In vitro effect of combinations of ampicillin and chloramphenicol on ampicillin sensitive and resistant haemophilus influenzae
Khurana, C.M.; Deddish, P.A.
Current Therapeutic Research 44(5): 759-765
ISSN/ISBN: 0011-393X Accession: 007438669
Five ampicillin-resistant (Amp-R) strains and one ampicillin-sensitive (Amp-S) strain of Haemophilus influenzae were tested against combinations of ampicillin (Amp) and chloramphenicol (Chlor) by a checkerboard dilution method. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined for each organism with drug combinations. Chlor alone was bactericidal for the H. influenzae strains at concentrations of 5-10 .mu.g/ml. MIC determinations showed effects ranging from indifference to synergisim between Amp and Chlor depending on the tested strain. No antagonism was seen. MBC determinations indicated that low levels of Chlor (< 2.5 .mu.g/ml) antagonized the bactericidal effect of Amp, but above this level, the antagonistic effect disappeared. Since cerebrospinal fluid levels of Chlor, even at 25 mg/kg/dose, generally exceed 2.5 .mu.g/ml, it would appear that initial therapy with Chlor and Amp combinations would be as effective as Chlor alone. Based on recent reports of clinical isolation of Chlor-R H. influenzae initial treatment with Chlor-Amp combinations would also provide coverage of either Amp-R or Chlor-R H. influenzae until specific sensitivities could be determined.