In vitro and in vivo synergistic activity of colistin, rifampin, and amikacin against a multiresistant Pseudomonas aeruginosa isolate

Multiresistant strains of Pseudomonas aeruginosa have become increasingly frequent causes of infections, especially in tertiary-care hospitals. Because of multiresistance, the available choices in the treatment of these infections are limited. In view of the high incidence of morbidity and mortality associated with severe infections caused by P aeruginosa, more effective treatment strategies are clearly needed. The observation that rifampin could be active in vitro against non-fermenting Gram-negative rods (NFGNR) [1Thornsberry C Hill BC Swenson JM McDougal LK Rifampin: spectrum of antibacterial activity.Rev Infect Dis. 1983; 5: S412-S417Crossref PubMed Scopus (67) Google Scholar] and could have synergistic activity with polymyxins against Acinetobacter baumannii [2Hogg GM Barr JG Webb CH In‐vitro activity of the combination of colistin and rifampicin against multidrug‐resistant strains of A. baumannii.J Antimicrob Chemother. 1998; 42: 413Google Scholar, 3Tascini C Menichetti F Bozza S Del Favero A Bistoni F Evaluation of the activities of two‐drug combinations of rifampicin, polymyxin B and ampicillin/sulbactam against A. baumannii.J Antimicrob Chemother. 1998; 42: 270-271Crossref PubMed Google Scholar] led us to study the potential therapeutic role of this combination in the treatment of a P aeruginosa infection in an immunodepressed patient. A 61-year-old patient with non-Hodgkin's lymphoma was admitted to the hospital because of fever, fatigue, and multiple abscesses of the lungs, perineum and gluteus. The patient received 60 mg daily of methylprednisolone because of hemolytic anemia. There was hypogammaglobulinemia, the IgG level in serum was 80 mg/dL (normal value, 700 ± 200 mg/dL) and there was a deficiency of CD4+ T-lymphocytes (536/μL, 20.8%). From all the infected sites, a P. aeruginosa isolate, resistant to all the available antibiotics except amikacin, colistin and fosfomicin, was isolated. The MICs of amikacin, colistin and rifampin were determined by a microbroth dilution method [4National Committee for Clinical Laboratory Standards Performance standards for antimicrobial susceptibility testing, Fifth international supplement. Document M100‐S5. NCCLS, Villanova, PA1993Google Scholar] using cationadjusted Mueller–Hinton broth (Oxoid, Basingstoke, UK) and an inoculum of 5 × 108 CFU/L. The synergistic activities of the different antibiotic combinations were evaluated by the checkerboard method with cationadjusted Mueller–Hinton broth (Oxoid) and an inoculum of 5 × 108 CFU/L [5Marques MB Brookings E Moser SA Sonke PB Waites KB Comparative in vitro antimicrobial susceptibilities of nosocomial combinations.Antimicrob Agents Chemother. 1997; 41: 881-885PubMed Google Scholar]. The fractional inhibitory concentration index (FICI) was calculated from the sum of the fractional inhibitory concentrations according to the following formula: On the basis of the FICIs, the results were categorized as follows: synergistic, FICI ≤ 0.5; partially synergistic, 0.5 < FICI < 1;additive, FICI = 1;indifferent, 1 < FICI ≤ 4; antagonistic, FICI > 4 [5Marques MB Brookings E Moser SA Sonke PB Waites KB Comparative in vitro antimicrobial susceptibilities of nosocomial combinations.Antimicrob Agents Chemother. 1997; 41: 881-885PubMed Google Scholar]. The results of antimicrobial susceptibility are reported in Table 1.Table 1Determination of relative bradycardiaCriteriaInclusive 1.Patient must be an adult, i.e. ≥13 years. 2.Temperature ≥102 °F. 3.Pulse must be taken simultaneously with the temperature elevation.Exclusive 1.Patient has NSR without arrhythmia, second/third- degree heart block or pacemaker-induced rhythm. 2.Patient must not be on β-blocker medication.Appropriate temperature-pulse relationshipsTemperatureBeats/min41.1 °f.f.s.C (106 °F)15040.6 °f.f.s.C (105 °F)14040.7 °f.f.s.C (104 °F)13039.4 °f.f.s.C (103 °F)12038.9 °f.f.s.C (102 °F)12038.3 °f.f.s.C (101 °F)110Reproduced with permission from Cunha BA. Diagnostic significance of relative bradycardia. Infect Dis Practice 1997; 21: 38–40. Open table in a new tab Reproduced with permission from Cunha BA. Diagnostic significance of relative bradycardia. Infect Dis Practice 1997; 21: 38–40. The addition of amikacin or rifampin strikingly decreases the MICs in combination with colistin against the P aeruginosa strain isolated from the patient studied. These combinations resulted in fully synergistic activity (FICI ≤ 0.5). On the other hand, the addition of amikacin to rifampin resulted in unchanged MICs in combination; therefore, their synergistic activity was indifferent (FICI = 2). On the basis of the antimicrobial susceptibility tests, treatment with colistin (2 million units/day) and amikacin (1 g/day) was started and the inhibitory activity of the serum was evaluated according to NCCLS guidelines [6National Committee for Clinical Laboratory Standards Methodology for the serum bactericidal test. Approved guidelines. Document M21‐A. NCCLS, Villanova, PA1998Google Scholar]. The serum titers of the patient, taken at the trough and peak during colistin plus amikacin treatment, were 1 : 4 and 1 : 16, respectively. When rifampin was added, at a dose of 600 mg daily, the peak and trough serum inhibitory titer increased to 1 : 32. After several days, amikacin was stopped and colistin plus rifampin was continued. After more than 6 weeks of this treatment, P. aeruginosa was eradicated from all the infected sites, skin lesions were cured, pulmonary lesions improved and general symptoms resolved. In summary, synergistic activity between colistin, rifampin and amikacin was demonstrated both in vitro and in vivo. Colistin has been recently evaluated as monotherapy for the treatment of serious infections caused by P aeruginosa and A. baumannii [7Levin AS Barone AA Penco J et al.Intravenous colistin as therapy for nosocomial infections caused by multidrug‐resistant P. aeruginosa and A. baumannii.Clin Infect Dis. 1999; 28: 1008-1011Crossref PubMed Scopus (497) Google Scholar]. Hu and Luh [8Hu FR Luh KT Pseudomonas aeruginosa isolated from corneal ulcer: susceptibility to antimicrobial agents tested alone or in combination.J Formos Med Assoc. 1992; 91: 190-194PubMed Google Scholar], in contrast to our findings, have observed an antagonistic in vitro effect of the combination colistin plus amikacin. Polymyxins are able to increase the permeability of the outer membrane of Gram-negative rods [9Vaara M Vaara T Polycations as outer membrane‐disorganizing agents.Antimicrob Agents Chemother. 1983; 24: 114-122Crossref PubMed Scopus (186) Google Scholar], so we postulate that they could be synergistic with antibiotics, such as aminoglycosides, that cross bacterial membranes with difficulty, resulting in increased intracellular concentrations. Furthermore, our experience suggests that adding rifampin and amikacin to colistin could be clinically useful. A prospective evaluation of this therapeutic approach seems to be warranted.