The human body is colonised by a diverse collective of microorganisms, including bacteria, fungi, protozoa and viruses. The smallest entity of this microbial conglomerate are the bacteriophages. Phages colonise all niches of the body, including the skin, oral cavity, lungs, gut and urinary tract. As such our bodies are frequently and continuously exposed to high numbers of phages, and we secrete more than several billion phages per gram of faeces.

Phages cannot infect eukaryotic cells in the same way they infect their bacterial host cells. Nevertheless, phages freely and profusely penetrate our bodies; having been found within the blood, serum, lung, liver, kidney and even within the brain. How phages are capable of crossing confluent epithelial cell barriers and accessing these ‘classically sterile’ regions of the body remained poorly understood.

The Barr Lab proposed bacteriophage transcytosis as a generalised mechanism for phage access to the body, whereby naturally occurring phages are endocytosed and transported across epithelial cell layers. In vitro studies demonstrated the rapid, directional transport of diverse phages across cell lines originating from across the body. Cell biology experiments revealed that phage particles were capable of accessing all endo-membrane components of the eukaryotic cell, with phage transit occurring through the Golgi apparatus before being functionally exocytosed on the contralateral cell membrane.

Based on these results we estimate that the average adult human body transcytoses approximately 31 billion phages from the gut into the body every day. Naturally occurring phages are capable of crossing the confluent epithelial cell layers of the gut, gaining access to the body and result in the accumulation and assembly of an ‘intra-body phageome’.

For more information on this transcytosis mechanism, see “Bacteriophage transcytosis provides a mechanism to cross epithelial cell layers”, or download the PDF here.

For a comprehensive review on bacteriophage biology within the human body, see “A bacteriophages journey through the human body”, or download the PDF here.

I am Marion Bichet a French student from Sorbonne University where I completed my Master’s degree in partnership with the Pasteur Institut in microbiology. I started as PhD student in the Barr Lab in February 2018 where I focus on – The interactions between bacteriophages and eukaryotic cells. In my project, I look at the transcytosis of phages across epithelial cell barrier with the help of microscopy and molecular biology to understand the effect of phages on the human body.

I am interested in different aspects of this transcytosis phenomenon. Firstly, how and when do phages enter epithelial cells? To answer this question, I use powerful microscopic tools to visualize and follow the entry of phages into the cells using a range of fluorescence stains where I can follow in real-time the entry and trafficking of the phages inside the cells (see Gif below: phages are labeled in green, cell membranes are in purple and nucleus in blue, time stamp at top left is minutes:seconds).

Secondly, I aim to extend the results obtain in the Nguyen et. al., 2017 paper using a transwells systems that mimics the epithelial cell layer. I use this technic to quantify how many phages are crossing the cell layer and how many phages are remaining within the cell cytoplasm. Using an absolute quantification technic called Digital Droplet PCR (ddPCR) I will analyze the number of phages that are transported during the transcytose process.

Third and finally, I aim to understand the immunology and cellular responses of eukaryotic cells that have transcytosed phages. These results will provide information on the impact of phages on eukaryotic cells and how the cells react with the foreign, proteinaceous phage particle. The data obtained from these experiments will allows us to better understand how phages interact with the cells and organs of the body without (seemingly) to stimulate an immune response.