It doesn’t. It’s E. coli.
Today we began Day 1 of the Microbial Genetics Lab for Undergrads here at Michigan State – we’re lucky this year with small class sizes of 32 and 28 students, with three instructors per section! Students arrive, usually without the pre-lab completed, hear a short technical lecture on the day’s tasks, and then set out to do the tasks. I find the entire thing to be a crazy fun bustle of confusion as we teachers tumble about the room correcting flaws in sterile technique, quizzing students about their conceptual understanding of the experiments, encouraging the especially curious. (Will there be any Martin Arrowsmiths this semester?) This is my second semester of teaching, so I'm still learning how to organize a classroom.
Today we spent an hour covering all the things micro students “should” know already: how to flame sterilize an inoculating loop until red hot, how to ethanol and flame a`hockey stick, how to put out an ethanol fire… (On this last point, we demonstrated and then had each student repeat this process: 1. Purposely lighting our ethanol containers on fire, and then 2. Calmly placing the container lid over the blaze, which extinguishes the fire.) The students seemed to get a kick out of this. No, lab safety doesn’t have to be boring. <<That said, you should read safety instructions like this and have someone experienced demo the technique before you before trying it out yourself.>>
Next I asked the students what they would be observing in the next lab period. Most said, “Whether or not the strains grow.” This answer, I think, is a residual response from the intro micro lab, where the students spend the semester streaking unknown isolates on various media, using growth to help identify species and strains – it’s typical flowchart work that doesn’t require much thinking. So, I next got to explain that we’ll be observing gene expression using color indicators and that we’ll be thinking a lot about genes and operons this semester, and welcome to microbial genetics. Today we used x-gal to indicate expression of the gene coding for B-galactosidase, which results in blue colonies.
Sometimes I ask my students questions like, “Why does E. coli want to produce blue pigments?” That gets them to think and provides a teaching moment that a) E. coli doesn’t really want or not want to do anything; it's E. coli, and b) In the natural environment B-galactosidase metabolizes the sugar lactose, not x-gal, which is a convenient substrate we use in the lab to produce blue colonies.
If everything makes more sense from an evolutionary perspective, we might ask, “Why did E. coli evolve to produce blue pigments?” It didn’t. The E. coli lac operon evolved as an efficient way to regulate expression of lactose metabolism. (Not everything is an adaptation, after all.)
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