Antibiotic heteroresistance is a common bacterial phenotype characterised by the presence of small resistant subpopulations within a susceptible population. During antibiotic exposure, these resistant subpopulations can be enriched and potentially lead to treatment failure. In this study, we examined the prevalence, misclassification, and clinical effect of heteroresistance in Escherichia coli bloodstream infections for the clinically important antibiotics cefotaxime, gentamicin, and piperacillin-tazobactam. We conducted a retrospective cohort analysis of patients (n=255) admitted to in-patient care and treated for Ecoli bloodstream infections within the Uppsala region in Sweden between Jan 1, 2014, and Dec 31, 2015. Patient inclusion criteria were admission to hospital on suspicion of infection, starting systemic antibiotics at the time of admission, positive blood cultures for the growth of Ecoli upon admission, and residency in the Uppsala health-care region at the time of admission. Exclusion criteria were growth of an additional pathogen than Ecoli in blood cultures taken at admission or previous inclusion of the patients in the study for another bloodstream infection. Antibiotic susceptibility of preserved blood culture isolates of Ecoli was assessed for cefotaxime, gentamicin, and piperacillin-tazobactam by disk diffusion and breakpoint crossing heteroresistance (BCHR) was identified using population analysis profiling. The clinical outcome parameters were obtained from patient records. The primary outcome variable was length of hospital stay due to the Ecoli bloodstream infection, defined as the time between admission and discharge from inpatient care as noted on the physician's notes. Secondary outcomes were time to fever resolution, admission to intermediary care unit or intensive care unit during time in hospital, switching or adding another intravenous antibiotic treatment, re-admission to hospital within 30days of original admission, recurrent Ecoli infection within 30days of admission to hospital, and all-cause mortality within 90days of admission. A total of 255participants with a corresponding Ecoli isolate (out of 500screened for eligibility) met theinclusion criteria, with 135female patients and 120male patients. One (<1%) of 255strains was BCHR for cefotaxime, 109 (43%) of 255strains were BCHR for gentamicin, and 22 (9%) of 255strains were BCHR for piperacillin-tazobactam. Clinical susceptibility testing misclassified 120 (96%) of 125heteroresistant bacterial strains as susceptible. The BCHR phenotypes had no correlation to length of hospital stay due to the Ecoli bloodstream infection. However, patients with piperacillin-tazobactam BCHR strains who received piperacillin-tazobactam had3·1times higher odds for admittance to the intermediate care unit (95% CI 1·1-9·6, p=0·041) than the remainder of the cohort, excluding those treated with gentamicin. Similarly, those infected with gentamicin BCHR who received gentamicin showed higher odds for admittance to the intensive care unit (5·6 [1·1-42·0, p=0·043]) and mortality (7·1[1·2-49·2, p=0·030]) than patients treated with gentamicin who were infected with non-gentamicin BCHR Ecoli. In a cohort of patients with Ecoli bloodstream infections, heteroresistance is common and frequently misidentified in routine clinical testing. Several negative effects on patient outcomes are associated with heteroresistant strains. Wallenberg Foundation, Swedish Research Council, and US National Institutes Of Health.
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