Unit 5: Purification of bacterial culture

I. Orientation

Purification of a bacterial culture is the separation of one microorganism from a mixed or contaminated population so that a culture contains only one type of organism. In the laboratory, purification is commonly achieved by separating individual cells on the surface of solid nutrient agar. A single viable cell, or a small group of identical cells, can multiply into a visible colony. The streak plate method uses this principle to obtain well-isolated colonies that can be subcultured and verified.

• Governing principle: Progressive dilution of cells across an agar surface reduces the number of cells deposited in later streaked areas.
• Pure culture: A population derived from one species, ideally from one isolated colony, and showing consistent cellular and colony characteristics.
• Mixed culture: A culture containing two or more microbial types, which may differ in colony appearance, staining, biochemical behavior, or growth rate.
• Aseptic technique: Procedures that prevent unwanted microorganisms from entering the culture, medium, instruments, or surrounding environment.
• Solid medium: Nutrient agar provides a firm surface on which separated cells can grow into discrete colonies.
• Colony: A visible mass of microbial cells produced by multiplication at one localized position on agar; it is not always guaranteed to originate from exactly one cell.
• Isolation versus purification: Isolation separates colonies physically; purification requires selecting an isolated colony and transferring it to fresh sterile medium.
• Incubation conditions: Temperature, atmosphere, time, and medium must support the organism being purified without encouraging excessive spreading or contamination.
• Laboratory safety: Unknown cultures are handled according to institutional biosafety procedures, with appropriate personal protective equipment, disinfection, and waste disposal.

II. Streak Plate Method — Surface dilution for obtaining isolated colonies

A. Purpose and principle

The streak plate method is a qualitative isolation technique in which a small inoculum is distributed over successive regions of sterile agar. Each new streaking region receives fewer cells than the previous one because the loop is sterilized or otherwise cleared between sectors and carries cells forward only from the edge of the preceding area.

• Primary purpose: To obtain separated colonies from a mixed culture or to detect contamination in a culture believed to be pure.
• Dilution mechanism: Cells are spread from a dense first region into progressively less dense second, third, and fourth regions.
• Growth result: Where individual cells become sufficiently separated, each develops into a distinct colony after incubation.
• Required materials: Sterile nutrient agar plate, inoculating loop or sterile disposable loop, bacterial culture, marker, disinfectant, and appropriate protective equipment.
• Agar surface: The plate should be dry enough to prevent pooling but not desiccated; excess moisture causes streaks to merge.
• Inoculum size: A small amount of culture is preferred. A heavily loaded loop may produce confluent growth throughout the plate.
• Important distinction: The method does not kill unwanted organisms or chemically sterilize the sample; it physically separates cells so that a desired colony can be selected.
• Quantitative limitation: A streak plate demonstrates isolation qualitatively. It does not directly determine viable cell concentration as a counted dilution plate does.

B. To purify the bacterial culture by streak plate method

To purify the bacterial culture by streak plate method, the culture is streaked through successive sectors of a sterile agar plate, followed by incubation, selection of an isolated colony, and confirmation by restreaking.

1. Prepare and label the plate.

   • Plate selection: Use a suitable sterile agar medium that supports the organism and permits colony development.
   • Labelling: Write the sample identity, date, medium, and initials on the base rather than the lid, because lids can be separated or exchanged.
   • Orientation: Keep the plate closed as much as possible and work near a properly disinfected laboratory area.

2. Mix the source culture gently.

   • Representative inoculum: Gently resuspend settled cells so that the loop does not collect only liquid above a sedimented population.
   • Avoid aerosols: Do not vortex or shake an unknown culture unnecessarily; use the approved local procedure for mixing.
   • Interpretation: If the source is mixed, different colony types may appear after incubation and can be separated by selection.

3. Sterilize and cool the inoculating loop.

   • Sterilization: Heat a reusable loop until the inoculating portion is sterile, or use a sterile disposable loop according to laboratory practice.
   • Cooling: Allow the loop to cool before touching the culture; a hot loop can kill cells and produce an unsuccessful inoculum.
   • Aseptic handling: Do not place the sterile loop on the bench or touch it to nonsterile surfaces.

4. Collect a small inoculum.

   • Inoculum control: Touch the loop to a small amount of culture rather than filling the loop.
   • Reason: A light inoculum helps achieve dilution across the plate and reduces confluent growth.
   • Contamination prevention: Keep the culture container open only briefly and close it immediately after sampling.

