Microbes get a pretty bad rap. We give them long, complicated names like Streptococcus thermophilus, or else we call them something negative like "germs." But we depend on microorganisms in every realm of life, from producing the food we eat to cycling energy in our ecosystems.

Meet Lactobacillus. This friendly microbe lives just about everywhere, including in dairy products and on fruits and vegetables. Most types of Lactobacillus wouldn't hurt a flea (though they might change your milk into yogurt). We use Lactobacillus to make yogurt, as well as cheese, buttermilk, soy sauce and kimchee.

Lactobacillus is an anaerobe, which means it grows best in environments lacking oxygen (though it has no trouble living with oxygen; it just slows down).

When you make kimchee, you set up a very friendly environment for Lactobacillus by filling a bottle with cabbage and adding salt, which helps to release water and sugars from the cabbage cells. By keeping the cabbage submerged in cabbage juice, you create an anaerobic environment.

This combination of no oxygen and lots of sugar is a paradise for Lactobacillus, which happens to be quite fond of sugar. It will happily eat up the sugars and churn out lactic acid, a habit that gives the microbe its name.

This activity is at the heart of kimchee fermentation. The more sugar Lactobacillus eats, the more lactic acid it produces, and this is why the pH of your kimchee drops over time. Lactobacillus grows best at a pH of about 5. The accompanying chart shows how pH, glucose (or sugar), and lactic acid all change over time in kimchee fermentation.

There are several species of Lactobacillus, which fall into two major groups depending on what they produce after eating sugars. The homofermentative bacteria produce one thing: lactic acid.