June 25, 2013
Proceedings of the National Academy of Sciences of the United States of
Sticky bacteriophage protect animal cells
Justin R. Meyer¹
Systems Biology Department, Harvard Medical School, Boston, MA 02115
For decades, biologists have discussed and experimented with bacterial
viruses, called phage, as a means of treating bacterial infections (1,
2). Remarkably, our own evolution may have beaten us to it. In PNAS,
Barr et al. present a compelling and unique hypothesis that animal
cells use phage as weapons against bacterial pathogens (3). Animal
cells ranging from cnidarian (coral) to human excrete mucus to protect
themselves from their external environment. Barr et al. propose a
unique function for mucus, to trap phage to intersect and destroy
invading bacteria before they reach the mucus-encapsulated tissue. This
hypothesis (termed BAM for bacteriophage adhering to mucus) suggests a
unique component of the animal immune system governed by a
cross-kingdom animal-phage mutualism.
The BAM model proposed by Barr et al. (3) is as follows: (i) Animal
epithelial cells secrete mucus that is rich with mucin glycoproteins,
which work like Velcro. These proteins have a unique structure that
incorporates hundreds of negatively charged glycan chains, which extend
nanometers into the surrounding environment (4). (ii) The other side of
the Velcro is the bacteriophage capsid, which has an Ig-like protein
domain that interacts with the glycoproteins. The capsid is the DNA
reservoir or "head" of the phage, from which the "tail" or other
structures that trigger infection extend. (iii) With the phage embedded
headfirst into the mucus matrix, their protruding tails may provide
protection by infecting and killing invading bacteria (Fig. 1).
| Fig. 1.
| Bacteriophage adherence to mucus. Ig-like properties of phage capsids
| (hexagon shape) adhere to glycan residues on mucin glycoproteins
| (branched structure) of epithelial cell's mucus layer. Once embedded
| within the mucus, phage defend animal cells against invading pathogenic
In this interaction, animal cells benefit by gaining protection against
bacterial pathogens, but the phage associating with the mucus layer may
also benefit. Bacteria tend to congregate in and near mucus layers
because the mucus provides a high resource habitat (5). Therefore,
phage embedded in the mucus have an increased probability of
encountering a host. In this model, chemical properties of mucus and
the Ig-like domains of capsid proteins have coevolved to interact and
reinforce this mutualistic relationship.
In support of this complex model, Barr et al. (3) identify two natural
patterns consistent with BAM. First, and most importantly, mucus is
enriched with phage. This finding is true for all mucus layers sampled,
which ranged from invertebrates to humans. Second, phage with Ig-like
domains were overrepresented in samples taken on or near mucus.
To provide direct causal evidence for BAM, Barr et al. (3) conducted
experiments that incorporated techniques from many biological
disciplines. The first set used a laboratory-based model for the
three-species interaction, with human tissue cultures, the bacterial
pathogen Escherichia coli, and the bacteriophage T4. Barr et al. began
by showing that T4 adheres to mucus and that these phage particles
remain viable and able to reproduce despite being tangled in a mucus
web. Next, T4 were shown to efficiently kill invading E. coli. Most
importantly, phage bound to mucus protected tissue culture cells from
mortality caused by E. coli. Barr et al. also ran experiments to
diagnose the molecular mechanisms that facilitate the phage-mucus
interaction. As predicted, adherence of the phage to the mucus layer is
a result of the interaction between glycans and Ig-like domains on the
Taken together, the experiments show that the physical mechanics
proposed by BAM work under controlled laboratory settings. Combined
with the natural patterns, the work strongly suggests that phage
enrichment in mucus provides defense from bacterial pathogens.
The next big challenge will be finding evidence that BAM adds an
additional layer to the animal immune system under natural conditions.
In a more natural environment, animal cells encounter tens or hundreds
of potentially pathogenic strains of bacteria. On average, phage are
extremely strain specific (6, 7); thus, how a mucus layer creates a
reservoir of phage extensive enough to protect against a myriad of
bacterial diseases remains to be seen.
