Differences in drug susceptibility between isolates of Pseudomonas cepacia recovered from patients with cystic fibrosis and other sources and its relationship to beta-lactamase focusing pattern

Salicylate-inducible antibiotic resistance in Pseudomonas cepacia associated with absence of a pore-forming outer membrane protein

The most common mechanism of antibiotic resistance in multiply resistant Pseudomonas cepacia is decreased porin-mediated outer membrane permeability. In some gram-negative organisms this form of antibiotic resistance can be induced by growth in the presence of weak acids, such as salicylates, which suppress porin synthesis. To determine the effects of salicylates on outer membrane permeability of P. cepacia, a susceptible laboratory strain, 249-2, was grown in 10 mM sodium salicylate. Antibiotic susceptibility and uptake, as well as outer membrane protein patterns, were compared between strain 249-2 grown with and without salicylates. The MICs of chloramphenicol, trimethoprim, ciprofloxacin, and ceftazidime were compared between organisms grown in standard and salicylate-containing medium and are as follows: chloramphenicol, 12.5 versus 100 micrograms/ml; trimethoprim, 0.78 versus 3.125 micrograms/ml; ciprofloxacin, 0.4 versus 1.56 micrograms/ml; ceftazidime, 3.125 versus 3.125 micrograms/ml. The permeability of beta-lactam antibiotics was calculated from the rate of hydrolysis of the chromogenic cephalosporin, PADAC. There was no significant difference between strains grown in the presence and absence of salicylate. By using high-pressure liquid chromatography quantitation of loss from culture medium, the effect of 10 mM salicylate on the cellular permeability of chloramphenicol was measured in strain 249-2 by introduction of a plasmid which encodes production of chloramphenicol acetyltransferase. After 1 h of incubation, 18.5% +/- 1.54% versus 70.1% +/- 3.52%, and after 2 h, 4.20% +/- 1.65% versus 41.90% +/- 2.16% remained in supernatants from organisms grown in the absence and presence of 10 mM salicylate, respectively. Outer membrane protein pattern analysis demonstrated the absence of a protein of apparent molecular weight of 40,000 when strain 249-2 was grown in the presence of 10 mM salicylate. To determine whether this protein functioned as a porin, reconstituted membrane vesicles were constructed to assess antibiotic permeability. Vesicles constructed with this salicylate-suppressible outer membrane protein (OpcS) were permeable to chloramphenicol but not to penicillin G. These findings suggest that OpcS is a selective, antibiotic-permeable porin which can be suppressed by growth in the presence of salicylate. Further investigation will be required to determine the biochemical effects of salicylate on porin synthesis.

Inducible beta-lactamases in clinical isolates of non-aeruginosa Pseudomonas

The incidence of antimicrobial resistance and expression of imipenem-inducible beta-lactamase were examined in 22 strains of non-aeruginosa Pseudomonas isolated from clinical specimens. The percentage of strains resistant to form one to eight antibiotics was 45. The most active antibiotics against all strains were norfloxacin, ciprofloxacin and imipenem. Eighteen out of the 22 strains were positive for beta-lactamase in a spectrophotometric assay using nitrocefin as substrate. A low inducible beta-lactamase specific activity (0.001-0.999 nmoles nitrocefin hydrolyzed/min/mg protein) was found in twelve strains whereas six strains had a relatively high specific activity (3.5-159.8 nmoles nitrocefin hydrolyzed/min/mg protein). Five strains representing different Pseudomonas spp. and showing high beta-lactamase activity were studied further. Crude enzymes from two species (Pseudomonas mendocina, Pseudomonas acidovorans) hydrolyzed cefazolin at a higher rate than penicillin and ampicillin. All enzymes from the five species were inhibited by cloxacillin and p-chloromercuribenzoate (1 mM), but were insensitive to inhibition by clavulanic acid, ethylenediamine acetic acid (EDTA) at the same concentration. The isoelectric point and molecular weight of the main beta-lactamase band from the 5 species were 6.5-6.8 and 47,000 respectively.

Subgrouping of Pseudomonas cepacia by cellular fatty acid composition

The cellular fatty acid compositions were determined for 42 strains of Pseudomonas cepacia from five cystic fibrosis centers in North America. All isolates contained significant (20%) amounts of hexadecanoic (C16:0), and cis-9 hexadecenoic (C16:1 cis9) acids and an isomer of octadecenoic acid (C18:1). None had hydroxy acids containing fewer than 14 carbon atoms. The quantitative data from the fatty acid analysis were highly reproducible and provided a basis for numerical analysis. Five subgroups comprising all the strains were obtained by cluster analysis and further characterized by principal-component analysis. With minor exceptions, the predominant subgroup identified in each center was different from that identified in other centers and accounted for one-half of the isolates within each center. Cellular fatty acid composition is a useful adjunct to biochemical characterization for the identification of P. cepacia isolated from cystic fibrosis patients. Numerical analysis of the fatty acid data can separate P. cepacia into subgroups, which may provide useful epidemiologic information or a basis for further analysis by more complex techniques such as DNA probe analysis.

