Carbapenem-resistant versus carbapenem-susceptible Acinetobacter baumannii bacteremia in a Greek intensive care unit: risk factors, clinical features and outcomes

BACKGROUND

There has been an increasing incidence of carbapenem-resistant Acinetobacter baumannii (CRAB) infections in recent years. The objective of this study was to determine specific risk factors for and outcome of bacteremia due to CRAB isolates among our ICU patients with A. baumannii bacteremia.

PATIENTS AND METHODS

Among 96 patients with ICU-acquired A. baumannii bacteremia, 30 patients with CRAB were compared with the remaining 66 with carbapenem-susceptible A. baumannii (CSAB) isolates.

RESULTS

Recent ventilator-associated pneumonia (VAP) due to CRAB (OR 16.74, 95% CI 3.16-88.79, p = 0.001) and a greater number of intravascular devices (OR 3.93, 95% CI 1.9-13.0, p = 0.025) were independently associated with CRAB bacteremia acquisition. Patients with CRAB bacteremia had a lower severity of illness on admission than those with CSAB. Although, by univariate analysis, patients with CRAB were more likely to have had exposure to colistin, carbapenems and linezolid, multivariate analysis did not revealed any significant association. The mortality was not different between patients with CRAB and CSAB bacteremia (43.3 vs. 46.9%, p = 0.740). Severity of organ failure (OR 1.42, 95% CI 1.20-1.67, p = 0.001), and increased white blood cell (WBC) count (OR 1.09, 95% CI 1.01-1.19, p = 0.036), at bacteremia onset were independently associated with mortality.

CONCLUSION

VAP due to CRAB and excess use of intravascular devices are the most important risk factors for CRAB bacteremia in our ICU. Severity of organ failure and WBC count at A. baumannii bacteremia onset are independently associated with mortality.

Characterization of the DsbA oxidative folding catalyst from Pseudomonas aeruginosa reveals a highly oxidizing protein that binds small molecules

Bacterial antibiotic resistance is an emerging global crisis, and treatment of multidrug-resistant gram-negative infections, particularly those caused by the opportunistic human pathogen Pseudomonas aeruginosa, remains a major challenge. This problem is compounded by a lack of new antibiotics in the development pipeline: only two new classes have been developed since the 1960s, and both are indicated for multidrug-resistant gram-positive infections. A promising new approach to combat antibiotic resistance is by targeting bacterial virulence, rather than bacterial viability. The bacterial periplasmic protein DsbA represents a central point for antivirulence intervention because its oxidoreductase activity is essential for the folding and function of almost all exported virulence factors. Here we describe the three-dimensional structure of this DsbA target from P. aeruginosa, and we establish for the first time that a member of this enzyme family is capable of binding small molecules. We also describe biochemical assays that validate the redox activity of PaDsbA. Together, the structural and functional characterization of PaDsbA provides the basis for future studies aimed at designing a new class of antivirulence compounds to combat antibiotic-resistant P. aeruginosa infection.

ramR mutations in clinical isolates of Klebsiella pneumoniae with reduced susceptibility to tigecycline

Five Klebsiella pneumoniae isolates with reduced susceptibility to tigecycline (MIC, 2 microg/ml) were analyzed. A gene homologous to ramR of Salmonella enterica was identified in Klebsiella pneumoniae. Sequencing of ramR in the nonsusceptible Klebsiella strains revealed deletions, insertions, and point mutations. Transformation of mutants with wild-type ramR genes, but not with mutant ramR genes, restored susceptibility to tigecycline and repressed overexpression of ramA and acrB. Thus, this study reveals a molecular mechanism for tigecycline resistance in Klebsiella pneumoniae.

Management of antibiotic resistance in the intensive care unit setting

Over the past few decades, an alarming increase of infections caused by antibiotic-resistant pathogens, including methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus species, carbapenem-resistant Pseudomonas aeruginosa, extended-spectrum beta-lactamase-producing Escherichia coli and Klebsiella spp., and multidrug-resistant Acinetobacter spp., has been observed, particularly in intensive care units. For clinicians, the rising resistance rate observed in nosocomial pathogens, when coupled with the lack of effective antimicrobials, represents the real challenge in the therapeutic management of critically ill patients. The contribution of clinicians in minimizing the increasing trend of resistance is represented by reduction of the patients' exposure to antibiotics, which reduces the resistance-selecting pressure, and by avoiding unnecessary antibiotic treatments. Recent issues on strategies to minimize resistance development and to appropriately manage critically ill patients with infections caused by multidrug-resistant organisms in the intensive care unit setting are discussed in this article.

