Comprehensive Microbiology Laboratory Manual and Definitive Study Guide

Institutional Framework and Platform Overview

• Platform and Dissemination:
  • The resource is disseminated via the Open Education Resource (OER) LibreTexts Project, supported by NICE CXone Expert.
  • Key supporting institutions include the Department of Education Open Textbook Pilot Project, the California Education Learning Lab, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot.
  • Supported by National Science Foundation (NSF) Grant Nos. 1246120, 1525057, and 1413739.
  • Content licensing is primarily designated under CC BY-SA 4.0 ($93.1\%$ of pages; $6.9\%$ undeclared).
  • Compilation Date: 08/22/2026.

Experiment 1: Introduction to the Microbiology Laboratory

• Learning Outcomes:

  • Articulate safety procedures specific to the microbiology laboratory.
  • Demonstrate proper handwashing and surface disinfection protocols.
  • Categorize and dispose of lab waste correctly across compost, biohazard, used slides, culture tubes, and Petri dish waste streams.
  • Identify required student supplies for purchase by Week 2.
  • Explain the necessity of in-person microbiology laboratory training for professional healthcare programs.

• General Laboratory Safety Rules:

  • Read all lab manual material prior to class to ensure thorough preparation.
  • Know the precise locations of emergency exits, eyewash stations (including ADA-compliant units), fire blankets, fire extinguishers, first aid kits, and broken glass containers.
  • Wear a long-sleeved lab coat covering below the waist whenever working with microbes; wash it regularly at home in hot water with a small amount of bleach.
  • Wear closed-toed shoes at all times (no flip-flops or open sandals permitted).
  • Wear disposable nitrile or latex gloves during microbial manipulation and staining procedures.
  • Turn off Bunsen burners immediately when not in active use.
  • Report pregnancy or other medical conditions requiring special precautions to the instructor.
  • Tie long hair back securely.
  • Conduct no unapproved original or optional experiments.
  • Never taste any chemicals or return unused chemicals to stock bottles.
  • Report broken glass immediately to the instructor; never handle or clean up broken glass independently.
  • Dispose of non-sharp items contaminated with blood, bacteria, or body fluids (e.g., plastic Petri dishes, plastic Pasteur pipettes) in an autoclave bag for steam decontamination, or immerse in a $10\%$ bleach solution prior to disposal/reuse.
  • Sanitize lab benches before and after each laboratory session using disinfectant from red squirt bottles; allow the disinfectant to air-dry completely via evaporation rather than wiping dry.
  • Clean and return all equipment to storage before leaving.
  • Report all spills, accidents, or injuries immediately.
  • Never sit on lab benches.
  • Wear safety goggles when staining bacteria or handling chemical reagents.
  • Keep backpacks and purses out of high-traffic walkways.

• Decontamination and Culture Protection Protocols:

  • Keep culture tubes in an upright position inside culture tube racks at all times; never lay them flat on the benchtop.
  • In the event of a culture spill on the bench, cover the spill with paper towels, soak the towels completely with disinfectant, allow a reaction time of $20\,\text{minutes}$, and discard the towels in the red biohazard bin.
  • If a microbial culture splashes into the eyes, immediately flush eyes at the eyewash station for a minimum of $15\,\text{minutes}$.

• Waste Categorization and Segregation:

  • Green Bins: Non-contaminated paper towels (compostable), including paper towels containing stain residues or sanitizing solutions.
  • Grey or White Trash Bins: Gloves and general non-recyclable, non-contaminated trash.
  • Metal Biohazard Bin / Biohazard Bag: Non-sharp biohazardous items (e.g., contaminated gloves, paper towels, plastic Petri dishes, cotton swabs).
  • Hard-Sided Sharps Container: All sharp objects, including lancets, needles, and microscope slides.
  • Glass Culture Tubes (Autoclave Bin): Remove all Sharpie markings from glass tubes using a cotton ball soaked in alcohol/acetone, then place tubes upright in designated metal autoclave bins to prevent liquid spillage.

• Biosafety Levels (BSL):

  • BSL-1: Microorganisms not known to cause disease in healthy adult humans (e.g., Escherichia coli lab strains, Bacillus subtilis, Lactococcus lactis).
  • BSL-2: Agents associated with human disease, presenting moderate environmental or health hazards. Transmission routes include ingestion, inhalation, or skin penetration (percutaneous). Treatments and vaccines are typically available (e.g., Salmonella enterica, Staphylococcus aureus).
  • BSL-3 & BSL-4: High-risk, exotic pathogens causing severe or lethal disease; require high-containment specialized facilities.
  • City College of San Francisco (CCSF) laboratory operations are strictly restricted to BSL-1 and BSL-2 organisms.

• Required Student Supplies (Required by Week 2):

  • Long-sleeved shirt or lab coat extending below the waist.
  • Nitrile or latex gloves.
  • Hair ties / rubber bands.
  • Colored pencils or pens (must include purple, pink, red, green, and blue).
  • Thick-point Sharpie permanent marker or wax pencil.
  • One box of microscope slides per student.
  • Blue plastic storage box for microscope slides (CCSF Bookstore).
  • Lens paper and bibulous (blotting) paper.
  • Lab safety goggles.

Experiment 2: The Microscope

• Learning Outcomes:

  • Set up and visualize specimens using bright-field optics under $4\times$, $10\times$, and $40\times$ magnifications.
  • Calculate total magnification across objective and ocular lens combinations.
  • Utilize scanning power ($4\times$) to locate specimens efficiently.
  • Apply depth of focus concepts to determine relative spatial orientation (top, middle, bottom) in crossed silk fiber samples.
  • Focus on fixed preparations of free-living protozoa (Paramecium, Euglena, Amoeba).

