Open Cytometry: discussing biological models and illumination needs

To build and field thousands of open-source cytometers capable of distinguishing deadly falciparum malaria for $1 per point-of-care field test requires starting with biology-first, science-first approach. We covered a brief history of lens making through 17th century Spinoza and Hooke’s cells. Then we crossed into more recent efforts (2011) to discriminate malaria based on hemozoin scattering alone with red and blue light.

For the host biology, hematopoietic progenitor cell (stem cells) have diploid DNA, lots of RNA. In the bone marrow, the stem cell reproduces itself, splitting into a new stem cell and future RBC (red blood cell). The nucleated red cell has lots of ribosomal RNA, transfer RNA and messenger RNA along with its own nucleus. As the RBC matures, it expells the nucleus and stops making RNA. The RBC needs to make hemoglobin to usefully carry oxygen– it makes ribosomal RNA and hemoglobal RNA and needs to hold peptide.

Hemozoin and falciparum malaria

Falciparum malaria feed on heme, crystallizing the toxic (to them) hemoglobin. The levels of hemozoin rise and fall, but can be detected via transmission or scattering measurements. Hemoglobin absorption drops off around 630 nm but for hemozoin 660 nm should be a good measurement wavelength. We use simple color glass filters to eliminate the longwave spectral tail of the LEDs. For our purposes and at the moment, we think of hemozoin measurements as a necessary but not sufficient condition to detect falciparum malaria.

Fluorescent dyes and falciparum high AT-pair ratio

Falciparum has the highest AT-pair percentage of common human-borne organisms. We estimate AT base pair percentage by using two dyes in a few microliters of peripheral blood. By measuring the fluorescence of AT-sensitive dye and then the fluorescence of a dye that binds equally well to AT and GC, we establish AT/(AT+GC) and since these are the only two base pair types, we have established how close each nucleus on the slide is to 80% AT-pairs, suggesting falciparum infection.