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ORIGINAL ARTICLE BACTERIOLOGY

Exposure to is possibly associated with resistance

M. J. Cohen, C. S. Block, A. E. Moses and R. Nir-Paz Department of Clinical Microbiology and Infectious Diseases, Hadassah-Hebrew University Medical Centers, Jerusalem, Israel

Abstract

The role of antibiotic exposure in the evolution and emergence of resistance is challenging to assess. We used -resistant Pseudomonas aeruginosa (PA) phenotypes to assess possible factors that are associated with the occurrence and prognosis of such a phenotype and to examine the possible contribution of antibiotic exposure to the evolution of antimicrobial resistance. We conducted a nested case-control study. Cases were defined as patients from whom carbapenem-resistant ureidopenicillin-sensitive PA (CRUS-PA) was isolated; matched controls were PA patients who did not have isolation of CRUS-PA. We analysed potential predictors of CRUS-PA isolation and assessed their clinical significance (mortality and eventual isolation of pan-resistant PA), taking into account antibiotic exposures. We matched 800 case-control pairs. Case patients were more likely to have been exposed to anti-PA (OR = 6.9; 95% CI, 2.5–18.6). This finding did not apply to the administration of other . The mortality among CRUS-PA patients was similar to that of the controls (HR, 0.8 95%; CI, 0.6–1.1). Subsequent isolation of pan-resistant PA was more frequent among case patients compared with non-pan-resistant controls (p-value <0.05). Among cases, the risk of eventual pan-resistant PA isolation was increased in ertapenem recipients, only after and not prior to the index specimen date (HR, 1.9, 95%; CI, 1.01–3.4). Therefore we suggest that the CRUS-PA phenotype may represent pan beta-lactam resistance and that antibiotic exposure is associated with evolution of PA resistance phenotypes. We demonstrate a novel association of ertapenem with sequentially appearing PA resistance patterns.

Keywords: Carbapenem antibiotics, Pseudomonas aeruginosa, Resistance Original Submission: 11 May 2013; Revised Submission: 29 July 2013; Accepted: 6 August 2013 Editor: R. Canton Article published online: 24 August 2013 Clin Microbiol Infect 2014; 20: O188–O196 10.1111/1469-0691.12362

The major classes of beta-lactam antibiotics available for the Corresponding author: R. Nir-Paz, Department of Clinical treatment of PA include: ureidopenicillins; third and fourth Microbiology and Infectious Diseases, Hadassah Hebrew University Medical Centers, Jerusalem 91120, Israel generation (e.g. and ); and E-mail: [email protected] carbapenems (such as , and ), though ertapenem is not considered to be an effective treatment [2]. Mechanisms of resistance to different beta-lac- tam agents vary and beta-lactam pan-resistance has been documented [1,3]. Introduction Recently, it was suggested that patient exposure to either imipenem or ciprofloxacin is associated with development of Pseudomonas aeruginosa (PA) is a known major bacterial PA infections with wider resistance profiles [4]. Patient-- pathogen. Although it occasionally causes community-acquired to-patient transmission of carbapenem-resistant PA can also infections, it is a significant medical burden in hospital settings, account for these infections in intensive care settings [5]. where it may be resistant to a wide range of antibiotics [1]. Antibiotic pressure is regarded by many as the culprit that Beta-lactam antibiotics are considered the mainstay of treat- induces antibiotic resistance at the single level as well ment for PA. In severe infections, some recommend double as an ecological pressure that promotes the spread of such antibiotic coverage including non-beta-lactam antibiotics [2]. bacteria. Measures to curtail inappropriate antibiotic

