Streak Plate Technique and Special Media

Introduction

The streak plate method and the use of special media are fundamental techniques in microbiology for isolating and identifying microorganisms. These methods allow for the separation of individual species from mixed cultures and the differentiation of bacteria based on their growth characteristics and metabolic properties.

Streak Plate Method

Purpose and Principle

• Streak Plate: A laboratory technique used to obtain pure cultures by spreading bacteria over the surface of an agar plate in a pattern that thins out the sample and separates individual cells.

• Pure Culture: Contains only one species of microorganism, essential for accurate identification and study.

• Each sector of the streak plate picks up fewer cells, leading to isolated colonies derived from single cells.

Example: Streaking a mixed culture of Escherichia coli and Staphylococcus aureus can result in isolated colonies of each species for further analysis.

Types of Special Media

Overview

• Selective Media: Inhibits the growth of unwanted organisms while allowing the desired organisms to grow.

• Differential Media: Distinguishes between organisms based on observable changes (e.g., color change) in the medium.

• Enrichment Media: Enhances the growth of a particular microorganism from a mixed population.

Common Media Used

Nutrient Agar

• General-purpose medium; not selective or differential.

• Supports the growth of a wide range of non-fastidious organisms.

• All four known cultures (E. coli, S. aureus, S. epidermidis, A. faecalis) grow on nutrient agar.

Mannitol Salt Agar (MSA)

• Selective: Contains 7.5% NaCl, which inhibits most bacteria except salt-tolerant Staphylococcus species.

• Differential: Contains mannitol and phenol red (pH indicator).

• Interpretation:

  • S. aureus ferments mannitol, producing acid and turning the medium yellow.

  • S. epidermidis grows but does not ferment mannitol; the medium remains red.

  • Other bacteria (e.g., E. coli, A. faecalis) typically do not grow or do not ferment mannitol.

Eosin Methylene Blue Agar (EMB)

• Selective: Eosin and methylene blue dyes inhibit the growth of Gram-positive bacteria.

• Differential: Differentiates lactose fermenters from non-fermenters.

• Interpretation:

  • Lactose fermenters (e.g., E. coli) produce a metallic green sheen.

  • Other lactose fermenters form pink to dark colonies.

  • Non-fermenters produce colorless colonies.

  • Gram-positive bacteria (e.g., S. aureus) are inhibited and show little or no growth.

Laboratory Procedures

Labeling and Inoculation

• Label the bottom (agar side) of each plate with name, date, class, and medium.

• Divide plates into quadrants for known cultures; label one plate as "Mixed" for the unknown culture.

• Inoculate each quadrant with a different known culture, flaming the loop between each.

• Streak the unknown mixed culture for isolation using the sector method, flaming the loop between sectors.

Post-Lab Procedures

• Label and stack plates, tape them together, and place them on the designated tray.

• Cap and return culture tubes to the appropriate bin.

• Sterilize loops, disinfect benches, and wash hands after completing the lab.

• Record expected results and hypotheses in the lab manual.

Recording and Interpreting Results

Observations

• Record growth (+/-) and colony appearance on each medium.

• Gram stain one colony of each organism from nutrient agar plates.

• Describe unknown colonies using standard morphological terms.

• Streak isolated unknown colonies to new plates to check for purity.

Critical Thinking: Identifying Bacteria

• Growth on EMB indicates Gram-negative bacteria; no growth indicates Gram-positive.

• Metallic green sheen on EMB is characteristic of E. coli.

• Yellow on MSA indicates mannitol fermentation (e.g., S. aureus); red indicates no fermentation (e.g., S. epidermidis).

Additional info: Table entries inferred from standard microbiological characteristics.

Reminders and Next Steps

• Complete Canvas Quiz 4 by the due date.

• Read results for Exercises 11 and 12 and prepare for Exercise 13 on Carbohydrate Catabolism.

• Bring your lab manual to the next class.