A study done at New castle University done in the elderly patients suffering from recurring urinary tract infection shows that bacteria causing the infection can lose their cell walls, which is the common target of many groups of antibiotics prescribed to combat the infection. As such the antibiotics can no longer kill the bacteria. Antibiotic resistance is one of the biggest threats to global health.
What they have discovered is that in the presence of antibiotics, the bacteria are able to change from a highly regular walled form to a completely random, cell wall-deficient form called the L-form state. As such, the bacteria can easily hide from our immune system as well as antibiotics.
In L-form the bacteria are weaker but some survive, hiding inside the body. They can then re-form their cell wall and the patient will be faced with recurring UTIs.