Carriage of resistant bacteria

Antibiotic treatment is associated with a risk to become a carrier and for suffering future infection with resistant bacteria.

Selection of antibiotic resistant bacteria in the microbiome

Any bacterium has the potential to evolve resistance. When you take an antibiotic, not only the infecting bacteria but also bacteria normally living in the body, the microbiome, will be reduced in numbers (for more on the microbiome, see Bacteria – Essential for human life). This leaves an open space and an opportunity for resistant bacteria to proliferate. Sometimes the resistant bacteria become established in the normal flora, and may prevail in the body for many years after ending the treatment (reviewed in). Suboptimal dosing, long duration of treatment and use of broad-spectrum antibiotics increases the risk for selection of resistant bacterial clones.

This can become a problem later in life; such bacteria may cause a serious and hard to treat infection in the future. Further, the resistance mechanisms may spread to other bacteria. The resistant bacteria can also spread to people in your family and in the community. Spread is fueled by for example poor water, sanitation and hygiene. That is, carriage can also be the result of exposure to for example food or environmental sources contaminated with resistant bacteria.

Colonization and future infection with resistant bacteria, examples:

• Exposure to antibiotics is followed by increased risk for colonization with resistant bacteria for several years, probably due to selection of resistant clones in the normal flora.
• Use of certain antibiotics in the preceding 1-2 months increased the risk 2-5 fold for nasal carriage of penicillin-resistant pneumococci among children. Also siblings to treated children had an increased risk for carriage, indicating spread within families .
• Exposure to antibiotics in the 3-month period before a community-acquired urinary tract infection was found to be a risk factor for infection with multidrug-resistant ESBL (extended-spectrum beta-lactamase)-producing E. coli.
• Prior use of antibiotics was also found to increase the risk for bloodstream infection (sepsis) with resistant Gram-negative bacteria .
• Athletes have been found to be at higher risk for becoming carriers of MRSA (methicillin-resistant Staphylococcus aureus). Importantly, being a carrier was associated with a 7-fold higher risk of developing MRSA infection.
• Antibiotic resistance spreads silently across the world. More than 60% of the populations in some areas carry multidrug-resistant bacteria as part of their normal bacterial flora. The exact routes of spread are not clear, but environmental contamination likely contributes.

Selected Resources