Bactericidal activity of cefoperazone with CP-45,899 against large inocula of beta-lactamase-producing Haemophilus influenzae

Effects of various test media on the activities of 21 antimicrobial agents against Haemophilus influenzae

As considerable variation in the antimicrobial susceptibility of Haemophilus influenzae has been reported, the effects of various test media on the susceptibility of H. influenzae were studied. MICs were determined by three laboratories for 21 antimicrobial agents against a panel of 100 selected isolates. Testing was performed using a reference NCCLS frozen broth microdilution method with Haemophilus test medium (HTM) broth and dried commercial MIC trays rehydrated with the following media: in-house and commercially prepared HTM broth, Mueller-Hinton broth with 2% lysed horse blood and NAD, IsoSensitest broth with 2% lysed horse blood and NAD, and IsoSensitest broth-based HTM. Overall, all results were very reproducible, with the MIC at which 50% of the isolates tested are inhibited (MIC(50)), MIC(90), and geometric mean MIC being within one doubling dilution by all six methods and at all three testing centers for 15 of the 21 agents tested. Interlaboratory differences were more marked than intralaboratory differences or differences among media. Cefprozil, cefaclor, and trimethoprim-sulfamethoxazole results differed the most, while results for ampicillin, amoxicillin-clavulanic acid, cefdinir, cefixime, ceftriaxone, and clarithromycin were the most reproducible. However, these variations in results caused considerable differences in susceptibility rates for agents for which NCCLS susceptible breakpoints were close to the geometric mean MIC, particularly for cefaclor and cefprozil. This was much less of a problem when pharmacokinetic-pharmacodynamic breakpoints were used. Reproducible susceptibility results were obtained for a wide range of agents against H. influenzae in three laboratories using a variety of media that support the growth of this fastidious species.

Characterization of the inoculum effect with Haemophilus influenzae and beta-lactams

An inoculum effect is defined as a four-fold or greater increase in MIC with an increase in bacterial inocula. Haemophilus influenzae was tested for an inoculum effect with ampicillin, cefuroxime, and amoxicillin/clavulanate using the standard initial inocula (5 x 10(5) CFU/mL) and a higher initial inocula (1 x 10(7) CFU/mL). An inoculum effect was observed with both beta-lactamase (TEM-1, ROB-1) positive and beta-lactamase negative strains of H. influenzae when MICs were determined based on turbidity. MICs based on viable cell counts however, demonstrated that only beta-lactamase positive strains of H. influenzae produced an inoculum effect. These observations suggest that MICs determined based on turbidity, using high initial inocula, are not reliable when examining the inoculum effect in H. influenzae. The magnitude of the inoculum effect with beta-lactamase positive strains was beta-lactam dependent (ampicillin > amoxicillin/clavulanate > cefuroxime). beta-lactam kill-curves confirmed the aforementioned results. Addition of the beta-lactamase inhibitor clavulanate completely reversed the inoculum effect in beta-lactamase (TEM-1 and ROB-1) positive strains of H. influenzae with all beta-lactams tested. Introduction of the beta-lactamase gene TEM-1 on plasmid vector pLS88 into a beta-lactamase negative strain of H. influenzae (Rd) produced an inoculum effect based on viable cell counts. In conclusion, our results suggest that the beta-lactam inoculum effect demonstrated by H. influenzae is the result of beta-lactamase production and is poorly assessed by turbidity.

Evaluation of Haemophilus influenzae isolates with elevated MICs to amoxicillin/clavulanic acid

