Low-dose antibiotics can boost bacterial biofilms

Low doses of antibiotics can stimulate formation of bacterial biofilms that lead to chronic lung, sinus, and ear infections, a new study shows. The biofilms can grow stronger instead of weaker in response to the antibiotics.

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Researchers exposed nontypeable Haemophilus influenzae (NTHi) bacteria, a common cause of acute otitis media and upper respiratory infection, to sublethal doses of ampicillin and other β-lactam antibiotics (amoxicillin and cefuroxime). They chose β-lactams because they are the drugs most often used to treat otitis media.

Some NTHi strains responded by producing glycogen, a common bacterial food source, enabling them to form biofilms in the laboratory. The biofilms strengthened on exposure to the antibiotics and were able to use stored glycogen for sustenance even after antibiotic exposure ended.

The amount of antibiotic needed to stimulate formation of biofilm depended on the particular antibiotic and strain of NTHi exposed to it. Antibiotic resistance or sensitivity didn’t predict how a specific strain would react. One resistant strain showed a stimulatory effect, whereas an antibiotic-susceptible strain didn’t.

Biofilms-communities of microorganisms that adhere to each other and surfaces-form a slimy polysaccharide cover that makes infections more difficult to treat by protecting embedded bacteria from lethal doses of antibiotics. They can lead to prolonged lung infections in adults and chronic ear infections in children. No treatments are currently approved for such infections. When bacteria don’t respond to antibiotics, biofilms rather than antibiotic resistance may be the culprit, the study suggests. 

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