Drug resistant isolates of Enterobacteriaceae, Pseudomonas aeruginosa, Acinetobacter spp. and Stenotrophomonas maltophilia

1.0 Introduction

These recommendations are produced under the auspices and authority of the Canadian Public Health Laboratory Network, Antimicrobial Resistance Working Group. They represent a consensus of peer reviewed information and expert opinion on the most appropriate ways to test for and report a multi-drug resistant phenotype in common Gram-negative pathogens. These recommendations were developed for use by all Canadian non-veterinary clinical microbiology laboratories to provide standardization for provincial and national surveillance programs.

2.0 Background

Antimicrobial resistance is a growing concern for human health as bacterial pathogens continue to accumulate genes and genomic alterations that confer resistance to antimicrobials. Most concerning is the occurrence of multiple resistance traits within individual key pathogens, which greatly limits, if not entirely eliminates the arsenal of effective treatment options for those infections, thereby leading to poor clinical outcomes. In Canada, we have observed these highly resistant strains in Enterobacteriaceae, Acinetobacter spp., Stenotrophomonas maltophilia,and Pseudomonas aeruginosa ^{Footnote 1Footnote 2Footnote 3}. There is a need for laboratories to classify organisms that are resistant to multiple antimicrobials in order to consistently and accurately share the information locally, nationally, and internationally with the medical community, public health authorities and policy makers. More specifically, classification as ‘multi-drug resistant’ is commonly an actionable finding within hospital Infection Prevention & Control programs. Recently, there has been a proposal to internationally standardize these definitions in selected Gram-positive and Gram-negative organisms ^{Footnote 4}, yet this proposal for interim definitions has not yet led to a revised definition or national recommendations.

The goal of this document is to provide Canadian laboratories with a framework for consistent reporting and monitoring of multi-drug resistant organisms (MDRO), extensively drug resistant organisms (XDRO), and pan-drug resistant organisms (PDRO). The recommendations were based on an interim international proposal published in 2012 for Gram-negative organisms ^{Footnote 4}. This document modifies the following for the Canadian setting: 1) Resistance was used instead of non-susceptibility (Intermediate and Resistant) to better match which antimicrobials will be clinically used for treating resistant infections; antimicrobials that are more easily tested in the laboratory; and those that would limit unnecessary reference testing. 2) MDRO rules are separated for commonly used antimicrobials in the community setting for urine infections and non-urine infections. 3) Rather than all classes of antimicrobials being considered in the definitions, only relevant classes that are commonly tested in Canadian clinical laboratories were considered. Also within a class of antimicrobials, resistance to the most commonly used antimicrobial for treating severe infections (i.e. meropenem or imipenem) was considered rather than an inferior drug for infections (i.e. ertapenem for the carbapenems). 4) Since XDRO definitions will fluctuate from country to country based on 2nd and 3rd line available antimicrobials, adjustments were made for antimicrobials available/approved for use in Canada rather than all drug categories listed in the Clinical Laboratory Standards Institute (CLSI) ^{Footnote 5}. The justification for these modifications can be found in Appendix 1. Over time as new antimicrobials become available, previously available antimicrobials lose effectiveness, or no longer available, the definitions will necessitate periodic review. The recommendations stated herein are considered interim and are open for stakeholder consultation such that future recommendations evolve to accommodate public health, community care, and acute care partners.

3.0 Recommendations for Antimicrobial Susceptibility Testing

3.1 A resistant interpretation of an isolate can be determined using disk diffusion, broth microdilution, or agar dilution following CLSI guidelines for the testing of Enterobacteriaceae, Pseudomonas aeruginosa, Acinetobacter spp. and Stenotrophomonas maltophilia ^{Footnote 5}. A Health Canada or Federal Drug Administration (FDA) approved automated method or gradient diffusion strips can also be used for the generation of the antimicrobial susceptibility data.

3.2 Current CLSI breakpoints (M100) for resistance should be used when determining the designations of MDRO, XDRO, and PDRO. It is understood that some laboratories use automated methods with Food and Drug Administration (FDA) breakpoints that may differ from the CLSI recommendations. A laboratory using FDA breakpoints should include the breakpoint difference in any report for MDRO, XDRO, and PDRO.

3.3 Certain species of Enterobacteriaceae should not be tested for particular antimicrobial agents because of intrinsic resistance to the agent (Table 1, Exceptions).

4.0 Definitions of Screening/Testing for MDRDO/XDRO/PDRO

These interim recommendations are to be applied only to clinical/diagnostic specimens. However, acute care and long term care facilities, and by extension health authorities, may choose to still apply the definitions of MDRO/XDRO/PDRO for screening purposes as determined by their own fiscal situation and local health resources. If isolates are part of a specialized surveillance program (e.g. in-patient screening), it should be clearly indicated in the laboratory report that the MDRO/XDRO/PDRO is pertinent for colonization or carriage status only.

