When a colony of bacteria circles its wagons it is emulating the defensive strategy of 19th-century American settlers moving west. The microorganisms ringing their protective components were constructing a pathogenic public health menace called biofilms.

"Biofilms are known to cause many chronic infections," observed microbiologist and adult pulmonologist Pradeep Singh, at the University of Iowa in Iowa City. "However," he continued, "they most often do so when the bodily surface has been damaged or compromised by disease. So normal mucosal surfaces, like the lung or the eye or the urogenital tract, are extremely resistant to forming biofilm infections. We wanted to see if some of the components of the secretions that bathe these surfaces might have antibiofilm effect."

Singh is lead author of a paper in today's Nature, dated May 30, 2002. Its title: "A component of innate immunity prevents bacterial biofilm development."

"Biofilm infections are a major medical problem," Singh told BioWorld Today," and our group is looking for new strategies to treat or prevent them. Resistance to antibiotics in chronic bacterial infections is a difficult and sometimes deadly medical threat. During the development of many chronic infections, the bacteria can forsake their free-living lifestyles as individual organisms, to form slime-coated community structures - biofilms - that are like microbial cities. There they prove extremely resistant to antibiotics, and impervious to the body's natural immune defenses. Biofilms typically include lethal lung infections by Pseudomonas aeruginosa bacteria in patients with cystic fibrosis [CF]. Biofilms also attack infections in diabetics and [those with] burn injuries, infect heart-valve infections with endocarditis, as well as cause most medical-device infections.

"Though the human body is constantly exposed to disease-causing bacteria," he went on, "biofilms do not naturally form unless a person's defenses have been compromised by disease. This lack of biofilm formation suggested to us that the body might have a natural antibiofilm defense mechanism.

"So we decided to look at lactoferrin, an iron-binding protein. Lactoferrin is extremely abundant in surface secretions. It's found in lungs, eyes, tears, saliva, breast milk, the urogenital tract - anywhere where such defense might be needed, lactoferrin is present in high concentrations."

In This Corner - Lactoferrin

"In lab models," Singh recounted, "what we found was that subinhibitory levels of lactoferrin would block the formation of biofilms. The mechanism of this biofilm inhibition wasn't killing or inhibiting the growth of bacteria, but rather interrupting the normal development pathway or process of their biofilm formation.

"For bacteria to form biofilms," Singh explained, "they have to aggregate in groups. Then these microcolonies go on through the process of cell division to grow up as mature biofilm structures. What we reported in Nature was that in the presence of lactoferrin, the bacteria are stimulated to move, to locomote, scoot across the growth surface, and do not settle into cell clusters, cannot form microcolonies, and therefore can't form biofilms.

"Pseudomonas has several types of motility. It can swim in liquids with traditional flagella-mediated locomotion. It also has specialized surface motion known as twitching motility in which the bacteria pull themselves along the surface, using a specialized appendage, a pilus, as a grappling hook to grab the surface and crawl along it. We showed that low concentrations of lactoferrin stimulate this twitching motility and cause the bacteria to wander aimlessly and incessantly over the surface, without forming biofilms.

"That's our main finding," Singh said, "and when the biofilms didn't form, the bacteria remained more sensitive to antibiotics and antimicrobial factors, and less resistant to being killed.

"Many different bacterial species grow in biofilms," Singh pointed out. "From a medical point of view, probably the other major player that causes biofilm infections would be Staphylococcus aureus. This bacterium commonly forms biofilms on medical devices like catheters, an artificial joint, a pacemaker, heart valve, or any foreign body.

"In medicine - and I treat cystic fibrosis patients - we're always trying to find some new strategy to try to prevent formation of biofilms. This particular finding in Nature may be difficult to translate into a therapy. I think the most promising avenue for therapeutic development might be to try to coat surfaces with lactoferrin or some other high-affinity iron-binder, in order to prevent biofilm formation.

"One of the things our study showed was that preformed biofilms are resistant to the effects of lactoferrin. That is, [lactoferrin] doesn't disrupt a biofilm once it's formed. It would have to be used as a preventative measure. Lactoferrin has two antibacterial functions: One, it can bind to components of bacterial outer membranes and disrupt them by punching holes, increasing their leakiness. Two, it's thought to keep the iron concentration low by binding iron, thus limiting bacterial growth. We demonstrated a new function of lactoferrin in preventing biofilm formation."

CF Therapy Far Off; Drug Target Closer?

"In our in vitro laboratory experiments," Singh recalled, "we were looking for a way to quantitate the behavior of large numbers of bacteria under two different conditions - the presence or absence of lactoferrin. So we came up with those arbitrary motility designations, to describe different bacterial behaviors. We call bacteria squatters, when the daughter cells remain localized at the site of their parents' cell division. Ramblers came after the parent divided; the daughters took off, hit the road, went off in different directions. And flyers detached from the surface and were carried away by the flow of culture media. The presence of lactoferrin significantly changed the behavior of bacteria by altering their patterns of movement.

"We used two drugs - hydrogen peroxide, an antiseptic, and tobramycin, a CF antibiotic - to show that when we inhibited biofilm formation by treating them with lactoferrin, the bacteria remained in a more-sensitive, less-resistant state to both agents.

"If we can interfere with early steps of this biofilm-formation developmental process," Singh speculated, "it might be possible to design pharmaceuticals that simulate lactoferrin's action, and prevent the formation of biofilms in CF lungs. But I think this is a long way off.

"Meanwhile, if we can make headway in understanding the mechanism of bacterial iron-sensing, then the possibility of exploiting that as a drug target might increase." Singh concluded: "We have recently submitted a patent application for this concept of preventing the formation of biofilms by iron chelation."