ª2013 The Authors Clinical Microbiology and Infection ª2013 European Society of Clinical Microbiology and Infectious Diseases CMI Cohen et al. Ertapenem and Pseudomonas aeruginosa O189

prescription and encourage prudent use are promoted at Methods institutional and national levels [6,7]. However, information on the effect that antibiotics have on a patient and their contribution to the development of resistant bacteria in that The study was performed at the Hadassah-Hebrew University same patient is not widely available. The ecological trends are Medical Centre, Jerusalem, Israel, a tertiary care medical population centred and not patient centred. In fact there is a institution comprising two hospital campuses and a total of dearth of evidence of the individual probabilities of benefits/ 1100 inpatient beds (the larger campus has 775 beds), and was harms patients might experience with regard to emergence of approved by the institutional ethics committee. The institution resistance in their flora. gives care to both admitted and daycare patients and does not In our institution, we have noticed that in recent years deliver primary outpatient care, which is delivered by the there has been an increased proportion of Pseudomonas community physicians as part of the patients’ Health aeruginosa isolates that display a unique susceptibility/resis- maintenance organizations. The clinical, microbiological and tance phenotype – carbapenem resistance and ureidopeni- pharmacy data included in this study were collected from the cillin sensitivity. Until 2002 they comprised no more than institutional database between January 1987 to August 2009. 5% of all Pseudomonas aeruginosa isolates. Since 2002 their Our institution has maintained an antibiotic stewardship prevalence has varied between 5% and 8.7% (see Table S2 programme for over 25 years. Part of this programme includes and Fig. S1). The possibility of ureidopenicillin therapy for a scrutiny and approval of selected antibiotic prescriptions by carbapenem-resistant pathogen is unique. However, if the in infectious diseases consultants. All relevant orders and vitro phenotype does not predict an in vivo response, other approvals have been computerized and documented since therapeutic options should be sought. Here we assess 1987. The microbiology laboratory computerized database whether this phenotype was unique to certain patients and also dates back to 1987. whether it might have different prognostic implications from Patients were included according to the protocol presented other clinical PA isolates. Additional aims for this study in Fig. 1. Using the WHONET software package [8] we were to evaluate whether specific antibiotic exposure can reviewed the entire clinical microbiological database and be related to the isolation of this unique phenotype or identified PA cultures that were tested for susceptibility to other PA resistance phenotypes. The main foundations for one or more carbapenems used in our institution (imipenem this study were a reliable and substantive dataset based on and meropenem) and ureidopenicillins (, both a long-standing antibiotic stewardship programme in and ). For some years only one carbapenem or our institution, which also documents all antibiotic ureidopenicillin was in use. prescriptions, and our computerized microbiological data- Carbapenem-resistant ureidopenicillin-susceptible PA base. (CRUS-PA) was defined as a P. aeruginosa that was resistant

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ͳ͹ͷͺͺ’ƒ–‹‡–• 956 had CRUS-PA at least once

FIG. 1. Flow chart depicting the selection process of the included cases and controls in the analysis.

ª2013 The Authors Clinical Microbiology and Infection ª2013 European Society of Clinical Microbiology and Infectious Diseases, CMI, 20, O188–O196 O190 Clinical Microbiology and Infection, Volume 20 Number 3, March 2014 CMI