• Microscope Mechanics and Optics:

  • Bright-field compound light microscopes refract light through multiple lenses to generate a magnified virtual image.
  • Light pathway: Base Light Source $\rightarrow$ Substage Condenser (focuses a cone of light onto specimen) $\rightarrow$ Specimen $\rightarrow$ Objective Lens $\rightarrow$ Ocular Lenses ($10\times$ magnification) $\rightarrow$ Retina.
  • Light intensity is regulated via the base light intensity control knob and the substage iris diaphragm.

• Objective Magnification and Applications:

  • Scanning Objective ($4\times$): Total magnification = $40\times$. Used for initial scanning and low-power field observation.
  • Low Power Objective ($10\times$): Total magnification = $100\times$. Used for medium-power observations.
  • High Dry Objective ($40\times$): Total magnification = $400\times$. Used for high-power detailed eukaryotic observation.
  • Oil Immersion Lens ($100\times$): Total magnification = $1000\times$. Highest power; required for bacterial visualization.
  • Total Magnification Formula: $\text{Total Magnification} = \text{Objective Magnification} \times \text{Ocular Magnification}$.

• Field of View, Depth of Focus, and Parfocality:

  • Field of View: The circular visible area when looking through oculars; inversely proportional to magnification.
  • Depth of Focus: The vertical thickness of the specimen that remains sharply in focus. Decreases as magnification increases.
  • Parfocal Lenses: Lenses calibrated so that when a specimen is focused with one objective (e.g., $4\times$), switching to higher objectives requires only minor adjustments using the fine focus knob.
  • Rule: Never use the coarse focus knob after moving past the $4\times$ scanning objective.

• Resolution vs. Magnification:

  • Resolution (Resolving Power): The minimum distance required between two distinct points for them to be distinguished as separate entities. Governed by light wavelength and numerical aperture.
  • Short wavelengths increase resolving power.
  • Compound light microscopes: Limit of resolution $\approx 200\,\text{nm}$. Suitable for typical bacteria ($\approx 10\,\mu\text{m}$ or $10,000\,\text{nm}$), but inadequate for viruses.
  • Electron microscopes: Resolving power down to $0.5\,\text{nm}$, utilizing high-energy electron beams with extremely short wavelengths under high vacuum conditions.

• Step-by-Step Letter "e" Alignment Protocol:

  1. Transport microscope by supporting the base with one hand and grasping the arm with the other.
  2. Ensure power switch is off and light intensity is set to minimum.
  3. Adjust substage condenser to position flush with or slightly below stage level; open iris diaphragm completely.
  4. Plug in power cord, turn on unit, and adjust light level.
  5. Click $4\times$ scanning objective into position.
  6. Secure letter "e" slide with stage clips and center the speciman over the light path using stage control knobs.
  7. Adjust binocular ocular lenses to merge fields into a single binocular circle.
  8. Focus using coarse adjustment knob, refine with fine focus knob, and optimize contrast via iris diaphragm.
  9. Center specimen, then rotate nosepiece to $10\times$ ($100\times$ total) and focus using fine focus only.
  10. Advance to $40\times$ ($400\times$ total) and adjust fine focus.

• Crossed Silk Fiber Depth Protocol:

  • Silk fiber slides contain overlapping red, yellow, and blue fibers.
  • View at $4\times$, $10\times$, and $40\times$. Move vertically in the Z-axis using the fine focus knob to evaluate which single fiber comes into sharp focus first (top layer), second (middle layer), and third (bottom layer).

Experiment 3: Examination of Living Microbes and Environmental Plates

• Learning Outcomes:

  • Prepare wet mounts of living microorganisms, including pond water organisms and termite gut symbionts.
  • Identify Euglena, Paramecium, Amoeba, and Trichonympha based on cell morphology and motility mechanisms.
  • Track live motile organisms across the microscopic field of view.
  • Inoculate nutrient agar plates with environmental and human body microbiota samples.

• Taxonomic and Morphological Characteristics of Protozoa:

  • Protozoans are single-celled eukaryotic organisms belonging to Domain Eukaryota; possess membrane-bound nuclei and organelles.
  • Cell size range: Protozoans ($\le 750\,\mu\text{m}$) are significantly larger than typical bacteria ($1\text{--}10\,\mu\text{m}$).
  • Amoeba proteus: Non-pathogenic relative of Naegleria fowleri ("brain-eating amoeba"). Moves and ingests prey via pseudopodia ("false feet") through amoeboid movement/phagocytosis (analogous to human neutrophils).
  • Paramecium: Covered in hair-like cilia that beat synchronously for rapid locomotion; referred to as the "white rat" of ciliates.
  • Euglena: Photosynthetic protist containing chloroplasts; utilizes flagellar rotation at its termini for motility.
  • Trichonympha: Symbiotic protist residing in termite hindguts. Termites lack cellulase enzymes to digest wood cellulose. Trichonympha and internal endosymbiotic bacteria hydrolyze cellulose into usable glucose. Transferred to juvenile termites via proctodeal trophallaxis (feeding on fluid excreted from the parental anus).