A 1994 to 1995 national Haemophilus influenzae surveillance study of 1910 strains showed that 13 strains (0.7%) were resistant to amoxicillin/clavulanic acid (MIC, > or = 8/4 micrograms/ml). These and other selected strains were investigated further in this study. Susceptibility of the surveillance study strains was determined with the commercial microdilution trays used in the original study and in triplicate with reference broth microdilution trays prepared by the investigators, as well as by Etest and disk diffusion. Amoxicillin/clavulanic acid resistance was confirmed for only one of the surveillance study strains. This strain produced double zones of growth with Etest and disk-diffusion methods, with the double zone containing spheroplasts. When the amoxicillin/clavulanic acid MICs of all beta-lactamase positive strains were compared, MIC results obtained with surveillance study trays and the Etest were one to two dilutions higher than MICs obtained with reference trays. The distribution and modal amoxicillin/clavulanic acid MICs of beta-lactamase-positive and -negative strains was essentially the same for a comparison group of strains using reference trays, in contrast to a fourfold higher modal MIC for beta-lactamase-positive strains using surveillance study reagents and strains. These differences in MICs could be attributed to variations in inoculum, the presence of spheroplasts, and/or a difference in potency of amoxicillin and/or clavulanic acid in the tray and Etest reagents used. Methods for assessing the adequacy of the clavulanic acid content are not adequate, amoxicillin control values and a beta-lactamase-positive H. influenzae control strain are required.

Efficacy of sulbactam/cefoperazone for the treatment of infections in patients with hematologic diseases

The efficacy and safety of sulbactam/cefoperazone (SBT/CPZ) was studied in 94 patients with severe infections and concomitant hematologic diseases. All of the study patients were included in the evaluation for safety, and 76 cases were evaluable for efficacy. Clinical efficacy was excellent in 13 cases (17.1%), good in 27 cases (35.5%), fair in seven cases (9.2%), and poor in 29 cases (38.2%). The bacteriologic eradication was 66.7% for Gram-negative bacilli and 50.0% for Gram-positive bacteria. The efficacy rate for neutropenic patients with counts less than 50 mm3 and 100 mm3 were 47.5 and 42.9%, respectively. Efficacy in patients for whom other antibiotic therapy before treatment with SBT/CPZ had been ineffective was 46.2%. Side effects were reported in one case (1.1%), and abnormal serum liver tests in five cases (5.3%); both returned to normal after discontinuation of the study medication. SBT/CPZ was an effective antibiotic for the treatment of severe infections in the presence of concurrent hematologic diseases.

In vitro antimicrobial spectrum, occurrence of synergy, and recommendations for dilution susceptibility testing concentrations of the cefoperazone-sulbactam combination

Broth microdilution tests and an antimicrobial interaction (synergy) studies using various combinations of cefoperazone and sulbactam were performed in an effort to determine the most appropriate in vitro dilution test system. The test results with cefoperazone and sulbactam were categorized as synergistic (complete or partial) for nearly 80% of the strains isolated from clinical trial patients. The results indicate that the cefoperazone-sulbactam fixed ratio (2:1) maximized the cefoperazone spectrum of activity and best approximated the parenteral formulation of the drug. The cefoperazone-sulbactam combination had a greater antimicrobial activity than did the other comparison beta-lactams, except for imipenem, tested against strains of the family Enterobacteriaceae. To be consistent with the National Committee for Clinical Laboratory Standards interpretive breakpoints for cefoperazone alone, the following MIC breakpoints should be applied to the combination (2:1 ratio): less than or equal to 16/8 micrograms/ml, susceptible; 32/16 micrograms/ml, moderately susceptible; and greater than or equal to 64/32 micrograms/ml, resistant.

Antimicrobial combinations in the therapy of infections due to gram-negative bacilli

Antimicrobial combinations are used most frequently to provide broad-spectrum coverage; however, they are also frequently employed to enhance antimicrobial activity (synergism). Although there is extensive in vitro documentation of synergism for many antibiotic combinations, a clear advantage for these combinations has been difficult to demonstrate in clinical studies. Several types of combinations have been useful in clinical medicine and frequently result in synergism. These include combinations of a cell wall-active agent with an aminoglycosidic aminocyclitol, combinations of a beta-lactamase inhibitor with a beta-lactam, and combinations of agents that inhibit sequential steps in a metabolic pathway. Given its spectrum of activity, aztreonam will often be used with clindamycin or a beta-lactam antibiotic. Combinations of beta-lactams may be synergistic via several mechanisms. However, these combinations also exhibit significant potential for antagonism when used against gram-negative bacilli and, therefore, require careful evaluation prior to clinical use.