to either imipenem or meropenem (most of the time only one and 2010 (1100 isolates). There were four isolates that were of the two was tested) and susceptible to ureidopenicillins. For assessed with piperacillin and azlocillin, all sensitive to both. the purpose of this analysis we classified intermediate resis- There were another four isolates that were assessed with tance as resistant. Carbapenem-resistant ureidopenicillin-resis- piperacillin and mezlocillin; all four were sensitive to both. All tant PA (CRUR-PA) was defined when a PA isolate was isolates that were assessed with azlocillin were also assessed non-susceptible to both antibiotic groups (see Table S1). We with mezlocillin. In 171 isolates (97%) there was complete defined a patient’s first CRUS-PA-positive specimen as the concordance between the isolates. There were six that were index specimen and the date of specimen collection as the mezlocillin resistant and azlocillin sensitive and were consid- index date. Patients with PA isolates, but from whom ered ureodopenicillin resistant in our analysis. CRUS-PA was never isolated, were used for the control pool. During the entire study period there has been an active The control pool contained any PA isolates that were antimicrobial stewardship programme encompassing all ser- carbapenem-sensitive and ureidopenicillin-sensitive (CSUS), vices in the hospital. In this programme each department is carbapenem-sensitive and ureidopenicillin-resistant (CSUR) visited by an infectious diseases specialist every 2 days and or carbapenem-resistant and ureidopenicillin-resistant antibiotics are distributed only if the infectious diseases (CRUR), but never contained isolates that were CRUS. Cases specialist approves dispensing by the hospital pharmacy. and controls were matched for sex, year of specimen During the study period, in the hospital formulary, at least collection, hospital campus and type of ward in which the one ureidopenicillin was available. Carbapenems were intro- specimen was collected. All potential controls were stratified duced in 1990. Ertapenem was introduced in 2003. Standard according to the matching criteria and randomly sampled to precautions were and are used when treating patients carrying/ generate the study controls. Specimens were included if all infected with multidrug-resistant Pseudomonas aeruginosa. such data were available. When a case and control were matched, the control’s matched specimen was considered his Statistical analysis index specimen and the specimen date, the index date. Continuous variables were compared using the paired t-test Data collection included demographic data (date of birth, whereas categorical variables were compared with the sex and date of admission) and laboratory data within 48 h of McNemar test. All comparisons were performed between specimen collection: white blood cell count, serum albumin, patients from whom CRUS-PA was isolated and those patients serum creatinine, serum urea and haemoglobin levels. Expo- from whom it was not. As the cases and controls were sure to all classes of antibiotics since 1987 at our institution matched, conditional logistic regression was performed in was assessed. Each patient’s exposure to antibiotics was order to assess risk factors for CRUS-PA vs. non-CRUS-PA estimated in two ways: whether the patient received an phenotype. All variables collected were entered into the initial antibiotic on a specific day (yes/no), and the patient’s antibiotic model. Additionally, only variables that were closely associated load, defined as the total number of days each patient had with the CRUS-PA phenotype on univariate analysis received an antibiotic in the 60 days prior to and following the (p-value <0.2) were assessed in a separate model. In cases of index date. The antibiotic resistance profile of each PA isolate multi-colinearity the variables with greater variability were was recorded as well as specimen source. Nosocomial included in the models. Prognosis as a major outcome was acquisition was determined if the specimen was collected measured with time from index date to death or censorship. A more than 72 h after admission. secondary outcome was a later isolation of CRUR-PA. Kaplan– All cultured bacteria were processed according to NCCLS/ Meier survival curves were generated and compared with the CLSI guidelines. [9] The cut-off values for sensitive/interme- log-rank test. Additionally, matched stratified Cox propor- diate/resistant designation did not change over the study tional hazard models were employed for multivariate survival period. In our laboratory, most often either imipenem discs analysis. In all analyses, p-values <0.05 were deemed statisti- (1302 isolates) or sometimes meropenem discs were used cally significant. Analyses were performed with SPSS version (303 isolates in the years 2003–2006) to assess carbapenem 19 (SPSS Inc., Chicago, IL, USA). susceptibility. Rarely, both were used (five isolates). In only one case was the isolate imipenem resistant but meropenem Results sensitive, and deemed carbapenem resistant. Ureidopenicillin susceptibility was assessed with three discs during the study period: azlocillin discs were used between 1987 and 1995 (177 During the 22-year study period, 65 226 PA-positive cultures isolates), mezlocillin discs were used between 1987 and 2002 were analysed. There were 57 518 (88.2%) PA culture (504 isolates) and piperacillin discs were used between 1991 records, obtained from 18 886 patients, that were tested for

