Run and Tumble

We did streak plates earlier this week and I had a blast.

I’d been looking forward to doing a streak plate since before I even fully decided to take a class. Streaking is really basic and easy, but it elegantly solves one of the earliest problems in microbiology, “How do you isolate bacteria from the hodge podge that is their environment?”

The early attempts used error-prone dilution techniques, but that was not ideal. There were tons of problems with contamination, it’s fidgety, and it doesn’t isolate bugs that have a low population count with respect to the total number of microbes.

The streak method is simple and clever. You divide your plate into a number of partitions, usually 3 or 4. The first partition gets a bunch of streaks from an inoculated loop. Subsequent streaks are started with a sterilized loop that begins by briefly passing through the previous set of streaks. This essentially reduces the microbial load. By the last streak, the loop should be depositing either single bacteria or a small amount glommed together (basically when something like a diplococci or tetrad won’t break up). The colonies that arise from these colony forming units (CFU) pretty much contain clones of the isolated bugs.

Koch devised it after observing that colonies growing on some old potato slices in his lab had different morphologies from each other. He figured out that each colony must have arisen from a single airborne CFU landing on the slice and that streaking in the manner described would simulate this. Once again, Pasteur’s remark that fortune favors the prepared mind was borne out.

I’ve read about this multiple times, but the description that best captures how thrilling it must’ve been occurs in De Kruif’s Microbe Hunters, which is a must-read for anyone even passingly interested in the history of microbiology.

We got our plates back tonight. I was able to get isolated colonies of both species from our test sample, but my technique was really sloppy.  I took some time to practice on a blank plate. I think I’ve got it down, now. The ‘a-ha!’ moment came when I lifted my elbow off the bench and started streaking from the shoulder, rather than from the wrist. I was able to go slower, with a lot less jitter, and much  more control.

There was also a little messing about with differential media (EMB and MSA). Not much to mention here except that whole idea behind differential media deeply appeals to my analytical engineering side and that E. coli is really pretty on EMB, like even prettier in real life than in a photo. (Thanks to Wearn for putting up that image).

We also gram stained samples from the isolated colonies. I’m happy to report that my stains worked much better this time and I had absolutely no trouble using the scopes. I found out I prefer rotating the ocular tube and arranging the microscope so that the stage, rather than the arm, is facing me. I even had another student say that the E. coli under my scope looked like it came from a textbook, so I’m really happy!

Next lab session is our first practical exam. 3 minutes per station. I feel like my technique is respectable for someone who’s only been doing this a short while. I worry a little bit about the ‘over the shoulder factor’ causing me to stress and do something dumb. Wish me luck.