Heteroresistance to Cephalosporins and Penicillins in Acinetobacter baumannii

Abstract

Heteroresistance to antimicrobial agents may affect susceptibility test results and therapeutic success. In this study, we investigated heteroresistance to cephalosporins and penicillins in Acinetobacter baumannii, a major pathogen causing nosocomial infections. Two A. baumannii isolates exhibited heteroresistance to ampicillin-sulbactam, ticarcillin-clavulanic acid, cefepime, and cefpirome, showing a distinct colony morphology of circular rings within the inhibition halos. Pulsed-field gel electrophoresis (PFGE) and outer membrane protein (OMP) analysis demonstrated that subpopulations around the disks/Etest strips and the original strains all belonged to the same PFGE type and OMP profile. Population analysis profile (PAP) showed the presence of heteroresistant subpopulations with high cefepime resistance levels in two isolates (008 and 328). Interestingly, A. baumannii 008 contained two peaks: one was grown in the presence of up to 1 g of cefepime/ml, the other apparently occurred when the concentration of cefepime was raised to 256 g/ml. After serial passages without exposure to cefepime, the PAP curve maintained the same trend observed for the original strain of A. baumannii 008. However, the PAP curve showed a shift to relatively lower cefepime resistance (from 256 to 64 g/ml) in A. baumannii 328 after 10 passages in antibiotic-free Mueller-Hinton agar plates. Convergence to a monotypic resistance phenotype did not occur. Growth rate analysis revealed that slower growth in resistant subpopulations may provide a strategy against antibiotic challenge. To our knowledge, this is the first report of heteroresistance to cephalosporins and penicillins in A. baumannii.

Antimicrobial Heteroresistance: an Emerging Field in Need of Clarity

“…This phenomenon, termed "Eagle-type" resistance, was similar to the Eagle killing by penicillin described earlier, in which the bactericidal action of penicillin paradoxically decreased at high antibiotic concentrations (17). Similar patterns of bimodal growth in population analysis profiles were observed for A. baumannii with cefepime, where growth inhibition after an initial peak of growth at a low antibiotic concentration was followed by another peak of growth at a higher concentration (18). Certain S. aureus strains displayed "thermosensitive" heteroresistance, where cultures growing at high methicillin concentrations at 30°C lost this ability within 30 min after shifting of the growth temperature to 37°C (19).…”

“…Disc diffusion assays (3,14,18,55,(86)(87)(88)(89)(90)(91)(92) and Etest strips have been used to detect heteroresistance as recommended for traditional in vitro susceptibility testing (6,15,18,50,63,64,66,68,71,73,76,80,86,(88)(89)(90)(91)(93)(94)(95)(96)(97)(98)(99)(100)(101). Special Etest strips were developed for glycopeptide resistance detection (GRD Etest) (69,74,75,81,102).…”

“…Differences in transcriptional levels may also underlie heteroresistance to carbapenems in P. aeruginosa; compared to the native populations, the resistant subpopulations had significantly increased transcription levels of the mexB and mexY genes, whose protein products are involved in multidrug efflux, and decreased expression of the oprD gene, encoding an outer membrane porin (57). Slower growth of ␤-lactam-resistant subpopulations of A. baumannii may protect against antibiotic challenge (18). In Enterobacter species, mutation of ampD, which is involved in the regulation of production of a class C ␤-lactamase, at rates as high as 10 Ϫ4 to 10 Ϫ6 , resulted in a heterogeneous population of bacterial cells with differing levels of ␤-lactam resistance (135).…”

“…Chemical communication of antibiotic resistance from the more resistant members of the population protecting less resistant bacteria is the major concern of such bacterial populations. tion, with 2-fold antibiotic increments, and by use of spread plate techniques for CFU counting (3,4,6,8,14,16,18,19,(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41). Counting of CFU by dropping smaller aliquots is as efficient as spread plate techniques (6,42).…”

Pharmacokinetics and Pharmacodynamics of Aerosolized Antibacterial Agents in Chronically Infected Cystic Fibrosis Patients

“…Although polymyxin heteroresistance was uncommon, several isolates presented heterogeneous subpopulations with increased and thus borderline susceptible MICs, which might cause treatment failures (92,95). Heteroresistance to carboxy-and ureidopenicillins, cephalosporins, carbapenems, aminoglycosides, colistin, fluoroquinolones, as well as the piperacillin-tazobactam and ticarcillin-clavulanate combinations in P. aeruginosa has been reported (96)(97)(98)(99)(100)(101).…”