Many reports focus on bacterial growth in water (especially nutrient-rich waste water), but bacteria also grow well – if not better – on surfaces, where they can form a biofilm community. Biofilms are surface-dwelling microbes growing in a single- or multispecies mixture, encased in an extracellular matrix. Scientists study many characteristics of biofilms – nutrient and oxygen gradation, gene expression patterns, adherence capabilities – but important to healthcare systems is the resilience of biofilms. Biofilms are extremely resistant to antibiotic treatment, and thus a common problem of hospital-acquired infections.
One reason biofilms are such a problem in hospitals is their ability to form on abiotic sources – think indwelling catheters, IVs, and drug pumps. Many microbes grow well on the plastics used for these devices. These biofilms can then seed systemic infections in the patients, who are already in the hospital for a primary reason (and may not be in prime health). To better understand how bacteria transition from planktonic to biofilm state, researchers Stephanie Cole and Vincent Lee used the pathogen Pseudomonas aeruginosa to investigate a mouse model of catheter-associated urinary tract infection (CAUTI), published online this week in the Journal of Bacteriology. CAUTI infections can lead to pyelonephritis and bacteremia. Approximately 30% of healthcare-associated infections are UTIs, and approximately 10% of these UTIs are caused by P. aeruginosa.
P. aeruginosa cells transition from planktonic growth in liquids to biofilm growth on a surface through detection of a signal molecule, c-di-GMP. Through interaction with other bacterial proteins, c-di-GMP decreases bacteria motility and increases production of exopolysaccharide (which is part of the biofilm extracellular matrix).
First the authors had to ask whether c-di-GMP was important in an in vivo model of infection – previous work had shown in vitro importance, but that doesn’t always translate to a more complicated system. The authors used mutant strains with increased or decreased c-di-GMP relative to normal (wild-type) levels. They tested these mutants for the ability to form biofilm and to colonize mice bladders via catheter tubes. The P. aeruginosa strains with increased c-di-GMP colonized the bladder and disseminated to the kidneys in greater numbers than those with normal or decreased c-di-GMP, demonstrating c-di-GMP signaling does contribute to CAUTI.
The authors wanted to pinpoint which enzymes increase or decrease c-di-GMP during CAUTI. The c-di-GMP molecules are generated through diguanylate cyclases (DGCs) and removed by phosphodiesterases (PDEs) (see schematic, taken from this review). By testing a library of mutants in these enzymes, the authors were able to find five mutants that had at least a ten-fold lower number of bacteria colonizing and two with a ten-fold higher number. This suggests a specific regulation of c-di-GMP in catheter biofilm growth.
The proper balance of c-di-GMP is important in infection: higher levels promote adhesion and biofilm formation, but if a bacterium is too sticky, it won’t leave the biofilm to seed a new infection/colonization (thus causing systemic disease). Decreasing c-di-GMP promotes this leaving effect, and demonstrates why proper regulation (and timing) of signaling molecules is important during infection. Interrupting this regulation is a potential target of antibiotic drugs, especially given than this signaling system has only been found in bacteria.
Potential inhibitors of c-di-GMP signaling have been identified in several bacterial species. Some have even been shown to work against in vitro P. aeruginosa biofilm formation. But just like the demonstration that c-di-GMP is important during in vivo biofilm formation, so must in vivo studies show any inhibitory function, either as a drug or impregnated into the plastics used for indwelling devices.
-- Julie Wolf