Probiotics: Better off dead?

Vast and diverse microbial ecosystems form within our guts. Trillions of bacteria strong, these communities help digest our meals, manufacture vitamins, kill pathogens, neutralize food-poisoning agents and boost our immune defenses. When antibiotic therapy or disease wipes out huge numbers of these intestinal squatters, bloating, diarrhea and more can ensue. Ewwwww.

Fortunately, medicine has developed starter cultures of beneficial germs to recolonize antibiotic-ravaged guts, to boost immunity and to sometimes act as a call to arms for other gut microbes. Because intentionally downing extra live germs doesn’t sound very appetizing, dietary supplement companies refer to these bacteria as probiotics — and even deliver some as part of the “live cultures” in a serving of yogurt.

But, as we reported last year, people have sickened or died after receiving probiotics. What role, if any, these bacteria played remains uncertain. Such events have, however, been giving some researchers and clinicians doubts about the safety of this ostensibly benign and “all natural” germ therapy.

A paper just published online in Nutrition Reviews now suggests a compromise tactic: Administer slain bacteria — microbial carcasses, if you will.

Joseph Neu of the University of Florida School of Medicine and his colleagues reviewed several studies that compared health impacts using live microbes versus those that had first been killed by heat or ultraviolet radiation. The dead bugs were just as effective as live ones, they report, but “considerably safer for the host.” For instance, they appeared less likely to provoke an overzealous immune response.

One study worked with Lactobacillus rhamnosus GG, better known as LGG. Researchers administered high doses of the bacterial strain to human cells that normally line the interior surface of the gut. When stimulated in a way that simulated the presence of a pathogen, the gut cells revved up a bigger inflammatory response than when the probiotics were absent. Both live and dead LGG induced fairly comparable responses — ones that would be expected to improve the gut’s ability to fight infection.

Then the researchers repeated the experiment, this time with gut cells that appeared healthy — i.e. not under siege by pathogens. Here, live LGG triggered significant inflammation. Which is not a beneficial response, since healthy cells might be slammed by deadly chemical shrapnel unleashed by immune cells. A similarly big dose of dead LGG caused only mild inflammation.

Probiotics are conventionally defined as live microbes, Neu’s team notes. However, these researchers argue, at least in some circumstances, dead microbes or elements of their carcasses “may be a safer alternative.”

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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