Microbiology Lab Techniques & Concepts

Vocabulary flashcards summarizing essential terms, structures, media, and tests discussed in the microbiology laboratory lecture.

Mycolic acid

Waxy lipid in acid-fast bacterial cell walls that makes them impermeable to most stains, including crystal violet.

Mycobacterium tuberculosis

Acid-fast bacterium that causes tuberculosis.

Capsule

Gelatinous outer layer that protects bacteria from phagocytosis, aids adhesion, and prevents dehydration.

Endospore

Dormant, highly resistant bacterial structure that ensures survival in heat, radiation, desiccation, and chemicals.

Flagella

Long, whip-like appendages used by bacteria for motility.

Acid-fast stain

Differential stain in which acid-fast cells appear fuchsia/red and non-acid-fast cells blue.

Schaeffer-Fulton endospore stain

Staining method that colors endospores green and vegetative cells pink/red.

Capsule stain

Negative stain revealing an unstained halo (capsule) around a stained cell against a dark background.

Flagella stain

Special stain that coats and thickens flagella so they appear as thin hair-like structures.

Streak Plate Method

Isolation technique that dilutes bacteria across agar surface to obtain individual colonies.

Pour Plate Method

Quantitative technique that mixes diluted bacteria with molten agar to count CFUs.

Colony-Forming Unit (CFU)

Single viable bacterium or clonal cluster that gives rise to one visible colony.

Pure culture

Culture containing only one known species of microorganism.

Mixed culture

Culture containing two or more known species.

Contaminated culture

Culture that has acquired unwanted or unknown organisms.

Selective media

Growth medium that favors some microbes while inhibiting others.

Differential media

Medium containing indicators that reveal metabolic differences via visible changes.

Mannitol Salt Agar (MSA)

Selective-differential medium with 7.5 % NaCl (selective) and mannitol + phenol red (differential).

Mannitol fermentation

Metabolism producing acid on MSA, turning phenol red indicator yellow.

Hemolysis

Bacterial lysis of red blood cells on blood agar.

Alpha-hemolysis

Partial hemolysis producing a greenish zone around colonies.

Beta-hemolysis

Complete hemolysis forming a clear zone around colonies.

Gamma-hemolysis

No hemolysis; red medium remains unchanged.

Salt preservation

High NaCl causes plasmolysis, inhibiting bacterial growth in foods like meat.

Acidophiles

Microbes that thrive at pH < 5.5.

Alkalinophiles

Microbes that thrive at pH > 8.

Neutrophiles

Microbes preferring near-neutral pH 6.5–7.5; includes most pathogens.

Psychrophiles

Cold-loving microbes growing optimally −5 °C to 15 °C; can grow in refrigerators.

Mesophiles

Microbes growing best at 25 °C–40 °C; most human pathogens.

Thermophiles

Heat-loving microbes with optima 45 °C–70 °C.

Thymine dimer

UV-induced covalent link between adjacent thymines causing DNA damage.

Obligate aerobe

Bacterium that requires oxygen and grows at the top of thioglycollate broth.

Facultative anaerobe

Bacterium that grows with or without O₂ but better with it; growth throughout broth, dense at top.

Obligate anaerobe

Bacterium harmed by oxygen; grows only at bottom of broth tube.

Aerotolerant anaerobe

Fermentative bacterium that ignores oxygen; uniform growth throughout broth.

Cytochrome C oxidase

Terminal electron-transport enzyme detected by oxidase test; purple color = positive.

Phenol red broth

Fermentation medium with pH indicator phenol red; yellow = acid production.

Durham tube

Inverted vial inside broth that traps gas produced during fermentation.

Catalase

Enzyme that degrades hydrogen peroxide into water and oxygen; bubbling = positive test.

Oxidase-positive

Bacterial trait indicated by rapid purple color on oxidase reagent.

Catalase-positive

Bacterial trait shown by immediate bubbling when H₂O₂ is added.

Bacterial transformation

Uptake of foreign DNA by a bacterium, leading to a new genotype/phenotype.

Plasmid

Small, circular, self-replicating DNA molecule carrying accessory genes.

Competent cell

Bacterium capable of taking up extracellular DNA.

bla gene

Plasmid gene encoding beta-lactamase, conferring ampicillin resistance.

gfp gene

Gene coding for green fluorescent protein, which glows under UV light.

araC gene

Regulatory gene whose protein activates gfp transcription in the presence of arabinose.

Beta-lactamase

Enzyme that hydrolyzes ampicillin, providing antibiotic resistance.

Green Fluorescent Protein (GFP)

Protein that emits green fluorescence under UV illumination.

Arabinose

Sugar that induces gfp expression by activating AraC regulator.

Ampicillin resistance

Phenotype allowing bacterial growth in the presence of ampicillin.

Calcium chloride treatment

Chemical step that increases membrane permeability during artificial competence.

Heat shock

Rapid temperature change facilitating plasmid entry into competent bacteria.

Thioglycollate broth

Medium creating an oxygen gradient to test bacterial aerotolerance.