Reservoirs of Antibiotic Resistance Network -- ROAR  
ROAR -- Reservoirs of Antibiotic Resistence Network

Commensal Science For Non-Scientists

Did you know your body contains more bacterial cells than human cells? About 100 trillion of these microscopic creatures live in your body and outnumber your 10 trillion human cells. Your intestines alone, where most of the bacteria in your body live, can contain up to 100 billion bacterial cells. These normally harmless bacteria are referred to as commensal bacteria. Commensal bacteria help keep our bodies healthy in many ways: they help us to digest foods and acquire nutrients such as vitamins B and K, encourage your immune system to develop and prevent the colonization of bacterial pathogens that cause disease by competing with them.

Click on the body to the right to find out more about where these beneficial organisms live in your body. 

Respiratory Tract

Commensal Science - MouthCommensal Science - Respiratory SystemCommensal Science - StomachCommensal Science - IntestinalCommensal Science - Urinary TractCommensal Science - Skin

Although these commensal bacteria are beneficial to our body, they can carry genes that cause them to become resistant to antibiotics.  Additionally, other harmless bacteria living in animals and the environment may carry these genes. When these bacteria are exposed to antibiotics these genes spread to more bacteria and become more common. In the body or in the environment, commensals may transfer these genes to pathogenic bacteria.  Once pathogens obtain these antibiotic resistance genes, it becomes difficult or impossible for doctors to treat the infections they cause.

This transfer of genes from one bacterium to another is called Horizontal Gene Transfer. In order to understand how commensals transfer antibiotic resistance genes to pathogenic bacteria, it is important to understand horizontal gene transfer and its role in antibiotic resistance.

Image generously provided by H. Hendrickson
at the University of Pittsburgh

Horizontal gene transfer can occur via three different mechanisms: transformation, conjugation and transduction. 

Transformation occurs when a bacterium picks up pieces of genetic material directly from its environment and inserts the genetic material into its own genome.  This genetic material can come from a ruptured bacterial cell from the same or unrelated species, as illustrated in the image to the left. 

Conjugation can be thought of as ‘bacterial sex’ and involves direct cell-to-cell contact as genetic material is transferred from donor to recipient.  The genetic material that is transferred is often in the form of a closed loop of DNA known as a plasmid.

Transduction is more complex and involves the transfer of genetic material from one bacterium to another by a virus. 

If a pathogen causing illness acquires an antibiotic resistance gene from commensal bacteria, the pathogen becomes resistant to the antibiotic. The illness can no longer be treated using that antibiotic and the doctor has to choose a different antibiotic. As a result, the illness is often longer, more severe, more expensive, and more often fatal. There are some pathogenic bacteria that are now resistant to every single available antibiotic! Untreatable infections caused by antibiotic resistant bacteria are now a common cause of death in the United States.

The current scientific evidence shows that the origin of this deadly resistance in pathogens may lie in harmless commensal bacteria. The Reservoirs of Antibiotic Resistance Network (ROAR) is working to understand the origin of resistance in pathogens by examining the abundance, diversity and distribution of antibiotic resistance in human, animal and environmental commensal bacteria.

Our goal is to understand how commensals acquire antibiotic resistance genes, how they pass these genes to pathogens, and use this information to predict and prevent the emergence of resistance in pathogens. As a citizen, consumer and patient, you can do many things to minimize the threat of antibiotic resistance. Visit the APUA website for more information.

  1. Albrecht T, et al.  “Bacteriology:  Normal Flora.”  Medical Microbiology, 4th Edition.  University of Texas.  1996
  2. Champonux JJ, Drew WL, Falkow S, Neidhardt FC, Plorde JJ, Ray CG.  Sherris Medical Microbiology:  An Introduction to Infectious Diseases.  Connecticut:  Appleton & Lange, 1994.
  3. Madigan MT, Martinko JM, Parker J.  Brock Biology of Microorganisms, 8th Edition.  New Jersey:  Prentice Hall, 1997. 
  4. Todar K.  “The Bacterial Flora of Humans.”   Todar’s Online Textbook of Bacteriology.  University of Wisconsin 2007

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ROAR is funded by grant AI50139 from the National Institute of Allergy and Infectious Diseases