Multidrug-resistant Acinetobacter baumannii has emerged as a significant clinical problem worldwide and colistin is being used increasingly as "salvage" therapy. MICs of colistin against A. baumannii indicate its significant activity. However, resistance to colistin in A. baumannii has been reported recently. Clonotypes of 16 clinical A. baumannii isolates and ATCC 19606 were determined by pulsed-field gel electrophoresis (PFGE), and colistin MICs were measured. The time-kill kinetics of colistin against A. baumannii ATCC 19606 and clinical isolate 6 were investigated, and population analysis profiles (PAPs) were conducted. Resistance development was investigated by serial passaging with or without exposure to colistin. Five different PFGE banding patterns were found in the clinical isolates. MICs of colistin against all isolates were within 0.25 to 2 g/ml. Colistin showed early concentration-dependent killing, but bacterial regrowth was observed at 24 h. PAPs revealed that heteroresistance to colistin occurred in 15 of the 16 clinical isolates. Subpopulations (<0.1% from inocula of 10 8 to 10 9 CFU/ml) of ATCC 19606, and most clinical isolates grew in the presence of colistin 3 to 10 g/ml. Four successive passages of ATCC 19606 in broth containing colistin (up to 200 g/ml) substantially increased the proportion of the resistant subpopulations able to grow in the presence of colistin at 10 g/ml from 0.000023 to 100%; even after 16 passages in colistin-free broth, the proportion only decreased to 2.1%. This represents the first demonstration of heterogeneous colistin-resistant A. baumannii in "colistinsusceptible" clinical isolates. Our findings give a strong warning that colistin-resistant A. baumannii may be observed more frequently due to potential suboptimal dosage regimens recommended in the product information of some products of colistin methanesulfonate.

Rapid detection of metallo-␤-lactamase (MBL)-producing gram-negative pathogens is critical to prevent their widespread dissemination. Thus far, no standardized phenotypic method is available, and previously reported techniques have poor sensitivity for detecting carbapenem-susceptible MBL-carrying isolates, an increasingly described phenomenon. We developed a phenotypic detection method using both a double-disk synergy test and a combined-disk test with imipenem and 292 g EDTA on one agar plate. Genotypic confirmation was used for validation. Of the 134 clinical isolates, 84 were confirmed to carry an MBL. Of these, 51 (61%) were susceptible to at least one carbapenem, and 22 (26%) were isolated from blood. The phenotypic method correctly differentiated all MBL-producing isolates (sensitivity, 100%). Fifty-one of the 52 MBLnegative isolates were correctly differentiated (specificity, 98%). This study reports the validation of a simple and accurate MBL detection method that can be easily incorporated into the daily routine of a clinical laboratory. Early detection of MBL-carrying organisms, including those with susceptibility to carbapenems, is of paramount clinical importance, as it allows rapid initiation of strict infection control practices as well as therapeutic guidance for confirmed infection.

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The aim of this study was to determine the diversity of Klebsiella pneumoniae capsular serotypes in an Australian setting. Consecutive (n ‫؍‬ 293) nonrepetitive isolates of K. pneumoniae from a large teaching hospital laboratory were analyzed. The majority of isolates were from urinary specimens (60.8%); the next most common source was sputum (14.3%), followed by blood (14%). Serotyping revealed a wide range of capsule types. K54 (17.1%), K28 (4.1%), and K17 (3.1%) were the most common, and K54 isolates displayed a high degree of clonality, suggesting a common, nosocomial source. In vitro, one K54 isolate was more adherent to urinary catheters and HEp-2 cells than four other tested isolates; it was slightly more resistant to chlorhexidine but was more susceptible to drying than heavily encapsulated strains. This is the first seroprevalence survey of K. pneumoniae to be performed on Australian isolates, and the high level of diversity of serotypes suggests that capsule-based immunoprophylaxis might not be useful for Australia. In addition there are significant differences in the predominance of specific serotypes compared to the results of surveys performed overseas, which has important implications for capsule-based immunoprophylaxis aimed at a global market.

The pharmacokinetic parameters determining antibiotic efficacy are peak concentrations (C max), minimum (trough) concentrations (C min), and area under the concentration-time curve (AUC). There is general agreement about the importance of C max and AUC for aminoglycosides, but this is not so for maintenance of C min. With in vitro exposures modelling in vivo administration,Pseudomonas aeruginosa reference strain ATCC 27853 (MIC, 1 mg/liter) and a higher-MIC (relatively resistant) clinical isolate (MIC, 4 mg/liter) were used to explore bacteriostatic and bactericidal outcomes. With P. aeruginosa ATCC 27853, kill followed a complete bolus profile with a 30-min postdistribution peak (C peak30) of 10 mg/liter. The clinical isolate required a C peak30 bolus profile of 20 mg/liter for kill, and there was no difference between the efficacies of the bolus and infusion exposures. Bolus profiles that were truncated at 8.5 h and producing sublethal effects were then combined with a wide range of C mins. With aC peak30 profile of 8 mg/liter, P. aeruginosa ATCC 27853 showed a graded bacteriostatic response until a C min of ≥0.8 mg/liter, when complete kill resulted. In contrast, bactericidal effects on the clinical isolate required a C peak30 profile of 18 mg/liter with a C min of ≥1.0 mg/liter. Therefore, C min also contributes to the bactericidal effect of tobramycin, with requirements showing minor variation with change in MIC. Dosing principles for relatively resistant (higher-MIC) organisms are suggested from the data. Relatively higher aminoglycoside doses via infusion regimens are likely to be needed to generate higher peak concentrations and higher AUC values necessary for bactericidal effect in resistant organisms. Maintenance of trough concentrations on the order of 1.0 mg/liter during the interdose interval will tend to guard against the possibility of inadequate peak and AUC exposures for kill.