Metallo-beta-lactamase or extended-spectrum beta-lactamase: a wolf in sheep's clothing

Plasmid interference for curing antibiotic resistance plasmids in vivo

Antibiotic resistance increases the likelihood of death from infection by common pathogens such as Escherichia coli and Klebsiella pneumoniae in developed and developing countries alike. Most important modern antibiotic resistance genes spread between such species on self-transmissible (conjugative) plasmids. These plasmids are traditionally grouped on the basis of replicon incompatibility (Inc), which prevents coexistence of related plasmids in the same cell. These plasmids also use post-segregational killing (‘addiction’) systems, which poison any bacterial cells that lose the addictive plasmid, to guarantee their own survival. This study demonstrates that plasmid incompatibilities and addiction systems can be exploited to achieve the safe and complete eradication of antibiotic resistance from bacteria in vitro and in the mouse gut. Conjugative ‘interference plasmids’ were constructed by specifically deleting toxin and antibiotic resistance genes from target plasmids. These interference plasmids efficiently cured the corresponding antibiotic resistant target plasmid from different Enterobacteriaceae in vitro and restored antibiotic susceptibility in vivo to all bacterial populations into which plasmid-mediated resistance had spread. This approach might allow eradication of emergent or established populations of resistance plasmids in individuals at risk of severe sepsis, enabling subsequent use of less toxic and/or more effective antibiotics than would otherwise be possible, if sepsis develops. The generalisability of this approach and its potential applications in bioremediation of animal and environmental microbiomes should now be systematically explored.

Predictability of Phenotype in Relation to Common β-Lactam Resistance Mechanisms in Escherichia coli and Klebsiella pneumoniae

The minimal concentration of antibiotic required to inhibit the growth of different isolates of a given species with no acquired resistance mechanisms has a normal distribution. We have previously shown that the presence or absence of transmissible antibiotic resistance genes has excellent predictive power for phenotype. In this study, we analyzed the distribution of six β-lactam antibiotic susceptibility phenotypes associated with commonly acquired resistance genes in Enterobacteriaceae in Sydney, Australia. Escherichia coli (n = 200) and Klebsiella pneumoniae (n = 178) clinical isolates, with relevant transmissible resistance genes (blaTEM, n = 33; plasmid AmpC, n = 69; extended-spectrum β-lactamase [ESBL], n = 116; and carbapenemase, n = 100), were characterized. A group of 60 isolates with no phenotypic resistance to any antibiotics tested and carrying none of the important β-lactamase genes served as comparators. The MICs for all drug-bacterium combinations had a normal distribution, varying only in the presence of additional genes relevant to the phenotype or, for ertapenem resistance in K. pneumoniae, with a loss or change in the outer membrane porin protein OmpK36. We demonstrated mutations in ompK36 or absence of OmpK36 in all isolates in which reduced susceptibility to ertapenem (MIC, >1 mg/liter) was evident. Ertapenem nonsusceptibility in K. pneumoniae was most common in the context of an OmpK36 variant with an ESBL or AmpC gene. Surveillance strategies to define appropriate antimicrobial therapies should include genotype-phenotype relationships for all major transmissible resistance genes and the characterization of mutations in relevant porins in organisms, like K. pneumoniae.

Resistance mechanisms in Enterobacteriaceae

Enterobacteriaceae are responsible for a large proportion of serious, life-threatening infections and resistance to multiple antibiotics in these organisms is an increasing global public health problem. Mutations in chromosomal genes contribute to antibiotic resistance, but Enterobacteriaceae are adapted to sharing genetic material and much important resistance is due to 'mobile' resistance genes. Different mobile genetic elements, which have different characteristics, are responsible for capturing these genes from the chromosomes of a variety of bacterial species and moving them between DNA molecules. If transferred to plasmids, these resistance genes are then able to be transferred 'horizontally' between different bacterial cells, including different species, and well as being transferred 'vertically' during cell division. Carriage of several resistance genes on the same plasmid enables a bacterial cell to acquire multi-resistance in a single step and means that spread of one resistance gene may be co-selected for by use of antibiotics other than those to which it confers resistance. Many different mobile genes conferring resistance to each class of antibiotic have been identified, complicating detection of the factors responsible for a particular resistance phenotype, especially when changes in chromosomal genes may also confer or contribute to resistance. Understanding the mechanisms of antibiotic resistance, and the means by which these mechanisms can evolve and disseminate, is important for developing ways to efficiently track the spread of resistance and to optimise treatment.

Accuracy of in vitro susceptibility tests for carbapenemase-producing Gram-negative bacteria

Accurate susceptibility results on antibiotic-resistant bacteria are essential for proper treatment of infections. In this study, 100 metallo-β-lactamase (MBL)-producing strains and 95 isolates with Klebsiella pneumoniae carbapenemase (KPC) were tested for carbapenem susceptibility using two automated platforms, the Phoenix and Vitek-2 systems, and a manual Etest. Phoenix showed higher categorical agreements (97% for imipenem and 94% for meropenem) compared with those from Vitek-2 (92 and 74%) and Etest (89 and 96%), respectively, when testing MBL strains. Categorical agreement for imipenem tests with KPC-producing strains was 88.4% with the Phoenix system, 83.2% with the Vitek 2 system and 90.5% with the Etest. Categorical agreement was 100% for all tests with ertapenem. In conclusion, the Phoenix system demonstrated a higher accuracy than Vitek-2 in testing carbapenemase-producing strains, particularly in MBL strains.