Synergy of new C-3 substituted cephalosporins and tobramycin against Pseudomonas aeruginosa and Pseudomonas cepacia

The effects of the combination of E-1040, a new cephalosporin, ceftazidime, cefpirome, or cefepime with tobramycin against 40 Pseudomonas aeruginosa and 16 P. cepacia isolates from cystic fibrosis patients were examined. Synergy fractional inhibitory concentration less than or equal to 0.5 was found against P. aeruginosa when tobramycin was combined with E-1040 32%, cefpirome 32%, cefepime 22%, and ceftazidime 22%. Fifty-two percent of isolates showed an additive response for E-1040, 60% for cefpirome, 45% cefepime, and 55% ceftazidime, p greater than 0.05. The differences were not statistically significant. Synergy was not more likely to be achieved if the isolates were susceptible or resistant to either the aminoglycoside or cephalosporin. None of the E-1040-resistant isolates, all of which were tobramycin-resistant, became susceptible when tested with an aminoglycoside, whereas 15 of 34 cefpirome-resistant, 13 of 30 cefepime-resistant, and seven of 14 ceftazidime-resistant isolates became susceptible. Synergy of aminoglycoside and the cephalosporins against P. cepacia was found for 25% with E-1040, 44% with cefpirome, 38% with cefepime, and 31% with ceftazidime. These differences were not statistically significant.

Outer membrane permeability in Pseudomonas cepacia: diminished porin content in a beta-lactam-resistant mutant and in resistant cystic fibrosis isolates

Since beta-lactam resistance is a feature of Pseudomonas cepacia isolates causing pulmonary infections in cystic fibrosis (CF), this study was undertaken to determine whether alterations in beta-lactam permeability mediate drug resistance in this species. A beta-lactam-susceptible non-CF isolate (strain 75-26), a resistant mutant derived from 75-26 by selection for cross-resistance to ciprofloxacin and ceftazidime, and two resistant CF isolates of P. cepacia were used. Permeability constants were calculated from the rate of nitrocefin hydrolysis in intact bacterial cells. Qualitative changes in outer membrane proteins were determined electrophoretically. The permeability constants of the mutant and the resistant CF isolates were lower than the value for the reference strain, 75-26. Whereas the lipopolysaccharide side chains were present in the test and reference strains, the resistant mutant and the CF isolates contained reduced amounts of the 36-kilodalton (kDa) outer membrane protein and failed to express the 27-kDa outer membrane protein. These observations suggest that the 27-kDa outer membrane protein may be a major porin or a major protein component of the porin complex in P. cepacia and that decreased expression of the 36-kDa outer membrane and loss of the 27-kDa porin are associated with high-level beta-lactam resistance in some CF isolates of P. cepacia.

Derepressed beta-lactamase production as a mediator of high-level beta-lactam resistance in Pseudomonas cepacia

A mechanism of high-level resistance to readily and poorly hydrolyzable beta-lactam substrates in Pseudomonas cepacia was identified using a hypersusceptible, beta-lactamase-inducible, non-CF clinical isolate, 75-26, and a drug-resistant mutant derived from this strain. Inoculation of 75-26 onto agar containing 16 micrograms/ml of ceftazidime produced a stable, beta-lactam-resistant mutant at a frequency of 1.7 x 10(-5). Baseline beta-lactamase production by a representative mutant isolate (75-26z) was almost 40-fold greater than the parent. Both strains produced major beta-lactamase bands with isoelectric points of 6.9, 7.8, 8.1, 8.5, and 9.2 by isoelectric focusing. Compared with the parental strain, multiple satellite bands associated with the major beta-lactamase bands were present in the mutant. Growth of an indicator strain, E. coli ATCC 25922, was inhibited by ceftazidime and piperacillin but was not inhibited after the compounds were preincubated with the beta-lactamase preparation from 75-26z. Preincubation of ceftazidime with beta-lactamase, followed by the addition of a beta-lactamase inhibitor, inhibited the growth of the indicator strain; piperacillin failed to inhibit growth of the indicator strain in a similar experiment. One mechanism of high-level resistance to both poorly and readily hydrolyzable beta-lactam substrates in P. cepacia is derepressed chromosomal beta-lactamase production.