Log Reduction & Log Kill
1. INTRODUCTION: First, Why The Terms “Disinfection”, “Sterilization” and “Decontamination” Are Important
In order to claim Disinfection a cleaning process, whether chemical or ultraviolet light (UV), must attain at least a 6 Log reduction of specific organisms, in a certain amount of time. Sterilization means a kill of at least 6+ Log of certain organisms, while leaving no growth or viable survivors.
The following are different United States Government Environmental Protection Agency (EPA) performance standards that must be met in order to claim disinfection or sterilization (in 10 minutes or less, except for sterilants):
- General Disinfection = 6 Log reduction of “Staphylococcus aureus” AND “Salmonella enterica”
- Hospital Disinfection = 6 Log reduction of “Staphylococcus aureus” AND “Pseudomonas aeruginosa”
- Disinfectant with Fungicidal claims = 6 Log reduction of “Trichophyton mentagrophytes”
- Sterilant with C-Diff. Spore Claims = 6 Log reduction of “Clostridium difficile (C. difficile) spores”
What is decontamination?
The Merriam Webster dictionary defines the term decontaminate as “to rid of contamination; to remove dirty or dangerous substances”. Many products on the market at this time do NOT rid or remove dangerous pathogens, and while they may reduce some, they do NOT remove or eliminate them.
CRITICAL: The terms Disinfection, Sterilant, Sterilize, and Decontamination, are specifically defined by the EPA, and have specific mathematical and scientific meaning, that have direct health and safety implications.
2. WHAT LOG REDUCTION MEANS
Log Reduction Basics
The term Log is short for "logarithm", a mathematical term for a power to which a number can be raised. For example, using 10 as the given number, a Log 2 increase can be shown as 10^2 or 10 x 10 = 100.
Alternatively, a Log Reduction is taking the power in the opposite direction. For example, a Log Reduction of 1.0 Log is equivalent to a 10 fold reduction or, stated another way, moving down one decimal place, or a 90% reduction.
Product efficacy testing is done by counting the number of "colony forming units" (CFU) of the given pathogen / bacteria at the start of the treatment, and then performing a count again after the required treatment time. The result of the difference between the start and end numbers is then expressed as a Log Reduction.
For example, if the number of bacteria or bacterial colony forming units (CFU) in the beginning was one million or 1,000,000 (or 10^6), and the end result after the treatment was 1,000 (or 10^3) survivors, that would be a 3.0 Log Reduction (Log 3 reduction) or a reduction of 99.9%.
The Importance of Log Reduction
It is important to understand what Log Reduction is and why it is important to the process of surface disinfection, surface sterilization, and surface decontamination.
Scientists, engineers, and other professionals who are responsible, or even legally responsible, for preventing illness and contamination, are concerned with Log Reduction or elimination of pathogenic bio-burden.
As a rule of thumb, for every additional Log Reduction number, you add the number 9 to the percentage reduction, so a Log Reduction of 3.0 Log is a 99.9% reduction compared with a Log Reduction of 6.0 Log which is equivalent to a 99.9999% reduction.
Below is an example of Log Reduction values using a starting point of one (1) million bacteria or 1,000,000 CFU's on a surface (ie: under bed rails in a hospital), as outlined below:
| Interpretation Scale for Percent Reduction, Log Reduction, Number of CFUs | ||
|---|---|---|
| Log Reduction | Number of CFUs | Percent Reduction |
| 0 log (Log 0) | 1,000,000 | 0% |
| 1 log (Log 1) | 100,000 | 90% |
| 2 log (Log 2) | 10,000 | 99% |
| 3 log (Log 3) | 1,000 | 99.9% |
| 4 log (Log 4) | 100 | 99.99% |
| 5 log (Log 5) | 10 | 99.999% |
| 6 log (Log 6) | 1 | 99.9999% |
3. WHY UNDERSTANDING LOG REDUCTION IS IMPORTANT
4. UNITED STATES GOVT. DEFINITIONS REGARDING DISINFECTION & STERILANTS
a) U.S. Regulatory Definition for “General Disinfection / Broad Spectrum Efficacy”
Reference: OCSPP 810.2200 (3)
DEFINITION: General or broad spectrum efficacy products When a disinfectant is represented in labeling as having efficacy against both Gram-negative and Gram-positive bacteria, the product is considered a “general or broad spectrum” disinfectant.
According to the United States Environmental Protection Agency (EPA), Disinfection is defined as set forth in EPA Product Performance Test Guidelines, OCSPP 810.2200.
The test microorganisms are:
- Effective against both Gram-negative and Gram-positive bacteria.
- Staphylococcus aureus (S. aureus)(ATCC 6538) for effectiveness against Gram-positive bacteria.
- Salmonella enterica (ATCC 10708) (S. enterica) for effectiveness against Gram-negative bacteria.
