What is the microbiome?
Microbes including bacteria and fungi are everywhere: on the surfaces we touch and in the air we breathe. Some are on our bodies, and some within. Some are transient, or temporary, members of our bodies while others stay much longer. The microbes frequently associated with our bodies make up what we call our microbiota, which has also been called our normal microflora. You may have also heard it called the microbiome, which includes culturable microbes that can be grown outside of the body, in addition to non-culturable microbes known to be associated with our bodies. According to a preprint article by Sender, Fuchs, and Milo, the number of microbes in our microbiome has been suggested to match the number of our cells. That’s a lot of microbes!
Because they are our normal microflora, do they ever make us sick?
Yes! Our normal flora can cause disease if they end up in a different location within the body. For example, a skin microbe can enter the bloodstream through an open wound and travel to other parts of the body where it may exhibit harmful effects. Another instance in which they cause disease is if a change in your body helps only certain members of your microflora grow better, thereby changing the balance of microbes present. Because these changes disrupt the balance of these populations, they may also alter your health. In fact, many diseases are linked to disruptions in our microbiome, including inflammatory bowel disease, diabetes, ulcers, and more!
Here are some other microbial diseases you might not have known are caused by otherwise normal microflora:
- Strep Throat (a.k.a. acute pharyngitis)
Figure 1. Colonies of Streptococcus pyogenes on a blood agar plate. The presence of clear zones around the colonies indicates these bacteria are capable of lysing (killing) blood cells. (Source: Richard R. Facklam, Ph.D., CDC)
Causative Agent: Streptococcus pyogenes (S. pyogenes) (Group A Streptococcus)
Location: Streptococci are normal inhabitants of our bodies, in which the Group A Streptococcus (GAS) organism S. pyogenes is commonly found in areas such as our nasal passages, throat, and skin.
How it causes disease: Typically, the development of strep throat results from inhalation of airborne droplets containing S. pyogenes from another individual. This new strain (or type) of S. pyogenes was able to establish an infection in your oropharynx (throat) and grow so well that it outgrew some of your resident microflora. S. pyogenes produces toxins, so more S. pyogenes means more damage.
- Toxic Shock Syndrome (TSS)
Figure 2. Light microscopy image of methylene blue stained **Staphlyococcus aureus. (Source: CDC)
Causative Agent: Staphylococcus aureus (S. aureus)
Location: S. aureus is found on the surface of 30% of healthy individuals, and at least once in the lifetime on 90% of people. The most prominent sites of encounter with S. aureus are the skin and soft tissue (the layers under the skin and lining the muscles). However, S. aureus can grow on almost any organ or tissue.
How it causes disease: Some strains of S. aureus produce toxins called toxic shock syndrome toxin 1 (TSST-1). These strains are not found in every individual, but if present, are low in number. In the case of menstruating women, use of a high absorbency tampon introduces oxygen into an otherwise anaerobic (no oxygen) environment, triggering TSST-1 production by these organisms. Your immune cells normally respond to S. aureus infection through temporary interactions with each other that induce the release of pro-inflammatory cytokines (chemical messengers that call more immune cells to the site of infection). The TSST-1 toxin is a superantigen that locks these immune cells in a permanent interaction, resulting in the massive release of cytokines and extensive damage to the tissues. In non-menstrual cases of TSS, rashes and peeling of the skin also occur.
- Yeast Infections
Figure 3. Microscopy image of **Candida albicans, the fungus that causes yeast infections.
(Source: Dr. Lucille K. Georg, CDC)
Causative Agent: Candida albicans (C. albicans)
Location: C. albicans can be found on the skin. It commonly manifests as a vaginal yeast infection in women but can also affect men.
How it causes disease: Changes in pH can alter the way organisms grow: some do better, others do worse. Bacteria called Lactobacilli produce a lot of lactic acid and keep C. albicans growth in check. Often, antibiotic use or hormonal changes disrupt this balance by killing these Lactobacilli, which enables C. albicans to outgrow your other microflora. That’s why probiotics (live bacteria) made with Lactobacilli and prebiotics (substances that enhance the growth of Lactobacilli) are increasingly used to maintain a healthy balance in our bodies.
As you can see, your microbiome is important for your health. Proper maintenance of our bodies such as a healthy diet and good hygiene can greatly affect our normal microflora, and thus, our overall health.
 Engleberg, N. C., T. Dermody, and V. DiRita. 2013. Mechanisms of microbial disease. Schaechter’s mechanisms of microbial disease. 5th ed. Baltimore, MD: LIppincott Williams & Wilkins.
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