In vitro activities of combinations of aztreonam, ciprofloxacin, and ceftazidime against clinical isolates of Pseudomonas aeruginosa and Pseudomonas cepacia from patients with cystic fibrosis

Abstract

The in vitro activities of two-drug combinations of aztreonam, ciprofloxacin, and ceftazidime were studied in 96 clinical isolates of Pseudomonas aeruginosa and in 20 clinical isolates of Pseudomonas cepacia from cystic fibrosis patients. Some synergy was observed with each combination used against P. aeruginosa, but synergy was rare when the combinations were used against P. cepacia.Pulmonary exacerbations of cystic fibrosis associated with antibiotic-resistant isolates of Pseudomonas aeruginosa and Pseudomonas cepacia continue to present a clinical problem in the care of affected patients despite the continuing introduction of new antipseudomonal agents. The concomitant use of two or more antibiotics is often necessary, especially in patients infected with multiple strains of P. aeruginosa or with both P. aeruginosa and P. cepacia. The objective of the present study was to explore the in vitro synergy of two-drug combinations of aztreonam, ciprofloxacin, and ceftazidime used against clinical isolates of these two pathogens.Ninety-six isolates of P. aeruginosa and 20 isolates of P. cepacia cultured from the sputa of cystic fibrosis patients at the University of Utah Intermountain Cystic Fibrosis Center were studied. All isolates were identified by standard microbiological methods and stored at room temperature on tryptic soy agar slants without glucose or were lyophilized and frozen until the time of the study. At the time of the study, all isolates were transferred to blood agar plates and checked for purity. They were then transferred to Trypticase soy broth (BBL Microbiology Systems, Cockeysville, Md.) and incubated overnight at 35°C. The turbidity of each bacterial broth suspension was then standardized to a 0.5 McFarland standard. From each of these suspensions, 0.1 ml was transferred to 9.9 ml of Mueller-Hinton broth which was supplemented with calcium and magnesium (final concentrations, 50 and 25 mg/ml, respectively), yielding a 1:100 dilution.The antibiotics studied were supplied as follows: aztreonam, E. R. Squibb & Sons, Princeton, N.J.; ciprofloxacin, Miles Pharmaceuticals, West Haven, Conn.; and ceftazidime, Glaxo, Inc., Research Triangle Park, N.C. Stock solutions were prepared from dry powders and used immediately or frozen at -32°C. All frozen solutions were used within 3 months of preparation. The antibiotic concentrations tested were twofold dilutions ranging from 1,024 to 0.0156 ,ug/ml. MICs were determined by broth microdilution. Microdilution plates (Dynatech Laboratories, Inc., Alexandria, Va.) were prepared with the antibiotic solutions (100 ,ul per well) and either used immediately or stored in * Corresponding author. 487 sealed plastic bags at -32°C for up to 1 month before use. Ten microliters of the 1:100 bacterial broth dilution was added to each microdilution well with an inoculator (Dynatech MIC-2000), producing a final inoculum of .5 x 105. The plates were resealed in plastic bags and incubated overnight at 35°C. Synergy was defined as a fourfold or greater reduction of the MIC...

Influence of High Mutation Rates on the Mechanisms and Dynamics of In Vitro and In Vivo Resistance Development to Single or Combined Antipseudomonal Agents

“…The opposite effect was documented for mice infected by PAO⌬mutS (a 2.6-log reduction of the total bacterial load compared to the bacterial load after CIP monotherapy was documented [P Ͻ 0.01]) (Table 4), although the reasons for this strain-specific synergy are clear: the high bacterial load of PAO⌬mutS-infected mice treated with CIP was due to the strong selection of resistant mutants, which is prevented (see below) by the combined regimen even if there is a tendency towards antagonism in terms of mortality rates. In vitro synergy between CAZ and CIP has been documented for approximately 30% of P. aeruginosa clinical isolates in various studies, and although synergy was generally observed in strains resistant to one or both antibiotics and not in those susceptible to both agents, antagonism was not reported to occur in any of the strains tested (2,25). Since the documented tendency towards antagonism did not reach statistical significance, further studies are needed to determine whether this observation may have any consequences in the selection of combined regimens for the treatment of chronic P. aeruginosa respiratory infections.…”

Routine susceptibility testing of four antibiotic combinations for improvement of laboratory guide to therapy of cystic fibrosis infections caused by Pseudomonas aeruginosa

“…Furthermore, the correlation between the two methods has not always been perfect; one explanation for this is that the definition of synergy is method dependent (21). Besides, no study in the field of CF pseudomonas infection has ever made a clear in vitro-in vivo correlation (2,8,10,14,16), but physicians still strongly rely on susceptibility testing for their therapeutic choices.…”

In vitro Synergy Studies Using Aztreonam and Fluoroquinolone Combinations against Six Species of Gram-Negative Bacilli

“…The FIC index was calculated by summing the individual FICs obtained for the two antimicrobial agents. Synergy was defined as an FIC index of ^0.5, additivity by an FIC index of 1 0.5 to ^4 and antagonism by an FIC index of 1 4 [10].…”

“…There are also published reports describing synergy against gram-negative isolates when aztreonam was tested in combination with either ciprofloxacin or levofloxacin [10][11][12][13]. As physicians seek effective antimicrobials for serious or complicated gramnegative infections, in vitro synergy studies may assist in the evaluation of new combinations.…”