How long can bacteria wait out antibiotics?

Agrowing number of pathogens are developing resistance to one or more antibiotics, threatening our ability to treat infectious diseases. According to a study published recently in Biophysical Journal, a simple new method for measuring the time it takes to kill a bacterial population could improve the ability of clinicians to effectively treat antimicrobial-tolerant strains that are on the path to becoming resistant.

“These findings allow measurement of tolerance, which has previously been largely overlooked in the clinical setting,” said senior study author Nathalie Balaban of the Hebrew University of Jerusalem. “Routinely measuring tolerance could supply valuable information about the duration of antibiotic treatments, reducing the chance of both under- and over-treatment. Furthermore, data compiled from such measurements could give an estimate of how widespread the phenomenon of tolerance really is, which is currently a complete unknown.”

According to the World Health Organization, antibiotic resistance is one of the biggest threats to global health and is putting the achievements of modern medicine at risk. Due to selective pressure, pathogens acquire resistance through mutations that make the antibiotic less effective, for example, by interfering with the ability of a drug to bind to its target. At present, clinicians determine which antibiotic and dose to prescribe by assessing resistance levels using a routine metric called minimum inhibitory concentration (MIC) – the minimal drug concentration required to prevent bacterial growth.

Although resistant strains continue to grow despite exposure to high drug concentrations, tolerant strains can survive lethal concentrations of an antibiotic for a long period of time before succumbing to its effects. Tolerance is often associated with treatment failure and relapse, and it is considered a stepping stone toward the evolution of antibiotic resistance. But unlike resistance, tolerance is poorly understood and is currently not evaluated in healthcare settings.

“The lack of a quantitative measure means that this aspect of the treatment relies largely on the experience of the individual physician or the community,” says first author Asher Brauner, a doctoral student in Balaban’s lab. “This can lead to treatment being either too short, increasing the risk of relapse and evolution of resistance, or much too long, unnecessarily causing side effects, release of antibiotic waste into the environment, and additional costs.”

To address this problem, Balaban and her team developed a tolerance metric called the minimum duration for killing 99% of the population (MDK99). The protocol, which can be performed manually or using an automated robotic system, involves exposing populations of approximately 100 bacteria in separate microwell plates to different concentrations of antibiotics for varied time periods, while determining the presence or lack of survivors.

The researchers applied MDK99 to six Escherichia coli strains, which showed tolerance levels ranging from two to 23 hours under ampicillin treatment. MDK99 also facilitates measurements of a special case of tolerance known as time-dependent persistence – the presence of transiently dormant subpopulations of bacteria that are killed more slowly than the majority of the fast-growing population. Like other forms of tolerance, time-dependent persistence can lead to recurrent infections because the few surviving bacteria can quickly grow to replenish the entire population once antibiotic treatment stops.

“A take-home message from this is that it is important to complete a course of antibiotic treatment as prescribed, even after the disappearance of the symptoms,” Balaban explained. “Partial treatment gives tolerance and persistence mutations a selective advantage, and these, in turn, hasten the development of resistance.”

In future studies, Balaban and her team will use MDK99 to study the evolution of tolerance in patients. Moreover, the ability to systematically determine the tolerance level of strains in the lab could facilitate research in the field.

“If implemented in hospital clinical microbiology labs, MDK99 could enable the efficient classification of bacterial strains as tolerant, resistant, or persistent, helping to guide treatment decisions,” Balaban said. “In the end, understanding tolerance and finding a way to combat it could significantly reduce the ever-growing risk of resistance.”

VISUAL ILLUSION COULD HELP YOU READ SMALLER FONTS

Exposure to a common visual illusion may enhance your ability to read fine print, according to new research published in Psychological Science, a journal of the American Association for Psychological Science.

“We discovered that visual acuity – the ability to see fine detail – can be enhanced by an illusion known as the ‘expanding motion aftereffect.’ While under its spell, viewers can read letters that are too small for them to read normally,” wrote psychological scientist Martin Lages of the University of Glasgow.

Visual acuity is normally thought to be dictated by the shape and condition of the eye but these new findings suggest that it may also be influenced by perceptual processes in the brain.

Interest in the intersection between perception and reality led Lages and co-authors Stephanie C. Boyle (University of Glasgow) and Rob Jenkins (University of York) to wonder about visual illusions and how they might affect visual acuity.

“The expanding motion aftereffect can make objects appear larger than they really are and our question was whether this apparent increase in size could bring about the visual benefits associated with actual increases in size,” Boyle explains. “In particular, could it make small letters easier to read?”

To find out, the researchers employed a tool that can be found in any optometrist’s office – the classic logMAR eye chart. On this chart, letters are arranged in rows and the letters become increasingly smaller and more difficult to read as you move down the chart. Optometrists calculate visual acuity based on the size at which a person can no longer reliably identify the letters.

In two related experiments, the researchers presented a total of 74 observers with a spiral pattern that rotated either clockwise or counterclockwise for 30 seconds followed by a set of letters, which participants were asked to identify. The font size of the letters became increasingly smaller over subsequent trials. The experiment revealed that participants’ visual acuity differed depending on which spiral they saw.

Participants who started with normal visual acuity and saw clockwise spirals – which induce adaptation to contracting motion and cause subsequent static images appear as if they are expanding – showed improved visual acuity. That is, they were able to identify letters at smaller font sizes after exposure to the clockwise spiral.

Those who saw counterclockwise spirals – which induce adaptation to expanding motion and cause later images to appear as if they are contracting – actually performed worse after exposure to the spirals.

A third experiment in which each participant saw both types of spirals over two sessions showed similar results: seeing clockwise spirals that induced an expanding motion aftereffect enabled participants to read letters at smaller font sizes.

“We were pretty impressed by the consistency of the effect. No matter how you break it down – by letter size, by letter position – the performance boost is there,” according to Jenkins. “And there was a correlation with initial ability; the harder people found the task, the more the illusion helped them.”

But don’t throw out your eyeglasses just yet: The researchers note that the overall boost to visual acuity is small and fleeting. Nonetheless, this common visual illusion reveals a fundamental aspect of how we see, showing us that our ability to discriminate fine detail isn’t solely governed by the optics of our eyes but can also be shaped by perceptual processes in the brain.