Exercise 1-4 Micro Lab: Streak Plate Methods of Isolation

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mixed culture

a microbial culture consisting of two or more species


pure culture

a microbial culture consisting of only a single species


What is the first step in identifying an organism?

isolation technique


isolation technique

obtaining isolation of individual species from a mixed sample


A commonly used isolation technique

streak plate


streak plate method of isolation

a bacterial sample (a mixed culture) is streaked over the surface of a plated agar medium. the cell density decreases, eventually leading to individual cells being deposited separately on the agar surface.



cells (on the streak plate that have been sufficiently isolated) that consist of only the original cell type grow into colonies


colony forming unit (CFU)

some colonies originate from individual cells and others from pairs, chains, or clusters of cells


What are streak patterns designed to do?

to separate deposited cells (CFUs) on the agar surface so individual cells (CFUs) grow into isolated colonies.


quadrant streak method

usually perfomed as the initial streak for isolation of two or more bacterial species in a mixed culture with samples suspected of high cell density


zigzag streak pattern

usually used with samples containing lower cell densities and with pure cultures when isolation is not necessary


The identification process of an unknown microbe relies on what?

obtaining a pure culture of that organism


How do you create a pure culture?

a portion of an isolated colony is transferred to a sterile medium to start a pure culture


When performing streak plate inoculation, what are the two ways you can hold the plate?

1. leave the sterile agar plate on the table and lift the lid slightly, using it as a shield from airborne contamination

2. place the plate lid down on the table and then remove the base and hold it in the air on an angle


What are the 4 steps in the quadrant streak method using a loop?

1. streak the mixed culture back and forth in one quadrant of the agar plate. flame the loop.

2. rotate the plate 90 degrees and touch the agar in an uninoculated region to cool the loop. flame the loop.

3. rotate the plate nearly 90 degrees and streak again using the same wrist motion. cool loop prior to streaking. flame loop.

4. after cooling the loop, streak one last time into the center of the plate. flame the loop and incubate the plate in an inverted position.


What 2 broth cultures will we use in class for practicing streaking methods?

1. micrococcus luteus
2. staphylococcus epidermidis


What 2 types of agar plates are generally used?

1. Trypticase soy agar
2. Nutrient agar plate