IOWA CITY, Iowa – A laboratory test developed by University of Iowa researchers indicates that the lungs of cystic fibrosis (CF) patients are infected primarily with bacterial biofilms, organized communities of bacterial cells that are extremely resistant to antibiotic treatment.
Infection by the bacterium Pseudomonas aeruginosa (P. aeruginosa) is the main cause of death in patients with CF. The Pseudomonas is able to set up permanent residence in the lungs of patients with CF where it is impossible to kill, even with powerful antibiotic treatment. In addition, CF patients’ own immune systems seem to overreact to the bacteria, damaging tissue and eventually destroying the lungs.
"This high level of antibiotic resistance and a host immune response that does more harm than good are the hallmarks of a bacterial biofilm," said E. Peter Greenberg, Ph.D., Virgil L. and Evalyn Shepperd Endowed Professor of Molecular Pathogenisis and UI professor of microbiology. He is an author of a study that appears in the October 12 issue of the journal Nature.
Biofilms are organized communities of bacterial cells encased in a self-produced slime. The bacterial cells produce signaling molecules, also called quorum-sensing molecules, that allow the cells to communicate with each other. At a critical cell density, these signals have accumulated and trigger the expression of a specific set of genes, which results in the formation of the biofilm. By growing as a biofilm, bacteria can survive and thrive in hostile environments.
Although the P. aeruginosa isolated from the lungs of CF patients looks like a biofilm and acts like a biofilm, until now there has not been an objective test available to confirm that it is a biofilm. The researchers also did not know what proportion of the P. aeruginosa might be in a biofilm in the lung.
"We needed a way to show that the Pseudomonas in CF lungs was communicating like a biofilm. That could tell us about the Pseudomonas lifestyle," said Pradeep Singh, M.D., UI assistant professor of internal medicine and a lead author on the study.
Greenberg and his colleagues have spent much of the last decade studying the inner workings of biofilms. They discovered that P. aeruginosa uses one of two particular quorum-sensing molecules to initiate the formation of biofilms. In November 1999, the researchers screened the entire bacterial genome, identifying 39 genes that are strongly controlled by the quorum-sensing system.
In this latest study, Greenberg, Singh and their colleagues have developed a sensitive new test which shows that Pseudomonas from CF lungs produce the telltale, quorum-sensing molecules that are the signals for biofilm formation.
"The fact that the P. aeruginosa in CF lung sputum are making the signals in the ratios that we see tells us that there is a biofilm and that most of the P. aeruginosa in the lung are in the biofilm state," Greenberg said.
P. aeruginosa secretes two signaling molecules, a long one and a short one. Using the new test, the researchers showed that a normal laboratory strain of Pseudomonas grown in broth, where the bacterial cells live in a planktonic, or free-moving form, produced more of the long molecule than the short. They then tested P. aeruginosa strains isolated from CF lungs. All of the strains produced the signaling molecules, but in the opposite ratio, more short than long. Even more interesting was the discovery that when several strains of the bacteria from CF lung sputum were grown as the free-moving form, the bacteria reverted to producing more long signal molecules than short ones.
Could this change in ratio indicate whether the bacteria were in the free-living or biofilm form?
To find out, the UI researchers took the bacteria from the broth and made them grow as a biofilm again. Sure enough, these strains of bacteria in their biofilm form produced more short signal molecules than long.
"These strains are biomarkers for biofilms," Greenberg said. "We now have a clear biochemical definition of whether these bacteria are in a biofilm."
Greenberg believes that the most exciting implication of this result is that it could be used to develop automated processes to test thousands of compounds for the ability to disrupt biofilm formation.
"I think this will attract interest from industry where they are very interested in being able to use high throughput, automated processes to rapidly identify compounds that inhibit biofilm formation," Greenberg said.
In addition to Greenberg and Singh, the other UI investigators on the study included Michael J. Welsh, M.D., a Howard Hughes Medical Institute investigator and Roy J. Carver Professor of Internal Medicine and Physiology and Biophysics at the UI, Amy L. Schaefer, a graduate student in microbiology, and Thomas O. Moninger, research assistant in the UI Central Microscopy Research Facility. Matthew R. Parsek, Ph.D., Louis Berger Junior Professor of Civil Engineering at Northwestern University, was also part of the research team. Singh, Schaefer and Parsek were considered the lead authors of the study.
The research was funded in part by grants from the National Institute of General Medical Sciences, the National Heart, Lung, and Blood Institute and the Cystic Fibrosis Foundation.
Journal
Nature