4.1 Enterobacteriaceae Multi-Drug Resistance Definition

It is recognized that laboratories may not test Gram-negative isolates for all classes of antimicrobial agents and therefore would not be able to determine MDRO, XDRO, and PDRO. Therefore, we have included a category of multi-drug resistant organisms (MDRO) that should be considered for screening isolates for XDRO or PDRO.

4.1.1 There are four rules for MDRO status in Enterobacteriaceae which takes into consideration the specific specimen type (Table 1).

Table 1: Rules for the determination of Multi-Drug-, Extensively Drug- and Pan-Drug Resistant Organisms in Enterobacteriaceae from clinical isolates^{Table 1 - Footnote 1}

Rule	Specimen Type	Antimicrobial Groups	Interpretation
1	Urine	Cefixime OR Amoxicillin-clavulanate	Resistance to THREE of the FOUR groups = MDRO
		Ciprofloxacin
		Trimethoprim-Sulfamethoxazole
		Nitrofurantoin
2	Non-Urine	(Cefixime OR Amoxicillin-clavulanate)	Resistance to THREE of the THREE groups = MDRO
		Ciprofloxacin
		Trimethoprim-sulfamethoxazole
3	All	MeropenemTable 1 - Footnote 2 AND (Ciprofloxacin OR Trimethoprim-sulfamethoxazole)	Resistance to a very broad spectrum antimicrobial and resistance to one of two commonly used and unrelated drug classes = MDRO
4	All	Tobramycin AND Gentamicin AND Piperacillin-Tazobactam AND (Ciprofloxacin OR Trimethoprim-sulfamethoxazole)	Resistance to two commonly susceptible drug classes and resistance to one of two commonly used and unrelated drug classes = MDRO
5	All	Tobramycin AND Gentamicin	Resistance to FOUR of the SIX antimicrobial groups = XDRO
		Piperacillin-Tazobactam
		Imipenem OR Meropenem
		Cefepime OR (cefotaxime/ceftriaxone) AND ceftazidime
		Ciprofloxacin
		Trimethoprim-Sulfamethoxazole
6	All	Same groups listed in rule #5	Resistance to SIX of SIX antimicrobial groups = PDRO

4.2 Acinetobacter spp. or Pseudomonas aeruginosa Multi-Drug Resistance Definition

4.2.1 An isolate should be considered MDRO if resistant to THREE of the FIVE antimicrobial agents listed below (Table 2):

1. Ciprofloxacin
2. Piperacillin-tazobactam OR piperacillin (specifically for P. aeruginosa)
3. Ceftazidime OR cefepime
4. Imipenem OR meropenem
5. Tobramycin

Table 2: Definitions for the determination of Multi-Drug-, Extensively Drug-, Pan-Drug Resistant Organisms in select organisms.

MDRO		XDRO / PDRO
Definition	Antimicrobial Groups	Definitions	Antimicrobial Groups
Organism: Pseudomonas aeruginosa
Resistance to THREE of the FIVE antimicrobial groups	Ciprofloxacin	Resistance to FOUR of the SIX antimicrobial groups = XDRO Resistance to SIX of the SIX antimicrobial groups = PDRO	Tobramycin
	Piperacillin-tazobactam OR piperacillin		Piperacillin-tazobactam OR piperacillin
	Ceftazidime OR cefepime		Imipenem OR meropenem OR doripenem
	Imipenem OR meropenem
			Cefepime OR ceftazidime
	Tobramycin
			Ciprofloxacin
			Colistin
Organism: Acinetobacter spp.
Resistance to THREE of the FIVE antimicrobial groups	Ciprofloxacin	Resistance to SIX of the EIGHT antimicrobial groups = XDRO Resistance to all groups = PDRO	Gentamicin OR tobramycin
	Piperacillin-tazobactam
			Piperacillin-tazobactam
	Ceftazidime OR cefepime
			Imipenem OR meropenem OR doripenem
	Imipenem OR meropenem
	Tobramycin		Cefepime OR ceftazidime
			Ciprofloxacin
			Colistin
			Doxycycline OR minocycline
			Trimethoprim-sulfamethoxazole
Organism: Stenotrophomonas maltophilia
Resistance to BOTH antimicrobial groups	Trimethoprim-sulfamethoxazole Minocycline OR levofloxacin	Resistance to the FIRST THREE antimicrobial groups = XDRO Resistance to all antimicrobial groups = PDRO	Trimethoprim-sulfamethoxazole
			Minocycline
			Levofloxacin
			Ceftazidime
			Chloramphenicol

4.3 Stenotrophomonas maltophilia Multi-Drug Resistance Definition

4.3.1 S. maltophilia is intrinsically resistant to all carbapenems and most cephalosporins. A clinical isolate should be considered an MDRO if it is resistant to trimethoprim-sulfamethoxazole and subsequent susceptibility testing indicates it is also resistant to an oral anti-microbial (minocycline or levofloxacin) [Table 2].

5.0 Confirmation of XDRO

5.1 Enterobacteriaceae XDRO Definition

5.1.1 An isolate that has been determined to be an MDRO should be considered an XDRO by testing/assessing resistance to other antimicrobial agents listed in this section.

