Pseudomonas maltophilia is resistant to most of the commonly used antimicrobial agents including those active against Pseudomonas aeruginosa. The susceptibility of 14 clinical isolates of P. maltophilia to 18 antimicrobial agents was determined by broth dilution testing. All organisms were susceptible to trimethoprim-sulfamethoxazole (TMP-SMZ), minocycline, and LY127935. A total of 87 and 79% of the organisms were susceptible in vitro to colistin and chloramphenicol, respectively. With the exception of sisomicin, the organisms were resistant to the aminoglycosides. Of 21 combinations of antimicrobials examined for synergy, only the combination of TMP-SMZ with carbenicillin was consistently (86%) synergistic in vitro. Supplementation of the testing media with calcium and magnesium increased the miniimal inhibitory concentrations for the aminoglycosides, the penicillins, and TMP-SMZ against P. maltophilia.
Combinations of two sets of antibiotics (gentamicin-carbenicillin-rifampicin and trimethoprim/sulphamethoxazole-carbenicillin-rifampicin) were tested against isolates of Pseudomonas maltophilia. An abbreviated checker-board method permitted tests with combinations of all three antibiotics and of each of the possible three pairs in each set. Triple combinations were usually synergistic, whereas 25% of combinations of pairs were antagonistic or additive. The modal degree of synergy was greater with combinations of three antibiotics than with combinations of any two. For any isolate, greater synergy was generally obtained with a triple than with a double combination, and the mean degree of synergy was always greater with triple than with double combinations. It was advantageous to add to double combinations even an antibiotic to which the organism was resistant. Organisms multiply resistant to three antibiotics were also resistant to 25% of combinations of pairs, but never resistant to combinations of all three. Synergy was more likely to be present at all antibiotic ratios with triple than with double combinations. Since ratios at the site of action may differ greatly from that of the administered combination, there would be substantial clinical benefit in identifying sets of antibiotics of which combinations at all ratios showed synergy. The search for these may entail exploring multicomponent combinations and, with conventional methods, this raises a considerable logistic problem. A strategy is therefore proposed, designed to find antibiotic sets showing synergy at all ratios.
An abbreviated three-dimensional checkerboard titration method was devised to determine whether synergistic interaction of three antimicrobial agents could be found against multidrug-resistant bacteria. Pseudomonas maltophilia was used as the test organism because of its resistance to most commercially available antimicrobial agents, including those active against Pseudomonas aeruginosa. Three-dimensional isobolograms with concave surfaces were formed when synergy occurred. Triple combinations of gentamicin-carbenicillin-rifampin (mean fractional inhibitory concentration index, 0.32) and trimethoprim/sulfamethoxazole-carbenicillin-rifampin (mean fractional inhibitory concentration index, 0.18) were consistently synergistic against 14 clinical isolates of P. maltophilia.
We report a case of peritonitis in a continuous ambulatory peritoneal dialysis patient with an unusual bacterium known as group Ve-2. This is the first reported case of peritonitis attributable to this organism and only the second well-documented case of infection with this organism in the English literature.
In 12 of 20 (60%) biopsy-proven cases of Pneumocystis carinii pneumonia, the diagnosis was first suggested by examination of routine Gram stains of impression smears made from infected lung tissue and later confirmed by methenamine-silver staining. The cysts appeared as 5- to 7-microns unstained spheres, each containing six to eight intracystic gram-negative bodies (sporozoites). Although the Gram stain does not appear to be as sensitive as more traditional staining techniques for the detection of P. carinii, clinical microbiologists should be aware of the morphology of this organism in gram-stained specimens because this relatively simple procedure gives quick results.
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