In our post on the origins of life in your refrigerator, we strayed into the area of friendly bacteria. Since this is the time of year when everyone seems to be sick, it seems like a good time to cover this topic.

Bacteria are often thought of as dirty, dangerous, denizens of the world, but we share our bodies with many of them. In fact, we are likely home to more bacterial cells than human cells! By recent estimates, an average adult human has nearly 4 × 10¹³ human cells. That’s forty trillion cells. As if that number isn’t staggering enough, your body is home to ten times more bacterial cells (Update: This estimate has been heavily criticized with an extremely rough estimate of 1.3:1 and a very detailed analysis published this year in Nature). Despite their staggering numbers, our bacterial cells are quite a bit smaller than our human cells, so they typically only add about 2 to 5 pounds to your body weight.

The bacteria normally in our bodies, the ones that comprise our microbiome, are both good and bad. Escherichia coli (E. coli) is a regular inhabitant of your gut when you are healthy, although its name is nearly synonymous with disease. Helicobacter pylori, the bacteria that causes stomach ulcers, contains DNA sequences which can be strongly anti-inflammatory as the dead bacteria pass into your intestines. These are both bacteria which can make us extremely sick when behaving badly, or they can help keep us healthy. The balance of our microbiome affects our everyday health.

But how do we get exposed to all of these bacteria in the first place? All of those bacteria we live with had to come from somewhere. We get some from our mothers in amniotic fluid, during birth, or from breast milk. We colonize ourselves with bacteria when we eat dirt as children or forget to wash our hands before eating. Everything we touch and the air we breathe affects our microbiome. Even if you think you live in a sterile environment, you may be surprised as to how much you are safely exposed to even from the food you eat. These innocuous bacteria are the reason a jug of unopened milk can go bad. Sure, it’s pasteurized to delay spoiling, but it’s not sterile. If you cooked a piece of meat until the bacteria in it are dead, there will likely be more on it by the time it is cool enough to put in your mouth. For people lacking immune systems, this is a constant struggle, but for the rest of us, this is how we live, even if we are unaware of our open invitations for bacteria to share our lives with us.

Homemade Yogurt

Some bacteria are even necessary for the foods we eat. If you eat cheese, yogurt, miso, or honey, you are eating living bacteria. Most of these foods cannot be made without the help of bacteria and we take advantage of some of these in the Citizenship in Science kitchen. Experimenting with food uses many of the same skills as experimenting in a lab, so we are always up for making new things. For anyone interested in growing healthy bacteria at home, one surprisingly simple food to make is yogurt; some helpful links are included below and we’ve included a picture of one of our batches. Give it a try before we get to part 2 of this topic where we address the bad and the ugly of bacteria.

http://www.culturesforhealth.com/
http://www.makeyourownyogurt.com/
http://www.npr.org/2013/11/06/243014945/yes-it-s-worth-it-to-make-your-own-yogurt