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High prevalence of carbapenem resistant Pseudomonas aeruginosa at a tertiary care centre of north India. Are we under-reporting?

Sir,

Pseudomonas aeruginosa is a common nosocomial pathogen, notorious for its multidrug resistance (MDR) and life threatening infections in critically ill patients. Lately, carbapenems are being used as the last resort antimicrobial to treat serious infections due to MDR P. aeruginosa. In a few Indian studies, the rate of carbapenem resistance in P. aeruginosa has been reported to vary from 12-37 per cent (1). However, these studies were done using the disc diffusion method of antimicrobial susceptibility testing on which problems have been highlighted in another studies (2). Therefore, we undertook a study to assess the prevalence of carbapenem resistance at a tertiary care and referral centre in north India where the rates of extended spectrum beta lactamase (ESBL) production in P. aeruginosa has been reported to be around 64 per cent and carbapenems have been in use to treat MDR P. aeruginosa in critical care units since 2002 (3).

The study was conducted at the All India Institute of Medical Sciences, New Delhi. A total of 91 consecutive isolates of P. aeruginosa obtained from various clinical samples of admitted patients [bronchoalveolar lavage (34), blood (21), tracheal aspirate (14), pus (14), urine (7), CSF (1)] obtained over a period of 1 month (April 1-30, 2007) were included in the study. Of these, 41 isolates were obtained from various intensive care units (medical, paediatric & surgical), 25 from the orthopedics ward, 13 from the various wards of neurosurgery, 8 from general paediatrics ward and 4 from radiotherapy. Only one isolate/patient was included in the study. The antibiotic susceptibility of the isolates was compared to exclude clonal origin of the isolates. The isolates were identified as P. aeruginosa by conventional methods 4 as well as by the VITEK-2 system (Biomeriux Vitek Systems Inc, Hazelwood, France) (5). Antimicrobial susceptibility of all the isolates was performed by the disc diffusion method according to the Clinical and Laboratory Standards Institute (CLSI) guidelines (6). The following antibiotics ([micro]g) were tested by the disk diffusion method, piperacillin (100), piperacillin/tazobactam (100/10), tic arcillin/clavulinic acid (75/10), ceftazidime (30), cefepime (30), cefoperazone (30), ceftriaxone (30), cefotaxime (30), amikacin (30), levofloxacin (5), ciprofloxacin (5) and imipenem (10) (Hi-Media Laboratories, Mumbai). Antibiotic susceptibility of all the isolates was also simultaneously done by the VITEK-2 advanced expert system ((Bio meriux Vitek Systems Inc, Hazelwood, France), as is the protocol in our laboratory, by which the minimum inhibitory concentrations (MICs) were determined for the following antibiotics: piperacillin, piperacillin/ tazobactam, ticarcillin/clavulinic acid, ceftazidime, cefepime, cefoperazone, ceftriaxone, cefotaxime, amikacin, levofloxacin, ciprofloxacin and imipenem. In all cases of discordant imipenem susceptibility results between disc diffusion and VITEK-2 system, the MIC of imipenem was determined by E-test method (AB Biodisk, Sweden) done according to manufacturer's instructions.

Of the 91 isolates of P. aeruginosa, 64 (70%) were resistant to ceftazidime, 68 (75%) to piperacillin, 54 (59%) to piperacilin/tazobactam, 81 (89%) to ticarcillin/ clavulinic acid, 75 (82%) to cefoperazone, 67 (74%) to amikacin, 74 (81%) to cefepime, 65 (71%) to levofloxacin, 72 (79%) to ciprofloxacin and 63 (69%) to aztreonam by the disc diffusion method.

The comparative results of imipenem sensitivity by disc diffusion and VITEK-2 are shown in the Table. The MIC of imipenem by E-test was determined for 25 isolates [15 of which were found to be resistant by VITEK/sensitive by disc diffusion, five were resistant by VITEK/intermediate by disc diffusion, three were intermediate by VITEK/sensitive by disc diffusion and one each was sensitive and intermediate by VITEK/ correspondingly resistant by disc diffusion method (Table)]. All the isolates (24), which gave resistant/ intermediate phenotype by VITEK, had an MIC >16 [micro]g/ml by the E-test, while one isolate, which was sensitive by VITEK and resistant by disk diffusion, had an imipenem MIC < 4 [micro]g/ml by the E-test. Thus, a total of 63 (69%) of isolates of P. aeruginosa were found to be resistant to carbapenems.

