Bacteria have gotten a bad reputation, and for good reason. Bacteria are behind a number of serious diseases — including pneumonia (Streptococcus pneumoniae), meningitis (Haemophilus influenzae), strep throat (Group A Streptococcus), food poisoning (Escherichia coli and Salmonella),and a variety of other infections.

These “bad” bacteria are the reason why we diligently disinfect our hands and wipe down our kitchen and bathroom sinks, as well as any other places where germs tend to congregate. We also have developed a wide range of antibiotics, which are drugs designed to kill the bacteria that cause disease.

Yet, not all bacteria are bad guys. In fact, our bodies are home to an estimated 100 trillion “good” bacteria, many of which reside in our gut. Not only do we live in harmony with these beneficial bacteria, but they are actually essential to our survival.

Good bacteria help our bodies digest food and absorb nutrients, and they produce several vitamins in the intestinal tract — including folic acid, niacin, and vitamins B6 and B12. According to research published in the journal Best Practice & Research Clinical Gastroenterology, beneficial bacteria may also protect us against their dangerous relatives that cause disease by crowding them out in the gut, producing acids that inhibit their growth, and stimulating the immune system to fight them off.

When helpful bacteria multiply and thrive in our bodies, they act as our protectors. But sometimes, we put the population of beneficial bacteria at risk. When we take antibiotics to treat an infection of harmful bacteria, we also kill helpful bacteria. This can cause an imbalance of bacteria in the body that can lead to diarrhea and other gastrointestinal problems.

The idea that certain types of bacteria can improve our health has been around since the early 20th century, when Nobel prize-winning Russian biologist Elie Metchnikoff first proposed that eating bacteria similar to those living in the body could have health benefits. More recently, companies started marketing products called probiotics (which means “for life”) containing these bacteria.

Probiotics are available in many forms, including supplement pills, suppositories, and creams. Many foods contain friendly bacteria, such as:

  • yogurt
  • buttermilk
  • cheeses with live active cultures

Other foods that contain friendly bacteria include fermented foods such as:

  • miso
  • tempeh
  • sauerkraut
  • beer
  • sourdough bread
  • chocolate
  • kimchi

Probiotics are proposed to prevent and treat a variety of health conditions, such as:

  • diarrhea (including diarrhea caused by
    antibiotics)
  • irritable bowel syndrome
  • ulcerative colitis and Crohn’s disease
  • tooth decay, gingivitis, and periodontitis
  • eczema

A few studies have hinted that probiotic pills might improve health, but many medical researchers such as those at the Cleveland Clinic report that there is not enough proof to say for sure.

Below are a few of the probiotics that are taken to treat or prevent disease, and how they’re thought to work.

Lactobacillus

In the body, lactobacillus bacteria are normally found in the digestive, urinary, and genital systems. You can also find them in yogurt and dietary supplements, as well as in suppositories.

More than 50 different species of lactobacillus exist, including:

  • Lactobacillus acidophilus, one of the
    most commonly used probiotics. It’s found in yogurt and fermented soy products
    such as miso and tempeh. Lactobacillus acidophilus has been
    used (in suppository form) to treat bacterial infections of the vagina. In pill
    form, it can be taken to prevent and treat diarrhea, including traveler’s
    diarrhea in adults and diarrhea caused by rotavirus in children.
  • Lactobacillus rhamnosus GG may help treat
    traveler’s diarrhea, or diarrhea that’s caused by Clostridium difficile
    (C. difficile) bacteria or by antibiotics in children. It’s also
    been found to help prevent eczema in infants.
  • Lactobacillus salivarius may help
    block the growth of Helicobacter pylori (H. pylori),
    the bacteria that cause peptic ulcers.
  • Lactobacillus plantarum can improve
    the immune system barrier against invading disease-causing bacteria.

Other uses for lactobacillus include:

  • preventing diarrhea caused by antibiotics and
    infection
  • preventing colic (inconsolable crying) in babies
  • preventing lung infections in young children
  • preventing diarrhea in adults who are in the
    hospital or receiving chemotherapy treatment for cancer
  • treating bowel conditions such as irritable
    bowel syndrome (IBS) and ulcerative colitis

Bifidobacteria

Bifidobacteria make up most of the “good” bacteria living in the gut. These bacteria begin colonizing the gastrointestinal system almost immediately after we’re born.

Bifidobacteria come in about 30 different strains, including:

  • Bifidobacteria bifidum may help
    protect against unhealthy bacteria. Research suggests they
    also can relieve IBS symptoms. When combined with Lactobacillus
    acidophilus    , Bifidobacteria bifidum might help prevent eczema in
    newborns.
  • Bifidobacteria infantis are thought
    to help relieve the symptoms of IBS, such as abdominal pain, gas, and bloating
  • Bifidobacteria lactis has been
    reported to improve cholesterol levels in women and in people with type 2
    diabetes.

Streptococcus thermophilus

These bacteria produce the enzyme lactase, which the body needs to digest the sugar in milk and other dairy products. Some studies suggest Streptococcus thermophilus can help prevent lactose intolerance.

Saccharomyces boulardii

Saccharomyces boulardii is actually a type of yeast, but it acts as a probiotic. Some studies have found it helpful for preventing and treating traveler’s diarrhea, as well as diarrhea caused by antibiotics. It may also be useful for treating acne, and reducing the side effects of antibiotic treatment for H. pylori bacteria.

Before you take any probiotic supplement, remember that the U.S. Food and Drug Administration (FDA) has not approved these products. That means you can’t be sure when you buy a product whether it’s safe and effective. There have been cases reported of people developing bacteria (bacteremia) or fungi (fungemia) in the blood after taking probiotics. More clinical studies are needed to confirm the benefits of probiotics, as well as the possible risks.

Let your doctor and pharmacist know before you take any probiotic supplement. Ask whether it’s safe for people with your health conditions, and if it will interact with other medications you are already taking. It’s especially important to tell your doctor before using these supplements if you are pregnant or nursing, have a weakened immune system from a condition such as HIV/AIDS, or are taking drugs that suppress your immune system.