Administered antibiotics initially work extremely well, killing more than 99.9% of the microbes they target. However, microbes have a high rate of mutation and because antibiotics are initially designed for a single specific target, they are unable to kill 100% of microbes during an infection. The surviving microbes that have successfully avoided antibiotics continue to replicate and spread, leading to new antibiotic resistant strains over time.
Dr. Schramm hypothesized that antibiotics do not necessarily have to kill the microbe itself, but rather, disrupt their infectious nature. In other words, if the microbe survives but does not replicate, then resistance to antibiotics will not occur.
In the Nature of Chemical Biology study, test microbes Vibrio cholerae, which causes cholera, and E. coli strain 0157:H7, the notorious food contaminant responsible for ~110,000 illnesses each year in the U.S. were used. The aim of the research study was to disrupt the infectious microbes' ability to communicate with each other.
The process by which bacteria communicate is called "quorum sensing." In quorum sensing, signaling molecules known as "autoinducers" are detected between bacteria. These autoinducers regulate expression of bacterial genes, including genes associated with virulence. Previous studies have shown that bacterial strains defective in quorum sensing are less infectious.
The bacterial enzyme "MTAN" is directly involved in synthesizing autoinducers and is crucial in catalyzing quorum sensing. The team's aim was to inhibit MTAN by designing a substrate to which the enzyme would preferentially bind to as opposed to its human host substrate.
In order to design an analog of MTAN's natural substrate, the Schramm lab obtained a picture of the enzyme's structure during its brief transitional state period (one-tenth of one trillionth of a second) in which the enzyme converts its substrate into a different chemical.
With knowledge of the enzyme's transitional structure, Dr. Schramm and his colleagues constructed and tested three MTAN transition state analogs. All three compounds developed were highly successful in disrupting the quorum sensing pathway in both V. cholerae and E. coli 0157:H7. Not only did MTAN preferentially bind to Schramm's analogs, but in fact, the analogs permanently inhibited the enzyme from initiating quorum sensing.
To test if the microbes would develop resistance over time, the researchers tested the analogs on 26 successive generations of both bacterial species. All 26 generations were as sensitive to the antibiotics as the first generation was.
"In our lab, we call these agents everlasting antibiotics," said Dr. Schramm.
Dr. Schramm also points out that many aggressive bacterial pathogens express MTAN and therefore would probably also be susceptible to these inhibiting analogs including: S. pneumoniae, Klebsiella pneumoniae, N. meningitides, and Staphylococcus aureus.
To date, Schramm's research team has developed more than 20 highly effective MTAN inhibitors, all of which are expected to be safe for human usage since MTAN is a bacterial enzyme and blocking it would have no effect on human metabolism.
The study, "Transition State Analogs of 5'-Methylthioadenosine Nucleosidase Disrupt Quorum Sensing" was published in the March 8, 2009 online edition of Nature Chemical Biology.