Team:Paris Bettencourt/Project/Old People Smell

Aims and Achievement

Figure 3. Putative 2-nonenal degradation by Aldehyde Dehydrogenase into 2-nonoic acid by skin bacteria
Figure 4. Human detection thresholds for 2-nonenal. 12 human smellers were asked to identify pure 2-nonenal in water at the following fold dilutions: 102 10⁴ 10⁶ and 10⁸. All samples were solubilized with tween 80 at a concentration of 0.05%. All smell tests were double-blind. Samples are scored by the percentage of smellers who were able to positively distinguish them from tween-only controls.

Box 1.Selected information about isolated bacterial strains groiwing in the presence of 2-nonenal as a major carbon source

Results

Isolation of strains on 2-nonenal

We sought to isolate bacterial strains adapted to use 2-nonenal as a carbon source. Such strains could hypothetically be used to actively scavenge 2-nonenal from the skin, neutralizing the smell (Fig. 3). We chose to look for these strains within the natural human skin flora. Human sweat is rich in fatty acids (Callewaert et al. 2014)], and lipophilic phenotypes are common among skin isolates. Natural skin bacteria are pre-adapted to the skin environment, so are more likely to be viable and metabolically active.

We prepared minimal 2-nonenal plates and inoculated them with human skin samples. M9 agar plates were prepared with 0.2% 2-nonenal solubilized with 0.05% tween 80. No other carbon sources were present. An inoculating loop was used to streak plates with samples collected from human forehead, hands, and outer nose. In total we sampled 3 body sites from 8 individuals.

We isolated tree bacteria strains capable of growing on 2-nonenal. Colonies appeared after 1 week. The strains were identified by 16S sequencing with universal primers (Box 1, information on each strain). We chose to focus further experiments on Micrococcus luteus as it performed the best in the M9 the 2-nonenal medium.

M. luteus is a natural human skin bacterium and a well-described oligotroph, or nutrient scavenger. Their genome sequence indicates a complete fatty acid degradation pathway, meaning they can plausibly degrade 2-nonenal.

We next sought to characterize the degradation of 2-nonenal by M. luteus. We first identified the smell detection threshold of 2-nonenal. Human smellers could consistently detect 2-nonenal at a concentration of 1000ppm (Fig. 4).

While this work was in progress, we learned that 2-nonenal is bactericidal for many species at concentrations as low as 1000 ppm (Cho et al. 2004) We therefore determined a threshold of toxicity of 2-nonenal for M. luteus. LB agar plates were prepared with varying concentrations of 2-nonenal. Growth of M. luteus was significantly inhibited at 2-nonenal concentrations above 1250ppm.


2-nonenal smell attenuation

M.luteus in medium containing 625 ppm 2-nonenal lowers the intensity of the ‘old people smell’ significantly (p-value: 1.04e-09, HSD turkey test), while C. flaccumfaciens lowers the 2-nonenal smell mildly (p-value: 9.72e-05, HSD turkey test) in comparison to the medium with 625ppm 2-nonenal medium only.

Fig. Word cloud describing 2-nonenal

Methods

The deterioration of personal odor due to aging may be a serious social problem for some people which would like to keep youth body odor. Moreover, the 2-nonenal smell is mostly perceived as unpleasant, thus people with body odor marked with this compound may undergo unfriendly reactions based on their odor. Therefore, iGEM Paris Bettencourt Team decided to study the 2-nonenal relationship with the human skin microbiome in order to help aged people neutralize their body odor.