Bacteria: the Good, the Bad, and the Ugly

by Jennifer Yttri, PhD

January 2013

Bacteria are everywhere, including your entire body. The bacteria in our body weighs as much as our brain–3 lbs! Bacteria can be harmful, but some species of bacteria are needed to keep us healthy. The bacteria on our skin, in our airways, and in our digestive system are the first line of defense against foreign “invaders” (pathogens) that can cause infection and other problems.

Bacteria also act as “tuning forks” for our body’s immune system, making sure it’s pitched just right. The immune system shouldn’t be too sensitive or too sluggish: it needs to respond quickly to an infection but it shouldn’t over-react. (If it does over-react and attacks the body itself, the result is an autoimmune disease, such as rheumatoid arthritis, lupus, or MS). Each person has a personalized collection of bacteria, called the microbiome.1 We acquire our first bacteria while being born, and every day our environment exposes us to more. Some of these bacteria will take up residence inside the body and help develop a robust immune system.

The Good

The species of bacteria that colonize our respiratory and digestive systems help set up checks and balances in the immune system. White blood cells police the body, looking for infections, but they also limit the amount of bacteria that grow there. Likewise, bacteria keep white blood cells from using too much force. Bacteria also help out by doing things cells are ill-equipped to do. For instance, bacteria break down carbohydrates (sugars) and toxins, and they help us absorb the fatty acids which cells need to grow. 2 Bacteria help protect the cells in your intestines from invading pathogens and also promote repair of damaged tissue. Most importantly, by having good bacteria in your body, bad bacteria don’t get a chance to grow and cause disease.

The Bad

Of course, some species of bacteria in your body can result in diseases, such as cancer, diabetes, cardiovascular disease, and obesity. 3  Usually, these diseases happen only when the normal microbiome is disrupted, but that can occur even from antibiotics. Antibiotics kill bacteria, and some of those will be good bacteria that we need to protect our health. When that happens, the bad bacteria that normally are kept in check have room to grow, creating an environment ripe for disease.

Bad bacteria can exist at low levels in your body without causing harm or can grow too much and wreak havoc. Staphylococcus aureus can cause something as simple as a pimple or as serious as pneumonia or toxic shock syndrome. P. gingivalis can cause gum disease, and was recently linked to pancreatic cancer (read our article find out more). Similarly, when not suppressed by good bacteria, Klebsiella pneumonia can cause colitis, and subsequently lead to colorectal cancer. 4

The Ugly

In addition to allowing disease-causing bacteria to flourish, the elimination of good bacteria throws  the immune system out of whack. The result can be simple allergies or very debilitating autoimmune diseases. Without the right balance of bacteria, your body might suffer from constant inflammation.

Inflammation is the body’s alarm system, which calls white blood cells to heal a wound or to get rid of infection. Chronic inflammation, however, can make the body more susceptible to autoimmune diseases and cancer, such as causing inflammatory bowel disease which if uncontrolled can cause colon cancer. 5

The Future

Research suggests that efforts to make a cleaner environment, free from bacteria, are contributing to the rise in obesity, cancer, and heart disease. 6 Experts are trying to figure out how “probiotics” (foods like yogurt with active cultures and dietary supplements that contain live bacteria) can improve our health. Research is underway so  that in the future, specific bacteria may be prescribed as individually tailored treatments for patients.

Our immune system needs the right combination of bacteria so we can stay healthy and rely less on medications. Antibiotics remain a powerful tool to keep us healthy but shouldn’t be used when they aren’t needed. The more we learn, the more we appreciate the power of the bugs inside of us—to heal and not just to do harm.

  1. Eckburg, P.B., et al. (2005) Diversity of the human intestinal microbial flora. Science. 308:1635-1638.  
  2. Bry, L,, et al. (1996) A model of host-microbial interactions in an open mammalian ecosystem. Science. 273:1380-1383.  
  3. Tlaskalova-Hogenova, H., et al. (2011) The role of gut microbiota (commensal bacteria) and the mucosal barrier in the pathogenesis of inflammatory and autoimmune diseases and cancer: contribution of germ-free and gnotobiotic animal models of human diseases. Cell Mol Immunol. 8:110-120.  
  4. Saleh, M. and G. Trinchieri (2011) Innate immune mechanisms of colitis and colitis-associated colorectal cancer. Nat Rev Immunol, 11:9-20.  
  5. Fox, J.G., et al. (2011) Helicobacter hepaticus infection in mice: models for understanding lower bowel inflammationand cancer. Mucosal Immunol. 4:22-30.  
  6. Backhed, F. (2010) 99th Dahlem conference on infection, inflammation, and chronic inflammatory disorders: the normal gut microbiota in health and disease. Clin Exp Immunol. 160: 80-84.  

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