Revenge of the bacteria

It’s not often that you find scary stories in scientific journals, but a new paper in the journal Microbiology offers a scary story indeed. Effect of subinhibitory concentrations of chloride on the competitiveness of Pseudomonas aeruginosa grown in continuous culture does not sound particularly menacing, but this paper raises the spectre that the antibacterial cleansers used every day to clean our homes and ourselves may lead to bacteria that are resistant not only to the cleansers but to powerful antibiotics as well.

Americans have become obsessed with “germs.” While there are certainly harmful bacteria and viruses that we would do well to avoid, the environment is full of bacteria and viruses that are harmless to humans. Antibacterial cleansers target all bacteria, regardless of whether or not they are harmful. That might be appropriate in the setting of the operating room, but it is excessive in non-medical settings. Yet manufacturers of anti-bacterial cleansers suggest otherwise:

Some days it seems like the kitchen is more than the center of your house—it’s the entire house. It’s the room where you and your family gather and where your “stuff” tends to end up too. No matter how much it becomes the hub of your home, the kitchen is still where you prepare and eat your meals—and where you are probably most concerned about bacteria and other germs spreading from surface to surface. That’s why we’ve invented a simple solution that will help you easily keep your kitchen clean and disinfected.

Clorox® Disinfecting Kitchen Cleaner kills 99.9% of common household the bacteria and other germs that can make your family sick. Plus, its bleach-free formula also cleans countertops, tabletops, and tough surfaces like stainless steel to a streak-free shine. It’s an effective formula you can use on almost any surface.*

Well, maybe antibacterial cleansers aren’t really necessary in the home, but there’s no harm, right? Actually, it seems like the widespread use of these cleansers has the potential to cause serious harm.

The active ingredient in Chlorox Disinfecting Kitchen Cleaner, and many other antibacterial cleansers, is benzalkonium chloride. The authors of the new paper suspected that Pseudomonas aeruginosa bacteria can become resistant to benzalkonium chloride if they are exposed to low concentrations of the chemical. In addition, they postulated that if bacteria became resistant to benzalkomium chloride,they would also become resistant against antibiotics that kill bacteria in similar ways.

If Pseudomonas is exposed to high concentrations of benzalkonium chloride, all the bacteria will die. However, if the bacteria are exposed to lower concentrations of benzalkonium chloride, some bacteria will die, but others will become resistant to the antibacterial cleanser. That, in itself, is worrying. If we continually wipe down our kitchen counters with benzalkonium chloride, Pseudomonas will eventually become resistant, making the antibacterial cleanser useless.

Even more concerning is the fact that the bacteria that became resistant to benzalkonium chloride also became resistant to the antibiotic ciprofloxin, even though the bacteria had not been exposed to ciprofloxin. It seems that the adaptation that allowed the bacteria to resist the effects of benzalkonium chloride also allows the bacteria to resist the action of ciprofloxin.

In other words, the use of the antibacterial cleanser eventually rendered the cleanser ineffective. That’s disturbing, but not surprising. What is surprising is that the bacteria that were resistant to the cleanser could no longer be killed by the antibiotic ciprofloxin. In attempting to make our environment safer, we may actually be making it far more dangerous.

The problem of antibiotic resistance has been known for decades and we have learned that antibiotics should only be used when absolutely necessary in order to limit the possibility of bacteria becoming resistant. This paper suggests that the same warning should apply to antibacterial cleansers as well. They should only be used when absolutely necessary, and not used indiscriminately to “keep your kitchen clean and disinfected.” The use of antibacterial cleansers is not merely unnecessary; it has the potential to be very harmful.