Question 3 of 6: Flow Cytometry

Question 3: Flow Cytometry (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Discriminating Whole Cells from Debris

Every event passing through the flow cell is first characterized by two light-scatter measurements taken off the same illuminating laser: forward scatter (FSC), collected roughly in line with the laser beam, which correlates with a particle's overall size/diameter, and side scatter (SSC), collected at ~90° to the beam, which correlates with internal granularity/complexity (membrane folding, organelle content). Plotting FSC (x-axis) against SSC (y-axis) as a dot plot for every event resolves distinct populations: intact whole cells form a coherent cluster at characteristically higher FSC (larger particles) and a granularity-appropriate SSC, while cellular debris (membrane fragments, dead-cell remnants, sub-cellular particles) is much smaller and appears as a separate, low-FSC/low-SSC cluster near the plot's origin. A gate (an operator- or software-defined region) is drawn around the whole-cell cluster, and only events falling inside that gate are carried forward into the fluorescence histograms shown in Figure 4 — excluding debris, doublets, and other non-cellular events from the analysis.

(b) How Cell Fluorescence Is Measured

Cells are hydrodynamically focused into a thin, single-file stream (the core of a sheath-fluid flow cell) so that they pass one at a time through the focal point of one or more lasers. As each cell transits the laser beam, any excited fluorophore inside or on the cell emits light at a longer (red-shifted) wavelength than the excitation light; this emitted light is collected by objective optics, passed through dichroic mirrors and bandpass optical filters that select the wavelength range of interest, and directed onto a photomultiplier tube (PMT) (or, in newer instruments, an avalanche photodiode). The PMT converts the light pulse into an electronic voltage pulse whose height is proportional to the amount of fluorescence emitted by that single cell; this value is digitized and logged as one data point, and accumulating tens of thousands of such single-cell measurements builds the fluorescence-intensity histogram (as in Figure 4), typically displayed on a logarithmic intensity axis because cellular fluorescence spans several orders of magnitude.

(c) Assessing Induction Effectiveness at 5 Hours

Figure 4 shows two overlaid distributions rather than two cleanly separated ones. The uninduced (pink, "No") population is concentrated as a fairly narrow peak at low fluorescence intensity (roughly the 10²–10³ range), consistent with baseline/leaky expression from the inducible promoter in the absence of IPTG. The induced (blue, "Yes") population is centred at substantially higher intensity (roughly one to two decades higher, in the 10³–10⁴ range), confirming that adding the inducer did drive a real, population-level increase in expression of the fluorescent reporter. Two features of the overlay, however, show that induction was not complete or fully synchronous across the population after 5 hours: (1) the induced distribution is noticeably broader than the uninduced one and its left flank overlaps substantially with the uninduced peak, meaning a real sub-population of "induced" cells remained at low, near-baseline fluorescence rather than shifting fully — i.e., a genuinely bimodal (or heavily overlapping) rather than a single cleanly shifted population; and (2) the two histograms are not fully resolved from one another, so a clean cut-off cannot separate "on" from "off" cells. Plausible biological reasons for this incomplete/heterogeneous response at the 5-hour timepoint include: cell-to-cell variability in plasmid copy number and promoter/operator occupancy (stochastic gene-expression noise), incomplete or uneven uptake/diffusion of IPTG across all cells in the culture, cells at different points in the cell cycle responding at different rates, and 5 hours simply being insufficient time for every cell to both transcribe/translate the reporter and mature its chromophore to full fluorescence (chromophore maturation itself takes time after translation). Overall: induction was partially effective — it clearly shifted a large fraction of the population to higher fluorescence — but it was incomplete and heterogeneous rather than uniform across all 10,000 cells, and a longer induction time or a stronger/more homogeneous induction protocol would be expected to sharpen the separation between the two populations.