5. Streak the first sector.

   • Initial deposit: Lift the lid only enough to work and streak a compact area near the edge of the agar using close, parallel lines.
   • Pressure: Use gentle pressure; gouging the agar can trap cells in grooves and interfere with isolation.
   • Pattern: Lines should be close enough to distribute the inoculum but not so crowded that the entire sector becomes a solid lawn.

6. Sterilize the loop and streak successive sectors.

   • Transfer principle: After completing the first sector, sterilize and cool the loop before entering the next sector.
   • Crossing step: Touch the loop once or twice into the edge of the previous sector, then streak into a fresh unused region.
   • Progressive dilution: Repeat the sterilize–cool–cross–streak sequence for the remaining sectors.
   • Sector arrangement: A four-sector pattern is common, but a T-streak or continuous zigzag may also work if each stage reduces cell density.
   • Critical error: Carrying the loop directly from a dense sector into every new region without sterilization transfers too many cells and prevents isolated colonies.

7. Incubate the inoculated plate.

   • Placement: Incubate the plate inverted when appropriate, so condensation does not drip onto the agar and spread colonies.
   • Conditions: Use the temperature, atmosphere, and duration specified for the organism and medium; these variables affect colony size and appearance.
   • Safety: Unknown or potentially pathogenic organisms must be incubated under the laboratory’s approved containment and handling requirements.
   • Observation timing: Examine the plate after sufficient growth appears, without repeatedly opening it.

8. Examine the growth pattern.

   • Dense region: The first sector may show confluent growth because it received most of the original inoculum.
   • Isolated region: Later sectors should contain separated colonies if the streak was performed effectively.
   • Colony features: Record size, shape, margin, elevation, surface, opacity, pigmentation, texture, and hemolysis if the medium permits such observation.
   • Mixed-culture evidence: Colonies with clearly different appearances may represent different organisms, although one organism can sometimes produce variable colonies under different conditions.

9. Select and transfer an isolated colony.

   • Selection criterion: Choose a well-separated colony with a clear margin and no visible contact with neighboring colonies or streak lines.
   • Representative choice: If the culture is expected to be pure, select the dominant, characteristic colony; if different types are present, process each type separately.
   • Transfer: With a sterile loop, touch the center of the selected colony and inoculate fresh sterile agar or broth.
   • Avoidance: Do not scrape agar surrounding the colony, since neighboring cells or contaminants may be carried over.

10. Restreak and verify purity.

    • Restreaking: Streak the transferred colony on a fresh plate using the same progressive-dilution principle.
    • Purity evidence: A purified culture should produce colonies with consistent morphology and no unexpected colony type.
    • Microscopic check: A smear and Gram stain can reveal whether cells have consistent morphology and Gram reaction, but microscopy alone cannot prove species identity.
    • Additional confirmation: Biochemical tests, selective or differential media, molecular tests, or other identification methods may be required to establish identity.
    • Culture record: Record the source, colony selected, medium, incubation conditions, observed morphology, and purification passage.

C. Interpretation, applications, and limitations

The value of a streak plate depends on correct interpretation of colony separation, careful colony selection, and confirmation that the transferred growth is pure.

• Successful outcome: The plate shows a dense inoculation region followed by sectors containing progressively fewer cells and discrete colonies.
• No isolated colonies: Possible causes include an oversized inoculum, insufficient loop sterilization between sectors, excessive moisture, overly dense streaking, or poor growth conditions.
• Growth only in the first sector: The inoculum may have been too small, the loop may have been hot, the organism may not tolerate the selected medium, or incubation conditions may be unsuitable.
• Colonies growing together: Merged colonies cannot be reliably selected as pure because cells from different populations may overlap.
• Unexpected contamination: Colonies outside the expected morphology may result from contaminated medium, instruments, environment, source culture, or handling technique.
• Purity verification: One isolated colony is not automatically proof of purity; restreaking and microscopic or biochemical checks strengthen the conclusion.
• Application to mixed cultures: Distinct colony types can be picked separately, producing individual subcultures for identification or further testing.
• Application to stock maintenance: A confirmed pure colony can be transferred to an appropriate maintenance medium and labelled with organism identity, date, and passage information.
• Limitation—nonculturable cells: Cells that are viable but unable to grow under the selected conditions will not appear as colonies.
• Limitation—similar morphology: Different species may form colonies that look alike, while one species may form multiple appearances because of age, medium, or environmental conditions.
• Limitation—selection bias: The fastest-growing organism may dominate the plate, while slow-growing organisms remain difficult to detect.
• Limitation—contamination control: The method separates organisms but does not replace sterile technique, proper disinfection, or safe disposal.
• Quality standard: A culture should be called purified only after isolated-colony selection produces consistent growth on restreaking and the observed characteristics support a single organism type.