Discrepancy between carbenicillin and ampicillin activities against enterococci and Listeria

For infections of the newborn, carbenicillin has theoretical advantages over ampicillin, when given with an aminoglycoside, because of its activity against Pseudomonas and many indole-positive Proteus strains. Agar and broth dilution susceptibility studies demonstrated comparable activities of carbenicillin and ampicillin against coliform organisms but greater resistance of enterococci and Listeria to carbenicillin. However, the significance of the higher inhibitory concentrations is questionable because many serum specimens containing concentrations of carbenicillin lower than the minimal bactericidal values for Listeria and enterococcus strains had demonstrable in vitro bactericidal activity. Carbenicillin and gentamicin combinations had greater antibacterial activities than either drug alone against all enterococci tested and against 50% of Listeria strains. The questions of efficacy raised by contradictory in vitro data may be answered by clinical studies of carbenicillin and gentamicin in infections of the newborn.