ª2013 The Authors Clinical Microbiology and Infection ª2013 European Society of Clinical Microbiology and Infectious Diseases, CMI, 20, O188–O196 CMI Cohen et al. Ertapenem and Pseudomonas aeruginosa O191

carbapenem and ureidopenicillin susceptibility (Fig. 1). A total TABLE 2. Sensitivity profiles of index specimensa of 9365 cultures (12% of all cultures) collected from 2499 Antibiotics Cases (%) Controls (%) p-Value patients (13% of all patients) were carbapenem resistant and of Gentamicin 481 (60) 558 (70) <0.001 these, 2426 cultures (25% of carbapenem-resistant PA cultures Amikacin 621 (78) 666 (83) 0.005 Ciprofloxacin 496 (62) 592 (74) <0.001 and 4.2% from all PA tested for both carbapenems and Ofloxacin 294 (37) 482 (60) <0.001 Colistin 796 (99) 795 (99) 0.31 ureidopenicillins) from 990 patients (39% of all patients with Ceftazidime 667 (83) 691 (86) 0.08 carbapenem-resistant PA) were ureidopenicillin susceptible. 382 (48) 423 (53) 0.04 Ureidopenicillin 800 (100) 584 (73) – Carbapenem resistance among PA emerged during the Meropenem 0 (0) (n = 150) 139 (92) (n = 152) – Imipenem 0 (0) (n = 650) 582 (90) (n = 648) – mid-1990s (Fig. S1a). Among the carbapenem-resistant PA, aThe table presents the number (%) of sensitive specimens. the proportion of CRUS-PA showed variability over the years (Fig. S1b; Table S2), ranging between 7% and 49%. After excluding patients with missing data, there were 956 patients In order to understand whether there was a possible (5.4% of all 18 886 patients) from whom CRUS-PA was association (either prior to or following the index date) isolated and were potential cases for the study (Fig. 1). We between antibiotic exposure (e.g. carbapenem administration) found 800 matched controls for CRUS-PA-positive patients and the CRUS-PA phenotype, we examined the antibiotic and these 1600 patients comprised the study sample for prescriptions given to the cases and controls before and after analysis. the isolation of the index culture. CRUS-PA cases were more Each group included 319 females (40%). In each group 93 likely to be exposed to carbapenems both prior to and after (12%) patients were hospitalized at the smaller campus. There the index date (Table 3). This was also true for exposure to was also matching in the in-hospital location of patients: 321 ureidopenicillins. The frequency of antibiotic exposure 60 days (40.1%) case-control pairs were in ICUs, 163 (20.4%) were in prior to and following specimen collection is presented in internal medicine wards, 163 (20.4%) in surgical wards, 51 Fig. 2. Antibiotic load was also higher in the CRUS-PA patients (6.4%) were from the emergency department, 48 (6%) were (Fig. S2). Assessment of possible risk factors for the occur- paediatric and the remaining 54 pairs were distributed amongst rence of CRUS-PA vs. non-CRUS-PA, using multivariate other clinical services. Characteristics of cases and controls analyses, confirmed that prior treatment with anti-PA carba- are presented in Table 1. The index specimens were more was associated with CRUS-PA isolation, with an odds often from urine and sputum, were less often designated as ratio of 6.9 (95% CI, 2.5–18.6). Age was not associated with community acquired and originated from younger patients and CRUS-PA isolation, nor was community acquisition (specimens those whose blood creatinine levels were slightly higher.

