The purpose of this sequencing project is to understand the genetic basis of a drug resistance mechanism as it emerges in the in-vivo environment under the selective pressure of the antimicrobial agent vancomycin. The proposal involves sequencing a pair of isogenic methicillin-resistant Staphylococcus aureus (MRSA) strains that were recently recovered from a patient undergoing extensive chemotherapy with vancomycin. The chronologically earliest isolate (JH1), recovered before the start of vancomycin therapy, is susceptible to vancomycin, and the “latest” isolate (JH9), recovered shortly before the death of the patient, has an increased vancomycin resistance. The range of change in antibiotic susceptibility and the altered physiological and biochemical properties of the strains indicate that strain JH9 represents the so-called VISA-type vancomycin resistance, which appeared in clinical specimens of MRSA in several countries during the last decade. Several types of tests indicate that the earliest (JH1) and latest (JH9) isolates, and also the additional isolates recovered with intermediate levels of vancomycin resistance between JH1 and JH9, are all “isogenic” bacteria (i.e., bacteria from the same genetic source) by the most exacting molecular criteria. Therefore, the comparison of sequence differences between the first (“parental”, JH1) and the last (“mutant”, JH9) isolates should allow one to identify the genetic basis of vancomycin resistance. The project should enable researchers to reconstruct–for the first time–the evolutionary history of a bacterial pathogen in vivo, in a patient undergoing chemotherapy. The result should provide detailed information about how S. aureus, one of the most important nosocomial pathogens, develops resistance to vancomycin, an antimicrobial agent most frequently used against it.
CSP project participants: Alexander Tomasz (proposer and PI, Rockefeller Univ.) and Eugene Myers (Univ. of California, Berkeley).