• Environmental and Body Part Plate Inoculation Protocol:

  1. Take two nutrient agar (NA) plates. Draw a line bisecting the bottom of each plate with a Sharpie to divide them into two sections.
  2. For dry environmental sampling: Dampen a sterile cotton swab with sterile water, swab the target surface, and gently streak across one assigned agar section. Discard swab.
  3. Body part sampling: Swab two selected body sites (excluding vaginas and anuses) and streak onto designated plate halves.
  4. Label plate bottoms with initials, full last name, and date.
  5. Incubate inverted at $37\,^{\circ}\text{C}$ for $48\,\text{hours}$ or at $30\,^{\circ}\text{C}$ for $5\,\text{days}$.

Experiment 4: Colony and Cell Morphology; Simple Stains

• Learning Outcomes:

  • Distinguish colony morphology from cellular morphology.
  • Prepare heat-fixed bacterial smears.
  • Perform simple staining techniques using basic dyes.
  • Visualize bacterial cell shapes and arrangements under $1000\times$ total magnification using oil immersion optics.

• Colony Morphology Assessment Parameters:

  • A single bacterial cell dividing on solid media produces a visible mass of millions of clonal cells called a colony.
  • Evaluated parameters: Form (overall shape), Elevation (side profile), Margin (edge distribution), Pigmentation (color production), Texture (moist, dry, mucoid), and Diameter (measured in millimeters).

• Bacterial Cell Shapes and Arrangements:

  • Coccus / Cocci: Spherical cells.
  • Bacillus / Bacilli: Rod-shaped cells.
  • Coccobacillus: Intermediate oval form (e.g., Lactococcus lactis).
  • Pleomorphic: Variable or irregular cellular shapes (e.g., Corynebacterium xerosis).
  • Swarming Growth: Concentric wave-like spreading growth over agar (e.g., Proteus vulgaris).

• Preparation of a Bacterial Smear:

  1. Circle the target colony on the bottom of the plate with a Sharpie.
  2. Label the edge of a clean glass slide with specimen identification.
  3. Place a single drop of water onto the slide surface.
  4. Aseptically collect a small fraction of a bacterial colony using a sterile inoculating loop or needle.
  5. Emulsify the bacteria thoroughly in the water droplet to form a thin, nickel- to dime-sized emulsion.
  6. Heat-fix the slide by placing it on a slide warmer/hot plate until evaporated, or pass through a Bunsen burner flame 2--3 times to kill organisms and adhere cells to the glass.

• Simple Staining Principles and Basic Dyes:

  • Simple stains utilize a single cationic (basic) chromophore dye.
  • Basic dyes carry a positive charge ($\text{Dye}^+$) which electrostatically binds to the negatively charged bacterial cell wall and cytoplasm.
  • Common Basic Dyes: Crystal Violet, Safranin, Methylene Blue.
  • Staining Times: Crystal Violet ($30\text{--}60\,\text{seconds}$), Safranin (up to $1\,\text{minute}$), Methylene Blue ($30\text{--}60\,\text{seconds}$).
  • Application Protocol: Cover heat-fixed smear with dye for designated time; rinse gently with distilled water directed above the smear; gently blot dry with bibulous paper (do not rub).

• Oil Immersion Optics Mechanics:

  • Air has a refractive index ($n \approx 1.000$) lower than glass ($n \approx 1.515$). Light passing through glass into air bends away from high-power objectives, causing light loss and numerical aperture loss.
  • Immersion oil possesses a refractive index matching glass ($n \approx 1.515$), eliminating light refraction at the slide-air gap and directing light rays into the $100\times$ objective.
  • Procedure: Focus specimen at $4\times$, $10\times$, then $40\times$. Rotate nosepiece half-way between $40\times$ and $100\times$. Place a drop of immersion oil directly onto the slide. Lock $100\times$ objective into place. Never rotate the $40\times$ high-dry lens back into immersion oil.

Experiment 5: Aseptic Technique and Streak Plates

• Learning Outcomes:

  • Define mixed vs. pure microbial cultures.
  • Demonstrate flawless aseptic transfer protocols in broths, slants, and deeps.
  • Perform three-sector streak plate procedures to isolate single pure colonies from mixed populations.

• Culture Terms and Bacterial Growth Curve Dynamics:

  • Pure Culture: Media containing only a single species or strain of microorganism.
  • Mixed Culture: Media containing two or more distinct microbial species.
  • Subculturing: Transferring microorganisms from an established culture into fresh sterile media to prevent cell death resulting from nutrient depletion and toxic metabolic acid accumulation.
  • Growth Curve Stages: Lag phase, Log (exponential growth) phase, Stationary phase, Death phase.

• Microbial Culture Media Formats and Physical Characteristics:

  • Nutrient Agar / Broth (NA/NB): Simple general-purpose media supporting non-fastidious microbes.
  • Trypticase Soy Agar (TSA): Enriched general-purpose media containing higher nutritional content for fastidious microbes.
  • Broth Patterns: Turbidity (cloudiness throughout), Pellicle (surface biofilm at liquid-air interface), Sediment (bacterial accumulation at tube bottom).
  • Agar Slants: Solid agar cooled at an angle in tubes; optimal for maintaining pure stock cultures.
  • Agar Deeps: Solidified flat agar tubes; used to analyze bacterial motility and oxygen requirement gradients.

