Let’s take an old and well-established type of antibiotic, the beta-lactams (penicillins, cephalosporins, etc.) How do they work?
Well, the standard answer to that one is that they disrupt the synthesis of peptidoglycan (PG), which is a key ingredient in the bacterial cell wall. And that’s enough of an explanation, most of the time, for most of us. But let’s keep going: how does that disruption kill bacteria? Do they end up making a weaker cell wall, which leaves them more vulnerable to other environmental stress? Does that weaker cell wall lead to key components leaking out, perhaps? Or is there something else about throwing a wrench into cell wall synthesis that causes trouble?
This new paper says that it’s the third option, at least in some cases. There’s been recent work pointing in that direction, and it’s been enabled by the specific beta-lactam drug mecillinam, which only inhibits one key enzyme in the pathway, penicillin-binding-protein 2 (PBP2). This earlier paper demonstrated that inhibiting peptidoglycan synthesis sets a bacterial cell off on a futile-cycle hamster wheel of attempted PG synthesis followed by degradation. Interestingly, if you interrupt that cycing, you find that you can inhibit PBP2 without killing the bacteria. Meanwhile, other work has shown that PBP inhibition in general seems to increase ATP synthesis in tuberculosis bacteria, and if you titrate in an ATP synthesis inhibitor, you can keep the beta-lactam drugs from killing them. All this suggests that inhibiting the targets of the beta-lactam drugs per se is not a lethal event. It’s what that inhibition does to the cell afterwards.
This latest work follows things further downstream by various chemical biology methods (analyzing time-course levels of various proteins and metabolic intermediates), and finds that in exposure of E. coli to beta-lactam agents speeds up protein synthesis to unsustainable levels. Several other cellular processes also get revved up at the same time, such as amino acid and nucleotide metabolic pathways, as well as ATP use in general. The cells end up in an ATP-starved state, and attempt to deal with that by increasing catabolic pathways, which can be a bit like heating your house by prying up the floorboards to put them into the furnace. This causes redox stress on the cell, which now has to fight off increased reactive oxygen species thrown off by the last-ditch attempts to make more ATP. Previous work had shown that adding glutathione (a classic reactive-intermediate scavenger) to bacterial growth media could attenuate the effects of beta-lactams, and that was confirmed here as well. Another notable finding was that if you grow the bacteria in a medium rich in amino acid and nucleotide precursors, mecillinam is even more lethal. That seems to allow the bacteria to go full-speed down the ultimately toxic metabolic pathways, whereas in nutrient-poor media they don’t have the resources to even get started. Protein synthesis rates can’t be increased enough to get them into trouble under those conditions.
I find it really interesting that we’re still finding out things about such an old class of antibiotics, but the last few years have really changed our ideas about how they work. And that can be useful in figuring out new compounds, new pathways to target, and new combinations to make existing drugs more effective. Doing the same sorts of chemical biology work on various kinds of beta-lactam-resistant strains will add to this story, and I would guess that’s where the field is going now. New antibiotics aren’t easy to find, so if we can make some of the older ones more lethal again, that could come in very handy indeed. . .
