Rapid diagnosis of experimental meningitis by bacterial heat production in cerebrospinal fluid

Background: Calorimetry is a nonspecific technique which allows direct measurement of heat generated by biological processes in the living cell. We evaluated the potential of calorimetry for rapid detection of bacterial growth in cerebrospinal fluid (CSF) in a rat model of bacterial meningitis. Methods: Infant rats were infected on postnatal day 11 by direct intracisternal injection with either Streptococcus pneumoniae, Neisseria meningitidis or Listeria monocytogenes. Control animals were injected with sterile saline or heat-inactivated S. pneumoniae. CSF was obtained at 18 hours after infection for quantitative cultures and heat flow measurement. For calorimetry, 10 mu l and 1 mu l CSF were inoculated in calorimetry ampoules containing 3 ml trypticase soy broth ( TSB). Results: The mean bacterial titer (+/- SD) in CSF was 1.5 +/- 0.6 x 10(8) for S. pneumoniae, 1.3 +/- 0.3 x 106 for N. meningitidis and 3.5 +/- 2.2 x 10(4) for L. monocytogenes. Calorimetric detection time was defined as the time until heat flow signal exceeded 10 mu W. Heat signal was detected in 10- mu l CSF samples from all infected animals with a mean (+/- SD) detection time of 1.5 +/- 0.2 hours for S. pneumoniae, 3.9 +/- 0.7 hours for N. meningitidis and 9.1 +/- 0.5 hours for L. monocytogenes. CSF samples from non-infected animals generated no increasing heat flow (< 10 mu W). The total heat was the highest in S. pneumoniae ranging from 6.7 to 7.5 Joules, followed by L. monocytogenes (5.6 to 6.1 Joules) and N. meningitidis (3.5 to 4.4 Joules). The lowest detectable bacterial titer by calorimetry was 2 cfu for S. pneumoniae, 4 cfu for N. meningitidis and 7 cfu for L. monocytogenes. Conclusion: By means of calorimetry, detection times of < 4 hours for S. pneumoniae and N. meningitidis and < 10 hours for Listeria monocytogenes using as little as 10 mu l CSF were achieved. Calorimetry is a new diagnostic method allowing rapid and accurate diagnosis of bacterial meningitis from a small volume of CSF.