Acinetobacter baumannii has emerged as a highly troublesome pathogen for many institutions globally. As a consequence of its immense ability to acquire or upregulate antibiotic drug resistance determinants, it has justifiably been propelled to the forefront of scientific attention. Apart from its predilection for the seriously ill within intensive care units, A. baumannii has more recently caused a range of infectious syndromes in military personnel injured in the Iraq and Afghanistan conflicts. This review details the significant advances that have been made in our understanding of this remarkable organism over the last 10 years, including current taxonomy and species identification, issues with susceptibility testing, mechanisms of antibiotic resistance, global epidemiology, clinical impact of infection, host-pathogen interactions, and infection control and therapeutic considerations.

Acinetobacter baumannii: Emergence of a Successful Pathogen

Metallo-β-lactamases (MBLs) have been increasingly recognized from clinical isolates worldwide, but the laboratory detection of these strains is not well defined. We report a study that developed an EDTA disk screen test and a molecular diagnostic assay for the detection of MBL-producing Pseudomonas aeruginosa. Using NCCLS disk methodology, inhibition zone diameters were determined in tests with imipenem (IPM) and meropenem (MEM) disks alone and in combination with 930 μg of EDTA. This test was compared with the MBL Etest. The duplex PCR assay showed 100% sensitivity and specificity for detecting MBL-producing control strains. Of the 241 clinical strains of IPM-nonsusceptible P. aeruginosa from the Calgary Health Region isolated from 2002 to 2004, 110/241 (46%) were MBL positive using phenotypic methods while 107/241 (45%) were PCR positive for MBL genes: 103/241 (43%) for blaVIM and 4/241 (2%) for blaIMP. The EDTA disk screen test using MEM showed 100% sensitivity and 97% specificity for detecting MBLs in control and clinical strains. The EDTA disk screen test is simple to perform and to interpret and can easily be introduced into the workflow of a clinical laboratory. We recommend that all IPM-nonsusceptible P. aeruginosa isolates be routinely screened for MBL production using the EDTA disk screen test and that PCR confirmation be performed at a regional laboratory.

Detection of pseudomonas aeruginosa producing metallo-beta-lactamases in a large centralized laboratory

The dramatic spread of antibiotic resistance is a crisis in the treatment of infectious diseases that affect humans Several studies suggest that wastewater treatment plants (WWTP) are reservoirs for diverse mobile antibiotic resistance elements This review summarizes findings derived from genomic analysis of IncP-1 resistance plasmids isolated from WWTP bacteria Plasmids that belong to the IncP-1 group are self-transmissible, and transfer to and replicate in a wide range of hosts Their backbone functions are described with respect to their impact on vegetative replication, stable maintenance and inheritance, mobility and plasmid control Accessory genetic modules, mainly representing mobile genetic elements, are integrated in-between functional plasmid backbone modules These elements carry determinants conferring resistance to nearly all clinically relevant antimicrobial drug classes, to heavy metals, and quaternary ammonium compounds used as disinfectants All plasmids analysed here contain integrons that potentially facilitate integration, exchange and dissemination of resistance gene cassettes Comparative genomics of accessory modules located on plasmids from WWTP and corresponding modules previously identified in other bacterial genomes revealed that animal, human and plant pathogens and other bacteria isolated from different habitats share a common pool of resistance determinants

Genomics of IncP-1 antibiotic resistance plasmids isolated from wastewater treatment plants provides evidence for a widely accessible drug resistance gene pool

Carbapenem resistance mediated by acquired carbapenemase genes has been increasingly reported, particularly for clinical isolates of Pseudomonas aeruginosa and Acinetobacter spp. Of 1,234 nonduplicate isolates of carbapenem-resistant Pseudomonas spp. and Acinetobacter spp. isolated at a tertiary-care hospital in Seoul, Korea, 211 (17%) were positive for metallo-β-lactamase (MBL). Of these, 204 (96%) had either the blaIMP-1 or blaVIM-2 allele. In addition, seven Acinetobacter baumannii isolates were found to have a novel MBL gene, which was designated blaSIM-1. The SIM-1 protein has a pI of 7.2, is a new member of subclass B1, and exhibits 64 to 69% identity with the IMP-type MBLs, which are its closest relatives. All SIM-1-producing isolates exhibited relatively low imipenem and meropenem MICs (8 to 16 μg/ml) and had a multidrug resistance phenotype. Expression of the cloned blaSIM-1 gene in Escherichia coli revealed that the encoded enzyme is capable of hydrolyzing a broad array of β-lactams, including penicillins, narrow- to expanded-spectrum cephalosporins, and carbapenems. The blaSIM-1 gene was carried on a gene cassette inserted into a class 1 integron, which included three additional cassettes (arr-3, catB3, and aadA1). The strains were isolated from sputum and urine specimens from patients with pneumonia and urinary tract infections, respectively. All patients had various underlying diseases. Pulsed-field gel electrophoresis of SmaI-digested genomic DNAs showed that the strains belonged to two different clonal lineages, indicating that horizontal transfer of this gene had occurred and suggesting the possibility of further spread of resistance in the future.

