> 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/advanced/measurements-explained/cell-size-range.md).

# Cell size range

BactoBox® works great for practically all bacteria. That said, a small proportion of bacteria are either too small or too large to be detected.

Bacteria come in many shapes and sizes. We compare their size by their sphere-equivalent diameter (SED). First, volume is calculated based on the overall width, length, and shape characteristics. Then, SED is calculated, assuming a spherical cell where diameter is a cube root function of the volume.

BactoBox® detects objects with sphere-equivalent diameter between 0.5–5.0 µm. Larger objects are too big to enter the measurement channel, and smaller objects often produce too subtle change in the electrical fields to be detected.

A pair of *Clostridium perfringens* cells are \~8.4 µm long. Nonetheless the object is readily detectable by BactoBox® measurements because the SED is \~3.5 µm, i.e. safely within the 5.0 µm limit.

<div data-with-frame="true"><figure><img src="https://2483617760-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FCaBGkox4Fd7NCFFCF47X%2Fuploads%2FrMl9puFZ0QUizi6cP56Z%2Fsed_plot_half_ticks.png?alt=media&#x26;token=e829bd9a-4679-4d17-8194-d095cb011802" alt=""><figcaption><p>Cells compatible with BactoBox® measurements have a sphere-equivalent diameter of 0.5–5.0 µm represented by the green region of scale bar. All objects are represented as single cells, except <em>Clostridium perfringens</em> (a pair of cells) and <em>Streptomyces griseus</em> (a filament of chains rolled up in a yarn-ball.</p></figcaption></figure></div>

{% hint style="info" icon="ant" %}

## Some cells with SED < 0.5 µm are compatible with BactoBox® measurements

The lower SED limit of 0.5 µm is a rule-of-thumb. It is based on measurement with silica beads. Electrical properties play a role and silica beads are non-conductive. In contrast, cells have conductive properties and objects smaller than 0.5 µm may may produce higher signal relative to their size and still be detectable in the measurement channel.

For example, BactoBox® works great for *Mycoplasma* species as demonstrated in our customer story with [HUN-REN Veterinary Medical Research Institute](https://sbtinstruments.com/customer-stories/hun-ren) even though most *Mycoplasma* species have an SED below 0.5 µm.

Don't fret if your cells have an SED slightly below 0.5 µm. They may still be compatible with BactoBox®. Try a head-to-head growth curve comparison of BactoBox® and your present enumeration method. You will often find that BactoBox® correlates beautifully with your current method.
{% endhint %}

## Cell size changes during a growth curve

The size of objects fluctuates throughout a growth curve. This is exemplified by the *E. coli* growth curve, where [CIZE](/software/access/pages/measurement-group/growth-curve-group-type.md), an approximation of SED, varies from \~0.9–2.1 µm.

<div data-with-frame="true"><figure><img src="https://2483617760-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FCaBGkox4Fd7NCFFCF47X%2Fuploads%2FlJ3xDH7jgTFiqNfaPMgS%2Fec_growth_curve.png?alt=media&#x26;token=bbcf5dc2-3798-4851-9a01-35ebc1f92f88" alt=""><figcaption><p>Cell size changes throughout a growth curve. <em>E. coli</em> shake flask culture.</p></figcaption></figure></div>

## Examples of too small objects

[Ultramicrobacteria](https://en.wikipedia.org/wiki/Ultramicrobacteria) are too small to be enumerated by BactoBox® measurements. For example, the SED of 0.25 µm for *Pelagibacter ubique* likely makes the bacterium too small. Viral particles are also too small to be detected.

## Examples of too big objects

A human lymphocyte has an SED of \~7 µm. This exceeds the size limit for the measurement channel and therefore lymphocytes will stay in the bypass channel. This is often an advantage as BactoBox® can detect the small bacteria even when the sample contains larger non-bacterial cells.

Actinomycetes like *Streptomyces griseus* form long chains (filaments) that bundle up into huge yarn-ball objects. Filaments with SED larger than 5.0 µm are not compatible with BactoBox® measurements. In contrast, individual actinomycetes cells and exospores are typically 0.5–5.0 µm and are easy to enumerate with BactoBox®.

Yeast cells like *Saccharomyces* species are often too big to enter the measurement channel. *Pichia pastoris* - a different yeast cell - is usually within the upper SED limit.


---

# 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/advanced/measurements-explained/cell-size-range.md?ask=<question>&goal=<endgoal>
```

`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.