Gunnar Kahlmeter

Multidrug-resistant, extensively drug-resistant and pandrug-resistant bacteria: an international expert proposal for interim standard definitions for acquired resistance

Many different definitions for multidrug-resistant (MDR), extensively drug-resistant (XDR) and pandrug-resistant (PDR) bacteria are being used in the medical literature to characterize the different patterns of resistance found in healthcare-associated, antimicrobial-resistant bacteria. A group of international experts came together through a joint initiative by the European Centre for Disease Prevention and Control (ECDC) and the Centers for Disease Control and Prevention (CDC), to create a standardized international terminology with which to describe acquired resistance profiles in Staphylococcus aureus, Enterococcus spp., Enterobacteriaceae (other than Salmonella and Shigella), Pseudomonas aeruginosa and Acinetobacter spp., all bacteria often responsible for healthcare-associated infections and prone to multidrug resistance. Epidemiologically significant antimicrobial categories were constructed for each bacterium. Lists of antimicrobial categories proposed for antimicrobial susceptibility testing were created using documents and breakpoints from the Clinical Laboratory Standards Institute (CLSI), the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the United States Food and Drug Administration (FDA). MDR was defined as acquired non-susceptibility to at least one agent in three or more antimicrobial categories, XDR was defined as non-susceptibility to at least one agent in all but two or fewer antimicrobial categories (i.e. bacterial isolates remain susceptible to only one or two categories) and PDR was defined as non-susceptibility to all agents in all antimicrobial categories. To ensure correct application of these definitions, bacterial isolates should be tested against all or nearly all of the antimicrobial agents within the antimicrobial categories and selective reporting and suppression of results should be avoided.

EUCAST Expert Rules in Antimicrobial Susceptibility Testing

EUCAST expert rules have been developed to assist clinical microbiologists and describe actions to be taken in response to specific antimicrobial susceptibility test results. They include recommendations on reporting, such as inferring susceptibility to other agents from results with one, suppression of results that may be inappropriate, and editing of results from susceptible to intermediate or resistant or from intermediate to resistant on the basis of an inferred resistance mechanism. They are based on current clinical and/or microbiological evidence. EUCAST expert rules also include intrinsic resistance phenotypes and exceptional resistance phenotypes, which have not yet been reported or are very rare. The applicability of EUCAST expert rules depends on the MIC breakpoints used to define the rules. Setting appropriate clinical breakpoints, based on treating patients and not on the detection of resistance mechanisms, may lead to modification of some expert rules in the future.

The global threat of antimicrobial resistance: science for intervention

In the last decade we have witnessed a dramatic increase in the proportion and absolute number of bacterial pathogens resistant to multiple antibacterial agents. Multidrug-resistant bacteria are currently considered as an emergent global disease and a major public health problem. The B-Debate meeting brought together renowned experts representing the main stakeholders (i.e. policy makers, public health authorities, regulatory agencies, pharmaceutical companies and the scientific community at large) to review the global threat of antibiotic resistance and come up with a coordinated set of strategies to fight antimicrobial resistance in a multifaceted approach. We summarize the views of the B-Debate participants regarding the current situation of antimicrobial resistance in animals and the food chain, within the community and the healthcare setting as well as the role of the environment and the development of novel diagnostic and therapeutic strategies, providing expert recommendations to tackle the global threat of antimicrobial resistance.