Novel Acquired Metallo-β-Lactamase Gene, blaSIM-1, in a Class 1 Integron from Acinetobacter baumannii Clinical Isolates from Korea

Optimal use of microbiology laboratories is essential to combat the spread of multiply antibiotic-resistant pathogens. This is vital for patient care, as advocated by Jarvis in stressing the importance of active detection and isolation to control methicillin-resistant Staphylococcus aureus and other resistant hospital-acquired pathogens (17). It is also vital for hospital accreditation in the United States, where the Joint Commission has set requirements for the control of acquisition and transmission of multidrug-resistant organisms (http: //www.jointcommission.org/NR/rdonlyres/31666E86-E7F4-423E -9BE8-F05BD1CB0AA8/0/HAP_NPSG.pdf-NPSG.07.03.01). These requirements cannot be met without excellence in diagnostic microbiology. The absence of new, effective anti-gram-negative antibiotics makes infection control the most important countermeasure against multidrug-resistant gram-negative pathogens. Infection control can prevent additional infections and the spread of resistant pathogens and thereby reduce the need to use antibiotics. Infection control is most effective when directed by rapid, accurate laboratory results. In short, excellence in diagnostic microbiology is critical to quality initiatives in hospitals. Some resistant pathogens may not be recognized because they are falsely susceptible in routine tests. This can lead to patients receiving ineffective antibiotics and contribute to the spread of the pathogens. Because the detection of such “hidden” resistance is so critical, this Commentary focuses on its detection in gram-negative pathogens. Because susceptibility tests may be unreliable, special tests are required to detect the resistance mechanisms involved. The mechanisms include extended-spectrum -lactamases (ESBLs), AmpC -lactamases, and carbapenemases of molecular classes A and B. ESBLs ESBLs are typically inhibitor-susceptible -lactamases that hydrolyze penicillins, cephalosporins, and aztreonam and are encoded by mobile genes. The most frequently encountered ESBLs belong to the CTX-M, SHV, and TEM families. ESBL producers are usually multiply drug resistant (5, 30), but their cephalosporin and aztreonam resistance is not reliably detected by susceptibility tests (33). Many labs have adopted CLSI recommendations and only attempted to detect ESBLs in Escherichia coli, Klebsiella pneumoniae, K. oxytoca, and Proteus mirabilis (8). Since ESBL genes are transmissible, it is important that ESBLs be tested for in other organisms in hospital and long-term care facility patient populations where ESBLs are encountered. This may be unnecessary for community isolates, which at this time appear to be predominantly CTX-M-producing E. coli. The need for ESBL detection is under challenge on the supposition that it is possible to set breakpoints for injectable cephalosporins and aztreonam that accurately discriminate which ESBL-producing isolates can and cannot be reliably treated with these drugs. This approach is controversial (19) but has been adopted in slightly different forms by the European Committee on Antimicrobial Susceptibility Testing and the CLSI. It is based on limited therapeutic outcome data (3, 31), pharmacokinetic/pharmacodynamic data (3), and the concept that the lower the cephalosporin MIC the greater the

