Antibiotic susceptibility of Gram-negative bacteria in cerebrospinal fluid compared to Mueller-Hinton broth and artificial cerebrospinal fluid.

INTRODUCTION Antibiotic resistance of Gram-negative bacteria has become an increasing problem in the therapy of critically ill patients, particularly in central nervous system (CNS) infections. In clinical routine, minimal inhibitory (MICs) and minimal bactericidal concentrations (MBCs) usually are determined in standard test media, not in respective body fluids.

METHODS MICs and MBCs were tested in human cerebrospinal fluid (CSF), Mueller-Hinton broth (MHB) and artificial cerebrospinal fluid (A-CSF) by microdilution for 24h at 37°C in room air supplemented with 5% CO2.

RESULTS Cefotaxime, ceftazidime and ciprofloxacin inhibited the growth of Pseudomonas aeruginosa, cefotaxime, ceftazidime and amikacin the growth of Morganella morganii, and amikacin the growth of Acinetobacter baumannii in the nutrient-deficient medium CSF at lower concentrations than the MICs determined in MHB. The differences in MIC between MHB and A-CSF were less pronounced than between MHB and human CSF.

CONCLUSION In CNS infections, when antimicrobial susceptibility testing in standard media does not result in adequate therapeutic options, we suggest to test the antimicrobial susceptibility of the causative bacterium in human CSF.