Team:Virginia/Project

Project Overview

Whooping cough, the infectious respiratory disease caused by Bordetella pertussis, is diagnosed in tens of millions of people and results in almost 300,000 deaths globally each year. Low-income and unvaccinated individuals as well as infants are especially susceptible. Current diagnostic procedures are complicated, costly, and can take up to a week, by which time the disease may have progressed or spread. The enormous impact of this disease urgently motivates the development of a faster, cheaper, and more reliable diagnostic. Our epidemiology models suggest that earlier diagnosis could drastically reduce the incidence and impact of the disease. We propose an engineered bacteriophage diagnostic system for rapid clinical detection of pertussis.

As a proof of principle, we worked on engineering the T7 bacteriophage to express a version of the beta subunit of human chorionic gonadotropin (hCG) which we codon-optimized for expression in E. coli. Due to host-pathogen specificity, bacteriophages will only replicate and produce intracellular hCG if the target bacteria is present in the sample. The bacteriophage lyses the cell, releasing the hCG, which can then be detected using a pregnancy test. Pregnancy tests are commonly available in all clinics and can detect very low concentrations of hCG, which is an advantage over currently-available methods for detecting phage amplification. The bacteriophage system also requires minimal training to be used in lab and does not require any high-tech machinery. This will make the diagnostic readily usable in developing countries, where whooping cough is a particular concern. Besides pertussis, this method can be used to detect the presence of any bacteria for which there exists a bacteriophage using a common pregnancy test.

Background

Limitations of Current Detection Methods

Bacteriophage Basics

Bacteriophage are viruses which specifically infect bacteria, hijacking the bacteria's own replication machinery to replicate themselves. Due to host-pathogen co-evolution, bacteriophage are often extremely specific to a particular genus, species, or even strain of bacteria. They may undergo the lytic cycle, in which the genome of the bacteriophage is used to produce new bacteriophage which are released from the bacterial cell following cell death and lysis, or the lysogenic cycle, in which the bacteriophage genome replicates along with the host genome but does not produce new viruses.

Bordetella Phage

Bacteriophage BPP-1 infects the genus Bordetella, which includes the human pathogen B. pertussis and its cousin B. bronchiseptica. It normally binds to the cell-surface receptor pertactin, which is only expressed during the Bvg+ phase of the Bordetella life cycle (during which the bacteria expresses virulence and colonization factors at high levels). However, BPP-1 also possesses a diversity-generating retroelement system which allows it to switch its tropism (Liu et al., 2002) (Liu et al., 2004).

T7 Phage

Human Chorionic Gonadotropin

Human chorionic gonadotropin is a naturally occurring glycoprotein in the human body, produced in the placenta during pregnancy. It has an alpha and a beta subunit, which are 92 and 145 amino acids long, respectively. In vivo, the large amount of oligosaccharide chains are highly glycosylated, and this highly glycosylated version of the protein is in a great proportion to unmodified hCG in early pregnancy because it is produced by cytotrophoblast cells at implantation of the egg (Cole 2003)(Cole 2006). The primary physiological function of the unmodified protein is known to stimulate progesterone manufacture by the corpus luteal cells (Cole 2003), whereas data on the glycosylated version suggests that it functions in signaling the growth of cytotrophoblast cells and placental implantation (Cole 2010).

Pregnancy Tests

The pregnancy test is an immunoassay, first developed in 1989 and has since expanded to production by 40 different producers, including several large companies such as INSIGHT pharmaceuticals and Procter & Gamble’s ClearBlue (Cole 2003)(Canfield 1989). Like in the first immunoassay developed, most pregnancy tests consists of an antibody against the beta-subunit of the dimer hCG that is immobilized on the test by a secondary antibody. When solution containing the beta-subunit of hCG is dropped onto the test, it is wicked through the immunoassay and causes a chemical change to indicate the presence or absence of the protein.

References

1. Liu, M., Deora, R., Doulatov, S. R., Gingery, M., Eiserling, F. A., Preston, A., Maskell, D. J., et al. (2002). Reverse Transcriptase – Mediated Tropism Switching in Bordetella Bacteriophage. Science, 2091(295). doi:10.1126/science.1067467
2. Liu, M., Gingery, M., Doulatov, S. R., Liu, Y., Hodes, A., Baker, S., Davis, P., et al. (2004). Genomic and Genetic Analysis of Bordetella Bacteriophages Encoding Reverse Transcriptase-Mediated Tropism-Switching Cassettes. Journal of Bacteriology, 186(5), 1503–1517. doi:10.1128/JB.186.5.1503