The risk factors for acquisition of imipenem-resistant Pseudomonas aeruginosa in the burn unit

Collateral sensitivity: An evolutionary trade-off between antibiotic resistance mechanisms, attractive for dealing with drug-resistance crisis

Background

The discovery and development of antimicrobial drugs were one of the most significant advances in medicine, but the evolution of microbial resistance limited the efficiency of these drugs.

Aim

This paper reviews the collateral sensitivity in bacteria and its potential and limitation as a new target for treating infections.

Results and Discussion

Knowledge mechanisms of resistance to antimicrobial agents are useful to trace a practical approach to treat and control of resistant pathogens. The effect of a resistance mechanism to certain antibiotics on the susceptibility or resistance to other drugs is a key point that may be helpful for applying a strategy to control resistance challenges. In an evolutionary trade-off known as collateral sensitivity, the resistance mechanism to a certain drug may be mediated by the hypersensitivity to other drugs. Collateral sensitivity has been described for different drugs in various bacteria, but the molecular mechanisms affecting susceptibility are not well demonstrated. Collateral sensitivity could be studied to detect its potential in the battle against resistance crisis as well as in the treatment of pathogens adapting to antibiotics. Collateral sensitivity-based antimicrobial therapy may have the potential to limit the emergence of antibiotic resistance.

Molecular epidemiology and carbapenem resistance of Pseudomonas aeruginosa isolated from patients with burns

OBJECTIVE

The aim of this study was to investigate the molecular epidemiology and carbapenem resistance mechanisms of Pseudomonas aeruginosa isolated from patients with burns in Azerbaijan, Iran.

METHOD

Pseudomonas aeruginosa was isolated from 38 patients with burns. Disk diffusion and agar dilution methods were used to determine antibiotic susceptibility patterns. The overproduction of AmpC β-lactamase and efflux pumps were detected by phenotypic methods. The presence of carbapenemase-encoding genes was detected by multiplex polymerase chain reaction (PCR). Expression of the OprD gene and MexAB efflux pumps were also evaluated with real-time PCR. Random amplified polymorphic DNA typing (RAPD-PCR) was used for genotyping of carbapenem-resistant Pseudomonas aeruginosa (CRPA).

RESULTS

Minimum inhibitory concentration (MIC) assays demonstrated high levels of resistance to all classes of antibiotics except colistin and polymyxin B. The initial screening by carbapenem disks indicated 24 isolates (63.15%) as CRPA. Different mechanisms of carbapenem resistance were observed, including carbapenemase production (8.4%), overexpression of AmpC (25%) and decreased expression of OprD (75%). The overexpression of MexAB efflux pumps was detected in 19 (79.1%) isolates by phenotypic assay or real-time PCR. The resistance to carbapenem was multifactorial in most cases (58.3%). The RAPD genotyping revealed different patterns with nine clusters.

CONCLUSION

According to our results, the prevalence of CRPA is at an alarming level. Our results did not demonstrate an epidemic clone. The most common mechanism of carbapenem resistance was decreased expression of OprD. Therefore, we suggest a reconsideration in the management of CRPA infections of patients in our burn care hospital in Azerbaijan, Iran.

Antimicrobial and antibiofilm activities of meropenem loaded-mesoporous silica nanoparticles against carbapenem-resistant Pseudomonas aeruginosa

The aims of the present study were the determination of antimicrobial and antibiofilm effects of meropenem-loaded mesoporous silica nanoparticles (MSNs) on carbapenem resistant Pseudomonas aeruginosa (P. aeruginosa) and cytotoxicity properties in vitro. The meropenem-loaded MSNs had shown antibacterial and biofilm inhibitory activities on all isolates at different levels lower than MICs and BICs of meropenem. The viability of HC-04 cells treated with serial concentrations as MICs and BICs of meropenem-loaded MSNs was 92-100%. According to the obtained results, meropenem-loaded MSNs display the significant antibacterial and antibiofilm effects against carbapenem resistant and biofilm forming P. aeruginosa and low cell toxicity in vitro. Then, the prepared system can be an appropriate option for the delivery of carbapenem for further evaluation in vivo assays.

Unnatural Amino-Acid-Based Star-Shaped Poly(l-Ornithine)s as Emerging Long-Term and Biofilm-Disrupting Antimicrobial Peptides to Treat Pseudomonas aeruginosa-Infected Burn Wounds

Peptide-based antimicrobial materials are recognized as promising alternatives to antibiotics to circumvent the emergence of antibiotic-resistant bacteria or to combat multiple resistant bacteria by targeting the bacterial cell membrane. The components and conformations of antimicrobial peptides are extensively explored to achieve broad-spectrum and effective antimicrobial activity. Here, star-shaped antimicrobial polypeptides are fabricated by employing homologs of poly(l-lysine)s (i.e., poly(l-ornithine)s, poly(l-lysine)s, and poly(l-α,ζ-diaminoheptylic acid)s) with the aim of modulating their charge/hydrophobicity balance and rationalizing their structure-antimicrobial property relationships. The in vitro antibacterial investigation reveals that unnatural amino-acid-based star-shaped poly(l-ornithine)s have remarkable proteolytic stability, excellent biofilm-disrupting capacity, and broad-spectrum antimicrobial activity, even against difficult-to-kill Gram-negative Pseudomonas aeruginosa. Furthermore, star-shaped poly(l-ornithine)s significantly reduce the microbial burden and improve the burn wound healing of mouse skin infected with P. aeruginosa. These results demonstrate that unnatural amino-acid-based star-shaped poly(l-ornithine)s can serve as emerging long-term and biofilm-disrupting antimicrobial agents to treat biofilm-related infections in burn, especially caused by notorious P. aeruginosa.

Characterization of carbapenem-resistant but cephalosporin-susceptible Pseudomonas aeruginosa

In this study, mechanisms of carbapenem resistance in carbapenem-resistant but cephalosporin-susceptible (Car-R/Ceph-S) Pseudomonas aeruginosa were investigated. A total of 243 P. aeruginosa isolates were studied. The disk diffusion and agar dilution methods were used for determination of antibiotic susceptibility patterns. AmpC and efflux pump overproductions were detected by phenotypic methods. The presence of carbapenemase-encoding genes was detected by polymerase chain reaction (PCR). The expression of OprD, MexAB-OprM, and MexXY-OprM efflux pumps was assessed by real-time PCR. According to disk diffusion method, altogether 116 P. aeruginosa isolates (47.7%) were carbapenem-resistant and among them, 23 isolates (19.8%) were cephalosporin-susceptible. Carbapenemase producer was not detected. Overexpression of AmpC was detected in one (4.3%) isolate that was ceftazidime-susceptible but cefepime-resistant. Overexpression of MexAB-OprM and MexXY-OprM efflux pumps was detected in 12 (60.9%) and 16 (68.8%) of isolates, respectively. A total of 16 (68.8%) isolates showed decreased expression of OprD. The Car-R/Ceph-S P. aeruginosa did not develop by carbapenemase production. The resistance to carbapenem was mediated in our clinical isolates by decreased expression of OprD and overexpression of MexAB-OprM and MexXY-OprM efflux systems or the combination of these mechanisms.