Persistence of related bla-IMP-4 metallo-beta-lactamase producing Enterobacteriaceae from clinical and environmental specimens within a burns unit in Australia - a six-year retrospective study

Background

To describe the clinical epidemiology, environmental surveillance and infection control interventions undertaken in a six-year persistence of bla-IMP-4 metallo-beta-lactamase (MBL) producing Enterobacteriaceae within a separately confined hospital burns unit in a tertiary hospital in Sydney, Australia.

Methods

MBL positive clinical and environmental isolates were collected from the Burns Unit, from the first detection of isolates in September 2006 to August 2012. Unit-acquired clinical isolates were included, and patient outcomes analyzed amongst those who acquired clinically significant infections. Environmental isolates were analyzed with regard to relationship to clinical isolates, bacterial species, and persistence despite cleaning efforts.

Results

Thirty clinical isolates detected from 23 patients were identified. Clinically significant infection developed in 7 (30%) patients – 2 bacteremias, 2 central venous catheter tip infections without bacteremia, and 3 wound infections. All patients survived at 30 days. Seventy-one environmental isolates were confirmed to be MBL-positive, with 85% sourced from shower facilities or equipment. MBL organisms persisted at these sites despite both usual hospital cleaning, and following targeted environmental disinfection interventions.

Conclusions

Clear association exists between environmental Burns Unit contamination by MBLs and subsequent patient colonization. Clinical infection occurred in a small proportion of patients colonized by MBLs, and with generally favorable outcomes. Its persistence in the Burns Unit environment, despite concerted infection control measures, pose concern for ongoing clinical transmission.

Detection of IMP metallo-β-lactamase in carbapenem-nonsusceptible Enterobacteriaceae and non-glucose-fermenting Gram-negative rods by immunochromatography assay

Metallo-β-lactamases (MBLs) are transmissible carbapenemases of increasing prevalence in Gram-negative bacteria among health care facilities worldwide. Control of the further spread of these carbapenem-resistant bacteria relies on clinical microbiological laboratories correctly identifying and classifying the MBLs. In this study, we evaluated a simple and rapid method for detecting IMP, the most prevalent MBL in Japan. We used an immunochromatography (IC) assay for 181 carbapenem-nonsusceptible (CNS) (nonsusceptible to imipenem or meropenem) strains comprising 74 IMP-producing and 33 non-IMP-producing strains of non-glucose-fermenting Gram-negative rods (NFGNR), as well as 64 IMP-producing and 10 non-IMP-producing Enterobacteriaceae strains. The IC assay results were compared to those from the double-disk synergy test (DDST), the MBL Etest, and the modified Hodge test (MHT) (only for Enterobacteriaceae). The IMP type was confirmed by specific PCR and direct sequencing. The IC assay detected all of the IMP-type MBLs, including IMP-1, -2, -6, -7, -10, -11, -19, -20, and -22 and IMP-40, -41, and -42 (new types), with 100% specificity and sensitivity against all strains tested. Although the sensitivity and specificity values for the DDST and MHT were equivalent to those for the IC assay, the MBL Etest was positive for only 87% of NFGNR and 31% of Enterobacteriaceae due to the low MIC of imipenem, causing an indeterminate evaluation. These results indicated that the IC assay might be a useful alternative to PCR for IMP MBL detection screening.

Expert systems in clinical microbiology

This review aims to discuss expert systems in general and how they may be used in medicine as a whole and clinical microbiology in particular (with the aid of interpretive reading). It considers rule-based systems, pattern-based systems, and data mining and introduces neural nets. A variety of noncommercial systems is described, and the central role played by the EUCAST is stressed. The need for expert rules in the environment of reset EUCAST breakpoints is also questioned. Commercial automated systems with on-board expert systems are considered, with emphasis being placed on the "big three": Vitek 2, BD Phoenix, and MicroScan. By necessity and in places, the review becomes a general review of automated system performances for the detection of specific resistance mechanisms rather than focusing solely on expert systems. Published performance evaluations of each system are drawn together and commented on critically.

Efficient direct extended-spectrum β-lactamase detection by multiplex real-time PCR: accurate assignment of phenotype by use of a limited set of genetic markers

The number and diversity of genes potentially complicate genetic approaches to the rapid detection of transmissible extended-spectrum β-lactamase genes. We developed a robust multiplexed real-time PCR assay based on targets identified in a prior survey and used this to detect relevant genes in 617 consecutive clinical isolates of extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae.

bla(IMP-4) in different genetic contexts in Enterobacteriaceae isolates from Australia

The IMP-4 metallo-beta-lactamase, originally recognized in Acinetobacter spp. from Hong Kong, more recently appeared simultaneously in isolates of the family Enterobacteriaceae from Sydney and Melbourne, Australia. The bla(IMP-4)-qacG2-aacA4-catB3 cassette array was found in isolates from both cities, but in different wider genetic contexts and on different plasmids, suggesting movement of this array by homologous recombination.