Bacteria Science Fair Questions - A few basic and complex questions about bacteria (Nov/04/2005 )
Any chemical (yes, NaCl and sucrose are chemicals) used to saturate a paper disk which is then placed on a agar plate will diffuse into the media, in this regard there's no difference between what you were going to do with other chemicals -- if the solution on the disk is growth inhibitory, and a sufficient concentration of it diffuses from the paper disc into the surrounding media, the bacteria will not grow there, and there will be a zone of clearing.
You can make the solutions exactly the same by making them on a molar basis. If the molarity of the solutions is the same, then there are the same number of molecules of the substance dissolved in each unit of liquid -- a one mole of any substance is 6.0221415 × 10²³ molecules.
So, how do you know how much of a substance to dissolve to be adding one mole? Look at the MW of the substance. This number tells you how many grams equal one mole. For NaCl, it's 58.44 g/mol, and for sucrose, it's 342.30 g/mol. Thus if you dissolve 58.44 grams of NaCl in a liter of water, and 342.30 grams of sucrose in a liter of water, the concentration of each solutions is one molar, and they have the same number of molecules per liter of water (or any onther measure: per ml, per teaspoon, per drop, etc.)
Alright, but I've thought of another method of comparing growth of bacteria in the different agars (sodium chloride and sucrose). By taking a circle of .25 centemeter grid paper thesame circumference of a Petri dish and taping/glueing it to the bottom so it is visible through the agar, I can see the surface area of growth. Of course, there will be the factor of the density of the growth, and I'm not sure what to do about that. But, if this doesn't work, I'll try your way, which is slightly more complicated, and the kit from Carolina.com that I've talked about before has already arrived, just not the living organisms yet, they come in 2 weeks. Alright, any suggestions are appreciated! Thanks again!
I’m not sure what you are trying to achieve with this now, you have the kit why over complicate things. An experiment to show at what concentration of sucrose or NaCl that does not support growth would be better than measuring the growth
Homebrews method looks like a better way to go if you must deviate from the kit you have. If you are using Molar concentrations as Homebrew suggested I would suggest a range of about 0.01M to 2M for Sucrose and 0.5M to 6M for NaCl, the higher end of both scales with be the overkill end and you will not see anything. I would prepare the higher concentrations than serial dilute down until you get the required concentrations of how many you want down to the lower end of the scale, from this you would need to make some agar plates up using the prepared volumes of NaCl and sucrose solutions, an easier way would be to immerse some filter paper in each solution as previously suggested and then inoculate each of the plates using the spread plate method so that you get a lawn of growth.
I would still however suggest using the kit as its meant to be used as you would get the same results.
Homebrews method looks like a better way to go if you must deviate from the kit you have. If you are using Molar concentrations as Homebrew suggested I would suggest a range of about 0.01M to 2M for Sucrose and 0.5M to 6M for NaCl, the higher end of both scales with be the overkill end and you will not see anything. I would prepare the higher concentrations than serial dilute down until you get the required concentrations of how many you want down to the lower end of the scale, from this you would need to make some agar plates up using the prepared volumes of NaCl and sucrose solutions, an easier way would be to immerse some filter paper in each solution as previously suggested and then inoculate each of the plates using the spread plate method so that you get a lawn of growth.
I would still however suggest using the kit as its meant to be used as you would get the same results.
What unit/type of measure would I use to measure growth using the kits instructions? The kit just shows that salt/sugar kills food spoilage bacteria, but thats why I'm altering the kit -- a science fair project can't be done with this kit, and you can use a kit's materials, but not do directly what the kit says. I could just easily(its not complicating things) measure the surface area in which the bacteria takes up to measure bacterial growth by using the grid paper. Its easy and shows growth; whats wrong with it?
I don’t understand why you cant do a project with this kit, cant you use it as its meant to be used then take samples from each plate to analyse under a microscope to say why you saw now grow or restricted grow etc.?
No... in Science Fair you can't just explain why your project happened that way, like the kit is intended, you have to have some type of measurement that you record your results and trials by, to make graphs and tables of your results. Therefore, I'm not sure what measurements to use when going by the kits instructions, it just "shows" and "explains" why osmosis can kill food spoilage bacteria by osmotic pressure. In my experiment, I need some kind of measurement to show comparisons and results, such as surface area, like in my suggestion by taping/glueing grid paper to the bottom of the Petri dishes and recording the cm2 that the bacteria takes up in that agar. So, with high concentrations of sucros-based and sodium chloride-based agars, the surface area of bacterial growth will be less than the more diluted agars. Any problems? Suggestions?
Maybe you grow them in liquid media supplemented with various amounts of preservatives and record optical density over time? Do you have a spectrophotometer and shaking incubator available?
I've looked up what a spectrophotometer is, but never found what it is in simple terms. I don't know of one available to me, but do I need one? I have an incubator available to me, I'm not sure about a shaking one, but why wouldn't measuring surface area work to compare bacterial growth?
If the surface area idea will work:
I've searched staples, and other locally found office supplies stores, and also have searched google multiple times, but I can't find .25 or .125 centimeter graph paper (to stick to the bottom of the Petri dishes[bottom outside]). Does anyone know where I can find one?
A spectrophotometer simply measures how much light passes through a culture -- the more dense (further grown) the culture is, the less light passes through. It is a quick way to estimate how far a culture has grown, and is recorded as the culture's optical density (OD) at whatever wavelength you measured it at (so you'll see things like "grow the culture to OD600 of 0.3")...
You can print your own graph paper with a utility like this graph paper printer.
I have my doubts about this approach, though...
You can print your own graph paper with a utility like this graph paper printer.
I have my doubts about this approach, though...
I'd use a spectrophotometer if I had one available, but I don't think I do.
What are your doubts about the surface area approach?