Changes of resistance rates in Pseudomonas aeruginosa strains are unrelated to antimicrobial consumption in ICU populations with invasive device-related infection

OBJECTIVE

To evaluate the relationship between antipseudomonal antibiotic consumption and each individual drug resistance rate in Pseudomonas aeruginosa strains causing ICU acquired invasive device-related infections (IDRI).

DESIGN

A post hoc analysis was made of the data collected prospectively from the ENVIN-HELICS registry.

SETTING

Intensive Care Units participating in the ENVIN-UCI registry between the years 2007 and 2016 (3-month registry each year).

PATIENTS

Patients admitted for over 24h.

MAIN VARIABLES

Annual linear and nonlinear trends of resistance rates of P. aeruginosa strains identified in IDRI and days of treatment of each antipseudomonal antibiotic family per 1000 occupied ICU bed days (DOT) were calculated.

RESULTS

A total of 15,095 episodes of IDRI were diagnosed in 11,652 patients (6.2% out of a total of 187,100). Pseudomonas aeruginosa was identified in 2095 (13.6%) of 15,432 pathogens causing IDRI. Resistance increased significantly over the study period for piperacillin-tazobactam (P<0.001), imipenem (P=0.016), meropenem (P=0.004), ceftazidime (P=0.005) and cefepime (P=0.015), while variations in resistance rates for amikacin, ciprofloxacin, levofloxacin and colistin proved nonsignificant. A significant DOT decrease was observed for aminoglycosides (P<0.001), cephalosporins (P<0.001), quinolones (P<0.001) and carbapenems (P<0.001).

CONCLUSIONS

No significant association was observed between consumption of each antipseudomonal antibiotic family and the respective resistance rates for P. aeruginosa strains identified in IDRI.

Modifiable risk factors for multidrug-resistant Gram-negative infection in critically ill burn patients: a systematic review and meta-analysis

BACKGROUND

We conducted a systematic review and meta-analysis to identify potentially modifiable risk factors for multidrug-resistant Gram-negative colonization or infection in critically ill burn patients.

METHODS

A systematic search was conducted of PubMed, Embase, CINAHL, Web of Science and Central (Cochrane). Risk factors including antibiotic use and hospital interventions were summarized in a random-effects meta-analysis. Risk of publication bias was assessed using the Grading of Recommendations Assessment, Development and Evaluation method and funnel plots.

RESULTS

A total of 11 studies met the inclusion criteria. We identified several potentially modifiable risk factors and were able to grade their importance based on effect size. Related to prior antibiotic exposure, extended-spectrum cephalosporins (pooled odds ratio (OR) 7.00, 95% confidence interval (CI) 2.77-17.67), carbapenems (pooled OR 6.65, 95% CI 3.49-12.69), anti-pseudomonal penicillins (pooled OR 4.23, 95% CI 1.23-14.61) and aminoglycosides (pooled OR 4.20, 95% CI 2.10-8.39) were most significant. Related to hospital intervention, urinary catheters (pooled OR 11.76, 95% CI 5.03-27.51), arterial catheters (pooled OR 8.99, 95% CI 3.84-21.04), mechanical ventilation (pooled OR 5.49, 95% CI 2.59-11.63), central venous catheters (pooled OR 4.26, 95% CI 1.03-17.59), transfusion or blood product administration (pooled OR 4.19, 95% CI 1.48-11.89) and hydrotherapy (pooled OR 3.29, 95% CI 1.64-6.63) were most significant.

CONCLUSION

Prior exposure to extended-spectrum cephalosporins and carbapenems, as well as the use of urinary catheters and arterial catheters pose the greatest threat for infection or colonization with multidrug-resistant Gram-negative organisms in the critically ill burn patient population.

Importance of control groups when delineating antibiotic use as a risk factor for carbapenem resistance, extreme-drug resistance, and pan-drug resistance in Acinetobacter baumannii and Pseudomonas aeruginosa: A systematic review and meta-analysis

BACKGROUND

Carbapenem-resistant (CR), extremely drug-resistant (XDR), and pan-drug-resistant (PDR) Acinetobacter baumannii and Pseudomonas aeruginosa pose a huge clinical threat. This study reviews the impact of control groups on the association of antecedent antibiotic use and the acquisition of CR/XDR/PDR A. baumannii and P. aeruginosa.

METHODS

Studies investigating the role of antibiotics as a risk factor for CR/XDR/PDR A. baumannii and P. aeruginosa acquisition in adult hospitalized patients from 1950 to 2016 were identified in the databases. These were divided into two groups: antibiotic-resistant versus antibiotic-sensitive pathogens (group I); antibiotic-resistant versus no infection (group II). A random-effects model was performed.

RESULTS

Eighty-five studies (46 A. baumannii, 38 P. aeruginosa, and one of both) involving 22 396 patients were included. CR was investigated in 60 studies, XDR in 20 studies, and PDR in two studies. Prior antibiotic exposure was associated with significant acquisition of CR/XDR/PDR A. baumannii and P. aeruginosa in both groups I and II (p<0.05). Antibiotic classes implicated in both groups included aminoglycosides, carbapenems, glycopeptides, and penicillins. Cephalosporin use was not associated with resistance in either group. Fluoroquinolone exposure was only associated with resistance in group I but not group II.

CONCLUSIONS

Control groups play an important role in determining the magnitudes of risk estimates for risk factor studies, hence careful selection is necessary. Antibiotic exposure increases the acquisition of highly resistant A. baumannii and P. aeruginosa, thus appropriate antibiotic use is imperative.

Risk factors for multidrug-resistant Gram-negative infection in burn patients

BACKGROUND

Infection with multidrug-resistant (MDR) Gram-negative organisms leads to poorer outcomes in the critically ill burn patient. The aim of this study was to identify the risk factors for MDR Gram-negative pathogen infection in critically ill burn patients admitted to a major tertiary referral intensive care unit (ICU) in Australia.

