In Vitro Additive Effect of Polymyxin B and Rifampin Against Serratia marcescens

Abstract

The combination of polymyxin B and rifampin resulted in an additive effect against all 12 clinical isolates of Serratia marcescens examined, including multiple-drug-resistant isolates.Very recently, Perez Urena et al. (5) demonstrated that the combination of rifampin and polymyxin B was synergistically effective against all strains of Proteus vulgaris, P. mirabilis, and Pseudomonas aeruginosa examined. As we had previously encountered several clinical isolates of Serratia marcescens (bacteriocin type 18) that proved resistant against all currently available antibacterial drugs, including gentamicin, tobramycin, nalidixic acid, and cotrimoxazole, it was of interest to determine whether the above combination of antibiotics was similarly effective. Furthermore, we wished to determine whether amikacin (BB-K8), a semisynthetic derivative of kanamycin (6), was effective against these problem isolates, since it had previously been shown that this latter drug, at a concentration of 6.25 ,ug/ml or less, inhibited 48 of 50 isolates of S. marcescens (2). (ii) Media. Brain heart infusion agar and broth, tryptic soy broth, and Mueller-Hinton agar and broth were purchased from Difco Laboratories, Detroit, Mich. The organisms were maintained on brain heart infusion agar slants at 4 C, and in a mixture of 1 volume of brain heart infusion broth plus 1 volume of heat-inactivated bovine serum (Behringwerke, Marburg, Germany) at -65 C.(iii) Antibiotics. Rifampin (lot no. 9124) was a gift from Ciba-Geigy AG, Wehr/Baden, Germany; stock solutions (2,000 zg/ml) were prepared in 20 vol% of aqueous methanol, membrane filter sterilized, and stored at 4 C in the dark. Amikacin (BB-K8) was obtained through the courtesy of Bristol Laboratories, Syracuse, N.Y. (lot no. 74F 1805); polymyxin B was a gift from Pfizer GmbH, Karlsruhe, Germany (no lot number given). Aqueous stock solutions of the latter two drugs (2,000 gg/ml) were filter sterilized and stored in small aliquots at -65 C. Antibiotic "high content" disks were procured from Difco.(iv) Antibiotic susceptibility tests. Disk susceptibility tests were carried out in accordance with the technique of Bauer et al. (1). Minimal inhibitory concentrations of rifampin, polymyxin B, and amikacin were determined in a microtiter system (Cooke Engineering Co., Alexandria, Va.). The antibiotic master dilutions had been prepared conventionally in Mueller-Hinton broth and were delivered with dropper pipettes (0.05 ml). Bacterial inocula (0.05 ml) were adjusted to yield a final number of 1.5 x 106 colony-forming units/ml at 0 time (10). The combined activity of polymyxin B and rifampin likewise was examined in the microtiter system; U-shaped wells received 0.05 ml of each respective 4x antibiotic dilution or Mueller-Hinton broth (control) and 2 x 0.05 ml of the adjusted bacterial inoculum. Thus,

Polymyxin B and Rifampin: New Regimen for Multiresistant Serratia marcescens Infections

“…The present study confirns previous reports of polymyxin B and rifampin synergy against Serratia (12,14) and extends those observations by demonstrating the efficacy of polymyxin B and rifampin in serious Serratia infections. Three of the four patients who failed to respond to polymyxin B and rifampin had severe underlying illnesses, an established risk factor in the treatment of gram-negative bacillary infections (6).…”

“…An endemic involving multiresistant Serratia at the Little Rock Veterans Administration Hospital (LRVAH) necessitated development of a novel therapeutic regimen. Polymyxin B and rifampin are synergistic in vitro for this species (12,14) but have not been used together in patients. This study was consequently undertaken to evaluate the extent of antibiotic resistance among Serratia at LRVAH, to determine their susceptibility to polymyxin B and rifampin, and to assess the efficacy and toxicity of therapy with polymyxin B and rifampin in patients seriously ill with infections due to multiresistant Serratia.…”

Further Studies on the Susceptibility of <i>Serratia marcescens </i>to the Bactericidal Activity of Human Serum

“…During the course of our investigations concerning the susceptibility of clinical isolates of Serrana marcescens, an opportunistic-pathogenic microorganism of considerable nosocomial significance, to the bacteri cidal activity of human serum, it was observed that so-called 'promptly serum-sensitive' (PS) strains were killed within minutes, whereas 'delayed serum-sensitive' (DS) isolates were killed only after several hours of in cubation [22], Chelation of fresh human serum with 0.01 m of magne sium ions (Mg) plus 0.01 M of ethylene glycol tetraacetic acid (EGTA), a procedure known to render the classical pathway of human complement activation nonfunctional [4,6], prevented 'prompt' killing activity. The formerly PS isolates, including Escherichia coli control strain C, were now killed only after extended incubation [23].…”

Microdilution Transfer Plate Technique for Determining In Vitro Synergy of Antimicrobial Agents

“…Several agar diffusion methods are also described (7,9,12). Microdilution methods have been described for determining synergy (3,5,6,15,16). Two methods (15,16) involve dilution of both drugs before pipetting each into the microdilution plate.…”

“…Microdilution methods have been described for determining synergy (3,5,6,15,16). Two methods (15,16) involve dilution of both drugs before pipetting each into the microdilution plate. The method of Kluge et al (6) requires diluting one drug in the microdilution plate, then adding the second drug (already diluted) to the wells containing the first drug.…”