Minimum bactericidal concentration of sulfamethoxazole-trimethoprim for Haemophilus influenzae: correlation with prophylaxis.

The inability of sulfamethoxazole-trimethoprim (SXT) to eradicate Haemophilus influenzae nasopharyngeal carriage in all asymptomatic patients in closed populations was examined in vitro. A broth medium was adapted for susceptibility testing of H. influenzae which permitted us to determine minimum inhibitory concentrations and minimum bactericidal concentrations (MBCs). The minimum inhibitory concentrations were all low, but the MBCs were bimodally distributed. Trimethoprim alone or the combination SXT either was bactericidal for H. influenzae isolates at low concentrations (i.e., low MBCs) similar to minimum inhibitory concentrations or showed no bactericidal activity (i.e., high MBCs). If trimethoprim was bactericidal when tested alone against H. influenzae, then the combination SXT was also bactericidal. H. influenzae carriage could not be eradicated from asymptomatic patients with SXT therapy when that combination was not bactericidal for these isolates in vitro. H. influenzae carriage was eradicated from patients when the activity of SXT was bactericidal in vitro. H. influenzae strains that are not killed by trimethoprim or SXT seem to occur at random.