Biological Costs and Mechanisms of Fosfomycin Resistance in Escherichia coli

ABSTRACT Fosfomycin is a cell wall inhibitor used mainly for the treatment of uncomplicated lower urinary tract infections. As shown here, resistance to fosfomycin develops rapidly in Escherichia coli under experimental conditions, but in spite of the relatively high mutation rate in vitro, resistance in clinical isolates is rare. To examine this apparent contradiction, we mathematically modeled the probability of resistance development in the bladder during treatment. The modeling showed that during a typical episode of urinary tract infection, the probability of resistance development was high (>10−2). However, if resistance was associated with a reduction in growth rate, the probability of resistance development rapidly decreased. To examine if fosfomycin resistance causes a reduced growth rate, we isolated in vitro and in vivo a set of resistant strains. We determined their resistance mechanisms and examined the effect of the different resistance mutations on bacterial growth in the absence and presence of fosfomycin. The types of mutations found in vitro and in vivo were partly different. Resistance in the mutants isolated in vitro was caused by ptsI, cyaA, glpT, uhpA/T, and unknown mutations, whereas no cyaA or ptsI mutants could be found in vivo. All mutations caused a decreased growth rate both in laboratory medium and in urine, irrespective of the absence or presence of fosfomycin. According to the mathematical model, the reduced growth rate of the resistant strains will prevent them from establishing in the bladder, which could explain why fosfomycin resistance remains rare in clinical isolates.

Development of the EUCAST disk diffusion antimicrobial susceptibility testing method and its implementation in routine microbiology laboratories

With the support of ESCMID and European countries, EUCAST has developed a disk diffusion test with zone diameter breakpoints correlated with the EUCAST clinical MIC breakpoints. The development of the EUCAST disk diffusion method and quality control criteria are described, together with guidance on quality control and implementation of the method in clinical microbiology laboratories. The method includes the use of Mueller-Hinton agar without supplements for non-fastidious organisms and with 5% mechanically defibrinated horse blood and 20 mg/L β-NAD for fastidious organisms, a standardized inoculum resulting in confluent growth, an incubation time of 16-20 h, a reading guide on how to read zone diameters on individual species-agent combinations and zone diameter breakpoints calibrated to the EUCAST clinical MIC breakpoints. EUCAST recommendations are described in detail and updated regularly on the EUCAST website (http://www.eucast.org).

Dissemination of Escherichia coli with CTX-M type ESBL between humans and yellow-legged gulls in the south of France

Extended Spectrum β-Lactamase (ESBL) producing Enterobacteriaceae started to appear in the 1980s, and have since emerged as some of the most significant hospital-acquired infections with Escherichia coli and Klebsiella being main players. More than 100 different ESBL types have been described, the most widespread being the CTX-M β-lactamase enzymes (bla CTX-M genes). This study focuses on the zoonotic dissemination of ESBL bacteria, mainly CTX-M type, in the southern coastal region of France. We found that the level of general antibiotic resistance in single randomly selected E. coli isolates from wild Yellow-legged Gulls in France was high. Nearly half the isolates (47,1%) carried resistance to one or more antibiotics (in a panel of six antibiotics), and resistance to tetracycline, ampicillin and streptomycin was most widespread. In an ESBL selective screen, 9,4% of the gulls carried ESBL producing bacteria and notably, 6% of the gulls carried bacteria harboring CTX-M-1 group of ESBL enzymes, a recently introduced and yet the most common clinical CTX-M group in France. Multi locus sequence type and phylogenetic group designations were established for the ESBL isolates, revealing that birds and humans share E. coli populations. Several ESBL producing E. coli isolated from birds were identical to or clustered with isolates with human origin. Hence, wild birds pick up E. coli of human origin, and with human resistance traits, and may accordingly also act as an environmental reservoir and melting pot of bacterial resistance with a potential to re-infect human populations.

Antimicrobial susceptibility of Escherichia coli from community-acquired urinary tract infections in Europe: the ECO·SENS study revisited

