Polymyxin B-Resistant <b> <i>Acinetobacter baumannii</i> Clinical Isolate Susceptible to Recombinant BPI ₂₁ and Cecropin P1 </b>

Abstract

Many strains of Acinetobacter baumannii have become resistant to a variety of clinically available antibacterial agents by both intrinsic and extrinsic mechanisms (4, 15). Several investigators have documented multidrug-resistant A. baumannii causing nosocomial infections and have demonstrated the in vitro and in vivo activities of imipenem, sulbactam, and the polymyxins (2, 9, 11, 14, 16). Increasing resistance to antibacterials, including sulbactam and the carbapenems, has prompted the use of polymyxin B and colistin as therapeutic agents, and within the last several years, the polymyxins have been used with increasing frequency to treat patients infected with multidrug-resistant, gram-negative bacteria, includingAcinetobacter (9, 14, 16). Although the literature, both clinical and microbiological, has shown that A. baumannii has retained susceptibility to the polymyxins despite resistance to all other antibacterial agents (2, 15), we document the isolation of a polymyxin B-resistant strain of A. baumannii from a patient who was given polymyxin B for treatment of a multidrug-resistant, polymyxin-susceptible strain of A. baumannii. The minimal inhibitory concentrations for the polymyxin-resistant strain (L1) were 48 μg/ml (polymyxin B) and 128 μg/ml (colistin) as determined by E-test methodology (AB Biodisk North America Inc., Piscataway, N.J.). More importantly, we have found that this strain is susceptible in vitro to rBPI21(Neuprex; XOMA Corporation, Berkeley, Calif.) (provided by S. Carroll), a recombinant form of the N-terminal domain of the human bactericidal/permeability-increasing protein (Table1). This isolate was also susceptible to cecropin P1 (Sigma, St. Louis, Mo.), an antibacterial peptide from pig intestine (3). The antibacterial effects of rBPI21 and cecropin P1 were manifest both in conventional MIC and minimal bactericidal concentration (MBC) assays with Mueller-Hinton broth and in bactericidal assays with nutrient broth. In the later type of assay, the antibacterial potency of rBPI21, but not of cecropin P1, toward A. baumannii was further increased nearly 100-fold in the presence of sublethal amounts of serum (data not shown).Table 1.Polymyxin B-resistant isolate of Acinetobacter baumannii is sensitive to rBPI21 and to cecropin P1aAntibacterial agentMIC and MBC (μM)LD90 (μM)L1D41L1D41A1 Polymyxin B&gt;100.5&gt;100.10.1rBPI213.01.00.30.30.2 Cecropin P12.01.00.50.3NTbaThe MIC and MBC of polymyxin B, rBPI21, and cecropin P1 for Acinetobacter baumannii were determined by incubation of bacteria (5 × 105 bacteria/ml) overnight at 37°C in Mueller-Hinton broth containing increasing concentrations of the antibacterial (poly)peptides. Under all conditions, the MIC and MBC were the same. The bactericidal activities of these agents for Acinetobacter baumannii were also measured by incubation with bacteria (106 bacteria/ml) in phosphate-buffered (pH 7.4) nutrient broth–0.9% sodium chloride for 2 h at 37°C. All results shown are the means of at least three independent determinations. LD90, 90% lethal dose.bNT, not tested.