Influence of pulmonary surfactant on in vitro bactericidal activities of amoxicillin, ceftazidime, and tobramycin

The influence of a natural pulmonary surfactant on antibiotic activity was investigated to assess the possible use of exogenous surfactant as a vehicle for antibiotic delivery to the lung. The influence of surfactant on the bactericidal activity of amoxicillin was tested against Staphylococcus aureus and Streptococcus pneumoniae, and the influence of surfactant on the activities of ceftazidime and tobramycin was tested against Klebsiella pneumoniae, Pseudomonas aeruginosa, S. aureus, and S. pneumoniae. In vitro antibiotic activity was determined by killing curve studies in media with and without surfactant. Amoxicillin and ceftazidime activities were not changed in the presence of surfactant, except for a decreased killing rate of S. pneumoniae by ceftazidime in medium with additional rabbit serum. In contrast, killing curves with low concentrations of tobramycin (0.25 x and 1x the MIC) showed a decreased level of activity of tobramycin against all pathogens tested in the presence of surfactant. With higher tobramycin concentrations (4x the MIC) killing rates were decreased less or were unchanged in the presence of surfactant. Concluding from the results of the study, both amoxicillin and ceftazidime can be combined with surfactant without the loss of activity. For mixing surfactant with tobramycin, dosages should be adjusted to overcome the partial inactivation of tobramycin by surfactant.