Why Can Washed and Dried Towels Still Smell?

A towel that has supposedly been washed and dried can still smell like a wet dishcloth. A 2012 study focused on Moraxella osloensis as a bacterium that produces a compound involved in this odor.

The researchers did more than find the bacterium in smelly laundry. When isolated bacteria were placed back onto used, sterilized towels, the characteristic odor and its main chemical component emerged. The study also examined resistance to drying and ultraviolet light to explore why the bacteria can persist on fabrics after laundering.

Tracking the “wet dishcloth” smell of laundry

Kubota and colleagues aimed to identify and characterize microorganisms that produce a particular laundry-malodor compound: 4-methyl-3-hexenoic acid, abbreviated 4M3H. Earlier research had identified it as a major contributor to laundry odors reminiscent of sweat or wet dishcloths.

The study combined bacterial identification, tests of actual odor-producing ability, surveys of household distribution, and tests of resistance to drying, ultraviolet light, and surfactants. Rather than attributing every laundry odor to a single cause, it focused on how 4M3H is produced.

Isolating candidate bacteria from 30 malodorous items

The researchers first collected 30 malodorous items from 15 Japanese households: 16 bath towels, seven hand towels, three T-shirts, and four other items. They extracted and cultured bacteria from small fabric pieces and grouped the frequently occurring bacteria.

The most commonly detected group occurred in 87% of these samples. This was the detection rate among 30 items collected because they smelled bad; it does not mean that the bacterium occurs in 87% of all laundry in Japan.

The team identified this group as Moraxella osloensis by examining morphology, biochemical characteristics, and 16S rRNA gene sequences. Ten representative isolates from different fabrics were assigned to the same species group.

Returning the bacteria to towels produced the odor compound

To investigate a causal connection, the researchers washed, dried, and sterilized used towels, then inoculated small pieces with the isolated bacteria and incubated them. Three assessors with at least three years of training evaluated odor, and gas chromatography–mass spectrometry was used to measure 4M3H.

The Moraxella group produced strong malodor and large amounts of 4M3H. Among the comparison groups, some produced no 4M3H above the detection limit, while others produced a weak odor and small amounts of 4M3H. Combining the observation that the bacteria were present with experimental evidence that they could produce the compound was central to the study.

Strong odor-producing ability was found not only in the 10 laundry isolates but also in stored reference strains of the same species. This suggested that the property might not be restricted to a few unusual strains found in laundry.

New towels did not support the same production

In the discussion, the authors also described an additional test comparing used and new towels. The tested Moraxella strain produced 4M3H on used towels but not new ones. Detailed data from this comparison, however, were not presented in the paper.

This observation led the authors to suggest that substances such as sebum and sweat left after washing might serve as raw materials for bacterial odor production. In this view, both bacteria and usable material need to be present.

The study did not identify the actual precursor of 4M3H. It cited an example of another bacterium producing the compound from linalool, but did not conclude that linalool was the cause in laundry. Identifying the starting material and metabolic pathway remained a task for future research.

Found on laundered fabrics and around household water facilities

In a separate survey, the researchers collected 59 items from 26 Japanese households that had been washed, dried, and stored in drawers or similar places. This set included items both with and without malodor.

Moraxella was frequently detected on towels, T-shirts, socks, pillowcases, and other items. For example, it was detected on approximately 86% of 21 towels. A survey of household surfaces also found it in places such as kitchen sinks, bathroom drains, and washing-machine filters.

However, detecting the bacterium is not the same as showing that an item will necessarily smell strongly. Odor production also depends on conditions that allow bacterial survival and activity, and on available starting materials.

Different resistance to drying, ultraviolet light, and two surfactants

The researchers stored inoculated cotton fabric under dry and humid conditions for 35 days and measured surviving bacteria. They also performed drying tests on filters and ultraviolet-lamp exposure tests on culture media.

The tested Moraxella strains survived drying and ultraviolet exposure better than comparison organisms such as Escherichia coli and Staphylococcus aureus. This resistance could help explain persistence on fabrics after washing and drying.

By contrast, they did not show high resistance to the two surfactants tested, SDS and ABS. The results did not show a bacterium uniformly resistant to every environmental stress.

These tests mainly tracked survival. They did not establish that the bacteria continuously produced malodor on dry cloth for 35 days. Moreover, laboratory-lamp exposure differs from sunlight and outdoor drying, so these results alone cannot establish whether line-drying in the sun is effective or ineffective.

Summary

Moraxella osloensis, frequently detected in malodorous laundry, produced 4M3H, a compound associated with dishcloth-like odor, on used, sterilized towels. It was also widely detected on washed fabrics and around homes. It showed high resistance to drying and ultraviolet light, but not to the surfactants tested.

Understanding laundry odor requires considering both bacterial persistence and the production of odor from remaining soils or other materials. The study’s contribution was to connect identification of the bacteria with their odor-producing ability and survival on fabric.

The paper lists seven of its eight authors as affiliated with Kao at the time of the research. No separate funding or conflict-of-interest statement was found in the main text consulted.

Reference

Kubota H, et al. Moraxella Species Are Primarily Responsible for Generating Malodor in Laundry. Applied and Environmental Microbiology. 2012;78(9):3317–3324. doi:10.1128/AEM.07816-11. Open-access full paper.