Antagonism by chloramphenicol of broad-spectrum beta-lactam antibiotics against Klebsiella pneumoniae.

Chloramphenicol combined with cefotaxime, moxalactam, cefoperazone, aztreonam, or imipenem was tested in vitro against clinical isolates of Klebsiella pneumoniae. By time-kill cultures (killing curves), chloramphenicol interfered with activity of all five beta-lactams. When chloramphenicol was added before the beta-lactams, the action of cefotaxime, moxalactam, or cefoperazone against all isolates was antagonized at all times tested. The action of aztreonam was antagonized against four of six isolates. With imipenem, antagonism occurred against half of the isolates at some time during 24 h when chloramphenicol was added simultaneously, provided that a sufficient inoculum of K. pneumoniae was employed. Generally, less antagonism resulted when chloramphenicol was added after the cephalosporins. Interference of bactericidal activity of three new cephalosporins by chloramphenicol has potential clinical relevance to the therapy of gram-negative bacillary meningitis. The lesser antagonism of aztreonam and imipenem by chloramphenicol is of uncertain clinical relevance but indicates that this in vitro phenomenon may apply to a wide range of beta-lactam antibiotics.