An additional limitation of BAM for providing immunity is that many
phage are not lethal and can donate beneficial genes to their hosts
(8). One example that challenges the effectiveness of BAM are
pathogenic enteric bacteria from the genera Escherichia and Shigella.
Many benign E. coli strains are made pathogenic after receiving toxin
genes from phage of the λ family (9, 10), which also possess Ig-like
domains on their capsids (11). Therefore, under some rare conditions,
BAM may increase the incidence of disease by improving the probability
of benign E. coli encountering a λ phage. Animal cells could avoid this
consequence by selectively binding with other phage. It will be
fascinating to know whether mucus is selective and if epithelial cells
can reap the benefits of BAM, but avoid this negative consequence.
Another promising line of research to explore will be the role of BAM
in the context of the animal microbiome. Recent work has revealed that
a healthy microbiome helps maintain host fitness (12, 13). One way
animal cells maintain a healthy microbiome is by producing compounds
that favor the proliferation of commensal bacteria in mucus (14, 15).
Perhaps BAM has a similar effect; phage may inhibit the growth of
certain microbes, clearing space and resources for commensals to
flourish. Unraveling the direct benefits of phage's ability to destroy
invading pathogens, and other indirect benefits moderated through the
microbiome will likely reveal new therapeutic possibilities for phage.
Overall, I believe BAM is a compelling hypothesis that has likely
revealed a hidden, but ubiquitous and beneficial interaction between
metazoans and phage. I look forward to seeing the important work this
report will undoubtedly instigate.
Author contributions: J.R.M. wrote the paper.
The author declares no conflict of interest.
1. Levin BR, Bull JJ
(2004) Population and evolutionary dynamics of phage therapy. Nat
Rev Microbiol 2(2):166–173.
2. Pirnay J-P, et al.
(2012) Introducing yesterday's phage therapy to today's medicine.
Future Virol 7(4):379–390.
3. Barr JJ, et al.
(2013) Bacteriophage adhering to mucus provide a non–host-derived
immunity. Proc Natl Acad Sci USA 110:10771–10776.
4. Cone RA
(2009) Barrier properties of mucus. Adv Drug Deliv Rev 61(2):75–85.
5. Martens EC, Chiang HC, Gordon JI
(2008) Mucosal glycan foraging enhances fitness and transmission of
a saccharolytic human gut bacterial symbiont. Cell Host Microbe
6. Flores CO, Meyer JR, Valverde S, Farr L, Weitz JS
(2011) Statistical structure of host-phage interactions. Proc Natl
Acad Sci USA 108(28):E288–E297.
7. Flores CO, Valverde S, Weitz JS
(2013) Multi-scale structure and geographic drivers of
cross-infection within marine bacteria and phage. ISME J
8. Daubin V, Lerat E, Perrière G
(2003) The source of laterally transferred genes in bacterial
genomes. Genome Biol 4(9):R57.
9. Barondess JJ, Beckwith J
(1990) A bacterial virulence determinant encoded by lysogenic
coliphage lambda. Nature 346(6287):871–874.
10. Cheetham BF, Katz ME
(1995) A role for bacteriophages in the evolution and transfer of
bacterial virulence determinants. Molec Microbiol 18(2):201–208.
11. Veesler D, Cambillau C
(2011) A common evolutionary origin for tailed-bacteriophage
functional modules and bacterial machineries. Microb and Molec Biol
12. Blaser M, et al.
(2013) The microbiome explored: Recent insights and future
challenges. Nat Rev Microbiol 11(3):213–217.
13. Bäckhed F, Ley RE, Sonnenburg JL, Peterson DA, Gordon JI
(2005) Host-bacterial mutualism in the human intestine. Science
14. Vaishnava S, et al.
(2011) The antibacterial lectin RegIII γ promotes the spatial
segregation of microbiota and host in the Intestine. Science
15. Schluter J, Foster KR
(2012) The evolution of mutualism in gut microbiota via host
epithelial selection. PLoS Biol 10(11):e1001424.