> 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/mpd/why-use-bactobox-r.md). # Why use BactoBox® Optical density and dry cell weight mix growth with changes in your cells. A cell count measures growth itself. Process development is about comparing conditions. A new medium, a new feed, a new induction point. Most labs make that comparison with optical density (OD), usually at 600 nm, and often convert it to dry cell weight. ## Growth is cell division A culture grows when its cells divide. BactoBox® counts every whole bacterial cell in your sample. The count rises only when cells divide, so its slope on a log scale is your specific growth rate, μ. * You need no conversion factor for each strain, medium or growth phase. * Cells that grow in your process but form no colonies on a plate still count. * You need no stain. Fluorescent flow cytometry needs a stain that is validated for your cells. ## Optical density and dry cell weight also measure cell size Optical density measures how much light your culture scatters. Dry cell weight measures the mass of your cells. Both rise when cells divide. Both also rise when cells grow larger, or fill up with product or storage compounds, without dividing. Your cells change size whenever their conditions change. Fast-growing cells are larger than slow-growing ones. In lag phase, cells grow before they divide. When nutrients run short, they shrink. In one *P. fluorescens* culture, the count held flat for more than three hours while the average cell grew to about eight times its volume. Optical density and dry cell weight would read that as growth. No cell had divided. In steady exponential growth, cell size holds, and every method gives the same growth rate. The methods part ways when conditions change. Lag phase, slowing growth, a new feed, induction, stress. Those are the moments process development is about. Dry cell weight is still the right number when you need mass, for example for a mass balance or a yield on substrate. ## Optical density does not work between conditions We grew one *E. coli* strain in four media. At OD600 = 0.6, the cell counts differed fivefold. At the end of the run, optical density and cell counts ranked the media in a different order.
Two bar charts of E. coli in four media. By OD600, CD medium ranks first, Terrific broth second, TSB third and LB fourth. By cells/mL, Terrific broth ranks first, CD medium second, LB third and TSB fourth.

E. coli in four media, shake flasks, read at 28 h. The medium with the highest optical density does not have the most cells. LB and TSB could not be told apart.

Read the full study in [Nobody measures growth accurately](https://sbtinstruments.com/knowledge/nobody-measures-growth-accurately?utm_source=help-center\&utm_medium=referral). Optical density still works to check that a run behaves like the last run of the same process, because the cells change the same way each time. It also works to drop the weakest candidates in a first screen. ## You also get cell size Every sample also gives you CIZE, the average size of your cells. In steady growth, faster-growing bacteria are larger, a link known since the 1950s ([Schaechter et al. 1958](https://doi.org/10.1099/00221287-19-3-592)). CIZE therefore shows the state of your cells, and it often moves before the count does. Next step: [Interpreting BactoBox® data](/mpd/interpreting-bactobox-r-data.md) Questions about your own process? [Contact us](https://sbtinstruments.com/contact). --- # 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 by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://help.sbtinstruments.com/mpd/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 what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. 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.