For some reason, we long assumed that #bacteria in the extra-systemic, #urinary cavity are subject to the same conditions as in other tissue infections, but they aren’t. #Antibiotics get to other tissue infections via blood whereas #cystitis #pathogens encounter antibiotics in #urine.
That’s a big difference, first of all, blood pH range is narrow, so an antibiotic’s MIC (minimum inhibitory concentration) is predictable there. The urinary pH range is very wide. Antibiotic MIC in urine varies with pH because pH influences its solubility and ionisation.
Secondly, the urine pool, especially if it is big, can dilute the entering antibiotic’s concentration enormously. #Bladders can be big urine reservoirs.
Thirdly, the antibiotic drip-feed into that pool is rate limited, so build up to urinary MIC can take many hours into days.
All those factors delaying the antibiotic reaching its urinary MIC enable the pathogens to build their defences. Sometimes they do, with results varying from treatment failure to infection recurrence and even AMR.
Intravesical instillation of a high concentration of a quick acting drug makes more therapeutic sense. That’s why #intravesical #gentamicin continues to gain traction in urology services, even though it remains off-label. Since #oralantibiotic treatments give bacteria time to build their defences, be they biofilms or AMR, we should not be surprised by the deleterious outcomes we’re seeing.