If antibiotics are becoming less effective, why not combine them?

It might seem logical to combine different antibiotics to create a more powerful weapon against resistant bacteria, but this approach can surprisingly backfire. Instead of becoming stronger, some antibiotic combinations can work against each other, a phenomenon known as antagonism. This means the mixture can be less effective than using a single, appropriate antibiotic.

One reason for this is that some antibiotics, called bacteriostatic agents, only stop bacteria from multiplying, while others, known as bactericidal antibiotics, actively kill them. If you mix the two, the bacteriostatic drug can prevent the bactericidal one from working properly as the latter often targets processes in actively dividing cells.

Further, every antibiotic use, including in combinations, puts pressure on bacteria to evolve and develop resistance. Using combinations without a clear benefit can inadvertently contribute to the creation of “superbugs” that are even harder to treat. This is why it’s crucial for doctors to carefully select the right antibiotic based on the specific infection, rather than simply mixing them.

A study published in Molecular Systems Biology on October 27 illustrated this problem in Escherichia coli bacteria. Researchers combined ciprofloxacin, an antibiotic that kills bacteria by damaging DNA, with tetracycline, which merely stops them from growing. They discovered that tetracycline actually slowed down the bacteria’s metabolism, preventing ciprofloxacin from killing them. Ultimately the survival rate of the bacterial cells increased.