Holiday lecture redux: Germs.
January 5, 2010 – 4:03 pmThe theme of last year’s holiday lecture, given twice at Harvard and once at Princeton, was Germs: A Detective Story.
The main part of the talk was about Dr. John Snow, who proved that Cholera is a water-borne illness caused by a bacterium. From the time he began to suspect this to the time he proved it scientifically was thirty years. In the meantime, he only drank distilled water.
The scientific consensus at the time was that a miasma caused cholera, that is, the stink of raw sewage and dead animals and garbage. Of course, conditions where “bad air” was present are also where the water would be contaminated, so the possible causes were confounded. London in the 1850’s was a funky place, but it turned out that water delivery was tightly controlled by a few companies.
One neighborhood had two delivery companies, one which took water from upstream Thames, and the other from downstream. The water downstream was contaminated with sewage, and also with small amounts of salt, from the tidal infiltration of sea water. To the eye and taste, the two waters were the same. The downstream water, John Snow thought, was causing cholera.
From house to house he went, asking about cholera in the household, and asking which water company delivered the water. If the respondent didn’t know which water company served the house, which was often the case, he asked for a sample of the water. Later, in the lab, he would add a few drops of silver nitrate solution to the water. The upstream water would remain clear, but the downstream water would form a precipitate of silver chloride, making the water cloudy.
From this survey and experiment, he found that 90% of the cholera cases came from houses using downstream water. Since all of the houses experienced the same “bad air”, John Snow demonstrated the connection between bad water and cholera.
For the holiday lecture, I started with two glasses of distilled water, and added salt to one, while the lecturer turned his back. With the water glasses shown up close on the video projection screen, I added a pipette full of silver nitrate solution to each. The salty water showed a massive precipitate of silver chloride, much more than John Snow would have seen, but perfect for a demonstration of how two identical looking glasses of water can be very different.
The story of John Snow can be found in the book, The Blue Death: The Intriguing Past and Present Danger of the Water You Drink by Robert D. Morris . He was on his way to being as big as Koch and Pasteur, but tragically died in his forties.
The other demos were microscope based. A video signal converted to VGA and then projected lets everyone see the microscopic world writ large. Working live made the experiment an active experience for everyone.
The first sample was saliva, which came from the kids who volunteered to come down and spit in a tube. Five kids came down and spat in 50 ml plastic centrifuge tubes, and then screwed on the tops. I picked one at random and swabbed some of the saliva on a microscope slide and dropped a cover glass on it. A 40x objective gave a 0.2 mm horizontal field of view. Cheek cells were visible, the size of (American) football on the screen. Motile bacteria were always there, wiggling and swimming about, and with thirty seconds of viewing everyone was ew-ing and aw-ing.
The other microscope demo looked at red blood cells and how they were affected by either saturated salt solution or distilled water. The blood came from my finger, pricked with the spring loaded needle from a blood sugar testing kit. The drop of blood went into a milliliter of normal saline, about 0.7% w/w, and mixed well. That diluted the red blood cells so that they were individually visible on the slide. A tiny drop of the diluted blood went on a cover glass, which was then inverted onto a prepared well slide. The drop didn’t fill the well, leaving room for added solution to wick in and act on the red blood cells.
Saturated salt solution collapsed the red blood cells, turning them from donuts into sickles and then crumpled blobs. Distilled water ballooned the red blood cells, which rapidly faded as they burst and leaked their contents. After that, the only cells easily visible were white blood cells, which also ballooned but are tougher, so it took longer for them to burst. I saw a few pop, very cool to see cell guts spewing out.
What did osmosis have to do with bacteria? Penicillin kills growing bacteria by installing pores into the cell membrane, so the same action that burst the fragile red blood cells could pop the tough bacteria. We showed this: Movie of staph being killed by penicillin.
I learned a lot, and got all jazzed about bacteria. Bacteria are all around us, and in us. I’m reading A Field Guide to Bacteria (Comstock Book) by Betsey Dexter Dyer which is just super. A few pointers on microscopy, but the main focus of the book is the macroscopic indications, or field marks, of various kinds of bacteria. I’m just loving it. Kimchee and yogurt in the fridge, wine turning to vinegar on the counter, all bacterial field marks.
The t-shirts were: Blue for Water, the wizard of osmosis. Purple and orange for Silver Chloride, if you’re not part of the solution, precipitate. Crimson for (cartoon image of lipid bilayer), osmosis, it’s swell. Volunteers got a green shirt with anthropomorphic staphylococcus, Volunteer Staph, our enthusiasm is contagious.
The t-shirts are used to differentiate the kids for playing molecules in short molecular simulations. The kid molecular demonstrations were of silver chloride precipitating, and of cells (ring of kids wearing lipid bilayer shirts) swelling and shrinking in hypo-tonic and hyper-tonic solutions. Pretty sophisticated stuff to act out, it was well done and fun to watch.