The test criteria states:
"Evaluation of confirmatory general or broad spectrum disinfectant success. The product should kill all the test microorganisms on all carriers in ≤ten minutes. In addition, per the 2009 AOAC revisions for the Use-Dilution Method, the mean log density for S. aureus is to be at least 6.0 (corresponding to a geometric mean density of 1.0 x 10^6 ); a mean log density <6.0 invalidates the test. For the Hard Surface Carrier Test, the dried carrier counts should be 0.5 –2.0 x 10^6 for Salmonella enterica and 1 – 5 x 10^6 for Staphylococcus aureus." (emphasis added) (4)
Summary: To meet the definition of “General Disinfection” a 6 log kill has to be obtained for both S. aureus and S. enterica in less than 10 minutes.
b) U.S. Legal Definition for “Hospital Disinfection”
Reference: OCSPP 810.2200 (5) & (6)
The EPA has a specific category established for the hospital and healthcare markets. For these markets, the following efficacy is required to meet the definition of disinfection as set forth in EPA Product Performance Test Guidelines, OCSPP 810.2200.
The test microorganisms are:
- Effective against both Gram-negative and Gram-positive bacteria.
- Staphylococcus aureus (S. aureus)(ATCC 6538) for effectiveness against Gram-positive bacteria.
- Pseudomonas aeruginosa (P. aeruginosa)(ATCC 15442) for effectiveness against Gram-negative bacteria.
The test criteria states:
“Evaluation of confirmatory hospital or healthcare disinfectant success. The product should kill all the test microorganisms on all carriers in ≤ten minutes. In addition, per the 2009 AOAC revisions for the Use-Dilution Method, the mean log density for S. aureus and P. aeruginosa is to be at least 6.0 (corresponding to a geometric mean density of 1.0 x 10^6); a mean log density <6.0 i nvalidates the test. For the Hard Surface Carrier Test, the dried carrier counts should be 1 –5 x 10^6 for both Staphylococcus aureus and Pseudomonas aeruginosa.” (emphasis added) (4)
Summary: To meet the definition of “Hospital Disinfection” a 6 log kill has to be obtained for both S. aureus and P. aeruginosa in less than 10 minutes.
c) U.S. Legal Definition for “Disinfectants With Fungicidal Claims”
Reference: OCSPP 810.2200 (9)(e)
The test microorganism is:
1) Trichophyton mentagrophytes (T.mentagrophytes)(ATCC 9533)
Two samples representing two different batches of the product should be evaluated for efficacy against Trichophyton mentagrophytes (T. mentagrophytes)(ATCC 9533). The inoculum employed should provide a concentration of ≥5 x 10^6 conidia/mL. Evaluation of fungicidal success. For the AOAC International Fungicidal Activity of Disinfectants test, all fungal spores at 10 and 15 minutes should be killed to support a 10 minute exposure time. For the AOAC International Use-Dilution Methods, all fungal spores on all 10 carriers should be killed in ≤ten minutes. (emphasis added) (4)
d) U.S. Legal Definition for “Sterilant w/ Clostridium difficile Claims”
Reference: OCSPPP 810.2100 (d)(2) and (g)
General Liquid Sterilants Claims Mandated Log Reductions:
5-6 Log reduction minimum for BOTH Bacillus subtilis (B. subtilis) spores and Clostridium sporogenes (C. sporogenes) spores, AND must reach at least 6 Log reduction minimum for Clostridium difficile (C. difficile) spores, to be classed as liquid Sterilant w/ Clostridium difficile (C. difficile) Claims. Kill everything, no growth, on ALL slides in less than XX minutes (time not specified).
The test microorganisms are:
- Effective against: (B. subtilis) and (C. sporogenes) and (C. difficile)
- Clostridium difficile (C. difficile) (ATCC 700792), (ATCC 43598) or (ATCC 43599)
- Bacillus subtilis (B. subtilis) (ATCC 19659)
- Clostridium sporogenes (C. sporogenes) (ATCC 3584)
Evaluation of sterilant success. The inoculum employed should provide a count of 1 x 10^5 – 1 x 10^6 spores per carrier. The product should kill the test spores on all 120 carriers without any failures (e.g., growth of test organism after carrier treatment constitutes failure). (5)
5. REFERENCES
1) Sreelatha Koganti, MD, and Curtis Donskey, MD "Evalution of Hospital Floors as a Potential Source of Pathogen Dissemination Using a Nonpathogenic Virus as a Surrogate Marker", Infection Control & Hospital Epidemiology, November 2016, Vol. 37, No. 11.
2) Jennifer L. Cadnum, BS, and Curtis Donskey, MD "Effect of Variation in Test methods on Performance of Ultraviolet-C Radiation Room Decontamination", Infection Control & Hospital Epidemiology, November 2016.
3) John M Boyce, MD "Impact of Room Location on UV-C Irradiance and UV-C Dosage and Antimicrobial Effect Delivered by a Mobile UV-C Light Device", Infection Control & Hospital Epidemiology, June 2016, Vol. 37, No. 6.
4) https://www.regulations.gov/document?D=EPA-HQ-OPPT-2009-0150-0021
5) https://www.regulations.gov/document?D=EPA-HQ-OPPT-2009-0150-0020