Testing Germicides: Disinfectants vs. Antiseptics vs. "Natural" cleaner

Continued from https://theorypracticelandscaping-kfrye.blogspot.com/2017/11/we-got-experiment-running-5-cleaners.html

• Lysol
• bleach
• hydrogen peroxide 
• isopropanol
• Thieves 

All tested on E. coli in a nutrient broth. Each cleaner was tested at three different concentrations (varied per cleaner) with 2 positive controls and 2 negative controls.

After being incubated for 48 hours at 37C, each tube was observed for growth determined through the amount of turbidity (or cloudiness). 

THIEVES (25, 50, 100 %)

 Growth was observed in all three concentrations, however the strongest concentration (100%) had significantly less turbidity and the bacteria was clumped. Controls were good: positive controls both had turbidity and the negative controls were clear (no growth). 

 Isopropanol also showed growth in all 3 concentrations and all controls were as expected, although it is anomalous that the most growth occurred in the highest concentration...a replication here would be ideal.

HYDROGEN PEROXIDE (0.1, 0.3, 3%)

Growth in all three concentrations and controls were as they should be. 

BLEACH (0.01, 0.1, 1.0%)

 Bleach showed growth in the 2 lower concentrations and no growth in highest concentration, which was only 1%. To provide some context, most labs use a 10% dilution as a standard cleaner.

LYSOL (25, 50, 100%)

No growth in any concentration. Controls here were good; the negatives were clear, however only positive control was run because of an error and lack of extra supplies. The only positive control that was run, did have the expected turbidity.

Each concentration was observed and rated for E. coli growth:
G for growth (turbidity aka cloudiness)
NG for no growth

Overall the Lysol is the most effective bactericide, but there are a couple of issues here:

1. The difference between DISINFECTANT and ANTISEPTIC
2. The difference between bactericidal and bacteriostatic

So the Lysol eliminated all observable growth at each concentration (I guess 99.9% according to the claims on the label) because its main biocidal ingredient is benzalkonium chloride, a compound with recognized toxicity as well as irritant to human skin, respiratory and gastrointestinal systems to name just a few down sides. This stuff is strong and good at making cell membranes no longer function as membranes.  To put it today's current political context, this would is your "nuclear" option. 

So if you need near sterilization - this is your choice. And the clearest choice from this experiment, no context!

Here's the more nuanced part however, Lysol is a disinfectant, so is bleach, and you can see from the results that even bleach at high enough concentrations killed the bacteria (also keep in mind the highest concentration of bleach used in this experiment was 1% compared to Lysol's 100%). These two germicides are meant for surfaces because again, you would experience adverse affects such as severe irritation and tissue damage for instance if you applied Lysol or bleach to try and treat an open wound.  

The isopropanol and the hydrogen peroxide are antiseptics and used intentionally on living tissue and at much lower concentrations than the Lysol (e.g. highest at 50% and 3% respectively vs. 100% Lysol) because they are not as caustic as the disinfectants; their biocidal capabilities increase with increased concentrations, lower concentrations however can still be bacteriostatic, that is prevent growth and reproduction of present bacteria rather than kill all the bacterial cells. 

Now the Thieves...the 2 dilutions (50% and 25%) definitely had typical-looking growth with high turbidity, but while the 100%  Thieves had some growth (the tube on the right in photo below), the growth did not cause much turbidity.  I also noticed in the 100% tube, the cleaner seemed to create a kind of floc out of the bacterial growth - its was clumped in the middle rather than being dissolved and suspended evenly through the broth.  

CONCLUSION

In terms of which cleaner is best, it just depends on what you want from your cleaner. If you are looking for near/nearest to sterile conditions you can get without antimicrobial gassing or using ultraviolet light in your kitchen, Lysol is easily the choice based on these results; you can even dilute the Lysol by as much as 25% and still maintain the biocidal effects. It is indeed the cleaner your grandparents said it was and there's a reason clinical settings use it today. 

If you are using Thieves and you want it to be the same as Lysol, you are out of luck. It's just not. 

Some folks however are more interested in just effectively reducing bacterial populations without going near sterile and in such cases, bacteriostatic agents may be a more appropriate choice. 

If though, you just want to tilt the landscape against the bacteria, Thieves is okay...just maybe don't dilute it to try and stretch it out. 

There are also people who choose not use an agent like Lysol in the home because of the implications of leaving behind the 0.1% of bacterial populations Lysol does not kill (remember Lysol only claims to be effective on 99.9%) and the potential effect this contributes to issues of antibiotic resistance. 

Keep in mind this is just one replication run by a small college class. We do this every semester and we can run it again or any other cleaners readers suggest! We could even try the raw ingredients listed in Thieves next time.

Notice also that we only ran this on bacterial species E. coli rather than adding a second bacterial species S. epidermidis as the lab manual lists on the data table (above), which brings up the issue of what type of bacteria you are interested in targeting with your cleaner. Both E. coli and S. epidermidis are normal microbiota found in the gut and on the skin respectively and require opportunistic situations to become pathogenic (e.g. introduction to abnormal place on the body, cuts and other wounds, altered levels of bacterial populations throughout the body, etc.). This supports the disinfectants in this experiment for use in the bathroom, but perhaps in something like a kitchen application, it may make more sense to test more commonly known food-borne bacterial species and/or fungal species instead of S. epidermidis. 

Just things to think about. Anyway, I predict those who were using Lysol before reading this continue to do so. Likewise for the Thieves camp!

Stay tuned, more on this come spring. One reader has already suggested adding the ever popular Mrs. Meyer's cleaner, so I've already put it on the list. 

Would you like me to run all the germicides at the same (at least more similar) concentrations next time? Let me know!