In vitro experiments on catheter-related infections due to gram-negative rods

Although many complications may arise with the use of central venous catheters, catheter-related bacteremia is considered to be the most serious complication. Microflora on the patient's skin (coagulase-negative staphylococci, Staphylococcus aureus) is thought to represent the major source of microorganisms causing catheter-related infections. Gram-negative rods are increasingly observed as causative agents. We therefore performed an in vitro study to elucidate the pathogenesis of polymer-associated infections caused by enterobacteriaceae and nonfermenters and to study the resistance of polymer surface-grown gram-negative rods against antibiotics. Using a modification of the semiquantitative method for culturing vascular cannulas on solid media described by Maki et al., we studied the adherence of various gram-negative rods to 1-cm segments of silastic catheters, a material used for Port-A-Cath-systems. Organisms were obtained from the American Type Culture Collection (Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, Pseudomonas aeruginosa, Enterobacter cloacae). All organisms but Klebsiella showed extensive irreversible adherence. Production of an extracellular slime substance was observed only with antimicrobially treated Pseudomonas. After 'infection', the silastic catheter segments were treated with various antimicrobial agents. The following antibiotics were used either individually or in combination: cefotaxime, ciprofloxacin, imipenem, fosfomycin and gentamicin. Despite the fact that the antibiotic concentrations used were many times greater than the respective MICs of the various antibiotics, none of the antibiotics tested succeeded in eliminating the organisms. E. coli was found to be the most susceptible organism. These in vitro data substantiate our clinical impression that it is hardly possible to successfully eliminate the colonization of central venous catheters with gram-negative rods by using the antimicrobial agents employed here.