Direct cell counts are new in many bacterial cultivation labs. When BactoBox® arrives in a workflow built around OD600 and CFU and produces a number called cells/mL, a fair early question is whether that number is anything more than another OD reading.
This article explains what a BactoBox® cell count is, how it compares to existing methods, and what direct cell counts make possible.
What is BactoBox® and impedance flow cytometry
BactoBox® is a benchtop instrument that counts bacterial cells one at a time as they pass through a microfluidic flow cell. It uses impedance flow cytometry: a pair of microelectrodes detects the way each particle perturbs an electrical field as it crosses between them. Every particle within the instrument's 0.5–5 µm detection range is recorded as a single event.
A bacterial cell with an intact lipid membrane and a conductive cytoplasm produces the characteristic signature[1]. A lysing cell whose envelope has been compromised does not produce that signature, so it is recorded as a particle rather than as a cell. Salt crystals, antifoam droplets, and cellular debris in the same size range are also recorded as particles.
The result BactoBox® reports as cells/mL is the concentration of structurally intact total cells in the sample. This is what the instrument is producing every time the reading appears.
One detail follows from the way the measurement works. Each particle that crosses the measurement zone is recorded as a single event. A doublet, a small chain, or a clump of cells therefore registers as one event with a larger electrical signature, rather than as several events. Size grows with aggregate size; the count does not. For samples that aggregate routinely, an accurate cell count requires sample preparation that breaks aggregates apart, and we work with users on the deaggregation steps appropriate for their organism and process.
Comparison to other enumeration methods
Before working through what cells/mL is useful for, it is worth being precise about how it relates to the methods most labs already run. A common misconception is that any total cell count is equivalent to any other, and that BactoBox® is, at best, a different way of producing the same kind of biomass-related signal that OD600 produces. This is not the case. Each method below answers a slightly different question, and once those differences are clear, the place BactoBox® fills in the measurement stack becomes clearer too.
One framing that recurs through this section is worth flagging in advance. None of the methods discussed here measure live cells. "Live" is not an experimentally defined property at the level of an individual bacterial cell, and no microbiological method, including BactoBox®, defines or measures it operationally[2]. What the various methods measure are specific, well-defined properties: structural integrity, membrane integrity, culturability under chosen conditions, and biomass-related optical signal. These are different properties, and the differences matter.
BactoBox® vs. OD600
OD600 is the optical density at 600 nm read on a spectrophotometer, and it is the workhorse real-time signal in most cultivation labs. It is not a cell count: it is a turbidity-based proxy for biomass that responds to cell number, but also to cell size, morphology, and any non-cell particulates in the light path. BactoBox® and OD600 therefore answer different questions, even when the curves they produce look similar in shape. The article Understanding OD600 covers the signal, its uses, and its limitations in depth.
BactoBox® vs. CFUs
A colony-forming unit (CFU) is, operationally, a unit in the sample capable of forming a visible colony on a chosen medium under chosen conditions. CFU counts are not counts of every cell present; they are counts of cells competent to grow into a colony in that specific assay. This is a narrower property than structural integrity. Cells that are physically intact but have lost the ability, perhaps temporarily, to grow do not register as CFUs, including cells in the viable-but-non-culturable (VBNC) state and cells that have suffered sub-lethal damage[3]. CFU also undercounts samples containing aggregates, where a clump of multiple cells grows up as a single colony[4], and is sensitive to the plating workflow itself: method-dependent differences between pour plating, spread plating, and dehydrated film methods such as Petrifilm have been documented on the same samples[5].
A BactoBox® count and a CFU count therefore measure overlapping but non-identical populations. In exponential and early stationary phase, where most structurally intact cells are also culturable, the two track each other closely, with R² values above 0.99 validated against CFU for E. coli and five additional bacterial species in those phases[6]. Outside that window, the two diverge.
The region where they diverge is classically described as the decline or death phase, but that label is itself an interpretation rather than an observation. From a comparison of structurally intact cell counts and CFU counts alone, it is not possible to determine whether cells that have lost culturability are dead or whether they have entered the VBNC state, in which they retain physical integrity but no longer form colonies on standard media. The accurate, more limited statement is that not all cells in the sample are culturable any more. The size of the gap between the two counts carries information about both the process and the analytical workflow regardless of which interpretation applies.
BactoBox® vs. direct microscopy
Direct microscopy with a counting chamber such as a Petroff-Hausser or hemocytometer is the textbook reference for counting cells directly. What is reported depends on the preparation. Brightfield or phase-contrast counts measure visible particles in a small sample volume. Fluorescence microscopy with a total-DNA stain such as DAPI or acridine orange, usually after fixation or mild permeabilisation, measures cells whose DNA is accessible to the dye. Fluorescence microscopy with a membrane-integrity stain such as PI paired with SYTO 9 reports cells whose membranes have been compromised, on the same logic as the PI/SYTO 9 dye combination used in fluorescent flow cytometry.
The advantage no other method on this list provides is that a human can see the cells: morphology, filament and aggregate structure, and sub-populations are all directly visible. This matters in particular for aggregating cultures, where microscopy resolves the structure that BactoBox® compresses into one event per particle. In practice, though, many labs do not use direct microscopy routinely. Throughput is slow, the counted sample volume is small (typically 1 to 100 nanolitres per field). Operator variability is usually the factor hardest to ignore: two analysts counting the same chamber will not always produce the same number, and the count depends on field selection, focus plane, and judgements about what counts as a cell. Petroff-Hausser counts of 1 µm microbeads, at bacterial size scale, have been measured 24% off from reference values[7], within the chamber manufacturer's own stated 20–30% expected count discrepancy.
