How to Kill Mice & Get Away With It: Griffith's Experiment

What a dashing man.

We know now that deoxyribonucleic acid (DNA) is the genetic material. But it wasn't so obvious back in the day. A few animal sacrifices were needed.

Frederick Griffith was a scientist who conducted one of the first notable sacrifices to determine the genetic material. He used a spherical bacterium (which he also killed for some trials) called Streptococcus pneumoniae. They look like this:

There are two kinds of Streptococcus strains: the smooth (S) strain, and the rough (R) strain. The S strain is harmful because it doesn't have antigenic markers to help immune systems detect and kill it. The R strain is relatively harmless because it's studded with antigens that make it nice and rough and conspicuous enough for our immune systems to #slay it.

So Griffith basically injected the R strain into mice for fun, and found out that it doesn't kill mice. No shock. Then he thought it would be fun to inject the S strain into mice. They died. No shock. He then injected a heat-killed S strain, which means that he boiled the S strain bacteria until they died. He injected the dead bacteria into the mice. The mice didn't die. No shock.

But then something shocking happened. He injected the live R strain mixed with a heat-killed S strain. The anticipated result is for the mice to survive, since the R strain by itself is harmless, and the heat-killed S strain by itself is harmless. But the mice died. How could this be?

Results of Griffith’s mouse sacrifices experiments.

Griffith postulated that something had to be transferred between the dead S strain and the live R strain such that the (normally harmless) R strain was capable of doing the actions / having the effects of the S strain, as if it were alive: an instruction manual, of sorts. This idea led to a revolutionary series of experiments that eventually created the understanding that DNA was the genetic material.

My next few articles will cover the rest of the relevant experiments. In the meanwhile, watch this video for a nice refresher of Griffith's experiment: https://youtu.be/ARHrYr_SqHs

Stay sciencey! <3