We have gained a detailed understanding of antibiotic action over the last 30 years or so, or at least we think so. Almost all the major current antibiotics are thought to work through a handful of processes that disrupt or halt bacterial growth: cell wall growth inhibition, protein synthesis inhibition, metabolism inhibition and DNA synthesis inhibition.
But a paper published a couple of weeks ago in Cell demonstrates a novel and previously unthought of mode of action for antibiotics- free radical generation and subsequent bacterial cell death. The surprising thing is that the authors of the paper don't demonstrate this for a particular antibiotic. They think that all antibiotics may work the same way. If this is true, it will lead to a reevaluation of our understanding of all antibiotic action. In their hands, three of the best known antibiotics around that have very different modes of action- penicillin, norfloxacin and kanamycin- showed the generation of free radicals and subsequent bacterial death of growth inhibition.
They observed hydroxyl radical production after bacterial cells were treated with all three antibiotics. The radical formation was visualized through a fluorescein derivative that fluoresces after reacting with hydroxyl radicals (apparently it's not possible yet to directly test for radical formation in-vivo). To substantiate the radical phenomenon (pun intended), they added an iron chelator that chelates Fe(II) thus inhibiting the well-known Fenton reaction that produces hydroxyl radicals from the reaction of Fe(II) and hydrogen peroxide. After the iron chelator was added, bacterial survival increased, indicating that radicals were playing a role in inhibiting the survival. Similar behaviour was observed after addition of a hydroxyl radical quencher.
The authors think that all three classes of antibiotics act by inhibiting the electron transport chain and depleting NADH which is converted to NAD+ in the reducing potential generating set of reactions. This impairs the chain and generates superoxide that disrupts iron-sulfur clusters in the chain, generating Fe(II). The Fe(II) reacts with the H2O2 produced by the reaction of superoxide with superoxide dismutase, thus producing toxic hydroxyl radicals.
Whether this study will be unique or general remains to be seen. As with other such biological studies, it is fraught with complications, most importantly that of distinguishing between cause and consequence. What else happens when the iron chelator and radical quencher are added? Are the radicals the cause of bacterial death or simply a byproduct of some other process induced by the antibiotics?
In any case, this paper is very interesting and sounds like one of those ideas that may simply indicate an incidentally exciting observation, or a real sea-change in a well-established paradigm. It was judged significant enough though to be included in C & EN's end of year issue as a 2007 chemistry highlight of the year.