Sweat and tears drive bugs apart

Natural antibacterial stops bugs finding strength in numbers.
30 May 2002

HELEN PEARSON

Protein protection against biofilm builder Pseudomonas aeruginosa. © SPL

A natural component of bodily secretions seems to stop bacteria amassing into tough drug-resistant sheets, shows new research. Drugs that mimic the protein could starve chronic infections into submission.

Biofilms - bacteria bunched into hardy layers - are a major medical problem. These antibiotic-resistant sheets can coat artificial joints and catheters, and can cause ultimately lethal infections in the lungs of cystic-fibrosis patients.

A protein called lactoferrin in tears, mucus and human milk stops bacteria getting together, say Pradeep Singh and his colleagues of the University of Iowa in Iowa City¹. The protein may help to keep healthy lungs and orifices biofilm-free, says Singh: "Every vulnerable human surface: lactoferrin is there".

"It's an exciting idea," says Tomas Ganz, who studies microbe defence at the University of California in Los Angeles. Although lactoferrin was known to fight infection, it was unclear why secretions contain such high levels of it.

The protein mops up traces of iron, explains Ganz, depriving bacteria of an essential nutrient. Faced with a shortage, they will "leave the biofilm - and go in search of greener pastures", he says.

Indeed, bacteria treated with just one-tenth as much lactoferrin as there is in lung sputum appear to go looking for more fertile ground: "They meandered aimlessly around the surface," says Singh.

On glass slides the team grew Pseudomonas aeruginosa, which thrives in the lungs of patients with cystic fibrosis. Left alone, the bacteria multiplied into pillars containing millions of bacteria within a week.

Film studies

Singh hasn't figured out why cystic fibrosis lungs are not protected by natural lactoferrin. Previous infections may have damaged the cells lining the lung, he speculates, reducing their ability to make the protective protein.

Treated with lactoferrin (top) bacteria do not form the harmful biofilms (bottom). © P.K. Singh

The effect of lactoferrin on bacteria offers a new way to eliminate persistent biofilm infections: by starving them. "Maybe the weak spot of biofilms is restricting their nutrients," says Ganz. Until now, many studies have focused on blocking the chemical signals that colony bacteria use to communicate.

Medical implants or wounds might be sprayed with lactoferrin or another drug that soaks up iron, speculates Singh, to prevent biofilms forming. And cystic-fibrosis patients might inhale such a drug.

Researchers are not yet clear what renders biofilms so resistant to antibiotics and natural immune mechanisms. The slimy coat they secrete may partly protect them - and the bacteria may switch to different metabolic processes that antibiotics don't affect.

1. Singh, P. K., Parsek, M. R., Greenberg, E. P. & Welsh, M. J. A component of innate immunity prevents bacterial biofilm development. Nature, 417, 552 - 555, (2002).