Vallo Varik will defend his doctoral thesis titled „Stringent response in bacterial growth and survival“ on Monday, 2 October 2017 at 14.15.
Professor Tanel Tenson
Senior Research Fellow Vasili Hauryiliuk
Gregory Bokinsky, PhD. Department of Bionanoscience, Delft University of Technology (The Netherlands)
Description of the problem
One of the most remarkable features of bacteria is the speed at which they proliferate. Rapid growth, however, inevitably leads to a change in growth environment and, then, we encounter the next remarkable feature of bacterial cells—withstand stasis and otherwise harsh environment. In rapidly growing bacterial cells, protein biosynthesis takes the largest toll on cellular energy. Sensing the status of ribosome, the molecular machinery of translation, is therefore important part of bacterial growth regulation. The process, which probes the capability of aminoacylation of tRNAs to keep up with the requirements of translation, is called the stringent response. Among other strong effects on cell physiology, the stringent response can stop the growth and thus interfere with the action of antibiotics. Bactericidal antibiotics, for example, require an active target to kill the bacterial cells and are therefore inefficient in case of non-growing dormant cells. It has been also proposed that in very few cells of growing bacterial population, for some reason, the stringent response is activated. Then, if the bactericidal antibiotic treatment is applied, while the rest of the bacterial population is eradicated, the few dormant cells survive and cause the recurrent infection, once the antibiotic regime is discontinued.
Result and benefit
Current work, therefore, set out to investigate the connections between bacterial growth, the action of antibiotics and the insult of immune system. It was found that serum mediated killing eradicates most of the growing population of uropathogenic Escherichia coli, only the very rapidly growing and the dormant cells, despite being recognized by the complement system, survive the insult. During simultaneous application of serum and various antibiotics from different classes, however, only dormant cells survive as antibiotics result in clearance of the rapidly growing cells. Since the non-growing state in growth supporting environment was protective against both antibiotic treatment and action of the immune system, the research group next set out to elucidate mechanisms controlling the growth resumption of E. coli. Of several plausible target genes initially studied, the stringent response factor RelA stood out. It was found that a culture of stringent response deficient E. coli, i.e. relaxed strain, is defective in growth resumption rendering cells non-growing for longer periods of time in growth supporting environment. The growth resumption defect of relaxed E. coli was a function of both the amino acid and carbon source composition of the medium. Curiously, compared to wild-type, relaxed strain survived ampicillin treatment better even if the growth resumption of the two strains was equal. As it was learned that the stringent response is a key player in growth resuscitation and given its reported importance to bacterial virulence, the research group set up a high-throughput search for specific inhibitors of the stringent response. If not an immediate value for medicine, they reasoned, such inhibitors would be a powerful tool for studies of bacterial physiology. A screening system was established, it failed to yield the desired compound, but resulted in identification of novel class of antibacterials.