METHODS

A retrospective case-control study of all adult burn patients admitted over a 7-year period was conducted. Twenty-one cases that cultured an MDR Gram-negative organism were matched with 21 controls of similar age, gender, burn size and ICU stay. Multivariable conditional logistic regression was used to individually assess risk factors after adjusting for Acute Burn Severity Index. Adjusted odds ratios (ORs) were reported. P-values < 0.25 were considered as potentially important risk factors.

RESULTS

Factors increasing the risk of MDR Gram-negative infection included superficial partial thickness burn size (OR: 1.08; 95% confidence interval (CI): 1.01-1.16; P-value: 0.034), prior meropenem exposure (OR: 10.39; 95% CI: 0.96-112.00; P-value: 0.054), Gram-negative colonization on admission (OR: 9.23; 95% CI: 0.65-130.15; P-value: 0.10) and escharotomy (OR: 2.66; 95% CI: 0.52-13.65; P-value: 0.24). For cases, mean age was 41 (SD: 13) years, mean total body surface area burned was 47% (SD: 18) and mean days in ICU until MDR specimen collection was 17 (SD: 10) days.

CONCLUSION

Prior meropenem exposure, Gram-negative colonization on admission, escharotomy and superficial partial thickness burn size may be potentially important factors for increasing the risk of MDR Gram-negative infection in the critically ill burn patient.

A prospective randomized trial comparing piperacillin/tazobactam with meropenem as empirical antibiotic treatment of febrile neutropenic children and adolescents with hematologic and malignant disorders

BACKGROUND

This randomized prospective study was designed to assess whether piperacillin/tazobactam (PIPC/TAZ) is as effective as meropenem (MEPM) as a first-line antibiotic treatment for febrile neutropenia (FN).

PROCEDURE

FN episodes were randomly assigned to receive either PIPC/TAZ (337.5 mg/kg per day in three doses, 1-hr DIV, maximum 13.5 g per day) or MEPM (120 mg/kg per day in three doses, 1-hr DIV, maximum 3 g per day). Clinical responses were evaluated 120 hr after the DIV.

RESULTS

A total of 434 febrile episodes in 105 patients (42 females and 63 males) with a median age of 8 years (range 0-25) were included in this trial. Blood cultures were positive in 47 out of the 434 episodes (10.8%). Regarding responses to the treatment, success rates between the PIPC/TAZ and MEPM groups were similar (62.4 vs. 65.9%, P = 0.484), even if patients were restricted to those with bacteremia (26.1 vs 37.5%, P = 0.534). Mortality rates did not significantly differ between the two groups (0.8 vs. 0%, P = 0.500).

CONCLUSION

Both PIPC/TAZ and MEPM appeared to be equally efficacious and safe. Carbapenems are now broadly used to treat FN; however, this may increase the prevalence of drug-resistant bacteria. In this regard, the treatment using PIPC/TAZ for FN is more beneficial.

Short Synthetic β-Sheet Antimicrobial Peptides for the Treatment of Multidrug-Resistant Pseudomonas aeruginosa Burn Wound Infections

Pseudomonas aeruginosa is often implicated in burn wound infections; its inherent drug resistance often renders these infections extremely challenging to treat. This is further compounded by the problem of emerging drug resistance and the dearth of novel antimicrobial drug discovery in recent years. In the perennial search for effective antimicrobial compounds, the authors identify short synthetic β-sheet folding peptides, IRIKIRIK (IK8L), IRIkIrIK (IK8-2D), and irikirik (IK8D) as prime candidates owing to their high potency against Gram-negative bacteria. In this study, the peptides are first assayed against 20 clinically isolated multidrug-resistant P. aeruginosa strains in comparison with the conventional antibiotics imipenem and ceftazidime, and IK8L is demonstrated to be the most effective. IK8L also exhibits superior antibacterial killing kinetics compared to imipenem and ceftazidime. From transmission electron microscopy, confocal microscopy, and protein release analyses, IK8L shows membrane-lytic antimicrobial mechanism. Repeated use of IK8L does not induce drug resistance, while the bacteria develop resistance against the antibiotics after several times of treatment at sublethal doses. Analysis of mouse blood serum chemistry reveals that peptide does not induce systemic toxicity. The potential utility of IK8L in the in vivo treatment of P. aeruginosa-infected burn wounds is further demonstrated in a mouse model.

Emerging Carbapenem-Resistant Pseudomonas aeruginosa Isolates Carrying blaIMP Among Burn Patients in Isfahan, Iran

BACKGROUND

Metallo-β-lactamase (MBL)-producing Pseudomonas aeruginosa is a significant pathogen in burn patients.

OBJECTIVES

The aim of this study was to determine the prevalence of carbapenem-resistant P. aeruginosa isolates, including those resistant to imipenemase (IMP), in a burn unit in Isfahan, Iran.

PATIENTS AND METHODS

One hundred and fifty P. aeruginosa isolates from burn patients were tested for antibiotic susceptibility by the disc diffusion method in accordance with CLSI guidelines. Production of MBL was identified with the EDTA disk method. DNA was purified from the MBL-positive isolates, and detection of the bla_IMP gene was performed with PCR.

RESULTS

Fifty-seven out of 150 (38%) isolates were multi-drug resistant (MDR), and 93 (62%) were extensively-drug resistant (XDR). Among all isolates, the resistance rate to ciprofloxacin, tobramycin, imipenem, meropenem, amikacin, ceftazidime, and cefepime was higher than 90%, while the resistance rates to piperacillin/tazobactam and aztreonam were 70.7% and 86%, respectively. Colistin and polymyxin B remained the most effective studied antibiotics. All of the imipenem-resistant P. aeruginosa isolates were MBL-positive, and 107 out of 144 (74.3%) of the MBL isolates were positive for the bla_IMP gene.

CONCLUSIONS

The results of this study show that the rate of P. aeruginosa-caused burn wound infections was very high, and many of the isolates were resistant to three or more classes of antimicrobials. Such extensive resistance to antimicrobial classes is important because few treatment options remain for patients with burn wound infections. bla_IMP -producing P. aeruginosa isolates are a rising threat in burn-care units, and should be controlled by conducting infection-control assessments.

Comparison between piperacillin/tazobactam and cefepime monotherapies as an empirical therapy for febrile neutropenia in children with hematological and malignant disorders: A prospective, randomized study

To evaluate the efficacy and safety of piperacillin/tazobactam (PIPC/TAZ) or cefepime (CFPM) monotherapy for febrile neutropenia (FN) in children, a total of 53 patients with 213 febrile episodes were randomly treated with either PIPC/TAZ 337.5 mg/kg/day, or CFPM 100 mg/kg/day. No significant differences were observed in the success rates of the PIPC/TAZ and CFPM treatments (62.1% vs. 59.1%, P = 0.650). Furthermore, no differences were noted in the rates of new infection and mortality, and no serious adverse effects occurred in either of groups. Both PIPC/TAZ and CFPM were effective and safe as an empirical therapy for FN in children. Pediatr Blood Cancer 2015;62:356-358. © 2014 Wiley Periodicals, Inc.

