Standardization of isothermal microcalorimetry in urinary tract infection detection by using artificial urine

Isothermal microcalorimetry (IMC) has recently been reported as a new method to rapidly detect urinary tract pathogens (UTP). However, further application of microcalorimetry in the clinical setting requires a standardized procedure. An important step toward such standardization is to use a reproducible growth medium. In this study, we investigated the potential of artificial urine in combination with microcalorimetry for detection of common UTP.

A microcalorimeter equipped with 48 channels was used. Detection was accomplished, and growth was monitored for four bacterial strains in artificial 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.

Bacterial growth was detected after 3-32 h with decreasing inoculums down to 1 CFU. The gram-negative strains grew and were detected faster than their gram-positive counterparts. The growth rates the different strains were 0.75 ± 0.11 for E. coli, 0.74 ± 0.10 for E. faecalis, 1.31 ± 0.04 for P. mirabilis, and 0.56 ± 0.20 for S. aureus. The shape of individual heat flow curves was characteristic for each species independent of its initial concentration.

IMC allows rapid detection of UTP in artificial urine. Clearly, different heat flow patterns enable accurate pathogen differentiation. UTP detection after only 4 h is realistic. The rapid detection of UTP tested in standardized artificial urine proves the diagnostic potential of IMC and warrants further microcalorimetric studies in the clinical setting of urinary tract infections.