Success of DNA fingerprinting after failure of biotyping, antimicrobial susceptibility testing, and plasmid analysis to reveal clonality of multiple blood and urine isolates from a patient with Escherichia coli urosepsis

Multiple isolates of Escherichia coli from the blood and urine of a 60-year-old woman with acute pyelonephritis exhibited different biotypes, antimicrobial susceptibility patterns, and plasmid profiles, suggesting the presence of polymicrobial bacteriuria and leaving in question the origin of the bacteremia. Only after bacterial restriction endonuclease analysis of total bacterial DNA was it discovered that all isolates represented the same strain, with plasmid instability possibly accounting for the varied antimicrobial susceptibility patterns observed. We conclude that the biotype, antimicrobial susceptibility profile, and plasmid profile are sometimes inadequate to clarify the relationships between different clinical isolates of E. coli from a single patient and can lead to erroneous epidemiologic conclusions. DNA fingerprinting can resolve dilemmas these less precise techniques leave unresolved.