Imipenem Resistance in Pseudomonas aeruginosa Emergence, Epidemiology, and Impact on Clinical and Economic Outcomes

Background.

Pseudomonas aeruginosa is one of the most common gram-negative hospital-acquired pathogens. Resistance of this organism to imipenem complicates treatment.

Objective.

To elucidate the risk factors for imipenem-resistant P. aeruginosa (IRPA) infection or colonization and to identify the effect of resistance on clinical and economic outcomes.

Methods.

Longitudinal trends in prevalence of IRPA from 2 centers were characterized during the period from 1989 through 2006. For P. aeruginosa isolates obtained during the period from 2001 through 2006, a case-control study was conducted to investigate the association between prior carbapenem use and IRPA infection or colonization, and a cohort study was performed to identify the effect of IRPA infection or colonization on mortality, length of stay after culture, and hospital cost after culture.

Results.

From 1989 through 2006, the proportion of P. aeruginosa isolates demonstrating resistance to imipenem increased from 13% to 20% (P< .001, trend). During the period from 2001 through 2006, there were 2,542 unique patients with P. aeruginosa isolates, and 253 (10.0%) had IRPA isolates. Prior carbapenem use was independently associated with IRPA infection or colonization (adjusted odds ratio [OR], 7.92 [95% confidence interval {CI}, 4.78-13.11]). Patients with an IRPA isolate recovered had higher in-hospital mortality than did patients with an imipenem-susceptible P. aeruginosa isolate (17.4% vs 13.4%; P = .01). IRPA infection or colonization was an independent risk factor for mortality among patients with isolates recovered from blood (adjusted OR, 5.43 [95% CI, 1.72-17.10]; P = .004) but not among patients with isolates recovered from other anatomic sites (adjusted OR, 0.78 [95% CI, 0.51-1.21]; P = .27). Isolation of IRPA was associated with longer hospital stay after culture (P<.001) and greater hospital cost after culture (P<.001) than was isolation of an imipenem-susceptible strain. In multivariable analysis, IRPA infection or colonization remained an independent risk factor for both longer hospital stay after culture (coefficient, 0.20 [95% CI, 0.04-0.36]; P = .02) and greater hospital cost after culture (coefficient, 0.30 [95% CI, 0.06-0.54]; P = .02).

Conclusions.

The prevalence of IRPA infection or colonization has increased significantly, with important implications for both clinical and economic outcomes. Interventions to curb this continued increase and strategies to optimize therapy are urgently needed.

Trends and correlation of antibacterial usage and bacterial resistance: time series analysis for antibacterial stewardship in a Chinese teaching hospital (2009-2013)

The purpose of this investigation was to describe the effect of antibacterial stewardship and evaluate the trends and correlation of antibacterial resistance and usage from 2009 to 2013 in a tertiary-care teaching hospital in northwest China. Antibacterial usage was expressed as defined daily doses per 100 patients per day (DDDs/100 PDs). Hospital-wide population-level data and time series analysis were used to evaluate the trends and determine associations between antibacterial exposure and acquisition of resistance. Yearly consumption of overall antibacterials significantly decreased from 66.54 to 28.08 DDDs/100 PDs (β = -10.504, p < 0.01). The resistant rates of the five most frequently isolated species (including Escherichia coli, Acinetobacter baumannii, Staphylococcus aureus, Pseudomonas aeruginosa, and Klebsiella pneumoniae) significantly decreased or remained stable, and none of them showed a statistically significant upward trend. The medical quality indicators got better or remained stable. Autoregressive integrated moving average (ARIMA) models demonstrated that the monthly resistance rate of P. aeruginosa to imipenem was strongly correlated with antipseudomonal carbapenems usage (β = 34.94, p < 0.001), as did the correlation of P. aeruginosa to meropenem with antipseudomonal third-generation cephalosporins usage (β = 32.76, p < 0.01) and K. pneumoniae to amikacin with aminoglycosides usage (β = 22.01, p < 0.001). The decreased antibacterial use paralleled the improved bacterial resistance without deteriorating medical quality indicators during antimicrobial stewardship. It also suggests that optimum antibiotic use is necessary to alleviate the threat posed by resistant microorganisms at the hospital level.

Antibiotic Susceptibility Patterns and Molecular Epidemiology of Metallo-β-Lactamase Producing Pseudomonas Aeruginosa Strains Isolated From Burn Patients

Background:

Failure in the treatment of burn patients infected with Pseudomonas aeruginosa could happen as a result of the acquisition of antibiotic resistance, including carbapenems.

Objectives:

The aim of the present study was to investigate the phenotypic and genotypic characteristics of the Pseudomonas aeruginosa strains, isolated from burn patients.

Patients and Methods:

During a 12 month period, in this cross-sectional study, two hundred seventy strains of Pseudomonas aeruginosa were isolated from the burn patients in Ghotbeddin Burn Hospital, Shiraz, Iran. Screening for the carbapenem resistance in the isolates was carried out by the E test method. Sensitivity patterns of metallo-β-lactamase (MβLs) producing strains of pseudomonas to eleven antibiotics were determined by the mentioned method. The epidemiological associations of these strains were determined by Pulsed-field gel electrophoresis (PFGE).

Results:

Of the 270 strains, 60 (22.2%) were resistant to imipenem and meropenem, classified as MβLs producing. MβLs producing strains of pseudomonas were completely resistant to five tested antibiotics while their sensitivities to the three most effective antibiotics including ceftazidime, amikacin and ciprofloxacin were 23.4%, 6.7 % and 1.7%, respectively. In PFGE, 37 patterns from the genome of Pseudomonas aeruginosa were observed. Majority of the strains (43; 71.6%) exhibited more than 80% similarity, based on the drawn dendrogram.

Conclusions:

According to the results, none of the tested antibiotics is safe to prescribe. As PFGE revealed, a limited number of Pseudomonas aeruginosa types are predominant in the hospitals which infect the burn patients.

Gram negative wound infection in hospitalised adult burn patients--systematic review and metanalysis-

BACKGROUND

Gram negative infection is a major determinant of morbidity and survival. Traditional teaching suggests that burn wound infections in different centres are caused by differing sets of causative organisms. This study established whether Gram-negative burn wound isolates associated to clinical wound infection differ between burn centres.

