Monday, September 28, 2009

Isolating bacteria through streak plate technique

My streak plate technique is off to a fairly good start, it seems. The streak plate method is a way of isolating bacteria into manageable colonies through a series of dilutions.

If you orient the Petri dish as a clock face, notice how thick the bacterial streaks are around the 6 o'clock position. That's zone 1, where I dipped a sterile loop into the broth containing Serratia marcescens, a pink- or red-colored bacterium often found in showers, feeding off phosphorus or lipids in soaps and shampoos. Zone 1 in my Petri dish is the growth that resulted directly from the broth.

Zone 2, which is the area at an angle to Zone 1, running from about 5 o'clock to about 3 o'clock, is less overgrown, though still pretty thick. This is due to taking the loop out of Zone 1, flaming it till it's sterile, then streaking out from Zone 1. So the bacteria in Zone 1 are correspondingly diluted over a larger area.

Zone 3, at an angle from Zone 2 from about 3 o'clock to about 12 o'clock is made in a similar way--flame the loop until sterile, and then dilute Zone 2 over a larger space. Finally, Zone 4--the lines coming out orthogonally from Zone 3--were made the same way, and usually by this point, the inoculum (sample) is diluted enough to where the bacteria can grow in individual colonies, rather than an overgrown "lawn" like in Zone 1.

My Zone 4's not perfect; I will continue to work on a lighter touch. But for an initial attempt, I'm pretty pleased with my colonies from it.



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