1. Culture and Isolation of Bacteria

3. 1. Streak Plate Method:
This method is used most commonly to isolate pure cultures of bacteria.
a small amount of mixed culture is placed on the tip of an inoculation loop/needle and is streaked across the surface of the agar medium
successive streaks “thin out” the inoculum sufficiently and the micro-organisms are separated from each other.

4. Isolation
If an individual bacterial cell is separated from other cells and has space on a nutrient surface, it will grow into a mound of cells— a colony. A colony consists of one species.

5. Isolation Techniques Streak plate technique Pour plate technique
Spread plate technique

6. Media: Providing Nutrients in the Laboratory
Media can be classified according to three properties:
Physical state – liquid, semisolid, and solid
Chemical composition – synthetic (chemically defined) and complex
Functional type – general purpose, enriched, selective, differential, anaerobic, transport, assay, enumeration

7. Physical States of Media
Liquid – broth; does not solidify
Semisolid – contains solidifying agent
Solid – firm surface for colony formation
Contains solidifying agent
Liquefiable and nonliquefiable

8. Agar The most commonly used solidifying agent
Solid at room temperature, liquefies at boiling (100oC), does not re-solidify until it cools to 42oC
Provides framework to hold moisture and nutrients
Not digestible for most microbes

9. Most Commonly Used Media
Nutrient broth – liquid medium containing beef extract and peptone
Nutrient agar – solid media containing beef extract, peptone, and agar

10. Chemical Content of Media
Synthetic – contains pure organic and inorganic compounds in an exact chemical formula
Complex or nonsynthetic – contains at least one ingredient that is not chemically definable
General purpose media – grows a broad range of microbes, usually nonsynthetic
Enriched media – contains complex organic substances such as blood, serum, hemoglobin, or special growth factors required by fastidious microbes

11. Examples of Enriched Media

12. Selective & Differential Media
Differential media: allows growth of several types of microbes and displays visible differences among those microbes
Selective media: contains one or more agents that inhibit growth of some microbes and encourage growth of the desired microbes

13. Concept Check:
CHROMagar contains several dyes and is used to diagnose Urinary Tract Infections. The patient’s sample is inoculated and based on the color of the colonies you can identify the pathogen. CHROMagar is best described as:
A. Enriched
B. Selective
C. Differential
D. Complex
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
© Kathy Park Talaro

14. Concept Check:
CHROMagar contains several dyes and is used to diagnose Urinary Tract Infections. The patient’s sample is inoculated and based on the color of the colonies you can identify the pathogen. CHROMagar is best described as:
A. Enriched
B. Selective
C. Differential
D. Complex
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
© Kathy Park Talaro

15. Some media can be both Selective & Differential

16. Miscellaneous Media
Carbohydrate fermentation medium – contains sugars that can be fermented, converted to acids, and a pH indicator to show this reaction 16 16