Biofilm stars: ‘Persister’ cells trigger relapse

Biofilm stars: Persister’ cells trigger relapse

Gene hunters trying to crack invincible bacteria

The relapse of infections, and the subsequent use and possible misuse of antibiotics that follows, appears to result from "persister" cells that remain impervious to treatment and gradually restore the pathogenic attack, researchers are finding.

They discovered that dense populations of bacteria produce considerable numbers of cells that are virtually insensitive to antibiotics. While the majority of the bacterial population dies, these "persister" cells remain alive in biofilms that protect them from the immune system, says Kim Lewis, PhD, professor of biology at Northeastern University in Boston. "The antibiotic will wipe out the majority of [a pathogenic] population, and if there are cells left, they will be mopped up by the immune system," he tells Hospital Infection Control. "Macrophages will take care of them."

But already immune to antibiotics, persisters will hide from the immune system’s agents by lurking in biofilm layers of polysaccharide mucous that are usually attached to the surface of a catheter or other object.

"This is a mucous layer that physically prevents the components of the immune system from penetrating into the biofilm," Lewis says. "The persisters remain there, and they are invulnerable not only to antibiotics but also to the immune system because the immune system cannot physically get to them."

The symptoms of the infection will diminish, and antibiotic therapy may be discontinued. However, persisters gradually bounce back and restart an infection of the attacking bacteria. That is how Lewis and colleagues theorize relapsing infections work, whether in the ear or on the tip of a catheter.

"The infection relapses because the moment the antibiotic is gone the persisters start growing again and rebuild the biofilm," Lewis says. Additional treatment is then necessary, which contributes to the growing problem of antibiotic resistance. "This whole process of relapsing infection increases the probability that these bugs will be picking up mutations or plasmids from other organisms and become very resistant to the antibiotics, which we are trying to use to treat them," he explains.

The existence of such cells was first reported more than 50 years ago, but they were considered little more than a curiosity because of their very low frequency — about one in a million — in a conventional cell culture. But while studying biofilms of Pseudomonas aeruginosa, Lewis found that persisters are present at much higher levels than generally assumed. Indeed, they accumulate in large numbers as the cell culture grows and becomes denser. That was true for all pathogens tested, including the nosocomial nemesis Staphylococcus aureus. "Why is it that persisters are not killed by antibiotics?" Lewis asks. "We do not know."

Whatever makes persisters different, it has to depend on gene expression, he says. The current thinking is that persisters have developed a method to turn off the "suicide" process in which cells disassemble after they have been damaged by toxins or overwhelmed by cancer, he says. With the exception of persisters, bacteria damaged by antibiotics undergo a similar process. "Persisters are cells that have turned off the suicide mechanism," he says. "Their function is to survive no matter what."

Lewis is using DNA assays to hunt for the genes that make persisters different. This approach allows researchers to compare expression of all genes in usual cells vs. persisters. By noting what the differences are, researchers may determine what genes are responsible for generating persisters. Understanding the nature of the invincible cells may allow researchers to turn them back into regular cells that will be sensitive to antibiotics. Ultimately, anti-persister drugs may allow clinicians to clear many infections that are currently untreatable.

Funded by a grant by the National Institutes of Health, the research was presented May 22, 2002, in Salt Lake City at the annual meeting of the American Society for Microbiology.