Urinary tract infections (UTI) are one of the most common bacterial infections in the world, with the primary cause being Uropathogenic E. coli (UPEC). UPEC infect, grow inside and finally rupture bladder epithelial cells, expelling the bacteria. A subpopulation of these bacteria has a highly filamentous morphology, where the bacteria have continued to grow without dividing.
Previous studies have shown the possibility of a urine component that triggers the bacteria to filament and our aim is to determine what this trigger may be. To achieve this, in vitro infection models have been used. The first model uses a flow cell system where the bacteria released from bladder cells are collected for microscopy and flow cytometry, to assess the extent of filamentation. We have also developed a microfluidic model which allows for the real time observation of the infection cycle by microscopy.
It was previously reported that concentrated urine applied to infected bladder cells was required to observe bacterial filamentation in experimental infections. We found that concentrated synthetic human urine did not induce filamentation, suggesting that another component of real human urine is important. The effect of the pH of urine, which is normally mildly acidic, was tested. Neutralised urine formed a precipitate which was removed and the resulting urine did not cause filamentation. Interestingly, when the pH of this urine was re-adjusted to normal (mildly acidic), the urine was able to induce strong filamentation. Finally, ultra-filtered urine (<3 kDa) was still able to induce filamentation, demonstrating that small molecules are responsible for triggering filamentation. Together, the results suggest that pH plays a significant role in a complex signaling pathway involving small molecule signals that trigger bacterial filamentation in UTI.