---
# System prepended metadata

title: Calculation tutorial
tags: [FST 104L]

---

# Calculation of total aerobic bacterial counts
After you have completed and recorded your counts, apply the rules here to decide which plates will be used for PCA calculation. The final counts need to be reported in **C**olony **F**orming **U**nit CFU or Log CFU. Depending the type of sample, we may further choose unit Log CFU/ ml or Log CFU/g.

### Criteria of unit selection
Students are usually confused about the difference between CFU/ml vs CFU/g 🤨. In fact, the difference between these two units are straightforward and simple. Just check below table . 
| Unit | Scenarios | 
| -------- | -------- | 
| CFU/ml     | When original sample is liquid (ie. milk, juice)      | 
| CFU/g     | When original sample is solid (ie. beef, dried fruit, oyster meat)    | 


### Dilution correction factor
$$
\frac{{\text{(Weight or volume of sample)} + \text{(volume of the diluent)}}}{{\text{(Weight or volume of sample)}}} = \text{Dilution correction factor DCF}\hspace{0.5cm}
$$

If the sample is liquid AND you directly use the stock solution for later serial dilution and plating, DCF in this case = 1.
u
* 1 ml of raw milk sample + 9 ml of peptone water = Dilution correction factor of 10
* 10 grams of cheese + 90 ml of peptone water = Dilution correction factor of 10
* 25 grams of ground beef + 225 ml of peptone water = Dilution correction factor of 10

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### Scenario 1 liquid sample (target unit Log CFU/ml)
$$
\text{CFU per ml} = \frac{{\left(\frac{{\text{Counts from plate 1} + \text{Counts from plate 2}}}{2}\right) \times \text{Total Dilution factor}}}{{\text{Volume plated}}}
$$

<center>where</center>

$$
\text{Total Dilution Factor} = \text{Dilution Correction Factor} \times \text{Serial Dilution Factor}
$$
$$
\text{Serial Dilution Factor is the one from serial dilution, which has 25-250 CFU on plate} 
$$

### Scenario 2 solid sample (target unit Log CFU/g)

$$
\text{CFU per g} = \frac{{\left(\frac{{\text{Counts from plate 1} + \text{Counts from plate 2}}}{2}\right) \times \text{Total Dilution factor}}}{{\text{Volume plated} \times \textbf{Sample weight}}}
$$

<center>where</center>

$$
\text{Total Dilution Factor} = \text{Dilution Correction Factor} \times \text{Serial Dilution Factor}
$$
$$
\text{Serial Dilution Factor is the one from serial dilution, which has 25-250 CFU on plate} 
$$