METHODS

Studies investigating adult hospitalised patients (2000-2010) were critically appraised and qualified to a levels of evidence hierarchy. The contribution of bacterial pathogen type, and burn centre to the variance in standardised incidence of Gram-negative burn wound infection was analysed using two-way analysis of variance.

PRIMARY FINDINGS

Pseudomonas aeruginosa, Klebsiella pneumoniae, Acinetobacter baumanni, Enterobacter spp., Proteus spp. and Escherichia coli emerged as the commonest Gram-negative burn wound pathogens. Individual pathogens' incidence did not differ significantly between burn centres (F (4, 20) = 1.1, p = 0.3797; r2 = 9.84).

INTERPRETATION

Gram-negative infections predominate in burn surgery. This study is the first to establish that burn wound infections do not differ significantly between burn centres. It is the first study to report the pathogens responsible for the majority of Gram-negative infections in these patients. Whilst burn wound infection is not exclusive to these bacteria, it is hoped that reporting the presence of this group of common Gram-negative "target organisms" facilitate clinical practice and target research towards a defined clinical demand.

A systematic review and meta-analyses show that carbapenem use and medical devices are the leading risk factors for carbapenem-resistant Pseudomonas aeruginosa

A systematic review and meta-analyses were performed to identify the risk factors associated with carbapenem-resistant Pseudomonas aeruginosa and to identify sources and reservoirs for the pathogen. A systematic search of PubMed and Embase databases from 1 January 1987 until 27 January 2012 identified 1,662 articles, 53 of which were included in a systematic review and 38 in a random-effects meta-analysis study. The use of carbapenem, use of fluoroquinolones, use of vancomycin, use of other antibiotics, having medical devices, intensive care unit (ICU) admission, having underlying diseases, patient characteristics, and length of hospital stay were significant risk factors in multivariate analyses. The meta-analyses showed that carbapenem use (odds ratio [OR] = 7.09; 95% confidence interval [CI] = 5.43 to 9.25) and medical devices (OR = 5.11; 95% CI = 3.55 to 7.37) generated the highest pooled estimates. Cumulative meta-analyses showed that the pooled estimate of carbapenem use was stable and that the pooled estimate of the risk factor "having medical devices" increased with time. We conclude that our results highlight the importance of antibiotic stewardship and the thoughtful use of medical devices in helping prevent outbreaks of carbapenem-resistant P. aeruginosa.

Surveillance and correlation of antimicrobial usage and resistance of Pseudomonas aeruginosa: a hospital population-based study

This retrospective study evaluated trends and association between resistance of Pseudomonas aeruginosa isolated from patients with hospital-acquired infections (HAIs) and hospital antimicrobial usage from 2003 through 2011 in a tertiary care hospital in northeast China. HAI was defined as occurrence of infection after hospital admission, without evidence that infection was present or incubating (≦48 h) on admission. In vitro susceptibilities were determined by disk diffusion test and susceptibility profiles were determined using zone diameter interpretive criteria, as recommended by Clinical and Laboratory Standards Institute (CLSI). Data on usage of various antimicrobial agents, expressed as defined daily dose (DDD) per 1,000 patients-days developed by WHO Anatomical Therapeutical Chemical (ATC)/DDD index 2011, were collected from hospital pharmacy computer database. Most of 747 strains of P. aeruginosa were collected from respiratory samples (201 isolates, 26.9%), blood (179, 24.0%), secretions and pus (145, 19.4%) over the years. Time series analysis demonstrated a significant increase in resistance rates of P. aeruginosa to ticarcillin/clavulanic acid, piperacillin/tazobactam, cefoperazone/sulbactam, piperacillin, imipenem, meropenem, ceftazidime, cefepime, ciprofloxacin, and levofloxacin except aminoglycosides over time in the hospital (P<0.001). The rates of carbapenem-resistant P. aeruginosa (CRPA) isolated from patients with HAIs were 14.3%, 17.1%, 21.1%, 24.6%, 37.0%, 48.8%, 56.4%, 51.2%, and 54.1% over time. A significant increase in usage of anti-pseudomonal carbapenems (P<0.001) was seen. ARIMA models demonstrated that anti-pseudomonal carbapenems usage was strongly correlated with the prevalence of imipenem and meropenem-resistant P. aeruginosa (P<0.001). Increasing of quarterly CRPA was strongly correlated at one time lag with quarterly use of anti-pseudomonal carbapenems (P<0.001). Our data demonstrated positive correlation between anti-pseudomonal antimicrobial usage and P. aeruginosa resistance to several classes of antibiotics, but not all antimicrobial agents in the hospital.

Epidemiological interpretation of studies examining the effect of antibiotic usage on resistance

Bacterial resistance to antibiotics is a growing clinical problem and public health threat. Antibiotic use is a known risk factor for the emergence of antibiotic resistance, but demonstrating the causal link between antibiotic use and resistance is challenging. This review describes different study designs for assessing the association between antibiotic use and resistance and discusses strengths and limitations of each. Approaches to measuring antibiotic use and antibiotic resistance are presented. Important methodological issues such as confounding, establishing temporality, and control group selection are examined.

Blood stream infections in patients in the burn intensive care unit

Background

The study on bacteremia helps empirically select the proper antibiotics before the results of culture test about causative pathogen. The purpose of this study is to investigate causative pathogen in bloodstream infection, changing aspects based on elapsed time after burn, relationship with other sites and resistance of important causative pathogen against antibiotics through analysis on bacteria isolated from blood culture of patients hospitalized in burn intensive care unit (BICU).

Materials and Methods

A retrospective study was conducted targeting patients hospitalized in BICU from January 2007 to June 2011. Changes of causative pathogen in bloodstream infection based on elapsed time after injury were analyzed. We would like to examine the relationship between bloodstream infection and infection on other body parts by comparing results of cultures in burn wound site, sputum, urine and catheter tip. Antibiotics resistance patterns of Pseudomonas aeruginosa, Acinetobacter baumannii, Staphylococcus aureus, Enterococcus species, and Klebsiella pneumoniae were studied.

Results

A total of 2,337 burn patients were hospitalized in BICU for 54 months. Causative pathogen was cultured in blood cultures from 397 patients (17.0%). P. aeruginosa (169, 30.1%) was the most cultured and A. baumannii (107, 19.0%) and S. aureus (81, 14.4%) were followed. It was confirmed that the relative frequency of A. baumannii tended to get lower as the period got longer after injury, but the relative frequency of K. pneumoniae got higher as the period got longer after injury. With comparison without bacteremia, P. aeruginosa bacteremia showed high probability in which the same bacteria were cultured in wound site, sputum and cathether tip, and A. baumannii bacteremia and candida bacteremia had high probability in sputum, and urine and catheter tip, respectively. 95.9% of P. aeruginosa and 95.3% of A. baumannii showed the resistance against carbapenem. 96.3% of S. aureus was methicillin resistant and 36.2% of Enterococcus species were vancomycin resistant. 75.0% of K. pneumonia were extended-spectrum beta-lactamase (ESBL)-producing bacteria.

