Almost half of all infections caused by a bacteria responsible for pneumonia, meningitis and ear infections will be virtually untreatable by next year, according to a study by researchers at the Harvard School of Public Health (SPH).

The study, published in the April edition of the journal Nature Medicine, pointed to the over-prescription of antibiotics as a likely cause for the growing number of cases of bacterial infections that are untreatable by these drugs.

“This is putting selective pressure on bacteria when it’s not needed,” said Alethea McCormick, the study’s principle author. “The use of antibiotics should be limited to infections that are truly bacterial.”

The study also revealed that while a new vaccine represents a temporary stop-gap measure, in the long-term, heavy use of antibiotics will continue to decrease their effectiveness.

McCormick said that there is a rising proportion of bacteria resistant to both penicillin and erythromycin—two of the most heavily prescribed antibiotics.

Researchers and clinicians have found that bacterial resistance to antibiotics has risen steeply in parts of the world where these drugs are commonly prescribed, she said, but there is little resistance to antibiotics that are rarely prescribed.

But McCormick added that until this study was published, nobody had explained how this process takes place.

The researchers looked at data from the 1990s which gave the number of infections caused by strains of the bacteria Streptococcus pneumoniae that were not treatable by certain antibiotics. They then employed a mathematical model of disease transmission to predict the prevalence of such drug-resistant strains in the future.

What they found confirmed earlier suspicions that the most likely cause of drug resistance is the overuse of antibiotics.

“The key point is that antibiotic use in different places is very likely driving substantial differences in which bacteria have resistance and which don’t have resistance,” said Marc Lipsitch, a researcher at the SPH and a co-author on the study.

This most likely occurs, according to McCormick, because the use of antibiotics to treat illness puts pressure on the population of bacteria that inhabit treated humans.

Heavy use of antibiotics causes more bacteria without the resistance to die than those with resistance, resulting in a growing proportion of bacteria that are resistant to whatever antibiotics are being used—a trait they can pass on to their offspring, she said.

And according to McCormick, this means that treatment will grow increasingly difficult, since the primary method of combatting bacterial infections is with antibiotics.

Lipsitch said that the bacteria they studied is very common.

“Somewhere between 30 and 100 percent of kids less than five have it at any given time,” he said, adding that the percentage decreases slightly in later years because of better hygiene and a new vaccine for children against some strains of the bacteria that was approved in 2000.

But the bacteria is not completely benign, said Lipsitch.

“It causes disease when it reaches a place where it doesn’t belong,” he said. “It travels from the upper respiratory tract to other places,” like the lungs, spinal cord, blood and ear.

He said that these infections can be fatal if left untreated, especially when they occur later in life.

Lipsitch also said the rise of drug resistance is not restricted to this bacteria.

“Resistance to penicillin... appeared within months of its first use. Drug resistance has been around as long as clinical use of antibiotics,” he said.

But according to Lipsitch, the genes responsible for drug resistance are much more ancient.

Since most of the antibiotics used today are derived from plants and other organisms that use them to naturally defend themselves from infection, bacteria have had plenty of time to develop resistance to these chemicals, he said.

McCormick said that the recent rise in resistance is because antibiotics are increasingly misprescribed, offered to patients for viral infections or other illnesses—for which antibiotics are ineffective.

There is hope that a vaccine released in 2000—effective against most of the strains with a high percentage of antibiotic resistance—will help to reduce the number of untreatable cases, she said.

“If most of the population is vaccinated, fewer people will be infected, and there will be a decrease in the number of people infected by resistant strains. There will be a decrease in the percentage who have resistant bacteria,” McCormick said.

However, her study found that the percentage of bacteria resistant to antibiotics will continue to increase so long as antibiotics are prescribed as frequently as they are now.

Lipsitch said that the prospects for a solution are slim.

“In the short term, the vaccine should reduce the number of resistant infections,” he said. “In the long term, bacteria will be smart enough to reestablish their resistance to treatment.”