> 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/explainers/correlation-with-plate-counts.md).

# Correlation with plate counts

BactoBox® shows strong agreement with plate count measurements for most [Active cultures](/mpd/cell-growth/active-cultures.md). I.e., when the culture is in exponential, deceleration, and stationary [Growth phases](/mpd/cell-growth/growth-phases.md).

## Correlation across a growth curve

The below figure compares plate counts (CFU/mL, yellow) with BactoBox® (lavender). Plate counts and BactoBox® curves are practically superimposed, and R<sup>2</sup> ≈ 1 for the linear regression on the log-log transformed data. This means that the methods are strongly correlated across exponential, deceleration, and early-stationary stage.

<figure><img src="/files/QFHF7Zaucu1Mh2EzPd0z" alt=""><figcaption><p><em>K. aerogenes</em> growth curve investigated with BactoBox® (lavender) and plate counts (yellow). Triplicate repeats were done for each data point. Additional head-to-head growth curves for <em>E. coli</em>, <em>A. baumannii</em>, <em>P. fluorescens</em>, <em>S. epidermidis</em>, and <em>L. innocua</em> are available in <a href="https://www.sciencedirect.com/science/article/pii/S0167701225002003">Journal of Microbiological Methods</a>.</p></figcaption></figure>

## Comparison of absolute numbers

The peer-reviewed publication in [Journal of Microbiological Methods](https://www.sciencedirect.com/science/article/pii/S0167701225002003) demonstrates full head-to-head growth curves for 6 bacteria. The chosen species are different in terms of size, morphology and envelope constitution. All growth curves showed strong correlation (R<sup>2</sup> ≈ 1.00) for active cultures.&#x20;

Correlation does not necessarily imply that the absolute numbers are the same. The table below shows the BactoBox®-to-CFU comparisons for the 6 bacteria. Most direct comparisons were within ±0.1 log units (i.e. within \~25% difference). The geomean cell-to-CFU ratio is also provided in the table. Geomean is generally close to 1, i.e. when a cell is detected on BactoBox® it practically corresponds to one colony on a plate.&#x20;

|                 |  S. epidermidis  |      E. coli     |   K. aerogenes   |   A. baumannii   |    L. innocua    |  P. fluorescens  |
| --------------- | :--------------: | :--------------: | :--------------: | :--------------: | :--------------: | :--------------: |
| Strain          |    ATCC 12228    |     ATCC 8739    |    ATCC 13048    |    ATCC 12457    |    ATCC 33090    |    ATCC 13525    |
| Approx. size    |   0.8 × 1.0 µm   |   1.0 × 3.0 µm   |   0.5 × 1.5 µm   |   0.6 × 1.0 µm   |   0.7 × 2.0 µm   |   0.5 × 1.2 µm   |
| Gram status     | Gram<sup>+</sup> | Gram<sup>-</sup> | Gram<sup>-</sup> | Gram<sup>+</sup> | Gram<sup>+</sup> | Gram<sup>-</sup> |
| Within ±0.1 log |    7 out of 7    |    5 out of 6    |    8 out of 8    |    3 out of 7    |    7 out of 7    |    7 out of 7    |
| Cells-to-CFU    |       1.09       |       1.12       |       0.99       |       1.35       |       0.89       |       0.90       |

## Correlation for dilutions of the same sample

The above plot demonstrates the correlation across a growth curve. Correlation between BactoBox® and CFU is also strong across different dilutions of the same sample as demonstrated on the plot below where the same sample was diluted and measured head-to-head with BactoBox® and plate counts.

The slope of the regression line is 0.9947. This is practically a perfect slope of one meaning that for every *S. epidermidis* detected on BactoBox® you will also get one colony by plate counts. The R<sup>2</sup> ≈ 1 signifies the strong linear relationship between the two methods. In most cases, BactoBox® demonstrates greater precision than the CFU method. For example the most concentrated sample has a coefficient of variation (CV) of 3 % for BactoBox®, while the CV is 35 % for the plate count method.

<figure><img src="/files/vkBkS4eMSVzijH1tc0It" alt=""><figcaption><p>Head-to-head comparison of BactoBox® and plate count method for dilutions of the same <em>Staphylococcus epidermidis</em> sample. X-axis represents plate count arithmetic mean and y-axis represents BactoBox® arithmetic mean. All measurements were performed in technical triplicates. The table displays the arithmetic mean and the coefficient of variation (CV) for each data point. CV ≤ 5 % are highlighted in green.</p></figcaption></figure>


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