Pure Culture Techniques

 Pure Culture Techniques (Streak Plate, Pour Plate and Spread Plate Methods)

 

1. Streak Plate Method

Aim To isolate pure colonies of bacteria from a mixed culture by streaking on a solid agar medium so that individual cells are spatially separated and form isolated colonies.

Principle The inoculum is progressively diluted by streaking across successive sectors of the agar plate using a sterile loop. In the later streaks, individual bacterial cells are deposited far apart. Each viable cell multiplies during incubation to form a visible isolated colony that can be picked for pure culture.

Materials Required

· Mixed bacterial culture (broth or colony)

· Sterile nutrient agar (or appropriate selective/differential medium) plates

· Inoculating loop (nichrome or platinum)

· Bunsen burner / spirit lamp

· Incubator (37 °C)

· Marker pen

· Sterile forceps (optional)

Procedure

1. Label the bottom of a sterile agar plate with specimen details, date and your initials. Divide the plate mentally or lightly into 3–4 sectors.

2. Sterilise the inoculating loop by flaming until red-hot and allow it to cool.

3. Take a loopful of the mixed culture.

4. First streak (Sector 1): Open the plate lid just enough, streak the inoculum in parallel lines or zig-zag pattern covering about one-quarter of the plate.

5. Flame and cool the loop again.

6. Second streak (Sector 2): Drag the loop once or twice through the end of the first streak and continue streaking into the second sector (without reloading inoculum).

7. Flame and cool the loop.

8. Third streak (Sector 3): Drag through the end of the second streak and streak the third sector.

9. (Optional) Flame, cool and make a final streak into the remaining area for maximum isolation.

10. Close the lid, invert the plate and incubate at 37 °C for 18–24 hours (or appropriate temperature/time).

11. Observe isolated colonies in the later sectors. Pick a well-isolated colony with a sterile loop and subculture onto a fresh slant or plate to obtain pure culture.

Result / Observation Isolated discrete colonies appear in the later streak areas. Colonies of different morphology indicate different species; pick one type for pure culture.

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 2. Pour Plate Method

Aim To isolate pure colonies and also to enumerate viable bacteria (colony-forming units) in a liquid sample by embedding diluted cells in molten agar.

Principle Serial dilutions of the sample are mixed with molten agar (cooled to 45–50 °C) and poured into sterile Petri dishes. Individual cells become immobilised and grow into colonies both on the surface and within the agar (sub-surface colonies). Colonies can be counted and pure cultures obtained by picking isolated colonies.

Materials Required

· Mixed bacterial suspension / sample

· Sterile molten nutrient agar (kept at 45–50 °C in water bath)

· Sterile Petri dishes

· Sterile pipettes (1 ml, 10 ml) or micropipettes

· Sterile dilution blanks (9 ml saline or broth)

· Water bath (45–50 °C)

· Incubator

· Colony counter (optional)

Procedure

1. Prepare serial dilutions of the sample (e.g., 10⁻¹ to 10⁻⁶) in sterile diluent.

2. Label sterile Petri dishes with dilution numbers.

3. Aseptically transfer 1 ml of the appropriate dilution(s) into each empty sterile Petri dish.

4. Pour 15–20 ml of molten agar (cooled to ~45–50 °C) into each dish containing the inoculum.

5. Immediately mix by gentle rotation or swirling of the plate (in a figure-of-8 or circular motion) so that the inoculum is evenly distributed.

6. Allow the agar to solidify completely (10–15 minutes) on a level surface.

7. Invert the plates and incubate at 37 °C for 24–48 hours.

8. Observe colonies growing on the surface and within the agar. Count colonies (preferably plates with 30–300 colonies) for enumeration if required.

9. Pick well-isolated surface or sub-surface colonies for pure culture.

Result / Observation Colonies appear both on the surface and embedded in the agar. Isolated colonies can be subcultured. Colony count gives viable count (CFU/ml).

3. Spread Plate Method

Aim To isolate pure colonies and enumerate viable bacteria by spreading a diluted liquid sample evenly over the surface of a solid agar medium.

Principle A measured volume (usually 0.1 ml) of a diluted bacterial suspension is placed on the surface of a pre-poured, solidified agar plate and spread uniformly with a sterile spreader. Individual cells are separated on the surface and grow into discrete colonies after incubation. This method yields only surface colonies and is preferred when oxygen-requiring organisms or easy colony picking is needed.