This study determined the antimicrobial susceptibility of Escherichia coli causing community-acquired, acute, uncomplicated, non-recurrent urinary tract infection in unselected women aged 18-65 years and compared the results with those obtained 8 years earlier in the first ECO·SENS study (1999-2000). During 2007-2008, urine samples were taken from 1697 women in Austria, Greece, Portugal, Sweden and the UK. The countries were chosen to represent areas of Europe indicated to have more (Greece and Portugal) or less (UK, Austria and Sweden) problems with resistance. Antimicrobial susceptibility testing of 903 E. coli isolates (150-200 isolates per country) to 14 antimicrobials was performed by disk diffusion using European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints. In E. coli, resistance to mecillinam, cefadroxil (representing oral cephalosporins), nitrofurantoin, fosfomycin trometamol, gentamicin and the third-generation cephalosporins cefotaxime and ceftazidime was <2%, with the following exceptions: gentamicin in Portugal (2.8%); fosfomycin in Greece (2.9%); and cephalosporins in Austria (2.7-4.1%). Resistance levels were higher for amoxicillin/clavulanic acid (2.0-8.9%) and ciprofloxacin (0.5-7.6%) and much higher to ampicillin (21.2-34.0%), sulfamethoxazole (21.2-31.3%), trimethoprim (14.9-19.1%) and trimethoprim/sulfamethoxazole (14.4-18.2%). Resistance to quinolones and trimethoprim increased between the ECO·SENS I (1999-2000) and ECO·SENS II (2007-2008): nalidixic acid 4.3% to 10.2%; ciprofloxacin 1.1% to 3.9%; and trimethoprim 13.3% to 16.7%. In the previous study, no isolates with extended-spectrum β-lactamase were found; however, in the present study 11 isolates were identified as having either CTX-M or AmpC.

Dissemination of multidrug-resistant bacteria into the Arctic

We show that Escherichia coli isolates originating from Arctic birds carry antimicrobial drug resistance determinants. This finding implies that dissemination of drug-resistant bacteria is worldwide. Resistance genes can be found even in a region where no selection pressure for resistance development exists.

Little evidence for reversibility of trimethoprim resistance after a drastic reduction in trimethoprim use

OBJECTIVES The worldwide rapid increase in antibiotic-resistant bacteria has made efforts to prolong the lifespan of existing antibiotics very important. Antibiotic resistance often confers a fitness cost in the bacterium. Resistance may thus be reversible if antibiotic use is discontinued or reduced. To examine this concept, we performed a 24 month voluntary restriction on the use of trimethoprim-containing drugs in Kronoberg County, Sweden. METHODS The intervention was performed on a 14 year baseline of monthly data on trimethoprim resistance and consumption. A three-parameter mathematical model was used to analyse the intervention effect. The prerequisites for reversion of resistance (i.e. fitness cost, associated resistance and clonal composition) were studied on subsets of consecutively collected Escherichia coli from urinary tract infections. RESULTS The use of trimethoprim-containing drugs decreased by 85% during the intervention. A marginal but statistically significant effect on the increase in trimethoprim resistance was registered. There was no change in the clonal composition of E. coli and there was no measurable fitness cost associated with trimethoprim resistance in clinical isolates. The frequency of associated antibiotic resistances in trimethoprim-resistant isolates was high. CONCLUSIONS A lack of detectable fitness cost of trimethoprim resistance in vitro together with a strong co-selection of other antibiotics could explain the rather disappointing effect of the intervention. The result emphasizes the low possibility of reverting antibiotic resistance once established and the urgent need for the development of new antibacterial agents.

Redefining extended-spectrum β-lactamases: balancing science and clinical need

The current beta-lactamase classifications have reached a high level of complexity, making them less accessible to clinicians, infection control professionals, hospital management and politicians. From the clinical perspective, a revised comprehensible nomenclature scheme is therefore needed. The term extended-spectrum beta-lactamases (ESBLs) has reached a broader audience over time, but is currently restricted to functional class 2be/molecular class A, clavulanic acid inhibited enzymes with activity against extended-spectrum cephalosporins. The proposed new classification expands the definition of ESBL to other clinically important acquired beta-lactamases with activity against extended-spectrum cephalosporins and/or carbapenems. The classical class A ESBLs have been designated ESBLA in this classification, whereas plasmid-mediated AmpC and OXA-ESBLs are classed as miscellaneous ESBLs (ESBLM). Lastly, the carbapenemases have been designated ESBLCARBA, ESBLs with hydrolytic activity against carbapenems. We believe that this simplified classification may encourage new groups of healthcare professionals to engage in the effort to prevent the spread of acquired beta-lactamases.