Carbapenem resistance not only has enormous therapeutic implications, but is also important from the point of view of infection control. Such stains are known for rapid intra-institutional spread and therefore, must be notified to infection control teams. Carbapenem resistance can be mediated by elaboration of metallo [beta]-lactamases (MBL) (7). The MBL genes are often hidden; thereby giving a sensitive phenotypes by the disc diffusion method (7). Detecting such sensitive phenotypes of P. aeruginosa is important since the only therapeutic option for these strains is the potentially toxic polymyxin B. In our study, we have used three methods of susceptibility testing for imipenem, two of which were based on detection of MIC. Thus, our study emphasizes that testing by disc diffusion alone might lead to under-reporting of carbapenem resistance. Therefore, every institution must carefully evaluate the screening method being used for carbapenem resistance in order to correctly report the sensitivity against this important antimicrobial.

Bijayini Behera *, Anupam Das **

Purva Mathur * ([dagger]) & Arti Kapil **

Departments of * Laboratory

Medicine & ** Microbiology

All India Institute of Medical Sciences

Ansari Nagar, New Delhi 110 029, India

([dagger]) For correspondence: purvamathur@yahoo.co.in

References

(1.) Gupta E, Mohanty S, Sood S, Dhawan B, Das BK, Kapil A. Emerging resistance to carbapenems in a tertiary care hospital in north India. Indian J Med Res 2006; 124 : 95-8.

(2.) Steward CD, Mohammed JM, Swenson JM, Stocker SA, Willams PP, Gajness RP. Antimicrobial susceptibility testing of carbapenems: multicenter validity testing and accuracy levels of five antimicrobial test methods for detecting resistance in Enterobacteriaceae and Pseudomonas aeruginosa isolates. J Clin Microbiol 2003; 41 : 351-8.

(3.) Mathur P, Kapil A, Das B, Dhawan B. Prevalence of extended spectrum beta lactamase producing Gram negative bacteria in a tertiary care hospital. Indian J Med Res 2002; 115:153-7.

(4.) Collee JG, Fraser AG, Marmion BP, Simmon A, editors. Mackie and McCartney practical medical microbiology. 14th ed. Edinburgh: Churchill Livingstone; 1996.

(5.) Nakasone I, Kinjo T, Yamane N, Kisanuki K, Shiohira CM. Laboratory-based evaluation of the colorimetric VITEK-2 Compact system for species identification and of the Advanced Expert System for detection of antimicrobial resistances: VITEK-2 Compact system identification and antimicrobial susceptibility testing. Diagn Microbiol Infect Dis 2007; 58 : 191-8.

(6.) Clinical and Laboratory Standards Institute (CLSI). Performance standards for antimicrobial susceptibility testing, 16th informational supplements. CLSI Document M2-A9, Wayne PA, 2006.

(7.) Franklin C, Liolios L, Peleg AY. Phenotypic detection of carbapenem-susceptible metallo [beta]-lactamase-producing Gram-negative bacilli in the clinical laboratory. J Clin Microbiol 2006; 44 : 3139-44.
Table. Comparative results of imipenem sensitivity of P. aeruginosa
isolates (n=91) by disc diffusion and VITEK methods

 VITEK-2
Disc
diffusion Resistant Sensitive Intermediate Total

Resistant 38 1 1 40
Sensitive 15 26 3 44
Intermediate 5 1 1 7
Total 58 28 5 91
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Title Annotation:Correspondence
Author:Behera, Bijayini; Das, Anupam; Mathur, Purva; Kapil, Arti
Publication:Indian Journal of Medical Research
Geographic Code:9INDI
Date:Sep 1, 2008
Words:1109
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