> For the complete documentation index, see [llms.txt](https://help.sbtinstruments.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.sbtinstruments.com/inoc/why-use-bactobox-r.md). # Why use BactoBox® Few things are more frustrating than wasted work. You set an inoculum specification, hit it on the photometer, and start the experiment. Days later the retrospective plate counts say you missed. Results turn inconsistent and the do-over costs you a week. Optical density (OD) and McFarland standards are the usual tools for standardizing inoculum. Both measure turbidity. Turbidity is not a cell count. ## OD and McFarland depend on cell size There is a simple explanation for why OD and McFarland often fail. Both methods measure scattered light, and scattering depends on object size as well as on concentration. Cells change size throughout a growth curve. So the factor that converts arbitrary turbidity units to colony forming units (CFU) changes with them. That leaves you guessing. Pick your calibration point early in the curve and every later number is too low. Pick it late and every earlier number is too high. The drift is one-directional, so more replicates do not average it away. We tracked a single *E. coli* culture with BactoBox®, plate counts, and OD₆₀₀ at the same time points. The OD-to-CFU factor rose 6.6× across the run. See [Drifts in OD to CFU ratios](/inoc/explainers/drifts-in-od-to-cfu-ratios.md).

Optical methods are strongly dependent on the size of objects. The two samples scatter the same amount of light, but the bacteria in exponential stage (left) are considerably bigger than the cells in stationary stage (right). Same OD but ~5× different cell concentrations.

## OD is blind at early growth phases At low cell concentration early in the growth curve, OD₆₀₀ readings are indistinguishable from a blank. BactoBox® quantifies 50–500× lower, depending on how much you dilute. See [OD is blind at low cell concentrations](/inoc/explainers/od-is-blind-at-low-cell-concentrations.md). The OD blindness problem becomes even more severe with extremely small cells e.g. *Mycoplasma* ssp and *Avibacterium* ssp. Sometimes the culture need to reach late exponential before the OD is above the limit of quantification. Standardize a dilute, early-stage culture by OD and you are reading noise. ## BactoBox® is virtually 1:1 with CFU in active cultures BactoBox® is a label-free flow cytometry in a benchtop format. Cells are detected one at a time, so cell size does not influence the result. You get sensitive and precise cell concentrations in real time. BactoBox® arrives calibrated. Unlike OD and McFarland you can skip building a standard curve for every strain. As shown in [Correlation with plate counts](/inoc/explainers/correlation-with-plate-counts.md) BactoBox® demonstrates great agreement with CFUs for active cultures.

*Klebsiella aerogenes* growth curve from shake flask with tryptic soy broth (TSB). BactoBox® cells (lavender) is practically superimposed on the plate counts (CFU, yellow).

## BactoBox® reveals the growth stage You get more than a cell concentration. The CIZE metric reports average cell size, and cell size tracks the growth stage. Map out your growth curve once. After that, a CIZE value tells you which stage a new culture is in. For the practical steps, see [Know the growth phase of your culture](/inoc/explainers/know-the-growth-phase-of-your-culture.md).

Illustration of cell size during a growth curve.

## Summary With BactoBox® you inoculate with confidence. The right dose of bacteria. The right growth stage. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://help.sbtinstruments.com/inoc/why-use-bactobox-r.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.