To grow a bacteria culture in a lab we normally use agar in a growth solution, such one formula I use is called Luria broth. It may be difficult to grow bacteria on only gelatin; they typically need additional nutrients (sugar or starch, amino acids vitamins and minerals) to grow. You can play around with some at home formulas that add sugar and beef bouillon cubes to the jello. One recipe could be:

1.5 packets of plain gelatin,
1 cup of water,
2 teaspoons of sugar,
4 beef cubes-
and follow the instructions for making jello!

The beef cubes will be a good source of the amino acids, vitamins and minerals. You can use sterile cotton swaps to collect your samples- be sure not to touch them to anything else! Mold spores and fungus may also be an issue in growing bacteria at home, so also cover your samples.

I also challenge you to think more about this question you are asking, and what other factors could influence the outcome. What outcome do you expect to observe? Will males have more bacteria than females, and why do you think so? What other factors could effect this, i.e. exercise, hair, where you collect the sweat (underarm, back of knee, hands, etc), use of deodorant or antiperspirant? You may find that there isn’t a relationship between sex and bacteria, and there may instead be a trend between whether or not the sample comes from someone who uses deodorant as opposed to antiperspirant. If you are so brave, you could also do a smell test! After all, smelly pits are caused by a buildup of bacteria; sweat is not what smells. I would suggest to you to do some research on body odor and how your body sweats.

The combination of sweat and dead skin cells are not too different than the food cocktail you are trying to make to grow these microorganisms- you have your salts and amino acids all in one warm and moist contained area!

Interesting question. My inkling is that sex will not play a factor in the amount of bacteria in sweat. Sweat itself does not actually contain bacteria and what you will pick up and grow is the bacteria that live on our skin. You could ask; do males or females have more bacteria on their skin? Or perhaps; how does sweating increase the amount of bacteria on the skin? You could sample someone when they are resting, then sample the same location after they have ran in place and started sweating! The amount of bacteria that you see growing could be attributed to many different things, but probably is a measure of how much someone showers or what soap they use to shower. The amount and types of bacteria on the skin differs greatly even when you compare individual fingers!

With this in mind, you should always make your collections from the same location on different individuals. Good luck!

You're going to want something to grow the bacteria on (like a petri dish with agar gel), and enough time to allow the bacteria to grow (a week, maybe?). I am certain that you will find some bacteria that will grow on your gelatin. What I don't know is how many kinds of bacteria will NOT grow on your gelatin, or how common they are in human sweat.

This is a very cool project. The bacteria that cause bad odor on our skin and exist in our sweat are a question that many scientists in industry want to tackle, find out more about and eventually control if not eliminate.

I would urge you to read these small articles from NRP or the Smithsonian which are particularly interesting. And very new! Just a few months ago this research was making headlines!

On to your project: Eccrine sweat glands (like the ones on our arms) produce water, electrolytes but also sufficient salts to make sweat acidic. Bacteria don't like it, so they don't live there. So, your best bet on which sweat to use is the one from the armpits (the "axillary vault") which comes not from eccrine glands, but from apocrine glands.

Here's a bacterial map, of which types of bacteria grown where. This can help you with how to identify them.

The pH is a very important parameter for their survival.

So in principle, you can grow your sweat bacteria on the agar gel but you may have to adjust the pH