5.1.2 Unlike the definition of MDRO for Enterobacteriaceae, the type of specimen does not need to be considered for the definition of XDRO.

5.1.3 An isolate of Enterobacteriaceae should be considered an XDRO when the isolate is resistant to FOUR of the SIX antimicrobial agents listed below (Table 1):

1. Tobramycin AND gentamicin
2. Piperacillin-tazobactam
3. Imipenem OR meropenem
4. Cefepime OR (cefotaxime/ceftriaxone) AND ceftazidime
5. Ciprofloxacin
6. Trimethoprim-sulfamethoxazole

5.2 Pseudomonas aeruginosa XDRO Definition

5.2.1 A P. aeruginosa should be considered an XDRO when the isolate is resistant to FOUR of the SIX antimicrobial agents listed below (Table 2):

1. Tobramycin
2. Piperacillin OR piperacillin-tazobactam
3. Imipenem OR meropenem OR doripenem
4. Cefepime OR ceftazidime
5. Ciprofloxacin
6. Colistin

5.2.2 A P. aeruginosa should be considered a PDRO when the isolate is resistant to ALL of the antimicrobial agents listed in 5.2.1.

5.3 Acinetobacter spp. XDRO Definition

5.3.1 An Acinetobacter spp. should be considered an XDRO when the isolate is resistant to SIX of the EIGHT antimicrobial agents listed below (Table 2):

1. Gentamicin OR Tobramycin
2. Piperacillin-tazobactam
3. Imipenem OR meropenem OR doripenem
4. Cefepime OR ceftazidime
5. Ciprofloxacin
6. Colistin
7. Doxycycline OR minocycline
8. Trimethoprim-sulfamethoxazole (note: intrinsically resistant to trimethoprim)

5.4 Stenotrophomonas maltophilia XDRO Definition

A S. maltophilia should be considered an XDRO if resistant to three oral antimicrobials (trimethoprim-sulfamethoxazole, minocycline, and levofloxacin). The isolate should be referred for complete antimicrobial susceptibility testing to exclude a PDRO (see Table 2).

6.0 Confirmation of PDRO

An Enterobacteriaceae, P. aeruginosa, Acinetobacter spp. should be considered a PDRO when the isolate is resistant to ALL antimicrobial agents listed in Table 1 (rule 6), section 5.2.1, or 5.3.1, respectively. S. maltophilia should be considered a PDRO if it is resistant to all of the following: trimethoprim-sulfamethoxazole, levofloxacin, ceftazidime, and chloramphenicol.

7.0 Reporting to Reference Laboratories

7.1 Any laboratory identifying a MDRO that cannot confirm an XDRO or PDRO using additional antimicrobial susceptibility tests should send the isolate to a reference (provincial) laboratory (See Appendix 2).

7.2 The reference (provincial) laboratory should be notified of any XDR or PDR organisms identified and the isolate should be forwarded to the reference laboratory, and should include the following information:

1. Age of patient
2. Gender of patient
3. Type of clinical specimen (blood, respiratory, skin/soft tissue, or urine)
4. Date of collection
5. Antimicrobial susceptibility testing results from submitting laboratory
6. Out of Canada travel history in the last 3 months. Travel history is dated from the time of the first isolation of the organism. This is highly recommended for inpatients and desirable for outpatients. All countries traveled should be listed.

7.3 If multiple clinical isolates of the same species and susceptibility pattern are recovered from the same patient, send the isolate from the most invasive site where possible. Additional isolates of the same species and susceptibility pattern should be reported/sent to a reference laboratory no more frequently than every 7 days after the first isolate. Annotating as an MDRO/XDRO/PDRO on the clinical report should continue for each isolate regardless number of isolates or time interval between specimens.

7.4 It is suggested that reports of clinical specimens found to contain XDRO or PDRO isolates incorporate the term Extensively Drug Resistant Organism or Pan-Drug Resistant Organism within the body of the clinical report.

7.5 Any XDRO or PDRO isolate identified should be reported to public health according to local, regional, and provincial regulations with the additional information outlined in 7.2.

7.6 The originating laboratory should retain the XDRO or PDRO isolates for at least six months, or as required by provincial or local regulations.

7.7 The reference (provincial) laboratory should report all of the data to the National Microbiology Laboratory as defined in 7.2.

Acknowledgements

The subcommittee would like to acknowledge the work of Dr. John Conly (University of Alberta), Dr. Charles Frenette (McGill University), and all the other members of the Canadian Infectious Disease Steering Committee Antimicrobial Resistance Surveillance Task Group. We also appreciate the support of Dr. George Zhanel (University of Manitoba) of the Canadian Antimicrobial Resistance Alliance for feedback on earlier versions of the document. We thank members of the Canadian Public Health Laboratory Network Laboratory Director’s Council for review and final approval of the document. We would also like to thank Ms. Sandra Radons-Arneson from our secretariat for her support.

Conflict of interest

None.

Funding

This work was supported in kind by all laboratories of the authors and the Canadian Public Health Laboratory Network Antimicrobial Resistance Subcommittee.