Materials Required

· Mixed bacterial suspension / sample, Sterile pre-poured nutrient agar plates (dried surface), Sterile dilution blanks,Sterile pipettes or micropipettes

· Sterile glass or plastic L-shaped spreader (or sterile bent glass rod)

· Turntable, Alcohol (for flaming spreader)

· Bunsen burner, Incubator

Procedure

1. Prepare serial dilutions of the sample.

2. Label the bottom of dried agar plates with dilution numbers.

3. Aseptically pipette 0.1 ml of the chosen dilution onto the centre of the agar surface.

4. Sterilise the spreader by dipping in alcohol and flaming; allow to cool.

5. Place the plate on a turntable (or hold it) and spread the inoculum evenly over the entire surface by rotating the plate while moving the spreader back and forth.

6. Allow the plate to dry for a few minutes (lid slightly ajar if needed) so that the liquid is absorbed.

7. Invert the plate and incubate at 37 °C for 18–24 hours.

8. Observe discrete surface colonies. Count colonies on plates with 30–300 colonies for viable count if required.

9. Pick isolated colonies for pure culture.

Result / Observation Only surface colonies are formed. Well-isolated colonies can be easily picked for pure culture. Colony count × dilution factor × 10 gives CFU/ml (because 0.1 ml was plated).

 

 

 

PREPARATION OF DIFFERENTIAL AND SELECTIVE MEDIUM

 

Preparation of Differential Medium and Selective Medium

AIM: To prepare differential medium (e.g., MacConkey Agar) and selective medium (e.g., Mannitol Salt Agar) for the isolation and identification of bacteria based on their growth characteristics and biochemical properties.

PRINCIPLE:

Differential Medium: A differential medium contains specific ingredients (dyes, sugars) that allow different bacterial species to be distinguished from one another based on visible changes in the medium — such as colour change, colony appearance, precipitate formation — even though multiple organisms may grow on it.

Selective Medium: A selective medium contains specific agents (antibiotics, dyes, high salt concentration, or bile salts) that inhibit the growth of unwanted/competing organisms while allowing the desired organism to grow. For example, Mannitol Salt Agar contains 7.5% NaCl, which inhibits most bacteria except salt-tolerant organisms like Staphylococcus aureus.

MATERIALS REQUIRED

Category	Items
Media/Chemicals	Nutrient agar base, MacConkey agar powder,  Mannitol salt agar powder, Distilled water
Glassware	Conical flask, Petri dishes, Measuring cylinder, Glass rod
Equipment	Weighing balance, Autoclave, Hot air oven / Laminar air flow, Water bath, pH meter/pH paper
Others	Cotton plugs/non-absorbent cotton, Aluminium foil/kraft paper, Inoculating loop, Bunsen burner/spirit lamp, Test cultures (e.g., E. coli, Staphylococcus aureus, Salmonella sp.), Labels/marker

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PROCEDURE

1. Weigh the required quantity of dehydrated medium as per manufacturer's instructions.
2. Dissolve completely in distilled water by heating; adjust pH if necessary.
3. Sterilize by autoclaving at 121°C, 15 lbs pressure for 15 minutes.
4. Cool to 45–50°C and Pour into sterile Petri dishes under aseptic conditions (laminar air flow) and allow to solidify.
5. Inoculate test organisms by streak plate method and incubate at 37°C for 24 hours.

 

 

 RESULT

Medium	Organism	Observation
MacConkey Agar (Selective + Differential)	E. coli	Pink/red colonies (lactose fermenter)
MacConkey Agar (Selective + Differential)	Salmonella sp.	Pale/colourless colonies (non-lactose fermenter)
Mannitol Salt Agar (Selective)	Staphylococcus aureus	Yellow colonies with yellow zone (mannitol fermented)
Mannitol Salt Agar (Selective)	Staphylococcus epidermidis	Pink/red colonies, no colour change (mannitol not fermented)

INTERPRETATION

• The differential medium helped distinguish between different bacterial species growing together based on visible biochemical reactions (fermentation-based colour change), aiding in preliminary identification.
• The selective medium successfully inhibited the growth of unwanted/competing flora and allowed only the specific target organism to grow, aiding in the isolation of that organism from a mixed sample.
• These media are extensively used in clinical microbiology laboratories for the isolation and presumptive identification of pathogens directly from clinical specimens (e.g., stool, throat swab, urine) without prior pure culture isolation.

