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Timing might be the key to evading destruction for bacteria facing antibiotics. Princeton researchers found that cells that repaired DNA damaged by antibiotics before resuming growth had a much better chance of surviving treatment. When antibiotics hit a population of bacteria, a small fraction of “persister” cells survive to pose a threat of recurrent infection. Unlike bacteria with genetic resistance to antibiotics, evidence suggests that persisters stay alive in part by stalling cellular processes targeted by the drugs.
Princeton researchers examined a class of antibiotics that target bacterial DNA. In bacterial populations, some cells repair damaged DNA before resuming growth, and others resume growth before making repairs. The researchers found that those that make repairs before resuming growth generally are the ones that survive as persisters. The research advances a long-term goal to make antibiotic treatment more effective.
Researchers analyzed the responses of E. coli bacteria to treatment with ofloxacin, an antibiotic that causes DNA damage by blocking enzymes needed for DNA replication and RNA transcription. Their work built on previous results from Brynildsen’s lab, which revealed that persisters to ofloxacin required DNA repair machinery to survive. The timing of DNA repair and the resumption of growth-related activities like DNA synthesis could impact the survival of persisters after treatment.
Researchers used a strain of E. coli bacteria that had been genetically engineered to allow researchers to control the cells’ growth. The researchers used the bacteria to create a uniform population of cells with stalled growth that could tolerate the ofloxacin antibiotic.
These non-growing cells, they found, experienced DNA damage similar to growing cells treated with ofloxacin. However, the non-growing cells showed delays in resuming DNA synthesis and repair following treatment.
By controlling the activity of a key DNA repair protein, RecA, the researchers tested the effect of further delaying DNA repair until after the resumption of DNA synthesis. This led to a sevenfold decrease in survival compared to cells that continuously produced RecA, demonstrating that persistence to ofloxacin depends on repairing DNA damage before synthesizing the new DNA necessary for growth.
They examined persistence in normal cells placed in a low-nutrient environment to stall their growth, simulating a condition that bacteria frequently encounter within an infected host. Indeed, following ofloxacin treatment, if cells were starved of carbon sources for at least three hours, they observed nearly complete tolerance to the antibiotic. This tolerance depended on effective DNA repair processes.
They also observed enhanced persistence toward ofloxacin with nutrient deprivation after treatment among cells growing in biofilms, which are groups of bacteria that stick to surfaces and are implicated in a majority of hospital-treated bacterial infections. Nutrient starvation is a stress that bacteria can routinely encounter at an infection site, after antibiotic treatment targeting some of these DNA repair processes may improve treatment outcome.
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