Can Odor-Producing Bacteria Move Between Towels in the Wash?

Do towels washed in the same machine end up sharing not only a cleaner appearance, but also a more similar community of bacteria?

A 2023 study examined cotton towels used by 30 people and found that bacterial communities that differed from person to person before washing became more alike afterward. Of the two detergents compared, the antibacterial formulation also left fewer bacteria and lower levels of malodor-related compounds after washing. After storage under highly humid conditions, its towels also received more favorable odor ratings from human assessors.

Investigating bacterial transfer during washing and the odors that follow

A team from P&G, the Chinese Academy of Sciences and other institutions investigated how bacteria move between used towels during laundering, and how these changes relate to odor. Simply adding one bacterial species to laboratory fabric cannot capture the relationships among the diverse bacteria found on household textiles.

The researchers therefore combined DNA-based analysis of bacterial community composition, counts of bacteria that could grow in culture, analysis of volatile odor compounds, and human odor assessments. They also examined communities at species level, a finer resolution than genus. Even bacteria within the same group do not necessarily have the same odor-producing properties.

Splitting towels from 30 people to compare two detergents

Thirty people in Osaka used new, sterilized cotton hand towels for 3 days. Each returned towel was cut in half: one half was washed with a comparison detergent without antibacterial action, and the other with an antibacterial detergent. This allowed comparisons between pieces used by the same person. The towels were divided into 3 wash batches of 10 people’s towels each.

Washing took place in a small commercial washing machine, using 6 L of purified water at 25°C and 1 g of detergent per liter. A 10-minute wash was followed by two 3-minute rinses. Before each wash, the machine was disassembled and disinfected with ethanol to minimize the influence of bacteria remaining in the drum.

The two formulations did not differ only in a single antibacterial ingredient. The comparison product had a pH of 8.7, while the antibacterial product included citric acid for pH adjustment and had a pH of 2.5. The results therefore need to be read as a comparison of these two particular formulations.

A total of 124 samples were collected to examine changes before washing, immediately afterward, and subsequently. In the simulated indoor-drying stage, the fabric was held for 24 hours at 25°C and more than 90% relative humidity. This was not a test of natural drying in a room at home, but a highly humid experimental condition in which the fabric remained damp.

Differences between individuals became smaller after washing

Before washing, the towels differed substantially in the types and proportions of bacteria present. After being washed together, their bacterial communities became more alike, and this pattern remained after the high-humidity stage. It was observed in all 3 wash batches and with both detergents.

Bacterial counts showed a similar pattern. Towels that initially carried many bacteria had fewer after washing, whereas some with low initial counts gained bacteria. Differences between towels consequently became smaller. These changes cannot be explained simply by imagining laundering as a process that removes the same proportion of bacteria from every piece of fabric.

The researchers also included newly sterilized fabric in the wash. It developed a bacterial community similar to those of the used towels. In another test, separate sterile fabric was immersed in rinse water for 2 minutes and acquired a community resembling that of the water. These findings support the interpretation that contact between fabrics and transfer through water during washing contribute to bacterial mixing. The study did not determine the proportion attributable to each route.

Lower bacterial counts with the antibacterial detergent carried through the drying stage

Counts of culturable bacteria were 1.45 log₁₀ lower on towels washed with the antibacterial detergent than on those washed with the comparison detergent. This statistically significant difference corresponds to about one twenty-eighth as many bacteria. A separate DNA-based measurement also supported the finding of a lower bacterial load.

However, bacteria increased with both formulations during the 24-hour high-humidity stage. The increases were similar: 1.51 log₁₀ with the comparison detergent and 1.41 log₁₀ with the antibacterial detergent. The antibacterial formulation tested here did not completely prevent subsequent growth. Its lower bacterial load at the end of washing also influenced what happened afterward.

Bacterial communities on towels washed with the antibacterial detergent did not become as uniform as those in the comparison group. Together with the lower counts on new fabric immersed in rinse water, this led the researchers to suggest that reducing bacterial numbers also limited transfer during washing.

Examining odor molecules and people’s impressions

Fewer bacteria do not, by themselves, establish that an odor is weaker. The study detected 6 bacterial species known to produce malodorous compounds on the towels. When 5 of these species were isolated and cultured, they produced dimethyl disulfide (DMDS). This sulfur-containing volatile molecule was used as an indicator of malodor associated with indoor drying.

Fabric washed with the antibacterial detergent contained fewer of these malodor-associated bacteria. Chemical analysis using 10 randomly selected used towels also found less DMDS than with the comparison detergent. This was an instrumentally measured difference in one particular odor compound.

Separately, 5 assessors evaluated the odors of fabric from used towels, sterile fabric included in the wash, and fabric immersed in rinse water. After the simulated indoor-drying stage, fabric treated with the antibacterial detergent received more favorable ratings. A key finding was that bacterial counts, odor compounds and human perception all showed differences in the same direction.

DMDS does not, however, determine the entire odor of a towel. The authors also discuss the involvement of multiple bacteria and the diverse compounds they produce in textile malodor.

How far can these findings be applied to household laundry?

Although the study began with textiles that people had actually used, the comparison involved disinfecting the washing machine each time and using purified water and fixed detergent doses. The same results cannot be assumed for older washing machines, other fibers, other detergents, or indoor drying with good airflow.

DNA-based community analysis also does not distinguish living bacteria from dead ones. Culture-based measurements were included, but not every bacterium detected through its DNA can be assumed to have been growing and producing odor there. The study examined bacteria and odor during laundering, not infections or health effects.

Product acidity offers a clue to the mechanism, but this was not an experiment in adding citric acid to detergent at home. The results concern complete detergent formulations, including their combinations of ingredients.

Conclusion

On used cotton towels, bacterial communities differed between individuals before washing and became more alike when washed together. Tests involving new sterile fabric and rinse water also supported bacterial transfer through fabric and water during laundering.

The antibacterial detergent tested left fewer bacteria after washing than the comparison detergent and reduced malodor-associated bacteria and DMDS. After simulated indoor drying at high humidity, human odor ratings also differed. Understanding odor after laundering therefore means considering not only the bacteria present beforehand, but also their movement during washing and their subsequent growth.

Ten of the authors are P&G employees. The study was supported by the P&G–Chinese Academy of Sciences iMicroCare joint research fund; support from China’s National Key Research and Development Program, the Chinese Academy of Sciences and the National Natural Science Foundation of China is also listed.

References

• Lam, T. H., et al. (2023). Impact of antibacterial detergent on used-towel microbiomes at species-level and its effect on malodor control. iMeta, e110. DOI · Full research article.