New Way to Measure How Long Bacteria Tolerate Antibiotics

In a new study published online in Biophysical Journal, Asher Brauner, from The Hebrew University, Jerusalem, Israel, and colleagues, describe a simple new approach for measuring how long it takes for an antibiotic to kill bacteria. These findings may hold promise for devising better treatments for bacterial infections.

Antibiotic resistance is a serious and growing public healthcare problem. In fact, some bacteria have become resistant to certain antibiotics through mutations that make the antibiotics less effective.

According to the authors, clinicians typically use the minimum inhibitory concentration (MIC) as a method to help guide their antibiotic prescribing. The MIC quantifies the level of resistance for each bacterial isolate, and is valuable in calculating the correct dose of an antibiotic. It represents the lowest concentration of the drug that prevents bacterial growth.

Corresponding author Nathalie Q. Balaban, PhD, also from The Hebrew University, discussed different strategies used by bacteria for survival. “We are mostly aware of resistance,” she told Contagion^®, “but bacteria can survive also by tolerance, which is a very different effect.”

The authors describe antibiotic tolerance as a poorly understood phenomenon that is rarely considered in healthcare; it is an extension of the length of time that bacteria can survive in lethal concentrations of an antibiotic, they say.

Antibiotic persistence is another phenomenon that clinicians rarely evaluate in healthcare settings. Persistence refers to the existence of subpopulations of bacteria in an overall population that are killed more slowly by an antibiotic than the majority population is killed.

“We were driven to define the terms ‘tolerance’ and ‘persistence,’ versus ‘resistance’ because there has been some confusion in the literature,” Dr. Balaban emphasized.

Although clinicians’ routine measurement of antibiotic tolerance could provide valuable information about the duration of antibiotic treatments, quantitative methods of assessing tolerance are lacking. Therefore, the researchers aimed to develop an approach to measure both tolerance and persistence.

They introduced a tolerance metric known as the MDK99, which represented the minimum length of time for killing 99% of the bacterial population. The technique they used to measure MDK99 can be performed manually or by using an automated robotic system, and involves exposing populations of bacteria to different concentrations of antibiotics for different lengths of time, and monitoring bacterial survival under these different conditions.

The researchers used this technique on six strains of Escherichia coli, showing that the different strains had various tolerance levels to ampicillin treatment, ranging from 2 to 23 hours. They also found that the MDK99 technique provided a way to detect antibiotic persistence.

“Here we develop a measurement, the MDK, that, together with the commonly used MIC, can help determine why a strain survives antibiotic treatments that should have killed it,” said Dr. Balaban. “By monitoring tolerance, and not only resistance, we may devise better treatments.”

“We propose that the MDK metric be implemented in hospital clinical microbiology labs. This would enable efficient classification of bacterial strains as tolerant, resistant, or persistent, and thereby provide a typical timescale for clinical implementation of effective treatment of tolerant strains,” the authors conclude.

Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.