• Step-by-Step Aseptic Transfer Protocol:

  1. Sterilize inoculating loop in Bunsen burner flame from base to tip until glowing red; cool completely.
  2. Hold source culture tube in non-dominant hand; remove cap using pinky and ring finger of dominant hand (never place caps on benchtop).
  3. Pass glass tube neck through flame 2--3 times to generate outward air currents that prevent dust/spore contamination.
  4. Insert sterile loop, collect microbial sample, flame tube neck again, and replace cap.
  5. Uncap target sterile tube, flame neck, inoculate media (zigzag streak for slants; central stab for deeps; swirl for broths).
  6. Flame tube neck, recap, and re-flame loop to red heat.

• Three-Sector Isolation Streak Method Protocol:

  1. Sector 1: Flame loop, cool, acquire inoculum, streak densely across Sector 1. Flame loop.
  2. Sector 2: Cool loop on fresh agar edge. Turn plate $90\,^{\circ}$, pass through Sector 1 streaks 1--5 times, then streak out Sector 2. Flame loop.
  3. Sector 3: Cool loop. Turn plate $90\,^{\circ}$, pass through Sector 2 streaks 1--5 times, then streak out Sector 3 in a wide pattern without touching Sector 1. Flame loop.
  4. Invert plates (lid down) during incubation at $35\text{--}37\,^{\circ}\text{C}$ to prevent condensation droplets from falling onto colonies.

Experiment 6: Gram Stains

• Learning Outcomes:

  • Differentiate between simple and differential staining strategies.
  • State the functional roles of primary stain, mordant, decolorizer, and counterstain.
  • Troubleshoot common technical errors in Gram staining.

• Structural Basis of the Gram Stain:

  • Gram-Positive Cell Wall: Thick multi-layered peptidoglycan meshwork that retains crystal violet-iodine complexes.
  • Gram-Negative Cell Wall: Thin single-layer peptidoglycan surrounded by an outer lipopolysaccharide (LPS) membrane easily dissolved by alcohol decolorizers.

• Reagent Sequence and Functional Mechanics:

  1. Primary Stain (Crystal Violet): Stains all cells deep purple ($1\,\text{minute}$ contact time).
  2. Mordant (Gram's Iodine): Forms insoluble crystal violet-iodine (CV-I) complexes inside peptidoglycan layers ($1\,\text{minute}$ contact time).
  3. Decolorizer ($95\%$ Ethyl Alcohol): Dehydrates Gram-positive peptidoglycan (trapping CV-I); dissolves Gram-negative outer membranes, washing out unbound CV-I complexes and leaving Gram-negative cells colorless ($20\,\text{seconds}$ strict contact time).
  4. Counterstain (Safranin): Stains decolorized Gram-negative cells light red/pink ($1\,\text{minute}$ contact time).

• Gram Stain Reagent Summary Table:

• Troubleshooting Gram Stain Errors:
  • Over-decolorization (Alcohol applied $> 20\,\text{seconds}$): Gram-positive cells appear uniformly pink/red.
  • Under-decolorization (Alcohol applied $< 20\,\text{seconds}$): Gram-negative cells appear uniformly purple.
  • Smear Too Thick: Decolorizer cannot penetrate thick cellular piles, trapping CV-I and making Gram-negative cells appear Gram-positive.
  • Culture Age / Gram-Variability: Older cultures ($> 24\,\text{hours}$), particularly Bacillus subtilis or Bacillus cereus, lose cell wall integrity and reliably exhibit variable/mixed Gram reactions.

Experiment 7: Acid-Fast and Endospore Stains

• Learning Outcomes:

  • Differentiate acid-fast from non-acid-fast bacteria.
  • Identify vegetative cells, central endospores, terminal endospores, and free spores.
  • Apply steam bath procedures to enhance dye penetration into resistant microbial structures.
  • Explain the clinical significance of acid-fast and spore-forming pathogens.

• Acid-Fast Staining Mechanics:

  • Target Microbes: Mycobacterium tuberculosis (tuberculosis), Mycobacterium leprae (leprosy), and Mycobacterium smegmatis (non-pathogenic model).
  • Cell Wall Feature: Outer envelope containing high concentrations of hydrophobic, waxy mycolic acids that repel aqueous dyes.
  • Primary Stain: Carbolfuchsin (lipid-soluble phenolic dye) applied for $10\,\text{minutes}$ over a steam bath to melt waxy walls and drive stain into cells.
  • Decolorizer: Acid Alcohol ($3\%\,\text{HCl}$ in $95\%\,\text{Ethanol}$); strips stain from non-acid-fast cells while acid-fast cell walls resist acid decolorization.
  • Counterstain: Methylene Blue ($1\,\text{minute}$ contact time).
  • Results: Acid-fast cells stain bright fuchsia/pink; non-acid-fast cells stain blue.

• Endospore Staining Mechanics:

  • Target Microbes: Genera Bacillus (central endospores) and Clostridium (terminal endospores).
  • Pathogens: Bacillus cereus (food poisoning), Clostridium botulinum (botulism), Clostridium tetani (tetanus), Clostridium perfringens (gas gangrene, food poisoning).
  • Structure: Dormant, highly resistant structures wrapped in tough keratin protein coats.
  • Primary Stain: Malachite Green applied over a steam bath for $10\,\text{minutes}$.
  • Decolorizer: Distilled Water; washes malachite green out of permeable vegetative cells while green stain remains trapped in keratin coated spores.
  • Counterstain: Safranin ($1\,\text{minute}$ contact time).
  • Results: Endospores and free spores stain green; vegetative cells stain red/pink.

