Size of a Paramecium (width and length)?
~20-50 um wide; 50-300 um long.
Stereo microscope - how does it work & use?
Low magnification, 3D surface view. Used for large specimens.
Compound microscope - how does it work & use?
Uses visible light + lenses. Used for bacteria and stained specimens.
Darkfield microscope - principle & use?
Light scatters → bright specimen on dark background. Used for thin/live organisms.
Confocal microscope - principle & use?
Laser scans specimen → 3D image reconstruction.
Fluorescent microscope - principle & use?
Uses fluorescent dyes + UV light to detect specific structures.
Transmission Electron Microscope (TEM) - use?
Electrons pass through specimen → internal ultrastructure.
Scanning Electron Microscope (SEM) - use?
Electrons scan surface → detailed 3D surface image.
What organisms produce pellicle, cloudy medium, or sediment?
Pellicle = obligate aerobe
Cloudy = facultative anaerobe
Sediment = obligate anaerobe
What is agar and why ideal?
Polysaccharide from red algae. Not digested by bacteria. Melts high, solidifies at incubation temp.
Defined vs complex vs enriched media?
Defined = exact composition known
Complex = extracts (unknown exact composition)
Enriched = added blood/serum for fastidious organisms
Why sterilize culture media?
Prevent contamination.
Direct vs indirect stains?
Direct = positive dye binds negative cell Indirect = negative dye stains background
Why make thin bacterial smears?
Allows proper staining and visualization.
Two functions of heat fixing?
Kills bacteria and adheres them to slide.
Why is blood agar not selective?
It supports growth of many organisms (rich medium).
What happens if agar contains vancomycin?
Selective against Gram-positive bacteria.
Does 4x objective magnify 4 times?
Yes (objective alone is 4x).
What are MacConkey and EMB used for?
Selective for Gram-negative; differential for lactose fermentation.
🧫 MacConkey agar
✅ Used for:
• Selecting for Gram-negative bacteria
• Differentiating lactose fermenters vs. non-lactose fermenters
🔬 How it works:
Selective components:
• Bile salts + crystal violet → inhibit Gram-positive bacteria
Differential component:
• Lactose + neutral red (pH indicator)
🧪 Results:
• Lactose fermenter (like Escherichia coli) → Pink/red colonies
• Non-lactose fermenter (like Salmonella enterica) → Colorless colonies
—————————————————
🧫 Eosin Methylene Blue agar (EMB)
✅ Used for:
• Selecting for Gram-negative bacteria
• Differentiating lactose fermentation
🔬 How it works:
Selective components:
• Eosin Y + methylene blue dyes → inhibit Gram-positive bacteria
Differential component:
• Lactose
🧪 Results:
• Strong lactose fermenter (Escherichia coli) → Metallic green sheen
• Weak fermenter → Purple/pink
• Non-fermenter → Colorless
Define Koch’s postulates.
Find it → Isolate it → Cause it → Re-isolate it
Steps to prove microorganism causes a disease in late 1800s.
The 4 Postulates:
1️⃣ The microorganism must be found in all organisms with the disease, but not in healthy organisms.
2️⃣ The microorganism must be isolated from the diseased host and grown in pure culture.
3️⃣ The cultured microorganism should cause disease when introduced into a healthy, susceptible host.
4️⃣ The same microorganism must then be re-isolated from the newly diseased host and shown to be identical to the original.
⸻
🧪 Why This Was Important
Koch used these principles to prove that:
• Bacillus anthracis causes anthrax
• Mycobacterium tuberculosis causes tuberculosis
This helped establish the Germ Theory of Disease.
Define etiological agent.
The cause of a disease.
Who defined abiogenesis?
(life arising from nonliving matter)
Francesco Redi – disproved spontaneous generation in meat experiments
Louis Pasteur – definitively disproved it with swan-neck flask experiments
Joseph Lister is known for?
Antiseptic surgical techniques.
Alexander Fleming is known for?
Discovery of penicillin.