Conclusions

Since the highly antibiotic resistant microorganisms were isolated from the patients hospitalized in BICU during early phase, the empirical selection of antibiotics targeting these pathogens should be considered before the results of microbiologic culture test. In addition, use of empirical antifungal agent after 1 week of injury can be considered for patients who have risk factor of fungal infection.

Correlation between the consumption of meropenem or doripenem and meropenem susceptibility of Pseudomonas aeruginosa in a university hospital in Japan

The appropriate use of carbapenems is essential in order to prevent resistance in Pseudomonas aeruginosa. We investigated the correlation between the consumption of meropenem or doripenem and the susceptibility of P. aeruginosa to meropenem in a Japanese university hospital from 2004 to 2009. The susceptibility data of P. aeruginosa and the annual consumption of meropenem or doripenem were analyzed. The consumption of meropenem or doripenem was calculated using the Anatomic Therapeutic Chemical classification and defined daily doses methodology. Meropenem consumption decreased and doripenem consumption increased and throughout the entire investigation period, total consumption of meropenem plus doripenem was stable. Although the annual number of isolated P. aeruginosa has not changed, the annual number of isolated multidrug resistant P. aeruginosa decreased by measures against nosocomial infection. The rate of meropenem resistant P. aeruginosa decreased in a time-dependent manner. Meropenem consumption was positively correlated with the meropenem resistance rate among P. aeruginosa (r = 0.9455, p<0.01). The total consumption of meropenem and doripenem was not correlated with the meropenem resistance rate (r = -0.6601, p>0.1). These results suggested that even if the total consumption of meropenem plus doripenem was not changed, meropenem susceptibility among P. aeruginosa improved by the decrease of meropenem consumption. Although meropenem and doripenem have been suggested to show cross-resistance with each other, the reduction of meropenem consumption might be effective for preventing an increase of meropenem-resistant P. aeruginosa.

Piperacillin/tazobactam versus cefozopran for the empirical treatment of pediatric cancer patients with febrile neutropenia

BACKGROUND

The aim of this study was to evaluate the efficacy and safety of piperacillin/tazobactam (PIP/TAZO) and cefozopran (CZOP) monotherapy in pediatric cancer patients with febrile neutropenia (FN).

PROCEDURE

A total of 119 febrile episodes in 49 neutropenic pediatric cancer patients (20 females and 29 males) with a median age of 6.8 years (range, 0.3-18.4 years) received randomized treatment either with PIP/TAZO 125 mg/kg every 8 hr or CZOP 25 mg/kg every 6 hr. Clinical response was determined at completion of therapy. Durations of fever and neutropenia, the need for modification of the therapy, and mortality rates were compared between the two groups.

RESULTS

The frequency of success without modification of treatment was not significantly different between PIP/TAZO (59.6%) and CZOP (53.2%). Durations of fever and antibiotic therapy did not differ between the treatment groups, and no major side effects were observed in either group.

CONCLUSIONS

PIP/TAZO and CZOP monotherapy were both effective and safe for the initial empirical treatment of pediatric cancer patients with FN.

Pseudomonas skin infection: clinical features, epidemiology, and management

Pseudomonas aeruginosa is a Gram-negative bacillus that is most frequently associated with opportunistic infection, but which can also present in the otherwise healthy patient. The range of P. aeruginosa infections varies from localized infections of the skin to life-threatening systemic disease. Many P. aeruginosa infections are marked by characteristic cutaneous manifestations. The aim of this article is to provide a comprehensive synthesis of the current knowledge of cutaneous manifestations of P. aeruginosa infection with specific emphasis on clinical features and management. The ability of P. aeruginosa to rapidly acquire antibacterial resistance is an increasingly well recognized phenomenon, and the correct application of antipseudomonal therapy is therefore of the utmost importance. A detailed discussion of currently available anti-pseudomonal agents is included, and the benefits of antimicrobial combination therapy versus monotherapy are explored. Rapid clinical recognition of P. aeruginosa infection aided by the identification of characteristic cutaneous manifestations can play a critical role in the successful management of potentially life-threatening disease.

Risk factors for hospital-acquired pneumonia caused by imipenem-resistant Pseudomonas aeruginosa in an intensive care unit

Imipenem-resistant Pseudomonas aeruginosa is a leading cause of hospital-acquired pneumonia. Aiming to determine the risk factors associated for hospital-acquired pneumonia due to imipenem-resistant Pseudomonas aeruginosa, we undertook a retrospective case-case-control study. Patients admitted to a 14-bed medical-surgical intensive care unit from a university-affiliated hospital with hospital-acquired pneumonia caused by imipenem-resistant Pseudomonas aeruginosa strains and by imipenem-susceptible Pseudomonas aeruginosa strains were matched to control patients by time under risk and comorbidities. A total of 58 resistant cases, 47 susceptible cases and 237 controls were evaluated. The risk factors independently associated to hospital-acquired pneumonia caused by imipenem-resistant Pseudomonas aeruginosa were: duration of hospitalisation, Acute Physiological and Chronic Health Evaluation II score, male gender receipt of haemodialysis, receipt of piperacillin-tazobactam and receipt of third-generation cephalosporins.

Emerging infections in burns

BACKGROUND

Patients who suffer severe burns are at higher risk for local and systemic infections. In recent years, emerging resistant pathogens have forced burn care providers world wide to search for alternative forms of treatment. Multidrug-resistant Staphylococcus aureus, Pseudomonas aeruginosa, Acinetobacter spp., and various fungal strains have been the major contributors to the increase in morbidity and mortality rates. Multi-drug-resistant S. aureus remains the major cause of gram-positive burn wound infections world wide. Treatment strategies include rigorous isolation protocols and new types of antibiotics where necessary.

METHODS

We reviewed 398 severely burned patients (burns >40% total body surface area [TBSA]) admitted to our hospital between 2000 and 2006. Patients who did not contract multi-drug-resistant gram-negative organisms during their hospital course and received our standard antibiotic regimen-vancomycin and piperacillin/tazobactam-served as controls (piperacillin/tazobactam; n = 280). The treatment group consisted of patients who, during their acute hospital stay, developed infections with multi-drug-resistant gram-negative pathogens and were treated with vancomycin and colistin for at least three days (colistin; n = 118).

