I’ve written here before about the possibility that the overuse of antibiotics and other anti-microbial measures may have side effects, by destroying some of the helpful bacteria that live on and in us. And we have all heard stories about the rise of of antibiotic resistant organisms, such as MRSA (methicillin resistant Staphylococcus aureus). The New Scientist has a report on some new research that suggests that incorrect use of antiseptics and disinfectants can not only make those agents less effective, but also contribute to antibiotic resistance.
The new study [abstract, PDF available], which was published in the journal Microbiology, was carried out by researchers at the School of Natural Sciences, National University of Ireland, in Galway. The researchers studied the effect of using over-diluted solutions of the disinfectant, benzalkonium chloride, on the bacterium Pseudomonas aeruginosa, which can be responsible for severe chest infections. They found that, under these conditions, the bacterium was able to evolve resistance to the disinfecting agent. Moreover, this resistant strain was also more than 200 times more resistant to the fluoroquinolone antibiotic ciprofloxacin, (You may remember a surge of publicity about ciprofloxacin at the time of the 2001 anthrax scare in the US.)
Benzalkonium chloride is a member of a family of similar chemicals, called quaternary ammonium salts, which are commonly used as antibacterial agents and antiseptics in cleaning products and personal care products, like mouthwash. (Other chemicals in this family include benzethonium chloride and cetylpyridinium chloride.) Because these chemicals are so widely used, the idea that their improper use may enable the development of antibiotic-resistant organisms is worrisome.
“If you use them wrongly by diluting, you’re asking for trouble,” says Gerard Fleming at the National University of Ireland in Galway, who led the research that revealed the problem. “The message is that you must use them properly, to the concentration stated on the bottle.”
Fleming further suggested that, for routine disinfecting purposes, chlorine bleach was probably the agent of choice, since no organisms are known to develop resistance to it. (This is in line with a general rule of thumb about anti-microbial substances: the lower the level on which the substance attacks the organism, the less likely the organism is to develop resistance to it. Chlorine bleach is a powerful oxidizing agent that attacks all kinds of biological molecules. Substances like benzalkonium chloride work at a higher level, by disrupting the cell wall of the organism.)
This is a further piece of evidence that the indiscriminate use of anti-microbial substances may do more harm than good.