protein casein on skim milk agar plates. Zone of proteolysis (clear area surrounding colony). Absence of clear zone / opaque media.

Catalase Test

Catalase enzyme breaking down reactive oxygen species (H2O2 into H2O and O2). Immediate bubbles form upon adding hydrogen peroxide. No bubbles formed.

Oxidase Test

Cytochrome C oxidase using dry slides as artificial electron donors. Blue / Purple color reaction. Yellow / Orange color.

Urease Activity

Urease enzyme attacking amide bonds in urea to form ammonia and CO2. Hot pink (alkaline pH rises from 6.5 to 8.4 using phenol red). Orange / no color change.

Nitrate Reduction

Nitrate reductase (anaerobic respiration using NO3 terminal acceptor). Gas in Durham tube (N2); or red color after Reagents A & B (NO2); or clear after zinc dust (NH3). Red color only after adding zinc dust (means nitrate was never reduced).

EnteroPluri Test

15 biochemical tests performed simultaneously using a compartmentalized wire system. Poking oxygen holes and adding indole reagent to the H2S compartment yields a bright red positive ring. Standard negative color reactions across individual compartmen