New information about the polymyxin/colistin class of antibiotics

Infections by multidrug resistant Gram-negative bacilli (MDR-GNB) have become a major threat for patients hospitalized in intensive care units, representing a prevalent cause of morbimortality in the critically ill, since these microorganisms have developed resistance to most available antimicrobial agents. In this respect, very few therapeutic innovations have been developed in recent years, and it is not foreseen that any new drugs will be commercialized in the near future. Tigecycline represents an effective alternative in this setting, but lacks activity against Pseudomonas aeruginosa, and its use has not been validated for all organ-specific infections. Frequently, only old antibiotics like colistin remain a valid option. New pharmaceutical formulations and dosage regimens of polymyxins have considerably reduced the toxicity previously attributed to these antimicrobials, and have made it possible to reintroduce them into clinical practice. Nonetheless, the effectiveness of polymyxins is still suboptimal, and the expansion of heteroresistance and pan-drug-resistant strains of gram-negative bacilli is of concern. Improvements in dosing, alternative methods of administration and different synergic antimicrobial combinations have been proposed in recent literature, among other measures, to enhance the effectiveness of polymyxins. The latest data regarding polymyxins and their clinical use are discussed in this review.

The systemic imprint of growth and its uses in ecological (meta)genomics

Microbial minimal generation times range from a few minutes to several weeks. They are evolutionarily determined by variables such as environment stability, nutrient availability, and community diversity. Selection for fast growth adaptively imprints genomes, resulting in gene amplification, adapted chromosomal organization, and biased codon usage. We found that these growth-related traits in 214 species of bacteria and archaea are highly correlated, suggesting they all result from growth optimization. While modeling their association with maximal growth rates in view of synthetic biology applications, we observed that codon usage biases are better correlates of growth rates than any other trait, including rRNA copy number. Systematic deviations to our model reveal two distinct evolutionary processes. First, genome organization shows more evolutionary inertia than growth rates. This results in over-representation of growth-related traits in fast degrading genomes. Second, selection for these traits depends on optimal growth temperature: for similar generation times purifying selection is stronger in psychrophiles, intermediate in mesophiles, and lower in thermophiles. Using this information, we created a predictor of maximal growth rate adapted to small genome fragments. We applied it to three metagenomic environmental samples to show that a transiently rich environment, as the human gut, selects for fast-growers, that a toxic environment, as the acid mine biofilm, selects for low growth rates, whereas a diverse environment, like the soil, shows all ranges of growth rates. We also demonstrate that microbial colonizers of babies gut grow faster than stabilized human adults gut communities. In conclusion, we show that one can predict maximal growth rates from sequence data alone, and we propose that such information can be used to facilitate the manipulation of generation times. Our predictor allows inferring growth rates in the vast majority of uncultivable prokaryotes and paves the way to the understanding of community dynamics from metagenomic data.

Microbial minimal generation times vary from a few minutes to several weeks. The reasons for this disparity have been thought to lie on different life-history strategies: fast-growing microbes grow extremely fast in rich media, but are less capable of dealing with stress and/or poor nutrient conditions. Prokaryotes have evolved a set of genomic traits to grow fast, including biased codon usage and transient or permanent gene multiplication for dosage effects. Here, we studied the relative role of these traits and show they can be used to predict minimal generation times from the genomic data of the vast majority of microbes that cannot be cultivated. We show that this inference can also be made with incomplete genomes and thus be applied to metagenomic data to test hypotheses about the biomass productivity of biotopes and the evolution of microbiota in the human gut after birth. Our results also allow a better understanding of the co-evolution between growth rates and genomic traits and how they can be manipulated in synthetic biology. Growth rates have been a key variable in microbial physiology studies in the last century, and we show how intimately they are linked with genome organization and prokaryotic ecology.

[Emerging multidrug-resistant microorganisms among travelers returning to France and persons repatriated from foreign hospitals]

The spread of multidrug-resistant bacteria has become a major problem in France in recent years, owing to increasing antibiotic exposure, growing international exchanges, repatriation of hospitalized French patients, and treatment of French and foreign travelers in French hospitals. This article examines how different pathogens may become endemic in France.

[Evolution of antimicrobial resistance against Pseudomonas aeruginosa in a French university hospital between 2002 and 2006]

AIM OF STUDY

Monitor evolution of antibiotic resistance of Pseudomonas aeruginosa from 2002 to 2006 in our hospital to optimize antibiotherapy.

