Antimicrobial susceptibility of 6685 organisms isolated from Canadian hospitals: CANWARD 2007

1Department of Medical Microbiology, Faculty of Medicine, University of Manitoba; 2Departments of Medicine; 3Clinical Microbiology, Health Sciences Centre, MS673-Microbiology; 4Nosocomial Infections Branch, National Microbiology Laboratory, Health Canada, Winnipeg, Manitoba Correspondence: Dr GG Zhanel, Clinical Microbiology, Health Sciences Centre, MS673-820 Sherbrook Street, Winnipeg, Manitoba R3A 1R9. Telephone 204-787-4902, fax 204-787-4699, e-mail ggzhanel@pcs.mb.ca CANWARD 2007

We recently reported on the antimicrobial activity of commonly used agents against 3931 organisms isolated from intensive care units in Canada (11). The purpose of the present study was to assess the in vitro activity (minimum inhibitory concentrations required to inhibit the growth of 50% and 90% of organisms [MIC 50 and MIC 90 ]) of commonly prescribed antimicrobials against the 20 most common organisms (6685 isolates) obtained from patients in hospitals across Canada.

Bacterial isolates
Study isolates were obtained as part of the Canadian Ward Surveillance Study (CANWARD 2007). The CANWARD study included 12 medical centres from all regions of Canada (www.can-r.ca). The precise methods of isolate collection are explained in detail in the first paper of the present supplement (12). In brief, from January 1, 2007, to December 31, 2007, inclusive, each centre collected and submitted clinical isolates from patients attending hospital clinics, emergency rooms, medical and surgical wards, and intensive care units. Each centre was asked to submit clinical isolates (consecutive, one organism per infection site per patient) from blood (360 isolates collected as 30 consecutive/month for each of the 12 months), respiratory (n=200), urine (n=100), and wound/ intravenous (n=50) infections. All organisms were identified at the originating centre using local site criteria and were deemed clinically significant. In total, 7881 isolates were collected. Isolates were shipped to the reference laboratory (Health Sciences Centre, Winnipeg, Manitoba) on Amies charcoal swabs, subcultured onto appropriate media, and stocked in skim milk at -80°C until MIC testing was carried out.

Antimicrobial susceptibilities
Susceptibility testing was carried out using microbroth dilution in accordance with the Clinical and Laboratory Standards Institute (CLSI) guidelines (11,13). For all antimicrobials tested, MIC interpretive standards were defined according to CLSI breakpoints (CLSI 2006). Susceptibility testing could not be performed with all agents due to lack of space on the susceptibility panels. Thus, susceptibility testing was not performed with P aeruginosa for ceftazidime, tobramycin and imipenem. The following interpretive breakpoints (Food and Drug Administration, USA) were used for tigecycline susceptible (S), intermediate (I) and resistant (R): S aureus (methicillin-susceptible [MSSA] and MRSA) 0.5 µg/mL or less (S); Enterococcus faecalis (vancomycin susceptible), 0.25 µg/mL or less (S); Enterobacteriaceae, 2 µg/mL or less (S), 4 µg/mL (I), and 8 µg/mL or greater (R). No breakpoints are presently available for dalbavancin and telavancin.
Characterization of MRSA, ESBL-producing Enterobacteriaceae and VRE MRSA: Potential MRSA isolates were confirmed and tested as previously described (10). All isolates of MRSA were typed using pulsed-field gel electrophoresis following the Canadian standardized protocol to assess whether the isolates were CA-MRSA or HA-MRSA (9,10,14,15). ESBL testing: Potential E coli or Klebsiella species. ESBL producers were identified and tested as previously described (10). VRE: Potential VRE isolates were confirmed using CLSI vancomycin disk diffusion testing and underwent vanA and vanB polymerase chain reaction as well as DNA fingerprinting to assess genetic similarity, as previously described (7,10).

Patient demographics and specimen types
A total of 7881 organisms (the 20 most common organisms, representing 6685 isolates, underwent susceptibility testing) were obtained from bacteremic, urinary, respiratory and wound specimens from hospitals across Canada. The patient demographics associated with these isolates have been described (12).

Most common organisms isolated from Canadian hospitals
The 20 most common organisms isolated from hospitals across Canada included 3178 Gram-positive cocci: MSSA, S pneumoniae, MRSA, coagulase-negative staphylococci/ Staphylococcus epidermidis, and Enterococcus species, as well as 3507 Gram-negative bacilli including E coli, P aeruginosa, Klebsiella pneumoniae, Haemophilus influenzae, Enterobacter cloacae and Proteus mirabilis (12).

