The molecular basis, and biological cost, of fluoroquinolone resistance in Salmonella enterica serovar Paratyphi A.

Vaccination against typhoid fever is well established and is now being implemented as a control measure for this major human disease. Paratyphoid fever, an indistinguishable disease, however is not covered by this vaccine and there is a danger that paratyphoid may replace typhoid fever. This is of particular concern because the bacterial cause, Salmonella Paratyphi A, has become resistant to all but one of the antibiotics available for treatment, including fluoroquinolones such as ciprofloxacin. Several reports from India and the Far East describe cases of paratyphoid fever resistant to ciprofloxacin, the main drug for treatment. We propose to examine in detail the development of resistance to ciprofloxacin in S. Paratyphi A as it emerges. This information will increase awareness among clinical practitioners about resistant strains and so allow patients to be treated appropriately and enhance our understanding of the fundamental process of acquisition of antibiotic resistance in pathogenic bacteria. Resistance to antibiotics is a major threat to modern medicine, and because isolates of S. Paratyphi A are very similar; only five genes differ between the two isolates sequenced. The development of resistance in S. Paratyphi A therfore provides an ideal opportunity to study the evolution of resistance in bacteria. By recreating the natural mutations in laboratory strains of S. Paratyphi A we hope to observe the effects of each mutation on fitness as well on the resistance phenotype.

Specific control measures for enteric fever; vaccination and antibiotic treatment, are designed to limit the spread of Salmonella Typhi (typhoid fever). An increasing number of cases of enteric fever however are being caused by S. Paratyphi A (paratyphoid fever). There is no evidence that typhoid fever protects against paratyphoid fever and so the currently available S. Typhi specific vaccines are not expected to protect against S. Paratyphi A. As well as a lack of a vaccine there has been an increase in antibiotic resistance. In particular a decrease in susceptibility to fluoroquinolones has been associated with an increase in cases of paratyphoid across Asia. Full fluoroquinolone resistance in S. Paratyphi A, has now emerged and is spreading. The two sequenced isolates of S. Paratyphi A, are very closely related at the nucleotide level and so a detailed analysis of antibiotic resistant and susceptible strains will allow us to investigate all of the genetic events involved in the emergence of fluoroquinolone resistance in this important human pathogen. Identification of genetic markers has allowed the early detection of resistant S. Paratyphi A strains and collaborative laboratories in China, Pakistan and India are collecting resistant strains as they are isolated. By phenotypic and genetic analysis of these isolates we will identify genetic factors which will be introduced into isogenic mutants using technology developed in the laboratory of principal applicant. This will allow a detailed study of the contribution of different mutations (or gene acquisition) to fluoroquinolone resistance in the S. Paratyphi A background. The co-applicants expertise in the biology of efflux mechanisms of resistance, and the quality of the two postdoctoral scientists is key to the success of this project which is of fundamental importance to understanding how resistance to the fluoroquinolones, a very important group of antibacterial agents, evolves in Salmonella enterica, an important human and animal pathogen.