Biochemical discordance of a urinary fluoroquinolone-resistant Escherichia coli ST1193 as a Shigella species: A case report.

Escherichia coli (E. coli) sequence type 1193 (ST1193) is an emerging pathogenic organism that is increasingly identified in urinary specimens and often demonstrates multi-drug resistance. Although ST1193 is a well-documented E. coli clone, its lactose-nonfermenting phenotype and other atypical biochemical features may complicate routine laboratory interpretation. We report a urinary isolate from a 38-year-old woman with asymptomatic bacteriuria and a history of recurrent urinary tract infections that was identified as E. coli by MALDI-TOF mass spectrometry and confirmed as E. coli ST1193 by whole-genome sequencing but was classified as Shigella group with 91% confidence by an automated biochemical identification panel. The isolate demonstrated delayed growth as pinpoint pale-tan colonies on CHROMagar, non-lactose fermentation on MacConkey agar, lysine decarboxylase positivity, and orange-yellow colony formation on Hektoen Enteric agar. Antimicrobial susceptibility testing demonstrated resistance to ampicillin, some 1st and 2nd generation cephalosporins, and fluoroquinolones, with susceptibility to nitrofurantoin, trimethoprim-sulfamethoxazole, later-generation cephalosporins, piperacillin-tazobactam, aminoglycosides, and carbapenems. This case highlights a practical diagnostic pitfall. E. coli ST1193 may be accurately identified by MALDI-TOF MS and genomic methods yet still generate misleading biochemical-panel results suggestive of Shigella due to their similar genomic profile. Awareness of this phenotype is important when interpreting discordant E. coli-Shigella biochemical profiles from urine cultures.