DISCUSSION
The CANWARD study was the first national, prospective surveillance study assessing antimicrobial activity against pathogens from Canadian hospitals, including hospital clinics,      (4,9,11). It must however be stated that no population analysis profiling was performed on any MRSA to assess for heteroresistant vancomycin-intermediate S aureus. Against MSSE and MRSE, vancomycin was less active compared with MSSA and MRSA. The MIC 90 s for both MSSE and MRSE were 2 µg/mL. This reduced vancomycin activity against MSSE and MRSE versus MSSA and MRSA has also been previously documented (9,16). In this study, as well as with previous data, vancomycin continues to be very active against all Streptococcus species, with all isolates displaying MICs of 1 µg/mL or less (9,17). Vancomycin was less active against E faecalis and E faecium with 0% and 11.7% of strains resistant, respectively. As has been reported elsewhere, the predominant VRE genotype in North America continues to be vanA (4,7).

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Linezolid was active against MSSA and MRSA with 100% of isolates demonstrating susceptibility with MICs 4 µg/mL or less (Table 1). No difference in linezolid activity was observed between HA-MRSA and CA-MRSA. Linezolid was more active against MSSE and MRSE in comparison with MSSA and MRSA, with all isolates demonstrating linezolid MICs of 1 µg/mL or less (Table 1). Linezolid's continued excellent activity against MSSA/MRSA and MSSE/MRSE has been previously documented (11,16,17). As has been previously documented, linezolid continues to be active against Streptococcus species with all isolates displaying MICs of 2 µg/mL or less (11,17). Linezolid was less active against E faecalis and E faecium, with 1.3% and 8.6% of strains demonstrating intermediate resistance, respectively. This rate of linezolid resistance in E faecium is consistent with previous reports (17)(18)(19).
Daptomycin was active against MSSA and MRSA with 100% of isolates demonstrating susceptibility, with MICs of 1 µg/mL or less (Table 1). No difference in daptomycin activity was observed between HA-MRSA and CA-MRSA. Daptomycin was equally active against MSSE and MRSE compared with MSSA and MRSA, with all isolates demonstrating daptomycin MICs of 0.25 µg/mL or less. Daptomycin's excellent activity against MSSA/ MRSA and MSSE/MRSE has been previously documented (11,16). As has been previously reported (11,16), daptomycin was active against Streptococcus species with isolates displaying MICs of 0.12 µg/mL or less. Daptomycin was active against E faecalis, E faecium and VRE, with 100% susceptibility and all isolates displaying MICs of 2 µg/mL or less (Table 1). Daptomycin-resistant enterococci species continue to be rare (18) and have not been documented in Canada. From these data, it is clear daptomycin is a very active agent against all Gram-positive organisms causing infections in Canadian hospitals.
Tigecycline was active against MSSA and MRSA with 100% of isolates demonstrating susceptibility, with MICs of 0.5 µg/mL or less (Table 1). No difference in tigecycline activity was observed between HA-MRSA and CA-MRSA. Tigecycline was equally active against MSSE and MRSE compared with MSSA and MRSA, with all isolates demonstrating tigecycline MICs of 0.5 µg/mL or less. Tigecycline's excellent activity against MSSA/ MRSA and MSSE/MRSE has been previously documented (11,19). As has been previously reported, tigecycline was very active against Streptococcus species, with all isolates displaying MICs of 0.12 µg/mL or less (11,19). Tigecycline was very active against E faecalis, E faecium and VRE, with all isolates displaying MICs of 0.5 µg/mL or less (Table 1). From these data, it is clear tigecycline is a very active agent against all Gram-positive organisms causing infections in Canadian hospitals.
Dalbavancin was active against MSSA and MRSA with 100% of isolates demonstrating MICs of 0.25 µg/mL or less (Table 1). No difference in dalbavancin activity was observed between HA-MRSA and CA-MRSA. Dalbavancin was equally active against MSSE and MRSE, with all isolates demonstrating  (11,20). As has been previously reported (11,20), dalbavancin was active against Streptococcus species with isolates displaying MICs of 0.12 µg/mL or less. Dalbavancin was active against E faecalis, but displayed less activity against E faecium and VRE (Table 1). Telavancin was active against MSSA and MRSA with 100% of isolates demonstrating MICs of 1 µg/mL or less (Table 1). No difference in telavancin activity was observed between HA-MRSA and CA-MRSA. Telavancin was equally active against MSSE and MRSE, with all isolates demonstrating MICs of 0.25 µg/mL or less. Telavancin's excellent activity against MSSA/MRSA and MSSE/MRSE has been previously documented (20,21). As has been previously reported (21), telavancin was active against Streptococcus species with isolates displaying MICs of 0.12 µg/mL or less. Telavancin was active against E faecalis, but displayed less activity against E faecium and VRE (Table 1). It has been previously documented that telavancin is active against VanB Enterococcus species, but not VanA Enterococcus species (21).
