Because you need one more fucking thing to worry about.

One of the consequences of a global pandemic is that we are forced to do things that will have harmful consequences to combat the pandemic. Using less mass transit exacerbates global warming, but probably lessens the spread of COVID-19, especially when it is not under control (as is the case in most of the U.S.). One thing that hasn’t been considered in terms of its harmful effects–but still needs to be done–is the use of disinfectants. Obviously, they need to be used in hospitals and other high-risk settings, such as nursing homes, even in the absence of a pandemic. And keeping your home and workplace clean is good!

But we’re also seeing the use of disinfectants in places where they’re typically not used (or used in such amounts) such as subways and outdoor areas (the latter is just general spraying disinfectants). One commonly used disinfectant, and which is effective against many viruses, is the class of compounds known as quaternary ammonium compounds (‘QACs’). When you hear about cleaning compounds that have a ‘lemon scent’, they’re usually QACs–not because QACs have a lemon scent, but because QACs smell…not good, and the lemon scent masks the smell (imagine a hybrid of mothballs and worn socks. Not good). If you’ve ever used a cleaner than has things in it named ‘benzalkonium chloride’, such as 409 cleaner*, then you’ve used QACs.

To be clear, QACs are very good at killing enveloped viruses, fungi, and bacteria. So you know what that means: bacteria evolved resistance to QACs. There are a bunch of genes that confer resistance to QACs, which biologists have cleverly called qac. One of the most common qac genes is qacEdelta1**, which is found in E. coli and relatives, and co-occurs with a particular set of sulfonamide, trimethoprim, and streptomycin resistance genes. Often, tetracycline resistance genes and erythromycin resistance genes travel along with qacEdelta1 (mercury resistance genes too. It’s a real party). While none of these genes are ‘last line’ antibiotics, they are still used, and we would like to keep those older antibiotics working for as long as we can.

It gets worse (you didn’t think I was finished, did you?). Looking at some genomic data, I estimate that about seventy percent of isolates that have a KPC carbapenemase–a gene that confers resistance to virtually of the penicillin derivatives including the last line carbapenemases–also have qacEdelta1. It’s typically not physically linked to KPC carbapenemases; it’s usually on a different plasmid–a mobile ‘mini-chromosome. But they are found in the same cell, so exposure to QACs provides these isolates a selective advantage, meaning it will be that much harder to reduce the frequency of KPC carbapenemases (and other antibiotic resistance genes) solely through less use of those drugs.

All that said, we do need to use disinfectants, including QACs, especially in the face of a massive viral outbreak for which we have no vaccine or proven therapies. But maybe spraying them on city streets isn’t the best idea ever.

*Not that you can find 409 or other QAC-based cleaners in most stores in the U.S. Got Non-medical PPE?

**If there’s any good news here, it’s that qacEdelta1 is one of the weaker qac genes.