What Causes MRSA Antibiotic Resistance?


  • MRSA is the acronym used for all strains of Staph bacteria that have developed resistance to methicillin, and related antibiotics. Staph bacteria’s repeated exposure to antibiotics encouraged the development of MRSA. 
  • It is important to understand that there is a spectrum of antibiotic resistance. One strain of MRSA may have partial resistance to one antibiotic and complete resistance to another antibiotic, whereas another strain of MRSA may have the opposite resistance to these same antibiotics. This is called the bacteria’s resistance pattern.

More Antibiotics, More MRSA

The risk for a MRSA infection is directly related to the number of antibiotic prescriptions to which one is exposed. More antibiotics lead to a greater risk for a MRSA infection.1

Repeated antibiotic exposure 

Antibiotics are used for specific purposes, and typically target more common and less resistant bacteria. (Note: no one antibiotic is able to kill all bacteria.) Therefore, repeated exposure to antibiotics kills off the more common bacteria leaving room for more resistant bacteria to invade or colonize (harbor) the body.

Examples that Lead To Resistance

  1. Patients who are frequently prescribed antibiotics, or are on chronic antibiotic therapy. 
    • Everyone is colonized with a variety of bacteria.  This is called bacterial flora and is a normal part of our physiology.
    • Antibiotics kill off our normal bacteria flora allowing for the invasion of more resistant bacteria, including MRSA.
  2. The full course of prescribed antibiotics is not completed.  
    • The last few days of antibiotic therapy kill the toughest or most resistant bacteria.  
    • If the antibiotic course is not completed, bacteria that have partial resistance to the antibiotic are able to survive, and develop increased or complete resistance.  
  3. Antibiotics used in our food source, such as poultry and beef.
    • Antibiotics are commonly given to animals to prevent infection, rather than treat infection.  This broad use of antibiotics provides a greater chance for bacteria to develop resistance. 

Mechanism for MRSA Antibiotic Resistance

  • Random mutations occur in the bacteria’s DNA, which create a new gene resistant to antibiotics. 
  • DNA/genes can be transferred between bacteria. The resistant gene may come from other Staph bacteria or non-Staph bacteria.

Future Concerns 

MRSA continues to develop resistance to new antibiotics, which presents future concern to us all. 


1 Antimicrobial drugs and community-acquired methicillin-resistant Staphylococcus aureus, United Kingdom. Schneider-Lindner V,et al. Emerg Infect Dis. 2007;13(7):994.