BactoBox® vs. fluorescent flow cytometry
Fluorescent flow cytometry shares its architecture with BactoBox®: cells pass one at a time through a sensing region, and each cell is detected as an event. The difference is the sensing. Fluorescent flow cytometry reads light scatter and the fluorescence of bound dyes, while BactoBox® reads electrical signals.
What the technique reports therefore depends on the dyes used. The most common viability-style combination in bacterial work is propidium iodide (PI) paired with SYTO 9. PI cannot cross an intact bacterial membrane, so cells that take up PI are reported as having compromised membranes[8]. PI flags as compromised any cell whose membrane has been damaged enough for the dye to enter. BactoBox®, by contrast, detects the gross electrical signature of a structurally intact cell. A cell with small membrane lesions that still maintains its overall electrical character can pass the BactoBox® classification while being flagged as compromised by PI. The two methods therefore both measure structural integrity, but at different sensitivities, and the populations they classify as structurally intact are not identical.
What direct cell counts make possible
Cells/mL from BactoBox® is a measurement of a specific, well-defined property: the concentration of structurally intact bacterial cells in the sample. The signal corresponds directly to what scientists usually mean by growth, namely cell division: a direct count rises when, and only when, division actually happens.
This makes a direct cell count uniquely well-suited to growth-related measurements. Any new object that appears in the sample over time can only be a new cell. A specific growth rate calculated from a series of cells/mL measurements is therefore an unambiguous measurement of how fast the population is dividing. OD600 cannot say this cleanly because increases can also come from cell elongation, morphology shifts, or storage compound accumulation. CFU is too laborious to run at the frequency that meaningful growth-rate measurements require.
Beyond growth rate, a direct count makes each phase of the growth curve more interpretable: lag, the true exponential phase, the onset of slowdown, peak cell concentration, plateau, and lysis. Lysis shows up as a drop in cells/mL, because cells that have lost structural integrity stop being counted. Loss of culturability often comes before lysis, however, so the drop does not, on its own, reveal whether or when those cells lost culturability; a direct cell count and a CFU count read together describe the transition from loss of culturability to loss of structural integrity cleanly.
Sequential measurements are where the value compounds. With a series of measurements through a run, the trajectory itself becomes the unit of analysis, and differences between strains, media, or runs become differences in trajectory shape rather than endpoint comparisons.
Conclusion
A direct count of structurally intact cells is a different kind of measurement than the methods most cultivation labs have built their workflows around. It is not a faster OD600 and it is not a faster CFU. It answers a question those methods were not designed to answer: how many structurally intact bacterial cells are in the sample right now.
BactoBox® produces that count in as little as two minutes per sample, with no operator-dependent counting step. CFU, by comparison, takes a day or more and adds variance from manual counting. The BactoBox® result is therefore fast enough to be repeated through a cultivation, and consistent enough that two operators running the same sample will report the same number.
What this enables, when integrated into a process development workflow, is a clearer view of the dynamics that drive most decisions at the bench: when exponential growth begins and ends, where the cell-count peak sits, when slowdown sets in, when a plateau is reached, and when lysis begins. Working from cells/mL is, at the same time, a new way of thinking about a process for many scientists. The growth curve has been read through OD600 and CFU for a generation, and re-anchoring interpretation in direct cell counts takes time.
We encourage close collaboration on that transition. Other articles in our help center walk through specific applications, including End of fermentation crosscheck, where the end-of-fermentation comparison between BactoBox® and CFU is used as an entry point for yield improvement. You are also always welcome to reach out to us directly.
References
- Bertelsen CV, Skands GE, González Díaz M, Dimaki M, Svendsen WE. Using impedance flow cytometry for rapid viability classification of heat-treated bacteria. ACS Omega. 2023;8(8):7714-21. https://pubs.acs.org/doi/10.1021/acsomega.2c07357
- Davey HM. Life, death, and in-between: meanings and methods in microbiology. Appl Environ Microbiol. 2011;77(16):5571-6. https://journals.asm.org/doi/10.1128/aem.00744-11
- Oliver JD. Recent findings on the viable but nonculturable state in pathogenic bacteria. FEMS Microbiol Rev. 2010;34(4):415-25. https://academic.oup.com/femsre/article/34/4/415/538375
- Martini KM, Boddu SS, Nemenman I, Vega NM. Maximum likelihood estimators for colony-forming units. Microbiol Spectr. 2024;12(9):e03946-23. https://journals.asm.org/doi/10.1128/spectrum.03946-23
- Linton RH, Eisel WG, Muriana PM. Comparison of conventional plating methods and Petrifilm for the recovery of microorganisms in a ground beef processing facility. J Food Prot. 1997;60(9):1084-8. https://pubmed.ncbi.nlm.nih.gov/31207841/
- Jordal PL, Díaz MG, Aalund F, Skands G. Performance qualification of impedance flow cytometry as a rapid in-process control proxy for colony-forming units in bacterial fermentation processes. J Microbiol Methods. 2025;238:107284. https://www.sciencedirect.com/science/article/pii/S0167701225002003
- Rahman KMT, Butzin NC. Counter-on-chip for bacterial cell quantification, growth, and live-dead estimations. Sci Rep. 2024;14(1):782. https://www.nature.com/articles/s41598-023-51014-2
- Stiefel P, Schmidt-Emrich S, Maniura-Weber K, Ren Q. Critical aspects of using bacterial cell viability assays with the fluorophores SYTO9 and propidium iodide. BMC Microbiol. 2015;15:36. https://bmcmicrobiol.biomedcentral.com/articles/10.1186/s12866-015-0376-x