Science Buddies: "Ask an Expert"

Hey Science Buddies,

I'm doing a project recommended by CMS (viewtopic.php?t=17962) , in which I find bacteria cultures capable of degrading oil in common water samples. I have a few questions.

1) How much crude oil should I add to my water samples? Is there a certain ratio you would recommend, say 2ml of crude oil every 10ml of water?
2) How long should I let the crude oil sit in the water for the bacteria to (possibly) start digesting the oil?
3) Why do I need to isolate the bacteria on the petri dishes? Am I counting the colonies?
4) How long would you say the crude oil + water bacteria would take to form colonies on the petri dish? 24 hours?

Thanks,
Marla

Hi, Marla! These are very good questions that you're asking before you get started on this experiment!

1) There are really two important factors to consider as you work this out. The first is, of course, that you don't want to add so much oil that you overwhelm and sicken even bacteria that potentially could survive in crude oil. Secondly, though, you want to add enough oil that if your water samples do contain oil-digesting bacteria, you would be able to reliably measure the difference in the amount of oil before and during/after the experiment duration. (For example, if you started by adding 2ml of oil, do you have equipment that is sensitive enough to accurately measure the potentially <2ml oil that remains later in the experiment?) Unfortunately, I don't have a direct answer for your questions, but searching through either the Science Buddies project ideas or published scientific literature for similar experiments would give you some reasonable starting points.

2) This is also a really good question for published project ideas or scientific papers, but my semi-educated guess (I am a molecular biologist) would be to try measuring the amount of oil in your cultures once a day for a few days to a week. The rate of change in the amount of oil will depend on a number of factors, including the temperature at which you perform your cultures and how much (if any) oil-digesting bacteria your water samples contain.

3) Swabbing the water on Petri dishes is not so much for counting colonies as it is for helping you identify what types of bacteria are living in your water samples. I'm not sure how elaborate the tools are that you have available to you, but at a minimum, you can compare what different bacterial colonies look like from each of your water samples:
http://microbiologyonline.org/teachers/ ... petri-dish

4) How long it will take to see colonies depends on factors including the temperature you do your cultures (warmer will grow faster) and how many bacteria were in the samples to begin with. A good estimate is 24-72 hours; if there are no visible colonies after several days, they are unlikely to appear with more time.

I hope this helps, and good luck!