Pseudomonas aeruginosa

Phage synergy with the immune system

24 August 2017 by Vincent Racaniello

Not longÂ after their discovery, viruses that infect bacteria – bacteriophages – were considered as therapeutic agents for treating infections. Despite many years of research on so-called phage therapy, clinical trials have produced conflicting results. They might be explained in part by the results of a new study which show that the host innate immune system is crucial for the efficacy of phage therapy.

When mice are infected intranasally with Pseudomonas aeruginosa (which causes pneumonia in patients with weak immune systems), the bacterium multiplies in the lungs and kills the animals in less than two days. WhenÂ a P. aeruginosa lytic phage (i.e. that kills the bacteria) is instilled in the nose of the mice two hours after bacterial infection, all the mice survive and there areÂ no detectable bacteria in the lungs. The phage can even be used prophylactically: it can prevent pneumonia when given up to four days before bacterial challenge.

The ability of phage to clear P. aeruginosa infection in the mouse lungs depends on the innate immune response. When bacteria infect a host, they are rapidly detected by pattern recognition receptors such as toll-like receptors. These receptors detect pathogen-specific molecular patterns and initiate a signaling cascade that leads to the production of cytokines, which may stop the infection. Phage cannot clear P. aeruginosa infection in mice lacking the myd88 gene, which is central to the activity of toll like receptors. This result shows that the innate immune response is crucial for the ability of phages to clear bacterial infections. In contrast, neither T cells, B cells, or innate lymphoid cells such as NK cells are needed for phage therapy to work.

The neutrophil is a cell of the immune system that is important in curtailing bacterial infections. Phage therapy does not work in mice depleted of neutrophils. This result suggests that humans with neutropenia, or low neutrophil counts, might not respond well to phage therapy.

A concern with phage therapy is that bacterial mutants resistant to infection might arise, leading to treatment failure. In silico modeling indicated thatÂ phage-resistant bacteria are eliminated by the innate immune response. In contrast, phage resistant bacteria dominate the population in mice lacking theÂ myd88 gene.

These results demonstrate that in mice, successful phage therapyÂ depends on a bothÂ the innate immune response of the host, which the authors call ‘immunophage synergy’. Whether such synergy also occurs in humans is not known, but should be studied. Even if observed in humans, immunophage synergy might notÂ be a feature of infections in other anatomical locations, or those caused by other bacteria. Nevertheless, should immunophage synergy occur in people, then clearly only those with appropriate host immunity – which needs to be defined – should be givenÂ phage therapy.

TWiV 368: Infected, you will be

20 December 2015 by Vincent Racaniello

On episode #368 of the science show This Week in Virology, aÂ plaque of virologists explores the biology of Zika virus andÂ recent outbreaks, and the contribution of a filamentous bacteriophage to the development of biofilms.

You can find TWiV #368 at www.microbe.tv/twiv.

Viruses help form biofilms

17 December 2015 by Vincent Racaniello

Bacteria frequently grow in communities called biofilms, which are aggregates of cells and polymers. An example of a biofilm is the dental plaque on your teeth. BiofilmsÂ are medically important as they can allow bacteria to persist in host tissues and on catheters, and confer increased resistance to antibiotics and dessication. Therefore understanding how biofilmsÂ form is crucial for controlling microbial infections. An advance in our understanding of biofilms formation is the observation that filamentous phages help themÂ assemble, and contribute to their fundamental properties.

Pseudomonas aeruginosa is an important human pathogen which is a particular problem in patients with cystic fibrosis. The ability of this bacterium to form biofilms in the lung is linked to its ability to cause chronic infections. Pseudomonas aeruginosa biofilms contain large numbers of filamentous Pf bacteriophages (pictured). These virusesÂ lyse cells and release DNA, which becomes oneÂ component of the biofilm matrix.

Mixing supernatants of P. aeruginosa cultures with hyaluronan, which is present in airways of cystic fibrosis patients, resulted in the formation of a biofilm – in the absence of bacteria. AÂ major component of P. aeruginosa biofilms was found to be Pf bacteriophages. When purifed Pf bacteriophages were mixed with hyaluronan, biofilms formed. Similar biofilms also formed when theÂ filamentous bacteriophage fd of E. coliÂ was mixed with hyaluronan.Â Mixtures of Pf bacteriophages and various polymers (alginate, DNA, hyaluronan, polyethylene glycol) formedÂ liquid crystals (matter in a state between a liquid and a solid crystal).

Pf phages were detected in sputum from patients with cystic fibrosis, but not in uninfected patients. Addition of Pf phage to sputum from patients infected with P. aeruginosa made the samples more birefringent, a property of liquid crystals. Compared with a strain of P. aeruginosa that does not produce Pf phage, colonies of virus-producing strains formedÂ liquid crystals. These observations indicate that Pf phage help organize the bacteria into a biofilm matrix.

Some features of biofilms include their ability to adhere to surfaces, to protect bacteria from dessication, and to increase resistance to antibiotics. Addition of phage Pf increased biofilm adhesion and tolerance against dessication. Such additionÂ also made the biofilm more resistant to aminoglycoside antibiotics, because these were sequestered in the biofilm. No phage-mediated increased resistance to ciprofloxacin was observed, probably because this antimicrobial does not interact with polyanions of the biofilm as do aminoglycosides.

These results show that presence of bacteriophage in a biofilm of P. aeruginosa helps organize the matrix while contributing to some of its fundamental properties. It seems likely that filamentous phages of other bacteria will play roles in biofilm formation, suggesting that targeting the phages in these matrices could be effectie strategies for treating biofilm infections.