Regulatory plasticity of the rdar biofilm morphotype in clinical uropathogenic Escherichia coli and its modulation by ciprofloxacin

Abstract

Background: Red, dry and rough (rdar) biofilm formation in Escherichia coli is characterized by the coordinated production of extracellular cellulose and amyloid curli fimbriae. Although rdar biofilm formation has been extensively characterized in laboratory strains, its clinical relevance and implications for antimicrobial treatment remain poorly understood. Methods: We screened 146 consecutive E. coli isolates from patients with urinary tract infections in Lahore, Pakistan (2017-2019) for Congo red morphotypes at 28 degrees C and 37 degrees C. Antimicrobial susceptibility was determined by disc diffusion and compared between morphotypes using the Fisher exact test. Fourteen isolates were whole-genome sequenced, the csgD-csgBA intergenic region was aligned, and csgD expression was quantified by qRT-PCR. The effect of 10 micrograms/mL ciprofloxacin on rdar morphotype expression, matrix ultrastructure and biofilm formation was examined in ciprofloxacin-resistant isolates (MIC at least 128 micrograms/mL). Results: Rdar-like colony morphotypes were expressed by 36.3% (53/146) of isolates at 28 degrees C and 29.5% (43/146) at 37 degrees C. Nucleotide polymorphisms in the csgD-csgBA intergenic region clustered within annotated OmpR, IHF and CsgD binding regions and were associated with, but did not individually predict, the mode of rdar morphotype expression. Isolates carrying the rdar morphotype were distributed across diverse E. coli phylogroups and sequence types. Resistance to aztreonam, cefuroxime and cephradine was more frequent among smooth and white isolates, whereas resistance to nitrofurantoin was more frequent among isolates rdar-positive at 28 degrees C (45.2% versus 20.2%, p = 0.0097); fluoroquinolone resistance did not differ between morphotypes. Subinhibitory ciprofloxacin (10 micrograms/mL) suppressed rdar biofilm formation, disrupted extracellular matrix architecture and abolished surface colonization on glass, in ciprofloxacin-resistant isolates. Conclusions: Our findings establish the rdar morphotype as a clinically relevant biofilm phenotype in uropathogenic E. coli and show that subinhibitory ciprofloxacin suppresses rdar biofilm formation in ciprofloxacin-resistant isolates. These results underscore the importance of considering antibiotic-mediated modulation of biofilm behavior when interpreting treatment responses and designing strategies to combat persistent urinary tract infections.