RESULTS

Gram-negative organisms continue to cause the most severe infections in burn patients. Colistin has re-emerged as a highly effective antibiotic against multiresistant Pseudomonas and Acinetobacter infections of burns. Patients who required colistin therapy had a significantly larger average total and full-thickness burn than patients treated with piperacillin/tazobactam and vancomycin, and the mortality rate was significantly higher in the colistin group (p < 0.05). However, there was no significant difference between the colistin and piperacillin/tazobactam groups in the incidence of neurotoxicity, hepatic toxicity, or nephrotoxicity. The main fungal pathogens in burn patients are Candida spp., Aspergillus spp., and Fusarium spp. A definitive diagnosis is more difficult to obtain than in bacterial infections. Amphotericin B and voriconazole remain the two most important anti-fungal substances in our practice.

CONCLUSIONS

Innovations in fluid management, ventilatory support, surgical care, and antimicrobial therapy have contributed to a significant reduction in morbidity and mortality rates in burn patients. Vancomycin and clindamycin are the two most important reserve antibiotics for methicillin-resistant Staphylococcus aureus infection. Oxazolidinones and streptogramins have showed high effectiveness against gram-positive infections. Colistin has re-emerged as a highly effective antibiotic against multiresistant Pseudomonas and Acinetobacter infections. Current challenges include Candida, Aspergillus, and molds. The development of new agents, prudent and appropriate use of antibiotics, and better infection control protocols are paramount in the ongoing battle against multi-resistant organisms.

Correlation between meropenem and doripenem use density and the incidence of carbapenem-resistant Pseudomonas aeruginosa

Optimal use of carbapenems is an important issue in the prevention of resistance in Pseudomonas aeruginosa. In this study, we investigated the correlation between antimicrobial use density (AUD) of carbapenems and imipenem/cilastatin (IPM/CS) or meropenem (MEPM) susceptibility of P. aeruginosa strains. The AUD of five carbapenems [IPM/CS, panipenem/betamipron, biapenem, MEPM and doripenem (DRPM)] was examined every 6 months between 2006 and 2008. The AUD was calculated using the defined daily doses methodology developed by the World Health Organisation. A minimum inhibitory concentration of IPM/CS or MEPM of < or =4 mg/L was considered to be sensitive. There was a significant negative correlation between MEPM susceptibility and the total AUD of MEPM and DRPM [r=-0.823, 95% confidence interval (CI) -0.035 to -0.980; P=0.044]. Furthermore, there was a significant correlation between MEPM susceptibility and IPM/CS susceptibility (r=0.839, 95% CI 0.084 to 0.981; P=0.037). Cross-resistance was therefore investigated and only 5.6% of MEPM-insensitive strains were susceptible to IPM/CS, although 43.3% of IPM/CS-insensitive strains were susceptible to MEPM. These results suggest that curtailing the use of MEPM and DRPM may curb the emergence not only of MEPM-resistant strains but also IPM/CS-resistant strains.

Imipenem-resistant Pseudomonas aeruginosa infection at a medical-surgical intensive care unit: risk factors and mortality

OBJECTIVES

The aim of this study was to evaluate the risk factors and attributable mortality associated with imipenem-resistant Pseudomonas aeruginosa (IRPA) infections in a medical-surgical intensive care unit (ICU).

METHODS

A retrospective case-control study was carried out at a 16-bed medical-surgical ICU in a 780-bed, university-affiliated hospital. All patients admitted from January 1, 2003, to December 31, 2004, who had nosocomial infection caused by IRPA, were included in the study.

RESULTS

Imipenem-resistant P. aeruginosa was recovered from 63 patients during the study period. One hundred eighty-two controls were matched with cases by period of admission, age, and time at risk. Urinary tract (34.9%) and respiratory tract (22.2%) were the main sources of IRPA isolation. In multivariate analysis, a previous stay in the ICU (odds ratio, 3.54; 95% confidence interval [CI], 1.29-9.73; P = .03) was the only independent risk factor for IRPA infection. The in-hospital mortality rate among case patients was 49% (31 of 63) compared with 33% (61 of 182) for control patients (odds ratio, 1.92; 95% CI, 1.07-3.44; P = .02). Thus, we had an attributable mortality of 16% (95% CI, 9.74%-22.3%; P = .03).

CONCLUSIONS

Our study suggests that IRPA infections are strongly related to previous ICU stay, and that IRPA infections significantly increase mortality in those critical patients.

Are topical antimicrobials effective against bacteria that are highly resistant to systemic antibiotics?

An increasing number of bacteria are resistant to multiple systemic antibiotics. The purpose of this study was to determine if topical antimicrobials are still effective against multi-drug resistant organisms (MDROs). MDROs, including Acinetobacter, Pseudomonas, Klebsiella, Staphylococcus, and Enterococcus, were collected from four burn hospitals. The sensitivity of 47 MDROs to 11 commonly used topical agents (mafenide acetate, nystatin, mafenide + nystatin, silver nitrate, Dakin's, polymyxin B, neomycin, polymyxin + neomycin, silver sulfadiazine, bacitracin, silver sulfadiazine + bacitracin) was tested using the agar well diffusion assay and compared with the sensitivity of 27 non-MDROs of similar genera. Overall 88% of the tests of the non-MDROs showed susceptibility to the topicals compared with 80% for the MDROs (P < .05). Specific findings included: all of the gram-positive non-MDROs were sensitive to bacitracin compared with only 67% of the MDROs (P < .05); 74% of the non-MDROs were sensitive to neomycin vs 26% of the MDROs (P < .01). Even for the susceptible isolates, the zones of inhibition were smaller for the MDROs than for the non-MDROs (P < .002), indicating decreased susceptibility of the MDROs. Specifically, while the MDRO Acinetobacter were sensitive to most of the topicals, the zones of inhibition for silvadene, silvadene + bacitracin, neomycin, and neomycin + polymyxin were significantly smaller (P < .001) for the Acinetobacter MDROs than the non-MDROs. Although many topicals are still effective against some MDROs, MDROs are more resistant to topicals than are non-MDROs. Some treatment assumptions based historically on the efficacy of topical antimicrobial agents against non-MDROs need to be re-evaluated for MDROs.

Influence of antipseudomonal agents on Pseudomonas aeruginosa colonization and acquisition of resistance in critically ill medical patients

OBJECTIVE

To assess the role of antipseudomonal agents on Pseudomonas aeruginosa colonization and acquisition of resistance.

DESIGN

Prospective cohort study.

SETTING

Two medical intensive care units.

PATIENTS AND PARTICIPANTS

346 patients admitted for >or= 48 h.

INTERVENTION

Analysis of data from an 8-month study comparing a mixing versus a cycling strategy of antibiotic use.

