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Timing is the key to bacteria that survive antibiotics



For bacteria confronted with a dose of antibiotics, time could be the key to destruction. In a series of experiments, the Princeton researchers found that cells that repaired DNA damaged by antibiotics before they resumed growth had much better chances of survival.

When antibiotics hit a population of bacteria, only small portions of "persister" cells often survive a risk of recurring infection. In contrast to bacteria with genetic resistance to antibiotics, there are indications that persisters partially survive by stopping the cellular processes that the drugs are targeting.

In a new study, Princeton researchers investigated a class of antibiotics against bacterial DNA. In bacterial populations, some cells repair damaged DNA before they re-grow, and others continue to grow before performing any repairs. The researchers found that those who make repairs before resuming growth are those who survive as Persists. Research is advancing a long-term goal to make antibiotic treatment more effective.

In results published on June 1

8 in the Proceedings of the National Academy of Sciences, Wendy Mok, a post-doctoral student, and Mark Brynildsen, an associate professor of Chemical and Biological Engineering analyzed the reactions of E. coli. Bacteria on treatment with ofloxacin, an antibiotic that causes DNA damage by blocking enzymes needed for DNA replication and RNA transcription. Their work builds on previous results from Brynildsen's lab, which revealed that persistence of ofloxacin needed DNA repair machines to survive.

"But that does not guarantee that they will necessarily survive," Mok said. "We hypothesized that the timing of DNA repair and the resumption of growth activities, such as DNA synthesis, could affect the survival of persists after treatment."

To test this hypothesis, Mok and Brynildsen used an E. coli bacterial strain genetically engineered to allow researchers to control the growth of cells. The researchers used the bacteria to create a consistent population of growth-arrested cells that tolerated the ofloxacin antibiotic.

These non-growing cells experienced DNA damage similar to growing cells treated with ofloxacin. However, the non-growing cells showed delays in resuming DNA synthesis and post-treatment repair.

Controlling the activity of a key DNA repair protein, RecA, researchers tested the effect of further delaying DNA repair until after resuming DNA synthesis. This resulted in a seven-fold decrease in survival compared to cells that produced continuous RecA, indicating that the persistence of ofloxacin depends on repairing DNA damage before the new DNA necessary for growth is synthesized.

Mok and Brynildsen then examined the persistence in normal cells Low-nutrient environment to stop their growth and to simulate a condition that bacteria often encounter in an infected host. Indeed, after treatment with ofloxacin, when cells were starved at carbon sources for at least three hours, researchers observed almost complete tolerance to the antibiotic. This tolerance depended on effective DNA repair processes. They also observed increased persistence of Ofloxacin with nutrient deprivation after treatment of cells growing in biofilms, which are groups of bacteria that stick to surfaces and are involved in the majority of hospital-treated bacterial infections.

Jan Michiels, Professor of Microbiology at the University of Leuven-VIB in Belgium, said the study uses "an elegant model system" to investigate the underlying mechanisms of persistence. Michiels, who was not involved in the research, said "this is a groundbreaking discovery that provides new basic insights into how persistent cells avoid death."

Ofloxacin and other similar antibiotics are included in the model list of Essential Medicines of the World Health Organization. a catalog of the most important medicines for the needs of health care. Curbing bacterial persistence could be a promising way to make these therapies more effective against urinary tract infections, staphylococcal infections, and other bacterial diseases.

"Nutrient deficiency is a stress that bacteria routinely encounter at an infection site," Mok said. "Our results suggest that in the period after antibiotic therapy, we may be considering whether to tackle some of these DNA repair processes and whether this will improve treatment outcomes." A counterintuitive approach could be to accelerate bacterial growth after antibiotic treatment, causing the cells to leave their repair mechanisms and die. The researchers added, however, that other approaches would probably be better than promoting the growth of a pathogen in a patient.

Brynildsen's group and others are interested in finding potential drug compounds that could interfere with bacterial DNA repair relationship between antibiotic tolerance and genetic resistance.

Support for this research was provided in part by the Army Research Center, the Charles H. Revson Foundation, and the Princeton University start-up fund.

Source: Princeton University


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