We compared the susceptibilities of the initial PA isolates in TABLE 3. Individual exposure to carbapenems and ureid- cases and controls (Table 2). Interestingly, the CRUS-PA openicillins in temporal relationship to index specimens isolates, as a group, were more resistant to a variety of Temporal Cases Controls non-beta-lactam antibiotics (including aminoglycosides and relationship Antibiotics (%) (%) p-Value fluoroquinolones) compared with the control group On index date Ertapenem 24 (3) 9 (1) 0.002 (p <0.005). They were more resistant to ceftazidime and Meropenem/ 119 (15) 30 (3.8) <0.001 imipenem aztreonam (p = 0.08 and 0.04). Ureidopenicillins 30 (3.8) 20 (2.5) 0.203 Prior to index Ertapenem 90 (11) 19 (2.4) <0.001 date – anytime Meropenem/ 336 (42) 77 (9.6) <0.001 imipenem TABLE 1. Characteristics of case and control patients and Ureidopenicillins 268 (33.5) 152 (19) <0.001 Prior to index date Ertapenem 70 (9) 16 (2) <0.001 their respective Pseudomonas aeruginosa-positive cultures – previous month Meropenem/ 204 (25.5) 37 (4.6) <0.001 imipenem Characteristics Case (n = 800) Control (n = 800) p-Value Ureidopenicillins 152 (19) 79 (9.9) <0.001 Prior to index date Ertapenem 87 (11) 19 (2.5) <0.001 – < Specimen type previous 6 months Meropenem/ 248 (31) 46 (5.8) 0.001 imipenem Blood 53 (6.6%) 56 (8%) 0.843 < Sputum 340 (43%) 274 (34%) 0.001 Ureidopenicillins 218 (27.3) 100 (12.5) 0.001 After index date Ertapenem 69 (9) 36 (4.5) <0.001 Wound 206 (26%) 185 (23%) 0.245 – < Urine 183 (23%) 236 (30%) 0.003 anytime Meropenem/ 289 (36.1) 115 (14.4) 0.001 imipenem Community 115 (24%) 157 (33%) 0.001 < acquired (3 days)a Ureidopenicillins 255 (31.9) 185 (23.1) 0.001 After index date Ertapenem 39 (5) 22 (2.8) 0.027 Age 55 24 58 25 0.002 – < Albumin 24 625 6 0.101 following month Meropenem/ 156 (19.5) 76 (9.5) 0.001 Haemoglobin 9.5 1.4 9.4 2.2 0.725 imipenem WBC 15.4 8.5 16.3 13.2 0.53 Ureidopenicillins 159 (19.9) 110 (13.8) 0.001 After index date Ertapenem 54 (6.8) 25 (3.1) <0.001 Urea 18.5 13.6 15.6 12.4 0.05 – < Creatinine 193 183 157 140 0.043 following 6 months Meropenem/ 192 (24) 86 (10.8) 0.001 imipenem < aData available for 477 pairs. Ureidopenicillins 201 (25.1) 131 (16.4) 0.001

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Imipenem/Meropenem Ertapenem Ureidopenicillins CRUR-PA positive CRUR-PA CRUR-PA positive CRUR-PA positive CRUR-PA

Days Days Days

Aminoglycosides Quinolones CRUR-PA positive CRUR-PA CRUR-PA positive CRUR-PA positive CRUR-PA

Days Days Days

FIG. 2. Frequency distribution of antibiotic exposure in temporal relation to the index specimen date. Cases are on the right pyramid (green) and controls are on the left (blue). (a) Distribution of antipseudomonal carbapenems (imipenem/meropenem). X-axis ranges between 0 and 220 days. (b) Distribution of ertapenem. X-axis ranges between 0 and 45 days. (c) Distribution of ureidopenicillin antibiotics. X-axis ranges between 0 and 45 days.

collected up to 72 h from admission or later) vs. nosocomial subsequent isolation of CRUR-PA, HR 0.8 (0.3–1.9), suggesting acquisition. Neither prior ureidopenicillin nor ertapenem that even when a CRUR-PA phenotype was subsequently regimens showed an association with CRUS-PA isolation. isolated from the CRUS-PA case patients, the survival Similarly, other antibiotic prescriptions (aminoglycosides, qui- outcome for those patients had not changed. nolones and colistin) were not associated with CRUS-PA As we found that prior use of carbapenems is associated isolation. with isolation of CRUS-PA, and a significant proportion of The mortality among CRUS-PA patients was similar to that CRUS-PA patients had later isolation of CRUR-PA, we of the controls (Fig. 3a). However, control patients with explored the possible association of antibiotic treatment with CRUR-PA index specimens had poorer survival compared the subsequent isolation of CRUR-PA among the CRUS-PA with all case patients and other control patients (Fig. S3). We patients. Surprisingly, patients who received ertapenem (a were intrigued by this because subsequent CRUR-PA isolation carbapenem that is not considered to be clinically effective for was more common among CRUS-PA patients compared with PA), as opposed to any other antibiotic, demonstrated a controls who did not start with CRUR-PA (Fig. 3b). Thus, in unique phenomenon. Specifically, the risk of eventual order to further understand if CRUS-PA was associated with CRUR-PA isolation was increased among CRUS-PA patients either increased mortality and/or increased likelihood of future who received ertapenem after the index specimen date and CRUR-PA isolation we used a stratified Cox proportional not prior to that date (HR, 1.9; 95% CI, 1.01–3.4). We did not hazard time-dependent regression model. The case patients find any similar pattern with other antibiotics, such as had a similar overall mortality rate to controls: hazard ratio imipenem/meropenem, ureidopenicillins, colistin, aminoglyco- (HR) of 0.8 (0.6–1.1). This was also true for patients with sides and quinolones (Fig. 4). This suggests a unique associa-

