Electronic Thesis and Dissertation Repository


Doctor of Philosophy


Microbiology and Immunology


In addition to their bactericidal effects, antibiotics are potent signal mediators at sub-inhibitory levels in the environment. The ability to modulate community structure in this niche raises concerns over their capacity to influence pathogenesis in patients during antibiotic therapy. This concept forms the basis of this thesis, and is explored using models of prophylactic therapy for recurrent urinary tract infection (UTI) management.

Sub-inhibitory ciprofloxacin, ampicillin, and gentamicin were found to augment virulence in vitro, increasing adherence and urothelial cell invasion in uropathogenic Escherichia coli (UPEC) and Staphylococcus saprophyticus. In addition, biofilm formation was increased, and swarming motility decreased. In UPEC, the effect of antibiotics on these processes was abolished in SOS-deficient strains.

Trans-urethral inoculation of mice with ciprofloxacin-primed S. saprophyticus or UPEC significantly increased bacterial burden in both bladders and kidneys at one and 14 days post-inoculation (dpi). Sub-therapeutic ciprofloxacin supplemented in the drinking water of chronically infected mice significantly increased bacterial urine load. In addition, mice previously infected but clinically resolved suffered recurrences. These mice had impaired urinary polymorphonuclear leukocyte infiltrates, in part due to antibiotic-dependent cytokine suppression during initial infection. Prophylactic intervention had no significant effect on UPEC clearance, but did significantly increase bacterial intracellular bladder reservoirs, raising concerns over the clinical efficacy of this management strategy and risks of promoting persistent infection.

The inability of antibiotics to clear infection in prophylaxis models was attributed to the presence of MDT persister cells. Sub-inhibitory antibiotic pre-treatments were found to increase persister fractions, but this effect was abolished in SOS-deficient strains. Conducting these assays with UPEC isolated from recurrent UTI patients revealed an enriched persister fraction compared to organisms cleared with standard antibiotic therapy, suggesting persister traits are either selected for during prolonged antibiotic treatment or initially contribute to therapy failure.

This work represents the first attempt to illustrate that observed sub-inhibitory, pathogen associated antibiotic-dependent changes in vitro have significant in vivo consequences. It is hoped that this research will lead to a re-examination of how antibiotics are administered for management of patients suffering from recurrent UTI and other chronic diseases.