en Nutrition Nutrition پژوهشي Research <strong>Background and Objectives: </strong>Recently, the use of probiotics for the treatment of Urinary Tract Infections has become more popular. The use of probiotics in therapy is useful as only a few side effects such as destruction of resistant bacteria or disturbance of the intestinal microbiota have been reported. The aim of this study was to evaluate the probiotic effects of lactic acid bacteria by co-aggregation of ciprofloxacin-resistant uropathogenic <em>Escherichia coli</em> strains using microbial techniques.<br> <strong>Materials and Methods: </strong>Three strains of <em>Lactobacillus plantarum</em>, <em>Lactobacillus casei and Lactobacillus acidophilus</em> were provided. Twenty isolates of uropathogenic <em>Escherichia coli</em> were collected from Shahid Labbafinejad hospital, Tehran. Eight samples with resistance to ciprofloxacin were selected using the disk diffusion method for the co-aggregation test. PCR was used to evaluate the presence of <em>qnrA</em> and<em> qnrS </em>genes in ciprofloxacin-resistant isolates<span dir="RTL">.</span> To evaluate the antimicrobial activity of complete culture and supernatants of lactobacilli, modified double-layer culture method and well diffusion methods were used, respectively. Co-aggregation of lactobacilli was evaluated by the co-aggregation test and microscopy test.<br> <strong>Results: </strong>Results showed that the eight human isolates were resistant to ciprofloxacin among other samples. Only one strain had both <em>qnrA</em> and <em>qnrS </em>genes simultaneously. <em>L. plantarum</em> with the average growth inhibition zone of 42 mm and with 65% of the co-aggregation had the best probiotic effects among all lactobacilli bacteria.<br> <strong>Conclusions: </strong>The probiotic lactobacilli had spectacular antimicrobial effects against the ciprofloxacin-resistant uropathogenic <em>Escherichia coli</em> strains. Also, lactobacillus spp. were aggregated with uropathogenic <em>Escherichia coli</em> strains and preventing from their adhesion to specific receptors on the Urethra, thus, the subsequent invasion to the host was prevented. Antimicrobial, Co-aggregation, Lactobacillus acidophilus, Lactobacillus casei, Lactobacillus plantarum 29 38 http://nfsr.sbmu.ac.ir/browse.php?a_code=A-10-588-1&slc_lang=en&sid=1 Mahsa Yeganeh 10031947532846003032 10031947532846003032 Yes Faculty of Biological Sciences, Tehran-North Branch, Islamic Azad University, Tehran, Iran Hedayat Hosseini 10031947532846003033 10031947532846003033 No Department of Food Technology Research, National Nutrition and Food Technology Research Institute, Faculty of Nutrition Sciences and Food Technology, Shahid Beheshti University of Medical Sciences, Tehran, Iran Sedigheh Mehrabian 10031947532846003034 10031947532846003034 No Faculty of Biological Sciences, Tehran-North Branch, Islamic Azad University, Tehran, Iran Elham Siasi Torbati 10031947532846003035 10031947532846003035 No Faculty of Biological Sciences, Tehran-North Branch, Islamic Azad University, Tehran, Iran Seyed Morteza Zamir 10031947532846003036 10031947532846003036 No Biotechnology Group, Faculty of Chemical Engineering, Tarbiat Modares University, Tehran, Iran