ª2013 The Authors Clinical Microbiology and Infection ª2013 European Society of Clinical Microbiology and Infectious Diseases, CMI, 20, O188–O196 CMI Cohen et al. Ertapenem and Pseudomonas aeruginosa O193

(a) CRUR-PA evolvement in CSUS-PA controls. Nevertheless, ureidopenicillin use was associated with the subsequent isolation of ureidopenicillin-resistant PA among these CSUS-PA patients (p 0.038). Not all patients (156, 16.3%) from whom CRUS-PA was isolated were included in the analysis due to lack of matching controls. We assessed whether the exclusion of those non-matched CRUS-PA patients had potentially biased our results. Indeed, non-matched CRUS-PA patients were com- monly from the Mt Scopus campus (42% vs. 12%, (b) p-value <0.001), the surgical subspecialty wards (24% vs. 16%) and pediatrics (22% vs. 6%), and less commonly from intensive care units (11% vs. 42%, p-value <0.001). The non-included CRUS-PA-positive patients were, on average, 12 years younger than the included CRUS-PA-positive patients (p-value <0.001). There was no difference in the sex and year of presentation between the included and non-included cases.

Discussion

(c) Carbapenem-resistant PA isolates emerged during the last decade of the 20th century. A subgroup of these demonstrated a susceptibility to ureidopenicillins (CRUS-PA). There are reports in which CRUS-PA was identified in as many as 40– 80% of carbapenem-resistant PA isolates [10–12]. In this report we used this PA resistance phenotype to study the effect of antibiotic selection pressure on the development of resistance patterns. We found that exposure to carbapenems is greater among cases, both before and after index specimens. Antibiotic exposure was associated with FIG. 3. Survival of patients and acquisition of resistant Pseudomonas subsequent isolation of CRUR-PA among CRUS-PA patients. aeruginosa (PA). (a) Kaplan–Meier survival curves. Cases (blue line) vs. Most notably, ertapenem exposure after the isolation of controls (orange line); p-values are with borderline statistical significance CRUS-PA, but not before, was predictive of CRUR-PA depending on method: log-rank 0.026, Breslow 0.071, Tarone-Ware isolation among the cases. 0.046. (b) Acquisition of carbapenem-resistant ureidopenicillin-resistant In line with our findings, it was previously shown that PA (CRUR-PA) since index date (CRUS-PA, blue line; carbapenem-sen- consumption of antibiotics is well associated with increased – sitive ureidopenicillin-sensitive PA, grey line; carbapenem-sensitive microbial resistance [13 17], including PA resistance [4,18]. ureidopenicillin-resistant PA, dark blue line). All three curves are Restriction of antibiotic use has also been shown to decrease significantly different one from the other (p-value <0.05). resistance [19,20]. That is a main reason for wide implementa- tion of antibiotic stewardship programmes [6,7]. PA is consid- tion between exposure to the non-PA carbapenem and future ered inherently resistant to ertapenem. Thus, it was presumed isolation of CRUR-PA. that the introduction of ertapenem into clinical use would delay As mentioned above, subsequent isolation of CRUR-PA was the emergence of carbapenem resistance in PA or even decrease far less common among control patients. Among control it [21–23]. Indeed, there are some studies suggesting that patients with a carbapenem-sensitive ureidopenicillin-sensitive ertapenem is not associated with increased carbapenem resis- (CSUS) PA (n = 554), there were 62 (11%) subsequent tance in PA [24–26]. However, those reports drew their ureidopenicillin-resistant isolates. Unlike the evolution to conclusions from analysis of collections of isolates and grouped CRUR-PA in a CRUS-PA patient, we did not found an antibiotic usage (ecological analyses), and there was no direct association between ertapenem and carbapenem use and analytical connection between the patients from whom PA

