Suppression of bacterial cell death underlies the antagonistic interaction between ciprofloxacin and tetracycline in Escherichia coli

Antibiotic combinations are an attractive strategy to maximise the efficiency of drug treatment and minimise resistance evolution, but we still lack a full understanding of their effect on bacterial cells. The interaction between DNA-targeting antibiotics, such as ciprofloxacin, and translation inhibitors, such as tetracycline, is antagonistic, resulting in a weaker effect on bacterial growth than expected from the effect of each single drug. This antagonism has been analysed in detail at the population level, but we lack a single-cell understanding of its effect and how it depends on nutrient availability. Here, we used a microfluidic device to quantify the antagonism between ciprofloxacin and tetracycline at the single-cell level in three nutrient conditions. We showed that improved growth is due to increased survival of cells under the drug combination compared to ciprofloxacin alone. This effect is growth-dependent, with better suppression in rich nutrient conditions. Quantification of the DNA damage response (SOS response) revealed two sub-populations among the cells that die upon ciprofloxacin treatment, with some cells reaching a very high level of SOS while others had a lower level of SOS, similar to surviving cells. The low-SOS cells were more frequent in fast growth conditions and showed increased survival under the drug combination but the high-SOS cells were hardly rescued by the drug combination. This result explains the stronger antagonistic effect of tetracycline on ciprofloxacin in fast growth compared to slow growth. Our results underscore the importance of single-cell quantification in understanding the bacterial response to antibiotic combinations and devising new treatment strategies.