|MadSci Network: Microbiology|
The way that heat is applied to water has very little to do with whether bacteria in the water live or die. What matters is the temperature. Most of the things in water that might make us sick (bacteria, viruses, parasites) are killed by boiling, but some bacteria form spores that are not killed by boiling. Fortunately, they are not common in water and are not likely to cause disease if water containing them is drunk. The key to your question is that water (pure, at sea level) boils at 212 degrees Fahrenheit (100 degrees Celsius), no matter how the heat is applied. If you put a pan of water to boil on a stove top burner and measure the temperature with the right kind of thermometer, it will be somewhere near 212 — depending on your altitude and the atmospheric pressure that day. If you put another pan of water in the oven at, say, 350 degrees Fahrenheit, the water won't get any hotter than it did on the stove top because it keeps itself "cool" by boiling. A thermometer will prove this. When all of the water has boiled away, the pan will get to 350. If we want to kill the bacteria that form spores, we do what the canning companies do — use a pressure cooker to raise the boiling point (temperature) of the water. So, you can show that the temperature of boiling water is the same as the temperature of "baking" water with a thermometer that will register such temperatures (say, a candy thermometer). If you dissolve a lot of salt or sugar in the water, you can raise its boiling temperature significantly. If you want to know whether the bacteria have all died, you can buy some sterile "petri plates" of medium (say, nutrient agar, for a common one) and spread a few drops on the agar surface. You can transfer the water, a drop at a time, from the pan to the agar with a metal or glass rod that has been wiped off with rubbing alcohol and let dry. You'll need to put a few drops of unheated water on one plate, a few drops of boiled water on another, and a few drops of "baked" water on a third. "Disinfect" your rod again with rubbing alcohol after you finish each plate. Technique is tricky, though; it's very easy to get bacteria from fingers, your breath, or the air onto the agar surface and ruin the experiment. You can touch the agar in another plate with just the "disinfected" rod (no water) to see whether anything dropped in that was not from the water. Put the lid back on each plate as soon as you have put maybe 4 drops of water at various places on the agar surface. After a half hour, carefully turn the plates over and put them somewhere where the temperature is comfortable (for you) and preferably away from light. Look at them the next morning, and daily till the agar dries out. You should see little clumps of bacteria ("colonies") growing on the agar surface where the water drops were. If there were a lot of bacteria, the whole area wet by the water drops will be covered by bacterial growth. If boiling or baking the water killed all the bacteria, you won't see anything where those drops landed. You may see a few after a couple of days, if there were spores in the water. You can try to look for water bacteria with a microscope, but that's not a really good way to do this kind of experiment because dead bacteria (killed by the heat) may stay around for some time and look just like the ones that were alive. Preparing bacteria to be seen with a microscope requires "fixing" them (mild heating kills them and sticks them to a glass slide). It is usually necessary to stain the bacteria and to look at them with a microscope lens that needs oil under it to magnify the bacteria enough to be seen. If you really want to see the bacteria, rather than just learn whether they are alive or dead, you will need to find a nearby science teacher to give you a hands-on lesson. Good luck!
Try the links in the MadSci Library for more information on Microbiology.