Extended-Spectrum-β-Lactamase, AmpC, and Carbapenemase Issues

Gram-negative bacilli with acquired metallo-β-lactamase (MBL) production have been increasingly reported in some countries, necessitating their detection. The aim of this study was to evaluate the performance of the Hodge test and those of the imipenem (IPM)-EDTA, ceftazidime (CAZ)-mercaptopropionic acid (MPA), and CAZ-sodium mercaptoacetic acid (SMA) double-disk synergy tests (DDSTs). The efficiencies of testing CAZ-resistant and IPM-nonsusceptible isolates were also compared. Strains used for the evaluation were known IMP-1 and VIM-2 MBL-producing isolates and consecutive and CAZ-nonsusceptible isolates of pseudomonads and acinetobacters. The performance of the Hodge test was improved by addition of zinc sulfate (140 μg/disk) to an IPM disk. In DDSTs, EDTA (ca. 1,900 μg) disks were better at detecting MBL-producing strains among pseudomonads, while MPA (3 μl) and SMA (3 mg) disks performed better for acinetobacters. EDTA (ca. 750 μg)-plus-SMA (ca. 2 mg) disks performed better than EDTA, MPA, or SMA disks with both organisms. CAZ-SMA DDSTs failed to detect 22 of 80 (28%) MBL-producing acinetobacters. In conclusion, use of an IPM disk and an EDTA (750 μg)-plus-SMA (2 mg) disk improves performance, and testing IPM-nonsusceptible isolates rather than CAZ-resistant isolates could reduce screening work. Further evaluation of the test is required for the detection of other types of MBL-producing gram-negative bacilli.

Evaluation of the Hodge Test and the Imipenem-EDTA Double-Disk Synergy Test for Differentiating Metallo-β-Lactamase-Producing Isolates of Pseudomonas spp. and Acinetobacter spp.

A chloramphenicol-resistant strain of Salmonella enterica serovar Typhi was first noted in Korea in 1992, when a resistant isolate was detected in a returned traveler. Continued isolation of multidrug-resistant (MDR) strains thereafter in other settings prompted a retrospective analysis of laboratory records and phenotypic and genotypic analyses of 12 chloramphenicol-resistant isolates. Among these, one isolate was resistant only to chloramphenicol, and the other isolates were also resistant to ampicillin and co-trimoxazole. MDR was transferred by conjugation from 9 of the 11 isolates. PCR showed that all isolates had an incompatible group HI1 plasmid, and oriT was detected in 10 isolates, which included strains with an unsuccessful transfer of resistance. All of the ampicillin-resistant isolates had a β-lactamase band of pI 5.4 and blaTEM alleles. A PCR amplicon from an isolate showed that the sequences were identical to those of blaTEM-1, suggesting that all isolates had a TEM-1 β-lactamase. All isolates had class 1 integrons: 10 isolates had integrons of ca. 1.2 kb with dhfr7 gene cassettes, and 1 isolate had an integron of ca. 2.3 kb with aacA4 and blaOXA-1-like gene cassettes. The pulsed-field gel electrophoresis patterns of 7 of 11 MDR isolates were identical and indistinguishable from those reported for isolates in India and Indonesia. In conclusion, some of the MDR strains in Korea are related to those in other Asian countries. Susceptibility testing became necessary for selection of antimicrobial agents for the optimal treatment of patients with the emergence of MDR Salmonella serovar Typhi in Korea.

Emergence of Multidrug-Resistant Salmonella enterica Serovar Typhi in Korea

CTX-M-14-type extended-spectrum β-lactamase was first detected in Salmonella enterica serovar London strains which were isolated from three hospitalized pediatric patients with gastroenteritis. The isolates had pulsed-field gel electrophoresis patterns identical to those of the previously isolated antimicrobial-susceptible strains from community-acquired gastroenteritis, suggesting the susceptible clone acquired the resistance.

Nosocomial Outbreak of Pediatric Gastroenteritis Caused by CTX-M-14-Type Extended-Spectrum β-Lactamase-Producing Strains of Salmonella enterica Serovar London

Young Sik Lim

Papers

Evaluation of the Hodge Test and the Imipenem-EDTA Double-Disk Synergy Test for Differentiating Metallo-β-Lactamase-Producing Isolates of Pseudomonas spp. and Acinetobacter spp.

Emergence of Multidrug-Resistant Salmonella enterica Serovar Typhi in Korea

Nosocomial Outbreak of Pediatric Gastroenteritis Caused by CTX-M-14-Type Extended-Spectrum β-Lactamase-Producing Strains of Salmonella enterica Serovar London

The first detection of CTX-M-14 extended-spectrum β-lactamase among diverse β-lactamase–producing Proteus mirabilis clinical isolates