• Steam Bath Setup Protocol:

  • Fill a $1\,\text{L}$ beaker with $\approx 500\,\text{mL}$ water; heat over Bunsen burner to maintain a low rolling boil.
  • Cut strips of bibulous paper to cover dried, heat-fixed slide smears.
  • Place slide across beaker mesh stand; saturate bibulous paper strip with primary stain (Carbolfuchsin or Malachite Green).
  • Steam for $10\,\text{minutes}$, re-applying stain periodically to keep paper moist.

Experiment 8: Gram Stain Unknown

• Learning Outcomes:

  • Integrate colony morphology, cell morphology, and Gram reaction data to identify unknown bacterial strains.
  • Navigate a dichotomous key based on phenotypic parameters.

• Target Organisms: Proteus mirabilis, Escherichia coli, Micrococcus luteus, Pseudomonas aeruginosa, Staphylococcus aureus, Bacillus subtilis.

• Dichotomous Key Logic Schema:

  • Dichotomous keys separate organisms using binary branch points ("Yes/No" phenotypic questions).
  • Key diagnostic traits used: Gram reaction ($\text{G}^+$ vs. $\text{G}^-$), cellular shape (coccus vs. bacillus), arrangement (clusters, chains, tetrads), swarming motility on agar (Proteus mirabilis), pigment generation (Micrococcus luteus yellow; Pseudomonas aeruginosa blue-green pyocyanin).

Experiment 9: Introduction to the Bacterial ID Project

• Learning Outcomes:

  • Distinguish selective media from differential media.
  • Explain functional parameters of TSA, PEA, and MacConkey (MAC) media.
  • Verify culture purity through rigorous Gram stain controls.

• Project Scope: A multi-week isolation and identification protocol using a mixed broth culture containing one Gram-positive organism and one Gram-negative organism.

• Potential Project Unknown Species List:

  • Gram-Positives: Bacillus subtilis, Staphylococcus epidermidis, Bacillus cereus, Corynebacterium xerosis, Bacillus circulans, Lactococcus lactis, Micrococcus luteus, Enterococcus faecalis, Staphylococcus aureus.
  • Gram-Negatives: Escherichia coli, Proteus mirabilis, Providencia stuartii, Salmonella enterica, Proteus vulgaris, Shigella sonnei, Klebsiella pneumoniae, Klebsiella aerogenes, Citrobacter freundii, Morganella morganii, Alcaligenes faecalis.

• Isolation Media Properties:

  • Trypticase Soy Agar (TSA): General-purpose, non-selective medium supporting growth of both Gram-positive and Gram-negative organisms.
  • Phenylethyl Alcohol Agar (PEA): Selective medium. Phenylethyl alcohol inhibits Gram-negative organisms by disrupting outer membrane permeability. Selects for Gram-positives.
  • MacConkey Agar (MAC): Selective and differential medium. Bile salts and crystal violet inhibit Gram-positive microbes (selects for Gram-negatives). Contains lactose and neutral red pH indicator. Lactose fermenters produce acid (drop pH below $6.8$) turning colonies pink/purple; non-fermenters remain colorless/tan.

• Three-Day Protocol Summary:

  • Day 1: Gram stain mixed culture; streak mixture onto TSA, PEA, and MAC plates. Incubate at $37\,^{\circ}\text{C}$ for $48\,\text{hours}$ or $30\,^{\circ}\text{C}$ for $5\,\text{days}$.
  • Day 2: Photograph plates. Select isolated single colonies from PEA (putative Gram-positive) and MAC (putative Gram-negative). Gram stain half of each selected colony. Use the remaining half to inoculate clean TSA slants.
  • Day 3: Gram stain TSA slant growth to confirm purity before executing biochemical testing.

Experiment 10: Carbohydrate Metabolism of Unknowns

• Learning Outcomes:

  • Analyze carbohydrate utilization profiles using biochemical test panels.
  • Interpret pH indicators including Phenol Red, Bromothymol Blue, and Methyl Red.
  • Relate enzyme production (amylase, citrate permease, catalase) to species differentiation.

• Assay Mechanics and Reactions:

  • Phenol Red Carbohydrate Broths (Glucose, Lactose, Sucrose, Mannitol):
  • Phenol Red indicator: Red at pH $6.8\text{--}7.4$; Yellow below pH $6.8$ (acid production via fermentation); Pink above pH $7.4$.
  • Durham Tube: Inverted glass vial inside broth to trap gas ($\text{CO}_2$ / $\text{H}_2$) bubbles.
  • Enzyme Note: Sucrose breakdown requires invertase enzyme.
  • Methyl Red (MR) & Voges-Proskauer (VP) Tests:
  • Inoculate single MR-VP tube; incubate $48\,\text{hours}$; split into two $1\,\text{mL}$ aliquots.
  • Methyl Red: Tests for mixed-acid fermentation (production of lactic, acetic, formic, succinic acids). Add 3 drops Methyl Red. Red color = Positive (pH $< 4.4$); Yellow = Negative.
  • Voges-Proskauer: Tests for 2,3-butanediol fermentation via acetoin intermediate. Add 15 drops VP Reagent A ($\alpha$-naphthol) and 5 drops VP Reagent B ($40\%\,\text{KOH}$). Mix vigorously to oxygenate. Red color within $10\text{--}60\,\text{minutes}$ = Positive.
  • Citrate Utilization Test:
  • Simmons Citrate Agar contains citrate as sole carbon source, ammonium phosphate as sole nitrogen source, and Bromothymol Blue pH indicator.
  • Requires citrate permease transport enzyme. Utilization generates basic sodium carbonate and ammonia, raising pH above $7.6$.
  • Results: Blue slant or visible growth = Positive; Green = Negative.
  • Starch Hydrolysis Test:
  • Starch is a macro-polymer composed of glucose monomers linked by $\alpha\text{-1,4}$ and $\alpha\text{-1,6}$ glycosidic bonds.
  • Extracellular amylase hydrolyzes starch into absorbable glucose molecules.
  • Flood starch agar plate with Lugol's Iodine. Iodine reacts with intact starch to form a dark blue/black complex. Clear zone around colony = Positive for starch hydrolysis.
  • Catalase Test:
  • Catalase converts toxic aerobic respiration byproduct hydrogen peroxide into water and oxygen gas: $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2(g)$.
  • Add $3\%\,\text{H}_2\text{O}_2$ directly to smear on glass slide. Immediate, vigorous bubbling = Positive.

