Looking at how body-odor compounds are produced reveals approaches other than killing bacteria. A 2014 study examined how cultured skin-associated bacteria make diacetyl from lactate and showed that two materials suppressed its production through different metabolic changes.
The focus was diacetyl, one component of body odor. Rather than testing a product applied to human skin, the study explored mechanisms of production and suppression in experiments combining bacteria with components found in sweat.
Which bacteria make it, and from which sweat components?
The researchers tested nine strains from the skin-associated genera Staphylococcus and Corynebacterium. Bacteria and potential precursor compounds were added to a specified culture medium, incubated under aerobic conditions at 36°C, and assessed by instrumental analysis for diacetyl and other compounds.
In the initial comparison using pyruvate, Staphylococcus aureus and S. epidermidis produced substantial amounts of diacetyl. Small amounts were detected for S. hominis; the other tested strains did not produce detectable amounts under these conditions.
The researchers next compared lactate, pyruvate, and several amino acids at the same concentration in the two higher-producing species. L-lactate yielded the most diacetyl. This narrowed down a specific combination through which a sweat component becomes an odor compound via bacterial metabolism.
From 156 materials to two different candidates
The researchers screened 156 materials, including plant extracts and cosmetic ingredients, and examined an extract of the root of the licorice species Glycyrrhiza glabra (GGR) and EPC-K1 in detail. EPC-K1 is a compound linking α-tocopherol, a form of vitamin E, and ascorbic acid, vitamin C, through a phosphate group.
In six-hour culture tests, adding GGR at 3.0 mg/mL strongly suppressed diacetyl production, while production became undetectable with EPC-K1 at 7.2 µM. Counts of viable bacteria showed no bactericidal activity under these conditions.
The two materials differed in their effects on bacterial growth, however. Subsequent time-course experiments showed that GGR inhibited growth, whereas growth was largely maintained with EPC-K1. Not killing bacteria is different from leaving bacterial activity unaffected, and these differences in activity were central to the study.
Licorice extract inhibits the entry point of lactate utilization
Detailed mechanistic analyses used S. aureus, which produced high levels of diacetyl under the test conditions. The bacteria were given carbon-labeled lactate, and changes in substances outside and inside the cells were tracked at 0, 3, 5, and 7 hours. Seventy-seven intracellular metabolites were measured.
Control bacteria used about 90% of the lactate within seven hours. In the presence of GGR, lactate utilization was inhibited and less pyruvate was produced. Less labeled carbon also entered intracellular metabolites.
These observations support an explanation in which GGR reduces downstream diacetyl production by inhibiting bacterial use of lactate. Bacterial growth was inhibited without killing the bacteria. Which molecules in the extract are chiefly responsible, and which enzyme or gene changes directly cause the effect, remain questions for further study.
EPC-K1 changes the flow of substances inside bacteria
With EPC-K1, lactate utilization continued, with roughly half consumed in seven hours. Even so, diacetyl was undetectable and bacterial growth changed little. This differed from GGR, which largely prevented use of the precursor.
Inside the bacteria, succinate and other compounds in the citric acid cycle, part of energy metabolism, accumulated. Meanwhile, malonyl-CoA, which is used in fatty acid synthesis, decreased. The authors proposed that carbon derived from lactate was redirected away from the pathway producing diacetyl and toward other metabolism.
To examine this explanation, they also tested HQNO, another substance that inhibits an enzyme involved in succinate metabolism. HQNO, however, did not reproduce the changes caused by EPC-K1 or completely inhibit diacetyl production. The mechanism was therefore not as simple as blocking succinate metabolism alone.
Changes affected bacterial fatty acids as well as the odor compound
Fatty acid measurements were another important finding. With EPC-K1, fatty acids per unit dry bacterial mass fell by about 32%, from 41.8 in the control to 28.5 nmol/mg. Fatty acid composition also changed, with the proportion of branched-chain fatty acids decreasing from 82.2% to 67.3%.
This supports the view that EPC-K1 also affects fatty acid synthesis and alters the balance of multiple bacterial metabolic processes. A distinctive finding was suppression of the flow producing the odor compound without large decreases in viable counts or an index of cellular energy status. The study did not fully identify the direct inhibitory target, but it came closer to a mechanistic explanation than measuring production alone would allow.
Summary
Under these culture conditions, S. aureus and S. epidermidis produced more diacetyl from lactate than from the other precursors tested. GGR and EPC-K1 suppressed production under conditions showing no bactericidal activity. Detailed analyses indicated that GGR inhibited lactate utilization and growth, whereas EPC-K1 largely maintained growth while altering metabolism involving the citric acid cycle and fatty acid synthesis.
The study shows why approaches to body-odor compounds should consider not only bacterial numbers but also what bacteria use as raw materials and what they produce. The magnitude, duration, and safety of any effect on human skin require testing beyond these culture experiments.
Mandom funded the research, and all three authors were employed by the company at the time. Patent applications related to GGR and EPC-K1 were also disclosed. The paper states that the employer had no role in study design, data acquisition, or the decision to publish.
Reference
Hara T, Matsui H, Shimizu H. (2014). Suppression of Microbial Metabolic Pathways Inhibits the Generation of the Human Body Odor Component Diacetyl by Staphylococcus spp. PLOS ONE, 9(11), e111833. Original research article and open full text