 

 

WINE PREPARATION

 

Preparation of Wine by Fermentation

Aim: To prepare wine from grape juice by alcoholic fermentation using Saccharomyces cerevisiae.

Principle: Wine is prepared by alcoholic fermentation of sugars present in fruit juice by yeast (Saccharomyces cerevisiae) under anaerobic conditions. The yeast converts fermentable sugars such as glucose and fructose into ethanol and carbon dioxide.

The lab-scale production of wine is a study of anaerobic metabolism.

·         Inoculum: While wild fermentation is possible via "bloom" (natural yeast on skins), lab practicals use a standardized pure culture of Saccharomyces cerevisiae (e.g., strain EC-1118) to ensure predictable kinetics.

·         Selective Inhibition: We use Potassium Metabisulfite (K₂S₂O₅). It releases Sulfur Dioxide ($SO_2$), which inhibits wild yeasts and acetic acid bacteria while the inoculated S. cerevisiae (which is sulfite-tolerant) takes over.

·         Substrate Chemistry: The "Must" must be balanced. If the sugar concentration is too high (hypertonic), it can cause osmotic stress on the yeast, leading to a "stuck" fermentation.

 

Chemical Reaction: C₆​H₁₂​O₆​→2C₂​H₅​OH+2CO₂​+Energy

Requirements: Materials:

• Fresh grapes – 1 kg
• Sucrose 20 g
• Distilled water – as required
• Active dry yeast (Saccharomyces cerevisiae) – 2–5 g
• Potassium metabisulphite – 0.1% (optional)

Equipment:

• Beakers, Conical flask, Measuring cylinder, Muslin cloth, Fermentation bottle with airlock, pH paper. Incubator (25–30°C)

Procedure:

1. Wash the grapes thoroughly with distilled water.
2. Crush the grapes to extract juice 500 ml. (must).
3. Filter the juice using muslin cloth.
4. Add a small quantity of potassium metabisulphite and allow to stand for 30 minutes to reduce contamination.
5. Activate yeast in water for 5–10 minutes.
6. Inoculate the 50 ml of yeast culture (10%) and 20 ml of sucrose into the grape juice and mix well.
7. Transfer the mixture into a fermentation bottle fitted with an airlock.
8. Incubate at 25–30°C for 7–14 days under anaerobic conditions.
9. After completion of fermentation (no bubble formation), filter and transfer the clear wine into a clean bottle for aging.

Observations:

• Formation of CO₂ bubbles during fermentation
• Alcoholic odor detected
• Sediment formation at the bottom
• Decrease in pH (acidic nature)

Result:

Wine was successfully prepared from grape juice by alcoholic fermentation using Saccharomyces cerevisiae.

 

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MEDICAL MICROBIOLOGY STUDY MATERIAL

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PRESERVATIVE EFFICACY TEST

 

Preservative Efficacy Testing (PET)

Preservative Efficacy Testing (PET) evaluates the effectiveness of a cosmetic or over-the-counter (OTC) pharmaceutical product’s preservative system, which is designed to inhibit the growth of microorganisms that could be introduced during manufacturing or by users during regular use.

By simulating these conditions, Preservative Efficacy Testing helps determine if products are capable of remaining free from harmful microbes, safeguarding consumer health and maintaining product integrity.

This testing is a critical safety and quality assurance measure for manufacturers in the health and beauty industries.

PRINCIPLE OF TEST:

Antimicrobial preservatives are added to products to prevent or limit microbial contamination, which can occur during normal conditions of storage and use.  The efficacy of an antimicrobial preservative may be enhanced or diminished by the active constituent of the preparation, or by the formulation in which it is incorporated, or by the container and/or closure being used as the final packaging material.

The test method must be qualified for the product under evaluation to ensure the correct diluent is used in assays for surviving microorganisms.

The product is inoculated with specified number of each challenge organism.  The inoculated product is held at room temperature for 28 days.  It is examined by the duplicate plate count method to determine the number of viable microorganisms which survive at each specified time interval.

