Molecules: You'd better learn to live with them.
So you know by now that negatively-charged soaps and detergents can mess up bacterial cell membranes nicely. The negatively-charged-yet-nonpolar detergents and soaps commingle nicely with similarly negatively-charged-yet-nonpolar lipids that comprise bacterial cell membranes, thereby ruining bacteria's days.
If you take a positively charged detergent, it'll work even better at disrupting the negatively-charged cell membrane. I've never been completely clear on why this is.
These lipid assemblies are pretty counterintuitive to begin with - something similar and equally amazing happens when you use soap. You're generating a bunch of spheres with negative charges packed closely together. All this is because the water wants to interact with itself so strongly that the greasy tails make their egress from bulk solution into the nonpolar interiors of self-assemblies ("micelles").
Some slightly different lipids with two "tails" form cell membranes - double layers with negative charges on either sides ("bicelles"). You'd think incorporating a few positive charges would be a good thing - packing like charges together usually is not a happy situation. However, these kill bacteria, and they're thought to do so because they disrupt exactly this "bicelle" structure.
Presumably this has something to do with the shape cells take; they self-assemble into these exquisite structures precisely because the negative charges decorating their surfaces repel one another. However, I'd welcome comments from anyone who knows a bit about the biophysics of cell membranes.
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Big water baths are notorious for growing stuff. A jot of benzalkonium chloride seemed like just the thing. We cleaned it out and added some borax instead. Works to spec (Blue Magic) in waterbeds.
If you suffer moss (most anything moving at less than 3 mph in British Columbia, Canukistan), benzalkonium chloride is a miracle.
Posted by: Uncle Al | January 22, 2009 11:58 AM
When certain soap says "anti-bacterial" is that for marketing purposes? I always thought pretty much all soap was anti-bacterial?
Sorry, I don't know anything about the biophysics of cell membranes, but love reading your blog!
Cheers
Candy
Posted by: Candy Minx | February 4, 2009 9:28 AM
When certain soap says "anti-bacterial" is that for marketing purposes? I always thought pretty much all soap was anti-bacterial?
Yes, all soaps kill bacteria. The "anti-bacterial" soaps just contain an additional antibiotic ingredient, most commonly triclosan, which inhibits fatty acid biosynthesis. I'm curious as to how much more effective anti-bacterial soaps are than regular soaps when applied for the average amount of time a person washes his hands. When left on for long periods of time, both should be nearly 100% effective I would think.
As for the action of benzalkonium chloride, I'm suspecting that the biophysical situation here is a competition between bilayers and micelles. Soaps and detergents would promote the transfer of lipids from the membrane bilayers of cells into micelles, creating voids in the membranes that eventually cause the cells to lyse. If cationic molecules form micelles in combination with membrane lipids more readily than anionic molecules, it seems that they should be more active.
Posted by: rosko | February 24, 2009 6:29 PM
I'm curious as to how much more effective anti-bacterial soaps are than regular soaps when applied for the average amount of time a person washes his hands.
The antimicrobials such as benzalkonium, benzethonium, and triclosan leave behind a residue that is bacteriostatic and fungistatic. Assuming you keep your hands debris-free, they will still be clean hours after you wash them.
Posted by: Rich | April 10, 2009 10:51 AM
Does anybody knows what are limits is dosage of Benzalkonium chloride in a formulation for hand sanitizer? We are using a 50% solution.
Thank you.
Posted by: JuanCarlos | May 10, 2009 6:52 PM