The most active (based on MIC only) agents against the 3507 Gram-negative bacilli obtained from Canadian hospitals were amikacin, cefepime, ertapenem (not P aeruginosa), meropenem, piperacillin-tazobactam and tigecycline (not P aeruginosa) ( Table 2). Amikacin was very active against E coli (including ESBL-producing strains) with 99.5% of strains testing susceptible with an MIC 90 of 4 µg/mL. Likewise, amikacin proved to be very active against all other Enterobacteriaceae tested ( Table 2). Against P aeruginosa, amikacin proved to be one of the most active agents tested, with 85.4% of strains testing susceptible with MIC 90 of 32 µg/mL. Against A baumannii, amikacin P aeruginosa was very active with 92.0% of strains being susceptible with MIC 90 of 2 µg/mL or less. The excellent activity of amikacin against both Enterobacteriaceae as well as nonfermenters isolated from patients in hospitals, including in the intensive care unit, is not surprising because the reduced usage of aminoglycosides in favour of fluoroquinolones over the past 15 years has resulted in maintained activity of aminoglycosides in the setting on increasing fluoroquinolone resistance (4,19,22). Thus, amikacin represents a potential option for the treatment of infections caused by Gram-negative bacilli resistant to other less toxic agents.
In the present study, we reported that cefepime, ertapenem, meropenem and piperacillin-tazobactam were very active against Gram-negative bacilli isolated from patients in Canadian hospitals. These agents were active against Enterobacteriaceae including against E coli (only ertapenem and meropenem were active against ESBL-producing strains). Against P aeruginosa, resistance was piperacillin-tazobactam 7.3%, meropenem 8.1% and cefepime 11.7%. Previous investigators have reported the ongoing excellent activity of these agents versus Gram-negative bacilli isolated from hospitalized patients (4,19,22). Colistin was found to be very active against E coli (including ESBL strains) with MIC 90 of 1 µg/mL. Colistin was also very active against Klebsiella species, E cloacae and P mirabilis. Against P aeruginosa, resistance to colistin was 12.4% with an MIC 90 of 4 µg/mL (Table 2). Against A baumannii, colistin was also very active, with an MIC 90 of 2 µg/mL ( Table 2). These data are consistent with other reports of the promising potential of colistin for Gram-negative bacilli such as P aeruginosa and A baumannii (23,24).
Tigecycline demonstrated 99.8% susceptibility versus E coli (100% versus ESBL-producing strains) and was also active against other Enterobacteriaceae including K pneumoniae, E cloacae, S marcescens and K oxytoca ( Table 2). Tigecycline was not active against P mirabilis and P aeruginosa. Tigecycline also proved to be active against S maltophilia and A baumannii organisms frequently resistant to other antimicrobial classes ( Table 2). The activity of tigecycline against Gram-negative bacilli (with the exception of P aeruginosa) has been previously reported and supports the potential to use this agent for the treatment of infections caused by non-Pseudomonas Gramnegative bacilli in hospitalized patients (11,19).
The present study has several limitations, including the fact that we can not be certain that all clinical specimens represented active infection. In the CANWARD study, we asked centres to obtain 'clinically significant' specimens from patients with a presumed infectious disease. Although all of the isolates may not represent actual infection from patients, we believe that most do because we excluded all surveillance swabs and duplicate swabs, as well as eye, ear, nose and throat swabs and genital cultures. In addition, we do not have admission date data for each patient/clinical specimen, thus were not able to provide a more accurate description of community versus nosocomial onset. Finally, susceptibility testing was not performed for all antimicrobial agents due to lack of space on the susceptibility panels utilized. It is recognized that data on antimicrobials such a ceftazidime, imipenem, tobramycin and others would be beneficial, because different hospital formularies stock these and other antimicrobials not tested in this study.

CONCLUSIONS
The most active agents versus Gram-positive cocci from Canadian hospitals were vancomycin, linezolid, daptomycin, tigecycline, dalbavancin and telavancin. The most active agents versus Gram-negative bacilli from Canadian hospitals were amikacin, cefepime, ertapenem (not P aeruginosa), meropenem, piperacillin-tazobactam and tigecycline (not P aeruginosa). Colistin was very active against P aeruginosa and A baumannii.