ª2013 The Authors Clinical Microbiology and Infection ª2013 European Society of Clinical Microbiology and Infectious Diseases, CMI, 20, O188–O196 O194 Clinical Microbiology and Infection, Volume 20 Number 3, March 2014 CMI

(a)

(b)

FIG. 4. Acquisition of carbapenem-resistant ureidopenicillin-resistant Pseudomonas aeruginosa (CRUR-PA) among carbapenem-resistant ureid- openicillin-susceptible PA (CRUS-PA) cases, according to the type of antibiotic exposure in a 6-month period. No antibiotic exposure prior to and after the index date, black line; antibiotic exposure only prior to the index date, orange line; antibiotic exposure only after the index date, blue line; antibiotic exposure before and after the index date, green line.

specimens were collected and those who received antibiotics. In that 82 multidrug-resistant PA-carrying patients were more contrast, our study overcame this limitation by using a often treated with prior broad-spectrum antibiotics [28]. In a patient-centred methodology and not an ecological analysis. study assessing different definitions of antibiotic exposure, Patient-centred analyses are more difficult to perform and ureidopenicillin resistance was associated with prior exposure are far less common than ecological analyses. Johnson et al. [5] to any antibiotics, depending on the cumulative burden of prior demonstrated that a significant proportion of ICU-associated exposures [29]. Our study is far larger than these previous imipenem-resistant PA was acquired via patient-to-patient analyses and presents patient-unique data spanning two transmission. There was no assessment of the role of antibiotic decades. exposure, though some imipenem-susceptible PA pulsotypes In vitro, antibiotic exposure has been linked to changes in became imipenem resistant, suggesting either evolutionary resistance mechanisms. Antibiotics have been shown to induce pressure or horizontal transfer [5]. Another study in a tertiary enzyme-mediated resistance [30], but other mechanisms may care centre demonstrated that among 142 imipenem-resistant exist. PA-carrying patients, quinolone exposure was more prevalent Our study has certain limitations. Molecular studies cannot than in patients with imipenem-susceptible PA [10]. In a be performed because most PA isolates are not preserved case-control study examining risk factors for acquisition of routinely. Such analyses might provide insight into mechanisms extended-spectrum drug-resistant PA, Park et al. [27] did not of resistance. Another possibility is that a substantial number find among the 33 cases that prior antibiotic exposure was of CRUS-PA specimens, for clinical purposes, are pan-beta-lac- more prevalent. A previous Israeli case-control report found tam resistant and ertapenem merely unveils this phenotype.

ª2013 The Authors Clinical Microbiology and Infection ª2013 European Society of Clinical Microbiology and Infectious Diseases, CMI, 20, O188–O196 CMI Cohen et al. Ertapenem and Pseudomonas aeruginosa O195

