Laboratory test: Can bacteria grow in soap?

Sara Idenfors

Short answer

RISE Research Institutes of Sweden investigated whether bacteria can grow in Soapa’s solid soap. First, the soap’s water activity (aw) was measured, after which the bacterium Escherichia coli (E. coli) was added to diluted soap. After seven days, the bacterial level had decreased from approximately 1,700 bacteria/ml to below 1 bacterium/ml. RISE concluded that the solid soap did not promote bacterial growth but instead killed the added bacteria under the tested conditions.

Why was the test conducted?

When a solid soap is used repeatedly, it comes into contact with hands, water, and the surrounding air. It is therefore natural to ask whether bacteria can survive or multiply in the product between uses.

This was precisely the question Soapa asked RISE to investigate. The assignment was not to test hand or surface disinfection, but to assess whether the solid soap provides an environment in which bacteria can grow.

About the laboratory

The test was conducted by RISE Research Institutes of Sweden, an independent Swedish research organization working in areas including microbiology and hygiene. The statement is dated June 14, 2022, and concerns Soapa’s solid soap.
Read the report here

How was the solid soap tested?

RISE conducted the investigation in two stages.

1. Measuring water activity

First, the soap’s water activity (aw) was measured. Water activity describes how much free water is available to microorganisms. Pure water has an aw of 1.0, and many bacteria need high water activity in order to grow.

The solid soap had a measured water activity of 0,95, while the liquid soap measured 0,995RISE also noted that the water activity itself was not low enough to prevent bacterial growth on its own.

2. Microbiological test

To obtain a more reliable answer, the solid soap was dissolved in sterile water and diluted both 10-fold and 100-fold. The bacterium Escherichia coli (E. coli), which is often used as an indicator organism in hygiene tests, was then added. The bacterial level was analyzed immediately after addition and again after seven days at room temperature.

Results

At the start of the test, each sample contained approximately 1,700 bacteria per milliliter.

After seven days, the level had decreased to below 1 bacterium per milliliter, both in the 10-fold and 100-fold diluted soap (the other soaps from Soapa contain more water than the solid soap, so this is also relevant to these products).

RISE summarized the results as follows:

  • the solid soap did not promote bacterial growth,
  • the added bacteria were instead killed under the test conditions,
  • the effect was also observed after 100-fold dilution.

Why does RISE think this happens?

RISE does not identify a single cause but considers the result likely to be due to a combination of:

  • the soap's water activity,
  • the product's high pH value.

Together, these factors are considered to make the environment unfavorable for bacterial growth under the conditions examined.

What does the laboratory test show?

The report shows that:

  • the bacterium E. coli was unable to reproduce in the tested product,
  • the bacterial count decreased sharply over seven days,
  • the same result was observed even after 100-fold dilution.

What does the laboratory test not show?

The report does not mean that:

  • the product is a disinfectant,
  • all bacteria respond in the same way,
  • viruses, fungi, or mold have been examined,
  • the product has been tested according to European standards for disinfection.

The report covers only the bacterium and test conditions described in the statement.

Frequently asked questions

Is the solid soap antibacterial?

RISE found that the solid soap did not promote growth of the tested bacterium (E. coli); instead, the bacterial count decreased under the conditions examined. However, the report does not mean that the product is a disinfectant or that it has been tested against all bacteria.

Why was E. coli used?

RISE describes E. coli as a bacterium found in the intestines and the environment that is often used as an indicator of hygiene in microbiological tests.

What is water activity?

Water activity (aw) is a measure of how much free water is available to microorganisms. It is the free water—not the total water content—that bacteria need in order to grow.

Was liquid soap also tested?

Yes. RISE measured the water activity of both solid and liquid soap, but the microbiological growth test was conducted on the solid soap.

Conclusion

The RISE report shows that, under the test conditions examined, VästerbottensSåpa's solid soap did not provide an environment in which the added bacterium E. coli could grow. On the contrary, the bacterial count fell from approximately 1,700 bacteria per milliliter to below the detection limit after seven days, even after substantial dilution. The report therefore provides a scientific basis for describing the product's microbiological properties, while it should not be interpreted as proof that the product is a disinfectant or has been tested against all types of microorganisms.

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