Protocol 1 - Contact Time Test: Antibacterial Soap Efficacy

Purpose & Scope

The Contact Time Test quantifies how rapidly a soap formulation reduces viable bacterial counts on a standardized inert surface or in a rinse simulation. It is intentionally practical: designed to compare consumer soap products (bar, liquid, and refill) for their rapidity of action against bacterial contributors to foot odor (notably Staphylococcus epidermidis, Corynebacterium spp., and representative Gram-negative commensals). Outcomes are expressed as log10 reductions at defined contact intervals (e.g., 10 s, 30 s, 60 s).

Use Cases: comparative ranking, informing "fast-acting" messaging, screening candidate formulations before deeper sweat-simulation tests.

Non-Use Cases: this protocol does not measure long-term microbiome shifts, fungal reduction (e.g., athlete’s foot), or clinical infection control efficacy.

Materials, Reagents & Setup

Aim to use reproducible, easily-sourced materials so independent labs or hobbyist test teams can replicate results. Below is a practical materials list used in CleanFormulation runs.

Environmental Conditions: perform tests at room temperature 20–24 °C and relative humidity 40–60% unless explicitly testing temperature sensitivity.

Stepwise Method

The method below is the reproducible "consumer-lab" variant used by CleanFormulation. Times and volumes are chosen to reflect real-world hand washing and soap use, while maintaining quantifiable outcomes.

1. Prepare Bacterial Inoculum: Grow test strains to mid-log phase in TSB (OD600 ~0.4–0.6). Dilute to ~1×10^8 CFU/mL in phosphate-buffered saline (PBS). For mixed-species panels, equalize counts before mixing.
2. Inoculate Test Surface: Apply 10 µL of inoculum onto each stainless coupon, spread evenly, and allow a short drying window (~5 min) to mimic natural adhesion without complete desiccation.
3. Prepare Soap Application: For liquids, use undiluted product. For bar soaps, create a 1:9 w/w slurry in sterile water (this mimics typical lather dilution). Record exact dilution used.
4. Apply Soap & Start Timer: Place 50 µL of soap formulation directly onto the inoculated coupon, ensuring coverage. Start stopwatch immediately. Contact intervals: 10 s, 30 s, 60 s, 120 s. Each time point should have triplicate coupons.
5. Neutralize Activity: At each time point, transfer coupon into 10 mL neutralizer (DE broth or Polysorbate 80 + lecithin solution) and vortex for 30 s to recover bacteria. This quenching step is critical - validate neutralizer against your test product.
6. Serial Dilution & Plating: Perform ten-fold serial dilutions in PBS, plate appropriate dilutions on non-selective agar, incubate at 37 °C for 18–24 h, and enumerate colonies.
7. Controls: Include negative (PBS only) and positive (benchmark antimicrobial like 0.12% chlorhexidine) controls. Also include "neutralizer control" where neutralizer is added to inoculum without soap to confirm no toxicity.
8. Replication: Each soap × time point should be performed with at least n = 3 independent coupons; for publication-grade work use n ≥ 5.

Small operational imperative: always validate that your neutralizer inactivates the soap within 5 s-this prevents carryover killing during counting. I once missed this step in an initial run and observed spuriously high kill rates. A quick neutralizer check saves hours later.

Endpoints, Metrics & Data Capture

Primary Endpoint: Log10 reduction in viable CFU compared to time-zero control (or compared to neutralizer-only control). Secondary Endpoints: percent reduction, time to achieve ≥2 log10 or ≥3 log10 reduction, recovery fraction.

Example Calculation: initial inoculum on coupon = 1×10^6 CFU recovered from time-zero control; recovered after 30 s with soap = 1×10^3 CFU → log10 reduction = 6 − 3 = 3 log10 (99.9% reduction).

Results Templates & Recommended Presentation

For clarity and reproducibility, present data in two complementary formats: (A) Time-series table of mean recovered CFU and log reductions, and (B) a line or bar chart showing log10 reduction vs. contact time with error bars (standard deviation).

Footnote: indicate limit of detection (LoD) for plating method (commonly 10 CFU/plate depending on dilution and plating volume).

Statistical Analysis & Interpretation

Use log-transformed CFU data for statistical comparisons. Recommended tests:

• Repeated measures ANOVA (for same soap across time) or mixed-effects model if runs span multiple days.
• Pairwise t-tests with Bonferroni correction when comparing two formulations at single time points.
• Non-parametric alternatives (Wilcoxon) if data are not normal after transformation.

Reporting: always provide mean ± SD of log10 reductions and confidence intervals for primary comparisons (95% CI). Effect size: report Cohen’s d for practical understanding of differences between formulations.

Example interpretive thresholds (practical):

• 1–2 log10 reduction - modest effect (90–99% reduction)
• ≥3 log10 reduction - strong immediate reduction (≥99.9%)

Limitations, Quality Controls & Reproducibility Notes

This protocol intentionally balances reproducibility with accessibility. Key limitations to state clearly in any public report:

• Model vs. Real Skin: Stainless coupons and slurry application approximate, but do not fully replicate, skin microenvironment, sebum presence, or mechanical rubbing in a wash.
• Neutralizer Reliance: Incorrect neutralizer selection or failure to validate leads to false positives. Always validate that neutralizer inactivates product within 5 s.
• Species Selection: Test strains are representative, not exhaustive. Foot odor is polymicrobial; consider mixed-species panels to better mimic reality.
• Temperature Sensitivity: Some formulations thicken at low temperature, which can bias contact efficacy by reducing active ingredient mobility.

Reproducibility Tips: record lot numbers, ambient conditions, neutralizer validation data, and operator initials for each run. Repeat key comparisons on at least two separate days to verify inter-day consistency.

Micro human insert - small observational note: in several repeat runs, citrus-scented liquid soaps sometimes gave marginally better early reductions (10–30 s) when freshly shaken; this likely stems from volatile co-solvents temporarily improving spread on the coupon. Note it, but don’t over-interpret without replication.

References

1. Russell AD, Hugo WB, Ayliffe GAJ. Principles and Practice of Disinfection, Preservation & Sterilization - classic methodology references for neutralizer validation.
2. EN 1500:2013 Chemical disinfectants and antiseptics - Hygienic handrub; provides contact-time benchmarking concepts (methodology inspiration, not used verbatim).
3. Dey-Engley Neutralizing Broth composition - technical supplier datasheet (Sigma-Aldrich / Thermo).
4. Scientific papers on Corynebacterium contribution to foot odor - e.g., James AG et al., Journal of Applied Microbiology (investigations into axillary/foot odor flora).
5. ISO 22196:2011 Measurement of antibacterial activity on plastics and other non-porous surfaces - used as conceptual guide for log-reduction reporting.

Summary of Findings

• Protocol Purpose: Designed to quantify time-dependent antibacterial action of soaps as a consumer-relevant comparative test.
• Primary Metric: Log10 reduction in viable CFU at defined time points (10 s, 30 s, 60 s, 120 s).
• Reproducibility: Validate neutralizer, use mixed-species panels for realism, and repeat on multiple days.
• Practical Note: This test measures short-term kill/ removal, not long-term microbiome changes or fungal activity.