Infectious disease is often untreatable, even when caused by a pathogen that is not resistant to antibiotics. This is the paradox, and the problem that we aim to solve. Microbial populations produce persisters, dormant cells that are not mutants, but phenotypic variants of the wild type that are tolerant to antibiotics. It seemed possible that the presence of persisters could explain the treatment paradox: an antibiotic eliminates most of the population, and once its concentration drops, the surviving persisters start dividing and reestablish the infection. However, prolonged treatment of persisters in vitro with antibiotics which should emulate the in vivo therapy eliminates these dormant cells. The hypothesis. We hypothesize that the agent responsible for untreatable infections is a super-persister cell which carries a high-persister mutation and has induced stress responses. Repeated application of high levels of antibiotics in vitro selects for E. coli hip (high-persister) mutants that have an increased level of persister cells. According to our data, the hip cells are also more drug-tolerant as compared to wild type persisters. We reasoned that periodic application of lethal doses of antibiotics to patients with chronic infections will similarly select for hip mutants. Analysis of longitudinal isolates from a cystic fibrosis patient infected with P. aeruginosa showed that late, but not early isolates are indeed hip mutants. It seems possible that therapy with repeated doses of antibiotic selects hip mutants in many if not all pathogens, and it is these presently overlooked tolerant (rather than resistant) mutants that are ultimately responsible for morbidity of the disease and for the death of a patient. Apart from hip mutations, there seems to be another overlooked, but potentially critical component contributing to tolerance - stress responses. So far, we have known of two seemingly opposite strategies of cell survival - dormancy, which shuts down functions and creates persister cells; and induction of stress responses (heat shock, DNA repair, oxidation stress, etc.) that actively protect the cell from noxious conditions. We propose that these two strategies actually complement each other. If a persister is formed in a population that had expressed stress proteins, then it will shut down antibiotic targets, while retaining protective proteins which will help it survive. In the body, a pathogen is exposes to oxidants, DNA damaging agents, membrane acting agents, and it seems that expression of several stress responses is a norm. The ultimate survivor is then a persister carrying a hip mutation which is formed in a population expressing stress responses. It is this super-persister that is probably responsible for much of untreatable disease and will be the focus of our investigation. The experimental plan will address the following interrelated questions: are hip mutants an important part of chronic infection? Are there super-persisters that combine hip mutations with expression of stress responses? Is tolerance, similarly to resistance, a transmissible trait?