Factors influencing the establishment and spread of R-plasmids in an experimental model of urinary tract infection.

An experimental model simulating infection of the urinary tract was used to investigate factors influencing the transfer of plasmid-mediated drug resistance in the human bladder in the absence of antibiotic therapy. When a small number of cells carrying plasmid-mediated drug resistance was added to an established population of sensitive cells, it was found that varying the physical conditions of the bladder model had no significant effect on the spread of the plasmid and, under all conditions used, exponential spread of the plasmid through the originally sensitive population occurred. When the initial donor cells were killed shortly after addition to the model, exponential transfer of resistance amongst the established sensitive population still occurred. Thus, even in the absence of antibiotic therapy, only a small number of initial transfer events was required for a fully resistant population to be developed. This was true of both actively multiplying and early stationary phase cultures. An initially greater proportion of resistant cells, or the use of an antimicrobial agent to which some of the cells are resistant, would ensure that this process occurred more rapidly.