Kill Rates and Log Reductions
Kill Rates
There has always been a big discussion about which disinfectant is best. We normally see in our home cleaning products “Kills 99% of bacteria” or even 99.9%. Ordinarily, 99% is a high percentage. However, with hospital-grade disinfectants this isn’t good enough.
According to New Scientist there are, on average, 10 million bacteria per square centimetre on an average adult person.1 If a disinfectant kills 99% of bacteria, then there are still 100,000 bacteria that remain. If it’s 99.9%, then there are still 10,000. It is also unlikely that a disinfectant will kill all bacteria at an equal rate. It’s much more likely that a disinfectant will be most effective against the weakest, oldest and most damaged bacteria first.2 So, the 100,000 or 10,000 that are left from a standard disinfectant are likely to be the strongest and generally most harmful bacteria.
Like us, bacteria need resources to survive. They have to absorb nutrients by breaking down chemical compounds, waste, and even dead organisms.3 If there are 100,000 bacteria that now have access to the resources that could support 10 million, they can now thrive because there’s no competition.
Log Reductions
A log reduction tells us what percentage of bacteria have been killed. Using a logarithmic scale, each log reduction decreases the Colony Forming Units (CFU) count by 10-fold, or an extra 90% of the remaining bacteria are killed in each step up. This is usually represented as 1 log10 or log-1 meaning 90%. Table 1 shows the % of bacteria eliminated with each log.
| Log Reduction | % Reduction | ||
|---|---|---|---|
| 1 | 90 | ||
| 2 | 99 | ||
| 3 | 99.9 | ||
| 4 | 99.99 | ||
| 5 | 99.999 | ||
| 6 | 99.9999 | ||
| 7 | 99.99999 |
Table 1 – Comparison of log reduction and % reduction
Contact Time
It is important when disinfecting that the biocide reaches the full contact time stated by the manufacturers.4 If the full contact time is not reached then the required log reduction will be lower, so if the solution states a log-4 reduction in 5 minutes and only remains on the surface for 2 minutes then, for example, the log reduction may only be a log-2 reduction. Therefore, if you started with 10 million bacteria, this could leave 1000 alive for a log-4 kill rate, and 100,000 for log-2 – a substantial difference!
Peracide
Choosing a disinfectant that has a short contact time and a high kill rates is a primary principal for the healthcare market, where hard to kill pathogens are present such as C. difficile spores, MRSA, Pseudomonas, and others. Peracide has been shown to have high log-reductions against a wide range of pathogens. Testing carried out by University College London Hospitals microbiology department has shown Peracide at 4000ppm strength capable of killing C. difficile spores in just 15 seconds to a log-7 reduction (99.99999%) in heavily soiled conditions, as shown in figure 1.
Figure 1 – Showing the contact time and log reduction of Peracide against C. difficile spores at a concentration of 4000ppm.
References:
1. Colin Barras. We contain microbes so deeply weird they alter the very tree of life. New Scientist 10th April 2019. https://www.newscientist.com/article/mg24232250-200-we-contain-microbes-so-deeply-weird-they-alter-the-very-tree-of-life/
2. R.N. Smith, Kinetics of biocide kill, International Biodeterioration, Volume 26, Issues 2–4, 1990, Pages 111-125, ISSN 0265-3036, https://doi.org/10.1016/0265-3036(90)90052-9
3. Bonnet M, Lagier JC, Raoult D, Khelaifia S. Bacterial culture through selective and non-selective conditions: the evolution of culture media in clinical microbiology. New Microbes New Infect. 2019 Nov 30;34:100622. doi: 10.1016/j.nmni.2019.100622. PMID: 31956419; PMCID: PMC6961714.
4. Voorn MG, Kelley AM, Chaggar GK, Li X, Teska PJ, Oliver HF. Contact time and disinfectant formulation significantly impact the efficacies of disinfectant towelettes against Candida auris on hard, non-porous surfaces. Sci Rep. 2023 Apr 10;13(1):5849. doi: 10.1038/s41598-023-32876-y. PMID: 37037898; PMCID: PMC10086017.
