Phages used to remove biofilm: The Good, the Bad and the Complicated
As the search for antibiotics repeatedly comes up empty handed, phage therapy has been touted as an alternative. The reality however, is normally quite nuanced and complex.
I want to highlight two recent papers that present some very nice work with respect to phages and biofilms. I also wanted to present them together because they help to highlight the complexity in using phage as therapeutics.
The first paper is from Lucía Fernández and a team of researchers from Asturias in the north of Spain. They showed that at low levels, lytic phage promote a unique biofilm phenotype in Staphylococcus aureus. Addition of the phage triggered the stringent response and led to thicker biofilms with a different morphology. Not good if you are trying to treat an intravenous catheter infection with phage, but being able to manipulate the biofilm does have it's uses, both in research and industry. Using phage to do this may be an interesting experimental approach in the future.
In a second paper, this time from Bruce Levin's lab in Atlanta, the researchers showed a specific and methodical approach to using phage and antibiotics to irradicate Pseudomonas aeruginosa biofilm. The team noted that individually, both antibiotics and phages were less effective at killing Ps. aeruginosa cells in a biofilm than they were in combination. Crucially though, the combination and the order of addition made a difference to the overall effectiveness of the treatment.
To me, the significance of this is that the situation is promising but complicated. It seems clear that we cannot simply go and empty phage cultures onto wound biofilms and hope everything is ok. However, by investigating the complexities of phage interactions with biofilms and with current treatments like antibiotics, maybe we can find new therapeutic strategies for the future.
Below are the abstracts to the two papers and links. I highly recommend you have a look.
Low-level predation by lytic phage phiIPLA-RODI promotes biofilm formation and triggers the stringent response in Staphylococcus aureus
An important lesson from the war on pathogenic bacteria has been the need to understand the physiological responses and evolution of natural microbial communities. Bacterial populations in the environment are generally forming biofilms subject to some level of phage predation. These multicellular communities are notoriously resistant to antimicrobials and, consequently, very difficult to eradicate. This has sparked the search for new therapeutic alternatives, including phage therapy. This study demonstrates that S. aureus biofilms formed in the presence of a non-lethal dose of phage phiIPLA-RODI exhibit a unique physiological state that could potentially benefit both the host and the predator. Thus, biofilms formed under phage pressure are thicker and have a greater DNA content. Also, the virus-infected biofilm displayed major transcriptional differences compared to an untreated control. Significantly, RNA-seq data revealed activation of the stringent response, which could slow down the advance of the bacteriophage within the biofilm. The end result would be an equilibrium that would help bacterial cells to withstand environmental challenges, while maintaining a reservoir of sensitive bacterial cells available to the phage upon reactivation of the dormant carrier population.
Lucía Fernández, Silvia González, Ana Belén Campelo, Beatriz Martínez, Ana Rodríguez & Pilar García
Scientific Reports 7, Article number: 40965 (2017)
Synergy and Order Effects of Antibiotics and Phages in Killing Pseudomonas aeruginosa Biofilms
In contrast to planktonic cells, bacteria imbedded biofilms are notoriously refractory to treatment by antibiotics or bacteriophage (phage) used alone. Given that the mechanisms of killing differ profoundly between drugs and phages, an obvious question is whether killing is improved by combining antibiotic and phage therapy. However, this question has only recently begun to be explored. Here, in vitro biofilm populations of Pseudomonas aeruginosa PA14 were treated singly and with combinations of two phages and bactericidal antibiotics of five classes. By themselves, phages and drugs commonly had only modest effects in killing the bacteria. However some phage-drug combinations reduced bacterial densities to well below that of the best single treatment; in some cases, bacterial densities were reduced even below the level expected if both agents killed independently of each other (synergy). Furthermore, there was a profound order effect in some cases: treatment with phages before drugs achieved maximum killing. Combined treatment was particularly effective in killing in Pseudomonas biofilms grown on layers of cultured epithelial cells. Phages were also capable of limiting the extent to which minority populations of bacteria resistant to the treating antibiotic ascend. The potential of combined antibiotic and phage treatment of biofilm infections is discussed as a realistic way to evaluate and establish the use of bacteriophage for the treatment of humans.
Waqas Nasir Chaudhry, Jeniffer Concepción-Acevedo, Taehyun Park, Saadia Andleeb, James J. Bull, Bruce R. Levin
PlosOne 2017 http://dx.doi.org/10.1371/journal.pone.0168615