Microbiology, streak plate technique

Streak Plate

The streak plate method is a rapid qualitative isolation method.

The techniques commonly used for isolation of discrete colonies initially require that the number of organisms in the inoculate be reduced.

 It is essentially a dilution technique that involves spreading a loopful of culture over the surface of an agar plate.

The resulting diminution of the population size ensures that, following inoculation, individual cells will be sufficiently far apart on the surface of the agar medium to effect a separation of the different species present.

In the streaking procedure, a sterile loop or swab is used to obtain an uncontaminated microbial culture. The process is called “picking colonies” when it is done from an agar plate with isolated colonies and is transferred to a new agar or gelatine plate using a sterile loop or needle.

The inoculating loop or needle is then streaked over an agar surface.

On the initial region of the streak, many microorganisms are deposited resulting in confluent growth or the growth of culture over the entire surface of the streaked area.

(C) procedure of streak plate T-Streak

The three-phase streaking pattern is known as the T-Streak.

The streaking is done using a sterile tool, such as a cotton swab, or commonly an inoculation loop.

The inoculation loop is first sterilised by passing it through a flame.

When the loop is cool, it is dipped into an inoculum such as a broth or patient specimen containing many species of bacteria.

The inoculation loop is then dragged across the surface of the agar back and forth in a zigzag motion until approximately 30% of the plate has been covered.

The loop then is re-sterilised and the plate is turned 90 degrees.

Starting in the previously streaked section, the loop is dragged through it two to three times continuing the zigzag pattern.

The procedure is then repeated once more being cautious to not touch the previously streaked sectors.

Each time the loop gathers fewer and fewer bacteria until it gathers just single bacterial cells that can grow into a colony.

The plate should show the heaviest growth in the first section.

The second section will have less growth and a few isolated colonies, while the final section will have the least amount of growth and many isolated colonies.

Quadrant method

  In the quadrant method, four equally sized sections are streaked. The continuous streaking method typically involves inoculating the top half of the plate, rotating it 180 degrees, and inoculating the other half of the plate without sterilising the loop or dragging bacteria from the previous section.