Experiment 11: Protein Metabolism of Unknowns

• Learning Outcomes:

  • Determine protein breakdown profiles and nitrogen metabolic capabilities.
  • Interpret indicators including Bromocresol Purple, Ferric Chloride, and Kovac's Reagent.
  • Analyze activity of decarboxylases, phenylalanine deaminase, cysteine desulfurase, tryptophanase, nitrate reductase, gelatinase, and urease.

• Assay Mechanics and Reactions:

  • Decarboxylase Tests (Lysine & Ornithine) (Gram-Negatives Only):
  • Evaluates removal of carboxyl group ($\text{-COOH}$) from amino acids.
  • Media contains glucose, amino acid, Bromocresol Purple indicator, and an overlay of $3\text{--}4\,\text{mm}$ sterile mineral oil to force anaerobic fermentation.
  • Acid from initial glucose fermentation turns media yellow. Decarboxylase induction generates basic amines, shifting media back to Purple = Positive; Yellow = Negative.
  • Phenylalanine Deaminase Test (Gram-Negatives Only):
  • Tests for removal of amino group ($\text{-NH}_2$) from phenylalanine.
  • Inoculate slant; after incubation, add drops of $12\%$ Ferric Chloride ($\text{FeCl}_3$).
  • Phenylpyruvic acid reacts with $\text{FeCl}_3$ to form a forest green color = Positive.
  • SIM Tests (Sulfur, Indole, Motility) (Gram-Negatives Only):
  • Inoculate SIM agar deep via straight central stab.
  • Sulfur Reduction: Cysteine desulfurase or thiosulfate reductase generates $\text{H}_2\text{S}$ gas, reacting with ferrous ammonium sulfate ($\text{FeSO}_4$) to produce a black ferric sulfide ($\text{FeS}$) precipitate = Positive.
  • Indole Production: Tryptophanase converts tryptophan into indole, pyruvate, and ammonia. Overlay media with Kovac's Reagent ($2\text{--}3\,\text{mm}$). Cherry-red ring = Positive.
  • Motility: Diffuse, cloudy growth radiating outward from central stab line = Positive (obscured if sulfur blackening occurs).
  • Nitrate Reduction Test:
  • Evaluates reduction of nitrate ($\text{NO}_3^-$) to nitrite ($\text{NO}_2^-$), ammonia ($\text{NH}_3$), or nitrogen gas ($\text{N}_2$).
  • Add 8 drops Nitrate Reagent A and 8 drops Nitrate Reagent B. Red color within $10\,\text{minutes}$ = Positive for nitrate reductase ($\text{NO}_3^- \rightarrow \text{NO}_2^-$).
  • If colorless, add powdered zinc dust. Zinc chemically converts remaining $\text{NO}_3^-$ to $\text{NO}_2^-$:
    • Red color after zinc = Negative (unreduced $\text{NO}_3^-$ was present).
    • Colorless after zinc = Positive ($\text{NO}_3^-$ was completely reduced past $\text{NO}_2^-$ to $\text{N}_2$ or $\text{NH}_3$).
  • Gelatin Hydrolysis Test:
  • Nutrient gelatin deep stab inoculation.
  • Secretion of extracellular gelatinase hydrolyzes gelatin protein matrix.
  • Chill tubes in an ice bath prior to reading; liquid medium state = Positive; solid state = Negative.
  • Urea Hydrolysis Test:
  • Urease hydrolyzes urea into carbon dioxide and basic ammonia: $\text{Urea} + \text{H}_2\text{O} \rightarrow \text{CO}_2 + 2\text{NH}_3$
  • Urea broth contains Yeast Extract, strong buffers, and Phenol Red indicator.
  • Bright hot pink color (pH $> 8.4$) = Positive; Orange/Yellow = Negative.

Experiment 12: Completion of the ID Project

• Learning Outcomes:

  • Perform confirmatory media assays to finalize species identification.
  • Differentiate $\alpha$, $\beta$, and $\gamma$ hemolytic reactions.
  • Apply specialized diagnostic media: Sheep's Blood Agar, MSA, Bile Esculin, EMB, and Hektoen Enteric agar.

