https://www.scientificamerican.com/article/combining-multiple-antibiotics-may-make-bacteria-more-likely-to-develop-resistance/

One way to address the growing problem of antibiotic resistance has been to use multiple drugs. Give patients two antibiotics, the thinking goes, and even if the microbes are resistant to one of them, the other will work. But a new study suggests that drug combinations can actually speed the development of resistance. .

In a paper published Thursday in Science, Israeli researchers showed that when a patient develops tolerance to a single antibiotic in the combination—meaning it kills bacteria more slowly—outright resistance to the second drug becomes more likely. Previous work by the same team and others had already shown the same effect in a lab dish: they found that slowing the killing rate can lead to resistance, in which the bacteria continue to grow even in the presence of an antibiotic. But this was the first study to demonstrate the process in people, according to its senior author Nathalie Balaban, a biophysicist at the Hebrew University of Jerusalem.