Rationale for optimal dosing of beta-lactam antibiotics in therapy for bacterial meningitis

This review considers the five major principles governing optimal dosing of beta-lactam antibiotics in therapy for bacterial meningitis: off the entry of passage of antibiotics into CSF, (2) the antimicrobial activity of beta-lactams within the purulent CSF in vivo, (3) the bactericidal activity within the CSF, (4) the route and mode of drug administration together with the postantibiotic effect, and (5) the duration of therapy. Special attention is paid to the third principle, bactericidal activity within the CSF, employing the model of the newer, third-generation cephalosporins used in the treatment of meningitis caused by gram-negative aerobic bacilli.

Activities of newer beta-lactam antibiotics against ampicillin, chloramphenicol, or multiply-resistant Haemophilus influenzae

The susceptibilities of singly or multiply-resistant clinical isolates of Haemophilus influenzae were determined by agar dilution to aztreonam, imipenem, and six third-generation cephalosporins. These included selected isolates that were resistant to ampicillin only, chloramphenicol only, and four isolates that were resistant to ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole. All of the isolates were highly susceptible to these newer beta-lactam antibiotics. Isolates resistant to trimethoprim-sulfamethoxazole and/or chloramphenicol had susceptibilities similar to those of strains resistant only to ampicillin. Ceftriaxone, ceftizoxime, and cefotaxime were the most active of the study antibiotics (MIC90 = 0.004-0.016 micrograms/ml), and were also bactericidal at concentrations no more than twice the minimum inhibitory concentration (MIC). Minimum inhibitory concentrations of cefoperazone increased dramatically with only a 10-fold increase in inoculum size of beta-lactamase producing strains, while MICs of the other new agents were not significantly affected by elevation of the inoculum. These new antibiotics appear to be promising candidates for therapy of infections due to resistant H. influenzae.

Evaluation of aztreonam in experimental bacterial meningitis and cerebritis

Aztreonam (SQ 26,776), a new monocyclic beta-lactam agent, was compared with ampicillin, ampicillin plus chloramphenicol, and gentamicin in rabbits with experimental meningitis induced by, respectively, ampicillin-susceptible Haemophilus influenzae, ampicillin-resistant H. influenzae, and Escherichia coli. Aztreonam was also compared with gentamicin in experimentally induced E. coli cerebritis in rats. Doses of the various agents were delivered that produced near-peak concentrations in serum comparable to those attained in humans on standard parenteral regimens. The percent penetration [( concentration in cerebrospinal fluid/concentration in serum] X 100) of aztreonam into purulent rabbit cerebrospinal fluid was 23% (versus 12, 27, and 21%, respectively, for ampicillin, chloramphenicol, and gentamicin). In experimental meningitis in vivo, aztreonam was more rapidly bactericidal than was ampicillin in ampicillin-susceptible H. influenzae meningitis, ampicillin or chloramphenicol in ampicillin-resistant H. influenzae meningitis, or gentamicin in E. coli meningitis. In the therapy of experimental cerebritis, the early stage of brain abscess formation, aztreonam reduced the numbers of E. coli in rat brain as rapidly as did gentamicin. Aztreonam deserves further evaluation in acute gram-negative bacterial infections of the central nervous system in both experimental animals and in humans.

Effect of inoculum size on Haemophilus influenzae type b susceptibility to new and conventional antibiotics

Thirty-three Haemophilus influenzae type b isolates, including beta-lactamase acetyltransferase-positive strains, were tested by microtiter broth dilution for susceptibility to eight beta-lactam compounds and chloramphenicol. All antibiotics except ampicillin and chloramphenicol were highly bactericidal against all isolates at an inoculum of 10(5) CFU/ml. However, at an inoculum of 10(5) CFU/ml, the minimal bactericidal concentrations of all drugs except ceftriaxone were above levels usually achievable in cerebrospinal fluid. Results of time-kill studies confirmed this inoculum effect. In vivo studies are needed to test the clinical impact of these observations.

