Can bacteria grow in solid soap?
Sara Idenfors
Short answer
That was precisely the question that RISE Research Institutes of Sweden investigated on behalf of VästerbottensSåpa. To answer the question, the soap's water activity (aw) was first measured. Then the bacterium Escherichia coli (E. coli) was added to diluted solid soap, and the bacterial count was monitored for seven days. When the test ended, the bacterial count had decreased from approximately 1,700 bacteria per milliliter to under 1 bacterium per milliliter. RISE therefore concluded that the solid soap did not promote bacterial growth under the conditions examined.
Why was the question raised?
Solid soap is often used for a long time. The same container is opened and closed repeatedly and comes into contact with hands, water, and air.
This raises a natural question:
Can bacteria begin to grow in the soap?
This product-hygiene question was the basis for RISE's investigation—not whether the product works as a disinfectant.
How was this investigated?
RISE conducted the investigation in two stages.
1. Water activity
First, the water activity (aw) was measured, meaning how much free water is available to microorganisms.
The solid soap had an average value of 0.95. RISE also noted that this value alone does not prevent bacterial growth. Therefore, a practical microbiological test was needed.
2. Microbiological test
The solid soap was diluted:
- 10 times
- 100 times
The bacterium Escherichia coli was then added.
The bacterial count was analyzed immediately after the addition and again after seven days at room temperature.
What did the test show?
At the beginning of the test, the samples contained approximately:
1,700 bacteria/ml
After one week, the remaining level was:
under 1 bacterium/ml
This was observed after both 10-fold and 100-fold dilution of the solid soap.
What conclusion did RISE draw?
RISE wrote:
"Solid VästerbottensSåpa did not promote bacterial growth; instead, it killed the bacteria that were added."
The laboratory also assessed that the combination of:
- water activity
- high pH
likely contributed to the result.
Does this mean that all solid soap is antibacterial?
No.
RISE examined a specific product – VästerbottensSåpa solid soap – under defined laboratory conditions.
The result therefore cannot be automatically applied to other products or other types of soap.
Does this mean that the soap is a disinfectant?
No. The report shows that the tested bacterium (E. coli) could not grow under the conditions examined.
However, it does not show that the product:
- kills all bacteria,
- kills viruses,
- kills mold or fungus,
- meets standards for disinfectants.
This was not examined in the report.
Why was E. coli used?
Escherichia coli (E. coli) is a bacterium that occurs naturally in the intestine and is often used as an indicator organism in microbiology and hygiene testing.
RISE therefore chose E. coli as a model organism to assess whether bacteria could survive and multiply in the solid soap.
What does the result mean for the user?
The result shows that, under the test conditions used, the solid soap did not provide an environment in which the tested bacterium could grow.
However, this does not mean that the product should be used as a disinfectant or replace hand sanitizer when required.
Frequently asked questions
Can bacteria live in solid soap?
RISE's laboratory test showed that the added bacterium, E. coli, did not grow in VästerbottensSåpa's solid soap under the conditions examined.
Was normal use tested?
No. The report describes a controlled laboratory test in which bacteria were added to diluted soap and monitored for seven days.
Why was water activity measured?
To investigate whether the amount of free water in the product could explain whether bacteria are able to grow. Since water activity alone did not provide the answer, a microbiological test was also conducted.
Does pH matter?
Yes. RISE assessed that the soap's high pH, together with its water activity, likely contributed to preventing the bacteria from growing.
Does the result apply to all bacteria?
No. The report is based on tests with E. coli and should not be interpreted to mean that all bacteria respond in the same way.
Conclusion
RISE's laboratory test shows that VästerbottensSåpa's solid soap did not support the growth of the tested bacterium, E. coli under the laboratory conditions examined. On the contrary, the bacterial count decreased sharply over seven days, even after 100-fold dilution. The result provides scientific support for the product's microbiological properties, but should not be interpreted to mean that the product is a disinfectant or that it has been tested against all types of microorganisms.