• Confirmatory Media Definitions and Interpretations:

  • Sheep's Blood Agar:
  • Evaluates hemolysin production.
  • $\beta$ (beta) Hemolysis: Complete lysis of red blood cells; clear transparent zone surrounding growth.
  • $\alpha$ (alpha) Hemolysis: Partial lysis of red blood cells; greenish/brownish discoloration surrounding growth.
  • $\gamma$ (gamma) Hemolysis: Non-hemolytic; unchanged red agar surrounding growth.
  • Mannitol Salt Agar (MSA):
  • Selective for salt-tolerant staphylococci ($7.5\%\,\text{NaCl}$ concentration).
  • Differential via mannitol fermentation and Phenol Red indicator.
  • Yellow media = Staphylococcus aureus (mannitol positive); Red/pink media = Staphylococcus epidermidis (mannitol negative).
  • Bile Esculin Agar:
  • Selective for Enterococcus and Group D Streptococcus (contains bile salts).
  • Differential via esculinase hydrolysis of esculin to esculetin. Esculetin reacts with ferric ions ($\text{Fe}^{3+}$) to produce a dark brown/black precipitate.
  • Eosin Methylene Blue (EMB) Agar:
  • Dyes Eosin Y and Methylene Blue inhibit Gram-positive organisms.
  • High-level lactose/sucrose fermentation drops pH below $4.9$, precipitating dyes into a metallic green sheen (characteristic of Escherichia coli). Moderate fermenters appear dark purple; non-fermenters appear colorless.
  • Hektoen Enteric (HE) Agar:
  • Selective against Gram-positives via bile salts, bromothymol blue, and acid fuchsin.
  • Differential for lactose/sucrose/salicin fermentation (orange/salmon pink colonies) vs. non-fermentation.
  • $\text{H}_2\text{S}$ production reacts with ferric ammonium citrate to produce black-centered colonies (differentiates Salmonella [black colonies] from Shigella [greenish/orange colonies]).

Experiment 13: Identification of White Blood Cells

• Learning Outcomes:

  • Prepare thin peripheral blood smears.
  • Perform a Wright's stain.
  • Identify erythrocytes, platelets, and leukocytes under oil immersion.
  • Conduct a 50-cell differential leukocyte count.

• Hematopoiesis and Cell Lifespans:

  • All blood components derive from hematopoietic stem cells in bone marrow.
  • Erythrocytes (Red Blood Cells): Lack nuclei; lifespan $\approx 120\,\text{days}$.
  • Leukocyte lifespans range from minutes to days or years.

• Leukocyte Identification and Differential Percentage Guide:

• Blood Smear and Wright's Staining Protocol:
  1. Clean finger with alcohol wipe; puncture with sterile lancet.
  2. Place small drop of blood near end of glass slide.
  3. Hold a second spreader slide at a $30\text{--}45\,^{\circ}$ angle; pull back until blood spreads along edge via capillary action.
  4. Push spreader slide smoothly forward to create a thin feather-edged smear; air dry.
  5. 1-Step Wright's Stain: Dip slide into Wright's stain for $30\,\text{seconds}$; dip into Beaker 1 distilled water for $45\,\text{seconds}$; dip into Beaker 2 distilled water for $25\,\text{seconds}$ (do not rinse between dips); air dry.
  6. Count and classify a minimum of 50 leukocytes systematically under $1000\times$ oil immersion.

Experiment 14: Antibiotics

• Learning Outcomes:

  • Execute Kirby-Bauer disk diffusion antimicrobial sensitivity assays.
  • Explain the physical and chemical properties of Mueller-Hinton (MH) agar.
  • Measure zones of inhibition (ZOI) and identify minimum inhibitory concentration (MIC) boundaries.
  • Classify organisms as Sensitive (S), Intermediate (I), or Resistant (R).

• Mechanisms of Antibiotic Action:

  1. Cell wall synthesis inhibitors (e.g., Penicillins).
  2. Protein synthesis inhibitors (e.g., Chloramphenicol).
  3. Nucleic acid (DNA/RNA) synthesis inhibitors.
  4. Metabolic pathway inhibitors (e.g., Sulfonamides).
  5. Plasma membrane disrupters (e.g., Polymyxin B).

• Mueller-Hinton (MH) Agar Specifics:

  • Non-selective, non-differential loose agar medium that facilitates uniform antibiotic diffusion.
  • Poured to a standardized depth ($4\,\text{mm}$) to ensure reproducible diffusion rates.
  • Formulated to be low in antibiotic-inhibiting compounds.
  • Contains starch to absorb toxic bacterial metabolites.

• Disk Diffusion Protocol and ZOI Measurement:

  1. Create a confluent bacterial lawn on MH agar using a sterile swab dipped in broth culture (Staphylococcus aureus, Escherichia coli, or Pseudomonas aeruginosa).
  2. Apply antimicrobial disks evenly spaced across agar using sterilized forceps.
  3. Incubate at $37\,^{\circ}\text{C}$ for $24\,\text{hours}$.
  4. Measure Zone of Inhibition (ZOI) diameters across disks in millimeters ($\text{mm}$) using a metric ruler.
  5. MIC Determination: The Minimum Inhibitory Concentration exists at the exact outer perimeter edge of the clear zone where growth recommences.
  6. Compare measured ZOI diameters against standardized lookup charts to determine S, I, or R status.
  • Critical Principle: Zone diameters cannot be directly compared between different antibiotics to determine potency without referencing standard interpretive tables.

Experiment 15: Oxygen Requirements

• Learning Outcomes:

  • Classify microbial aerotolerance categories using anaerobic jars and Brain-Heart Infusion (BHI) shake tubes.
  • Monitor anaerobic conditions using chemical test indicators.