A preservative efficacy test (PET), also known as an antimicrobial effectiveness test (AET) or challenge test, is a crucial procedure used to evaluate the ability of a product's preservative system to prevent the growth of microorganisms that may be introduced during manufacturing or consumer use.

The goal of a PET is to ensure that a product remains safe for its intended shelf life by effectively controlling microbial contamination. This is particularly important for multi-dose products (e.g., lotions, creams, eye drops) where repeated opening and use can introduce microorganisms.

Here is a general overview of a typical preservative efficacy test protocol:

1. Preparation of Test Materials

·         Test Sample: The product to be tested is divided into separate containers, one for each test microorganism. It is often recommended to use the final container closure system to simulate real-world conditions.

·         Test Microorganisms: A panel of standard microorganisms is used to represent common contaminants. The most common strains include:

o    Pseudomonas aeruginosa (ATCC 9027)

o    Staphylococcus aureus (ATCC 6538)

o    Escherichia coli (ATCC 8739)

o    Candida albicans (ATCC 10231)

o    Aspergillus brasiliensis (ATCC 16404)

o    Note: Other relevant environmental isolates from the manufacturing facility may also be included.

·         Preparation of Inoculum: The microorganisms are grown under specific conditions to a desired concentration, typically to produce a suspension of about 1×108 colony-forming units (CFU)/mL.

2. Inoculation and Incubation

·         Inoculation: A small volume (e.g., 0.5% to 1.0% of the product volume) of the prepared microbial suspension is added to each container of the test product. The final concentration of microorganisms in the product should be between 1×105 and 1×106 CFU/mL.

·         Mixing: The inoculated product is thoroughly mixed to ensure a homogeneous distribution of the microorganisms.

·         Incubation: The inoculated samples are incubated at a prescribed temperature, typically in the range of 20−25∘C or 22.5±2.5∘C, for a period of at least 28 days.

3. Sampling and Microbial Enumeration

·         Sampling Intervals: Samples are withdrawn from each container at specific time intervals to determine the number of surviving microorganisms. Standard time points often include:

o    Initial count (time 0)

o    Day 7

o    Day 14

o    Day 28

o    Some protocols may include additional time points, such as Day 2, Day 21, or re-challenge on a later date.

·         Neutralization: A crucial step is to neutralize the antimicrobial activity of the product's preservative system before plating the samples. This ensures that the microorganisms are not killed on the agar medium, allowing for accurate enumeration of the surviving population. Neutralizers such as polysorbate 80 and lecithin are commonly used.

·         Plate Counting: The neutralized samples are serially diluted and plated onto appropriate agar media. The plates are then incubated, and the colonies are counted. This process allows for the calculation of the number of viable microorganisms per gram or milliliter of the product at each time interval.

4. Calculation and Acceptance Criteria

·         Log Reduction Calculation: The log reduction is calculated by comparing the initial microbial count to the count at each sampling interval. The formula is as follows:

o    Log Reduction=log10​(Initial Count)−log10​(Count at Time t)

·         Acceptance Criteria: The product is deemed adequately preserved if the microbial counts meet the specific criteria outlined in the relevant pharmacopoeia or standard. The criteria vary depending on the product category (e.g., injections, topical products, oral preparations).

o    For example (simplified criteria):

§  Bacteria: A certain log reduction (e.g., 1 or 2 log) is required at Day 7, followed by a further reduction or no increase at Day 14 and Day 28.

§  Yeast and Mold: The count should not increase from the initial count, or a specified log reduction may be required.

Important Considerations

·         Suitability Testing: Before the official test, a "suitability of recovery" or "neutralizer validation" test is performed. This confirms that the chosen neutralization method effectively inactivates the preservative without harming the test microorganisms, ensuring accurate enumeration.

·         Environmental Isolates: Incorporating microorganisms isolated from the manufacturing environment can provide a more robust test, as these strains may have a higher resistance to the preservatives used.

·         Product-Specific Factors: The effectiveness of a preservative can be influenced by the product's pH, the presence of other ingredients, and the packaging. A thorough PET should account for these factors.

·         Shelf-Life and Stability: PET is often performed as part of a product's stability study to ensure the preservative system remains effective over its entire shelf life.