The model for bacterial death by antibiotics was fairly simply until recently. Antibiotics work by targeting a certain area of the bacteria; beta-lactams target the cell wall, Rifamycins target RNA synthesis, tetracyclins inhibit protein synthesis etc. The theory was that by inhibiting these processes, a certain vital function within the bacteria would be stopped, leading to its death.
However due to research done by Kohanski (references below) the story is looking a bit more complicated. Looking at three different classes of antibiotics they found that no matter what the site of action, all the antibiotics induced hydroxyl radicals. This was in bactericidal drugs, which actually kill bacteria, rather than bacteristatic ones (which just prevent cell growth). They also demonstrated that this mechanism of hydroxyl radical production was the end product of a chain of reactions involving damage to the TCA cycle (aka the Krebs cycle - which is a major part of respiration) which lead to damage to iron-sulphur clusters and subsequent production of the DNA-damaging hydroxyl radicals. This is shown diagramatically below, and this first paper was covered by Jim at Mental Indigestion with some great follow-up comments and discussion.
They've recently put out a review (second reference below) of which I find the most exciting parts are the two little extra-information boxes. One of them covers drug synergy and the second covers synthetic biology, both of which I'm getting increasingly more interested in.
One of the most useful things about modelling drug actions is it can help to show which drugs would work most effectively in pairs. Using two drugs together can have many potential effects; it can make the treatment more effective, sometimes is can make the treatment less effective and of course some can be dangerous for the patient. Work on drug synergy showed that aminoglycoside antibiotics (which affect RNA synthesis) become more affective when given simultaneously with B-lactam antibiotics (which lead to cell wall breakdown) as the increased cell wall breakdown helps the aminoglycosides to get inside the cell. Conversely, drugs that inhibit protein synthesis are less effective when given at the same time as drugs which inhibit DNA synthesis as making it harder to synthesise proteins from sub-optimal DNA actually makes the cell more able to survive.
These interactions will affect the dosage of drugs used during synergistic treatments, and it is hoped that using two different types of antibiotics at low doses might be more healthy for the patient, and might help to combat against antibacterial resistance to one of the drugs.
Another interesting concept the paper brings attention too is the potential use of synthetic biology to aid in both the study and application of antibiotic-related death systems. By using synthetic genes to disrupt or alter the proposed antibiotic network novel drug targets could be discovered. If turned into a high-throughput system this would be far more useful than the current screening system which tests for a potential drugs interaction with a target, rather than the ability of this interaction to lead to cell death.
Synthetic genes can be delivered into the bacterial cell via bacteriophages. Adding a synthetic gene into a bacteriophage for bacteria cell delivery has been attempted successfully before when they were used to enhance E. coli cell death by delivering genes for proteins that disrupted the DNA-repair system within the bacteria. This allowed faster and more effective killing of the bacteria at lower doses of antibiotic.
At a time when bacteria are fast becoming resistant to even the front line drugs, research that suggests novel ways of killing bacteria can produce some very useful outcomes. Using combinations of drugs at lower concentrations, or aiding antibiotics by introducing them along with synthetic genes in bacteriophages allows an increased shelf-life of the drugs that we currently possess as well as providing potential systems to aid the discovery of new antibiotics.
Kohanski MA, Dwyer DJ, Hayete B, Lawrence CA, & Collins JJ (2007). A common mechanism of cellular death induced by bactericidal antibiotics. Cell, 130 (5), 797-810 PMID: 17803904
Kohanski MA, Dwyer DJ, & Collins JJ (2010). How antibiotics kill bacteria: from targets to networks. Nature reviews. Microbiology, 8 (6), 423-35 PMID: 20440275
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