Think like a scientist to learn like a scientist

This week the microbiology students are studying prokaryotic cell anatomy, so this is the week I spend a lot of time talking about Gram-positive and Gram-negative bacteria.

For the uninitiated, the nutshell description is that bacteria can be classified as Gram-positive (G+) and Gram-negative (G-) based on whether they retain a particular stain under the Gram staining process. Here’s a video. (I didn’t make it.)

Differential staining isn’t all that interesting in itself, but the anatomical differences underlying the differential staining behavior (and also underlying differences in pathogenicity and vulnerability to drugs) are. These anatomical differences come down to how thick is the peptidoglycan layer, whether there is a lipopolysaccharide layer, and stuff having to do with flagella. As always in comparative anything, they’re frequently laid out in tables (like this) and diagrams (like this) which appear in every microbio textbook there is.

Now, I do like textbooks, and I do like the figures and tables that summarize large amounts of information for quick reference. Where problems arise is in students’ tendency to zero in on them and spend all their energy memorizing them.

Scientists don’t memorize big lists of facts. Instead, they fit new facts into the knowledge they already have. In the case of G+/- bacteria, it comes down to what’s happening during staining.

When you add the first stain, the stain particles enter all bacteria. Then you add Gram’s iodine, which forms a complex with the stain particles. Then you wash the slide with alcohol, and the stain-iodine complex leaves some cells but not the others. The cells that retain the complex are G+, and the G- cells can be stained with something else.

That’s a narrative, and this business about particles entering cells, forming complexes, and then being too big to exit might remind you of other narratives. (Bananafish, anyone?) I know that my own brain takes better to stories than to lists, so I use this narrative to anchor all the facts I need to retain about bacterial cell-wall anatomy.

Like this: Is it G+ or G- that has the thicker peptidoglycan layer? Well, it’s the G+ that retains the first dye, so it’s the G+ whose cell wall is impermeable to the stain-iodine complex. It therefore seems that the G+ must have the thicker peptidoglycan layer. Here’s another: Which has the lipopolysaccharide layer? It turns out that the alcohol wash disrupts the LPS layer, and that disruption promotes permeability to the complex. So it’s the G- that has it.

Learning biology means learning a lot of facts, but it can help to connect them together. Form conceptual complexes. In this case, I grouped some facts about structure to some facts about function, and I used the narrative of the staining procedure to give me a handle on it all. Grouping facts into complexes reduces the sheer number of things you have to remember — and more important, it’s understanding rather than simply remembering.