Rapid detection of urinary tract pathogens using microcalorimetry: principle, technique and first results

Abstract

What's known on the subject? and What does the study add? Microcalorimetry has been applied in several microbiological studies, but never in a clinical urological context. In addition, basic knowledge on the growth of urinary pathogens in urine is still scarce and data regarding the growth rate of many urinary pathogens in urine are still not available. The study demonstrates that this innovative application of microcalorimetry is useful in (i) investigating the growth of urinary pathogens in sterilized urine and (ii) as a rapid tool for diagnosis of urinary infection as well as for further identification of the causative infectious agent.

Objective: To investigate the value of isothermal microcalorimetry (IMC) in the detection and differentiation of common urinary tract pathogens in urine. IMC is a non-specific analytical tool for the measurement of heat in the microwatt range.

Patients and methods: A microcalorimeter equipped with 48 channels was used. Detection was accomplished, and growth was monitored for four bacterial strains in sterilized urine at 37 °C by measuring metabolic heat flow (µW = µJ/s) as a function of time. The strains were Escherichia coli, Proteus mirabilis, Enterococcus faecalis and Staphylococcus aureus.

Results: Bacterial growth was detected after 3.1 to 17.1 h with decreasing inocula. The detection limit was 1 colony-forming unit (CFU)/mL for E. coli, 10 CFU/mL for P. mirabilis and E. faecalis and 10(3) CFU/mL for S. aureus. The total heat was highest in P. mirabilis ranging from 10 to 12 J, followed by E. coli (3-4 J), S. aureus (2-3 J) and E. faecalis (1.3-1.5 J). The shape of the heat flow curves was characteristic for each species independent of its initial concentration.

Conclusions: IMC allows rapid detection of bacteriuria, much faster than conventional culture. Urinary tract pathogen detection after only 3.1 h is realistic. Clearly different heat flow patterns enable accurate pathogen differentiation. Due to expeditious identification of urine samples that contain only low colony counts (i.e. less than 10(3) CFU/mL), IMC may become a valuable screening tool for detecting the presence of significant bacteriuria.