Isolation Technique

In nature microbial cultures are mixed

Identification relies upon isolating individual colonies

Testing requires pure cultures

As a result isolation technique provides an essential microbiological tool

Mixed Culture from Raw Poultry

Streak Plate Isolation Principle

An original inoculum containing a mixture of bacteria is spread into 4 quadrants on solid media.

The goal is to reduce the number of bacteria in each subsequent quadrant.

Colonies are masses of offspring from an individual cell therefore streaking attempts to separate individual cells.

Discrete colonies form as the individual cells are separated and then multiply to form isolated colonies in the later quadrants.

The Goal -Isolated Colonies to Start Pure Cultures

Can an isolated colony be considered pure?

This is generally assumed, however….

some colonies are very slow growers and may be too small to see.

some colonies may be growing under another colony

selective media may be preventing reproduction of some bacteria so they may be present but not visible

condensed water, capsules, slime, all represent areas where individual contaminant cells hide out.

Any special considerations?

Different species of microbes represent challenges….

Encapsulated bacteria are sticky and don’t separate well.

Some species are motile and do not stay where you streak them spreading across the plate.

Fungal spores easily contaminate cultures within a plate.

Organisms can gain entrance to a Petri dish through water or the edges, or from the air currents while you are streaking.

Microbes will surprise you each chance they get !

Isolation Requires Aseptic Technique

Isolation Requires Aseptic Technique

Streaking the Quadrants

Quadrant 1- Streak with broad narrow strokes in the upper half of the first quarter of the plate.

Incinerate and cool the loop between the quadrants

Quadrant 2 – Rotate the plate, enter the previous streak mark one or two times and then streak the upper portion of the second quarter of the plate with broad strokes.

Quadrant 3 – Rotate the plate, enter quadrant 2 one or two times and then streak with shorter more separated strokes from the top of the quadrant to the center.

Quadrant 4 – Enter quadrant 3 and then streak with broad S-shaped motions through the center of the plate.

Streaking the Quadrants

Isolated Colonies

Gram Stain

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