Differential epidemiology and antibiotic resistance of lactose-fermenting and non-fermenting Escherichia coli: Is it just a matter of taste?

Urinary tract infections (UTIs) are some of the most common infections affecting humans worldwide. Occurrence of atypical, lactose non-fermenting, biochemically "inactive" strains of E. coli in clinical material has been described in the literature, which may cause a significant diagnostic challenge. The present retrospective microbiological study was carried out using isolates and data collected between January 1, 2013, and December 31, 2017, at the Institute of Clinical Microbiology. n = 24,285 positive urine samples were noted during the study period, out of which, samples positive for either lac + and lac- E. coli were included in the analysis. E. coli represented n = 7075 (55.8% ± 4.6%) of outpatient and n = 4916 (42.4% ± 3.6%) of inpatient isolates. n = 401 (3.3%; 80.2 ± 14.6/year) lac- E. coli isolates were identified from urinary tract infections. The ratio of lac- E. coli isolates was significantly higher in outpatient samples (262 vs. 139). Resistance levels of lac- isolates for antibiotics commonly used for treating UTIs were significantly higher for both inpatient and outpatient isolates: norfloxacin, ciprofloxacin, fosfomycin and nitrofurantoin. It is essential to pay attention to the presence of lac- strains, and their omission from clinical material during diagnostic procedures may have significant consequences for epidemiological studies and therapy.