PATIENTS AND METHOD

The infections/colonizations with P. aeruginosa have been identified by the hospital's informatic database. Bacteriological samples realized 48hours after patient's admission was considered as nosocomial. A Cochran-Armitage test was conducted to assess the evolution of resistance.

RESULTS

During this period, 2098 infections/colonizations with P. aeruginosa have been identified. Bacteriological samples (68.5%) were nosocomial. Among the beta-lactam antibiotics, ceftazidime and imipenem were the most active (R=16.8% and 15.2%, respectively), followed by piperacillin and piperacillin-tazobactam (R=24.8%, 18.4%, respectively). Amikacin and tobramycin were more active than gentamicin (R=19.9%; 22.2% and 40.6%, respectively). 28.9% of strains were resistant to ciprofloxacin. Nosocomial strains were significantly more resistant than non-hospital strains: ceftazidime: 17.9% versus 14.2%, p=0.0346; ticarcillin-clavulanic acid: 47.5% versus 39.6%, p=0.0009; piperacillin-tazobactam: 20.0% versus 14.8%, p=0.0046; ciprofloxacin: 30.7% versus 25.2%, p=0.0112. A significant increase in the resistance of nosocomial strains to ceftazidime, ticarcillin-clavulanic acid and piperacillin-tazobactam was noted. Resistance from non-hospital strains to fluoroquinolones, aminoglycosides, ceftazidime, piperacillin and ticarcillin-clavulanic acid decreased significantly.

CONCLUSION

P. aeruginosa is a predominantly nosocomial microorganism. There is a decrease of resistance for non-hospital strains. But the resistance of nosocomial strains to antibiotics widely prescribed in hospital is worrying.

In vitro activity of tigecycline and comparator agents against a global collection of Gram-negative and Gram-positive organisms: tigecycline Evaluation and Surveillance Trial 2004 to 2007

The Tigecycline Evaluation and Surveillance Trial began in 2004 to monitor the in vitro activity of tigecycline and comparator agents against a global collection of Gram-negative and Gram-positive pathogens. Against Gram negatives (n = 63 699), tigecycline MIC(90)'s ranged from 0.25 to 2 mg/L for Escherichia coli, Haemophilus influenzae, Acinetobacter baumannii, Klebsiella oxytoca, Enterobacter cloacae, Klebsiella pneumoniae, and Serratia marcescens (but was > or =32 for Pseudomonas aeruginosa). Against Gram-positive organisms (n = 32 218), tigecycline MIC(90)'s were between 0.06 and 0.25 mg/L for Streptococcus pneumoniae, Enterococcus faecium, Streptococcus agalactiae, Staphylococcus aureus, and Enterococcus faecalis. The in vitro activity of tigecycline was maintained against resistant phenotypes, including multidrug-resistant A. baumannii (9.2% of isolates), extended-spectrum beta-lactamase-producing E. coli (7.0%) and K. pneumoniae (14.0%), beta-lactamase-producing H. influenzae (22.2%), methicillin-resistant S. aureus (44.5%), vancomycin-resistant E. faecium (45.9%) and E. faecalis (2.8%), and penicillin-resistant S. pneumoniae (13.8%). Tigecycline represents a welcome addition to the armamentarium against difficult to treat organisms.

Pseudomonas aeruginosa: a formidable and ever-present adversary

Pseudomonas aeruginosa is a versatile pathogen associated with a broad spectrum of infections in humans. In healthcare settings the bacterium is an important cause of infection in vulnerable individuals including those with burns or neutropenia or receiving intensive care. In these groups morbidity and mortality attributable to P. aeruginosa infection can be high. Management of infections is difficult as P. aeruginosa is inherently resistant to many antimicrobials. Furthermore, treatment is being rendered increasingly problematic due to the emergence and spread of resistance to the few agents that remain as therapeutic options. A notable recent development is the acquisition of carbapenemases by some strains of P. aeruginosa. Given these challenges, it would seem reasonable to identify strategies that would prevent acquisition of the bacterium by hospitalised patients. Environmental reservoirs of P. aeruginosa are readily identifiable, and there are numerous reports of outbreaks that have been attributed to an environmental source; however, the role of such sources in sporadic pseudomonal infection is less well understood. Nevertheless there is emerging evidence from prospective studies to suggest that environmental sources, especially water, may have significance in the epidemiology of sporadic P. aeruginosa infections in hospital settings, including intensive care units. A better understanding of the role of environmental reservoirs in pseudomonal infection will permit the development of new strategies and refinement of existing approaches to interrupt transmission from these sources to patients.