We are not aware of previous observations demonstrating Transparency Declarations selection of resistance to other drugs by ertapenem. Selective information bias poses a threat to the validity and reliability of retrolective analyses such as ours. All data used The authors declare no conflicts of interest. for analyses were based on computerized records, reducing the potential for non-systematic information bias. Supporting Information Computerized records themselves are limited and patient file review might have led to insights into the clinicians’ reasoning for choice of specific antimicrobials. Another possible draw- Additional Supporting Information may be found in the online back could result from the fact that antibiotic dispensing in the version of this article: emergency room in our hospitals was not documented on the Figure S1. Distribution of Pseudomonas aeruginosa isolates computerized database. These accounts for no more than a by year. day of undocumented care and continuation of treatment in Figure S2. Distribution of antibiotic load (total antibiotic the inpatient wards have always been monitored by the days) per patient. antibiotic stewardship programme. Most of the antibiotics Figure S3. Patient survival curves per specific resistance examined are not available outside the hospital setting, with phenotype. the exception of oral fluoroquinolones. Thus, our assessment Table S1. Original resistance pattern of index case and of the effect of quinolone might be flawed to some degree. control specimens. The patients included in this study might have received Table S2. Distribution of ureidopenicillin and carbapenem antibiotic medications, prior to hospital admittance and after sensitivities among Pseudomonas aureginosa clinical isolates. discharge, which we could not account for. Unaccounted for antibiotic exposure could lead to confounding of the presented References results; however, antibiotic dispensing would need to be differently distributed between the cases and the controls, which is unlikely as they had similar demographic and medical 1. Breidenstein EB, de la Fuente-Nunez C, Hancock RE. Pseudomonas – profiles. aeruginosa: all roads lead to resistance. Trends Microbiol 2011; 19: 419 426. Another result worth noting is the fact that patients, in both 2. Traugott KA, Echevarria K, Maxwell P, Green K, Lewis JS II. case and control groups, had received antibiotics to which the Monotherapy or combination therapy? The Pseudomonas aeruginosa PA isolated was resistant after the index date. This might conundrum. Pharmacotherapy 2011; 31: 598–608. 3. Lister PD, Wolter DJ, Hanson ND. Antibacterial-resistant Pseudomonas reflect consideration of other isolates that were felt to be aeruginosa: clinical impact and complex regulation of chromosomally clinically significant and, possibly, medical errors. However, we encoded resistance mechanisms. Clin Microbiol Rev 2009; 22: 582–610. have no reason to believe that these additional isolates were 4. Miliani K, L’Heriteau F, Lacave L, Carbonne A, Astagneau P. Imipenem differentially distributed between the cases and the controls, and ciprofloxacin consumption as factors associated with high incidence rates of resistant Pseudomonas aeruginosa in hospitals in given their matching. northern france. J Hosp Infect 2011; 77: 343–347. To conclude, we have conducted a large and comprehensive 5. Johnson JK, Smith G, Lee MS et al. The role of patient-to-patient study assessing patient-specific relationships between antibi- transmission in the acquisition of imipenem-resistant Pseudomonas aeruginosa colonization in the intensive care unit. J Infect Dis 2009; 200: otic exposure and PA resistance phenotypes. We confirmed 900–905. that antibiotics are associated with differential resistance 6. Royal decree of April 26, 1999. Creation of belgian antibiotic coordi- phenotypes and also provide a possible association of ertape- nation committee (bapcoc). Belgisch staatsblad July 31, 1999. 1999. 7. Dellit TH, Owens RC, McGowan JE Jr et al. Infectious diseases society nem with sequentially appearing PA resistance patterns. of america and the society for healthcare epidemiology of america Clinicians should be aware that CRUS-PA may be pan-beta-lac- guidelines for developing an institutional program to enhance antimi- tam resistant and the use of antibiotics other than ureidopen- crobial stewardship. Clin Infect Dis 2007; 44: 159–177. icillins might be prudent. 8. Stelling J, O’Brien TF. Whonet 5.4: software for the management and analysis of microbiology laboratory results. Boston, MA: WHO Collabor- ating Centre for Surveillance of Antimicrobial Resistance, 2007. http:// www.who.int/drugresistance/whonetsoftware. Acknowledgments 9. Clinical and Laboratory Standards Institute Performance standards for antimicrobial susceptibility testing; twentieth informational supplement; m100-s20. Wayne, PA: Clinical and Laboratory Standards Institute, This study was approved by the Hadassah-Hebrew University 2010. ethics committee. Parts of this paper were presented at the 10. Lautenbach E, Weiner MG, Nachamkin I, Bilker WB, Sheridan A, Fishman NO. Imipenem resistance among Pseudomonas aeruginosa 51st ICAAC, Chicago, Illinois.

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