Chloramphenicol kills Haemophilus influenzae more rapidly than does ampicillin or cefamandole

The bactericidal effects of chloramphenicol and three beta-lactams (ampicillin, cefamandole, and penicillin G) were measured for 27 strains of Haemophilus influenzae type b isolated from the blood or cerebrospinal fluid of infected infants. Of the ampicillin-susceptible strains, 75% were killed by less than 2.0 micrograms of each antibiotic per ml; however, the concentration of the beta-lactam agents required for bactericidal activity was higher than that required for inhibitory activity. Chloramphenicol was the only agent which had no marked discrepancy between inhibitory and bactericidal concentrations regardless of beta-lactamase production. Importantly, chloramphenicol was more rapidly bactericidal than either ampicillin or cefamandole. The bactericidal requirement of ampicillin was increased by the presence of chloramphenicol for about one-third of the isolates examined. Neither the inhibitory nor the bactericidal activity of chloramphenicol was influenced by ampicillin. Synergy occurred for only two beta-lactamase-positive isolates. The more rapid bactericidal action of chloramphenicol persisted even in the presence of ampicillin. The rapid bactericidal action of chloramphenicol with or without ampicillin supports the use of chloramphenicol alone or with ampicillin for H. influenzae infections.

Comparison of cefoperazone with penicillin, ampicillin, gentamicin, and chloramphenicol in the therapy of experimental meningitis

Cefoperazone was compared with penicillin against Streptococcus pneumoniae, gentamicin against Escherichia coli, and ampicillin and chloramphenicol against Haemophilus influenzae in the therapy of experimental meningitis in rabbits. Meningitis was produced by intracisternal inoculation into cerebrospinal fluid, and all antibiotics were administered intravenously over 8 h in dosages that would achieve serum levels comparable to those found in humans. The mean percent penetration into purulent cerebrospinal fluid, expressed as (cerebrospinal fluid concentration/serum concentration) x 100%, was 2.6% for penicillin, 22.0% for gentamicin, 12.1% for ampicillin, 23.8% for chloramphenicol, and 6.4% for cefoperazone. The mean cerebrospinal fluid antibiotic concentrations exceeded the minimum bactericidal concentration for the test strain in each experimental model, except for ampicillin in experimental meningitis due to the beta-lactamase-producing H. influenzae. Cefoperazone produced a significantly faster bactericidal effect after 4 h of treatment when compared with penicillin (P = 0.037) and ampicillin (P = 0.01) in meningitis caused by S. pneumoniae and H. influenzae (ampicillin susceptible), respectively. In meningitis caused by E. coli, cefoperazone was significantly (P = 0.006) more rapidly bactericidal after 8 h of treatment when compared to gentamicin. In addition, cefoperazone was significantly more rapidly bactericidal than either ampicillin or chloramphenicol in experimental meningitis due to beta-lactamase-producing H. influenzae. Cefoperazone deserves further evaluation in the therapy of bacterial meningitis in humans.

Antimicrobial activity, pharmacokinetics, adverse reactions, and therapeutic indications of cefoperazone

Cefoperazone is a parenteral cephalosporin antibiotic that is pending approval by the U. S. Food and Drug Administration. Compared to most other cephalosporins cefoperazone has a greatly expanded spectrum of bactericidal activity that encompasses most aerobic gram-positive bacteria except enterococci, most aerobic gram-negative bacteria, including a majority of Pseudomonas aeruginosa strains, and a number of pathogenic anaerobic bacteria. Its long serum half-life, approximately two hours, permits a twelve hourly dosing schedule. No dosage modification is required in patients with renal insufficiency, and only minor modification is needed in patients with hepatic insufficiency or biliary obstruction. Clinical trials have established cefoperazone's efficacy in lower respiratory tract infections, urinary tract infections, and a variety of other bacterial infections. Adverse reactions have been infrequent, and few serious reactions have been identified. Cefoperazone is a promising new agent for the treatment of gram-negative bacillary and polymicrobial infections, especially in settings that require empiric therapy.