• Aerotolerance Classifications:

  • Obligate Aerobe: Requires $\text{O}_2$ for respiration; grows only at the top of BHI shake tubes (e.g., Pseudomonas aeruginosa).
  • Obligate Anaerobe: Poisoned by $\text{O}_2$; lacks superoxide dismutase and catalase; grows only at the bottom of BHI shake tubes (e.g., Clostridium sporogenes).
  • Facultative Anaerobe: Prefers $\text{O}_2$ (higher ATP yield), but grows throughout media via fermentation/anaerobic respiration in $\text{O}_2$ absence; dense surface growth with lighter growth below (e.g., Escherichia coli, Staphylococcus aureus).
  • Aerotolerant Anaerobe: Indifferent to $\text{O}_2$; does not use $\text{O}_2$ but possesses detoxification enzymes; uniform growth top-to-bottom (e.g., Enterococcus faecalis, Lactococcus lactis).

• Anaerobic Chamber Mechanics:

  • Sealed jar containing a chemical GasPak envelope and a methylene blue indicator strip.
  • GasPak generates $\text{H}_2$ and $\text{CO}_2$; $\text{H}_2$ reacts with free $\text{O}_2$ over a palladium catalyst to produce $\text{H}_2\text{O}$.
  • Methylene Blue Strip: Blue in aerobic conditions; turns white under true anaerobic conditions.

• BHI Shake Tube Method:

  • Melt BHI agar deeps; cool and equilibrate to $60\,^{\circ}\text{C}$ in a water bath.
  • Inoculate molten agar with 3 drops of organism; roll/shake gently to distribute without introducing $\text{O}_2$ bubbles; solidify in ice.
  • Establishes a natural vertical oxygen gradient from atmospheric $\text{O}_2$ at the top to absolute anoxia at the bottom.

Experiment 16: Germicides and UV Light

• Learning Outcomes:

  • Evaluate physical (UV light) and chemical (germicides) methods of environmental microbial control.
  • Explain mechanisms of action and factors limiting control efficacy.
  • Compare structural resistance differences between vegetative cells and bacterial endospores.

• Ultraviolet (UV) Light Radiation Mechanics:

  • Non-ionizing, short-wavelength electromagnetic radiation.
  • Induces pyrimidine dimers (specifically thymine dimers) in cellular DNA, halting replication and transcription.
  • Limitation: UV light has low penetrating power; blocked by plastic Petri dish lids, paper, glass, and organic material.
  • Endospore Resistance: Endospores of Bacillus subtilis exhibit dramatically higher UV resistance than vegetative Escherichia coli cells, requiring significantly longer exposure times for inactivation.

• Chemical Germicides Testing Protocol:

  • Disinfectant Categories: Low-level (soaps), intermediate (alcohols), high-level (bleach, hydrogen peroxide).
  • Ceramic Bead Disinfection Assay:
  1. Coat sterile ceramic beads with target bacteria (S. aureus, P. aeruginosa, or E. coli) for $5\,\text{minutes}$.
  2. Dry beads on sterile filter paper to absorb excess culture.
  3. Submerge beads in target disinfectant solutions for $5\,\text{minutes}$.
  4. Transfer disinfected beads into Nutrient Broth tubes; incubate at $37\,^{\circ}\text{C}$ for $48\,\text{hours}$.
  5. Turbidity = Bacterial survival / disinfectant failure (+); Clear broth = Successful disinfection / bactericidal efficacy (-).

• Tested Disinfectant Concentrations:

  • Bleach: $0.1\%$, $1\%$, $10\%$
  • Lysol Brand II: $0.1\%$, $1\%$, $10\%$
  • Hydrogen Peroxide: $0.1\%$, $1\%$, $10\%$
  • Isopropyl Alcohol: $10\%$, $50\%$, $70\%$

Experiment 17: Food Microbiology

• Learning Outcomes:

  • Calculate Colony Forming Units per gram ($\text{CFU/g}$) of food samples using serial dilution standard plate counts.
  • Differentiate quantitative plate counts from qualitative diagnostic media assays (EMB/Hektoen).
  • Evaluate food safety parameters.

• Serial Dilution Mathematical Calculations:

  • $1\,\text{g}$ of food mixed into $99\,\text{mL}$ water blank yields a $1/100$ ($10^{-2}$) dilution.
  • Transfer $1\,\text{mL}$ sequentially through $99\,\text{mL}$ water blanks to establish $10^{-4}$ and $10^{-6}$ dilutions.
  • Plating $1.0\,\text{mL}$ yields $10^{-2}$, $10^{-4}$, and $10^{-6}$ plate dilutions.
  • Plating $0.1\,\text{mL}$ yields $10^{-3}$ and $10^{-5}$ plate dilutions.

• Standard Plate Count Rules:

  • Statistically Valid Range: Only count plates containing between $30$ and $300$ colonies.
  • TNTC: Too Numerous To Count ($> 300 colonies).\n - **TFTC**: Too Few To Count ($< 30 colonies).
  • Calculation Formula: $\text{CFU / g} = \text{Colony Count (CFU on plate)} \times \frac{1}{\text{Dilution Factor of Plate}}$
  • Example: $250\,\text{CFU}$ counted on a $10^{-4}$ dilution plate $\rightarrow 250 \times 10^4 = 2,500,000\,\text{CFU/g}$ ($2.5 \times 10^6\,\text{CFU/g}$).

• Food Microbiological Qualitative Safety Screen:

  • Inoculate $0.1\,\text{mL}$ of $10^{-2}$ food dilution onto EMB and Hektoen plates.
  • Screen for fecal coliforms (Escherichia coli green metallic sheen on EMB) and foodborne enteric pathogens (Salmonella enterica black colonies on Hektoen).