MEASUREMENTS AND RESULTS

Surveillance cultures from nares, pharynx, rectum, and respiratory secretions were obtained thrice weekly. Acquisition of resistance was defined as the isolation, after 48 h of ICU stay, of an isolate resistant to a given antibiotic if culture of admission samples were either negative or positive for a susceptible isolate. Emergence of resistance refers to the conversion of a defined pulsotype from susceptible to non-susceptible. Forty-four (13%) patients acquired 52 strains of P. aeruginosa. Administration of piperacillin-tazobactam for >or= 3 days (OR 2.6, 95% CI 1.09-6.27) and use of amikacin for >or= 3 days (OR 2.6, 95% CI 1.04-6.7) were positively associated with acquisition of P. aeruginosa, whereas use of quinolones (OR 0.27, 95% CI 0.1-0.7) and antipseudomonal cephalosporins (OR 0.27, 95% CI 0.08-0.9) was protective. Exposure to quinolones and cephalosporins was not associated with the acquisition of resistance, whereas it was linked with usage of all other agents. Neither quinolones nor cephalosporins were a major determinant on the emergence of resistance to themselves, as resistance to these antibiotics developed at a similar frequency in non-exposed patients.

CONCLUSIONS

In critically ill patients, quinolones and antipseudomonal cephalosporins may prevent the acquisition of P. aeruginosa and may have a negligible influence on the acquisition and emergence of resistance.

Epidemiology of metalloenzyme-producing Pseudomonas aeruginosa in a tertiary hospital in Greece

A total of 132 infections of Pseudomonas aeruginosa (including 112 imipenem resistant, 32 of them producing VIM-2 beta-lactamase) were identified during a one-year period (June 2002-June 2003). PFGE molecular typing revealed that P. aeruginosa clinical isolates sensitive to imipenem, P. aeruginosa isolates resistant to imipenem but VIM-negative, and P. aeruginosa-resistant and VIM-positive isolates could be allocated to three different clusters with approximately 70% similarity. A case control study of patients infected with an MBL-producing imipenem-resistant P. aeruginosa isolate and controls (patients hospitalized in the same time period with no infection), revealed that only the number of catheters present at the time of the infection was strongly associated with the development of infection due to VIM-producing P. aeruginosa (OR 4.83, 95% CI: 1.94-12.0). In conclusion, the results of the molecular typing combined with the results of the case control study indicate that in the specific hospital setting, infection control, addressed specifically to critically ill patients, is an important part of any strategy to reduce imipenem-resistant infections.

Identification of patients with Pseudomonas aeruginosa respiratory tract infections at greatest risk of infection with carbapenem-resistant isolates

OBJECTIVE

To create a clinical tool based on institution-specific risk factors to estimate the probability of carbapenem resistance among Pseudomonas aeruginosa isolates obtained from infected patients. By better estimating the probability of carbapenem resistance on the basis of patient-specific factors, clinicians can refine their empirical therapy for P. aeruginosa infections and potentially maximize clinical outcomes by increasing the likelihood of appropriate empirical antimicrobial therapy.

DESIGN

A retrospective, cross-sectional study.

SETTING

Tertiary care academic hospital.

PATIENTS

All adult inpatients who had a respiratory tract infection due to P. aeruginosa between January 2001 and June 2005.

INTERVENTION

Data on demographic characteristics, antibiotic history, and microbiology were collected. Log-binomial regression was employed to identify predictors of carbapenem resistance among P. aeruginosa isolates and to devise the clinical prediction tool.

RESULTS

Among 351 patients with P. aeruginosa infection, 44% were infected with carbapenem-resistant P. aeruginosa strains. Independent predictors of carbapenem resistance were prior receipt of mechanical ventilation for 11 days or more, prior exposure to fluoroquinolones for 3 days or more, and prior exposure to carbapenems for 3 days or more.

CONCLUSIONS

With carbapenem resistance rates among P. aeruginosa isolates on the rise at our institution, the challenge was to identify patients for whom carbapenems would remain an effective empirical agent, as well as the patients at greatest risk for infection with carbapenem-resistant strains. The clinical prediction tool accurately estimated carbapenem resistance among this risk-stratified cross-sectional study of patients with P. aeruginosa infection. This tool may be an effective way for clinicians to refine their selection of empirical antibiotic therapy and to maximize clinical outcomes by increasing the likelihood of appropriate antibiotic treatment.

Differential effects of levofloxacin and ciprofloxacin on the risk for isolation of quinolone-resistant Pseudomonas aeruginosa

Due to the greater in vitro activity of ciprofloxacin than that of levofloxacin against Pseudomonas aeruginosa, the likelihood of isolating a clinical strain of quinolone-resistant (QR) P. aeruginosa might be greater after exposure to levofloxacin than ciprofloxacin. We examined the risk of isolating QR P. aeruginosa in association with prior levofloxacin or ciprofloxacin exposure. A case-case-control study was conducted. Two groups of cases, one with nosocomial QR P. aeruginosa infections and one with nosocomial quinolone-susceptible (QS) P. aeruginosa infections, were compared to a control group of hospitalized patients without P. aeruginosa infections. Bivariable and multivariable analyses were used to determine risk factors for isolation of QR P. aeruginosa and QS P. aeruginosa. One hundred seventeen QR P. aeruginosa and 255 QS P. aeruginosa cases were identified, and 739 controls were selected. Exposures to ciprofloxacin were similar among all three groups (8% for controls, 9.4% for QR P. aeruginosa cases, and 7.5% for QS P. aeruginosa cases; P >/= 0.6). Levofloxacin use was more frequent in the QR P. aeruginosa cases than in the controls (35.9% and 22.1%, respectively; odds ratio [OR] = 2.0; 95% confidence interval [CI] = 1.3 to 3.0) and less frequent in QS P. aeruginosa cases (14.1% of QS P. aeruginosa cases; OR = 0.6; 95% CI = 0.4 to 0.9). In multivariable analysis, levofloxacin, but not ciprofloxacin, was a significant risk factor for isolation of QR P. aeruginosa (OR for levofloxacin = 1.7 [95% CI = 1.0 to 2.9]; OR for ciprofloxacin = 1.2 [95% CI = 0.6 to 2.5]). Levofloxacin was associated with a reduced risk of isolation of QS P. aeruginosa (OR = 0.6; 95% CI = 0.4 to 0.9), whereas ciprofloxacin had no significant effect (OR = 1.0; 95% CI = 0.6 to 1.8). In conclusion, the use of levofloxacin, but not ciprofloxacin, was associated with isolation of QR P. aeruginosa.