Available online at www.annclinlabsci.org Annals of Clinical & Laboratory Science, vol. 41, no. 1, 2011 39 Time-Kill Synergy Tests of Tigecycline Combined with Imipenem, Amikacin, and Ciprofloxacin against Clinical Isolates of Multidrug-Resistant Klebsiella pneumoniae and Escherichia coli Haejun Yim,1 Heungjeong Woo,2 Wonkeun Song,3 Min-Jeong Park,3 Hyun Soo Kim,3 Kyu Man Lee,2 Jun Hur,1 and Man-Seung Park4 Departments of 1Burn Surgery, 2Internal Medicine, 3Laboratory Medicine, and 4Microbiology, Hallym University Medical Center and College of Medicine, Seoul and Chuncheon, Korea Abstract. This study evaluated the activity of tigecycline combined with imipenem, amikacin, and ciprofloxacin against clinical isolates of multidrug-resistant Klebsiella pneumoniae and Escherichia coli co-producing extended- spectrum β-lactamases and acquired AmpC β-lactamases. Broth microdilution tests were performed for cefotaxime, ceftazidime, cefepime, imipenem, amikacin, ciprofloxacin, and tigecycline. Time-kill synergy studies were tested for tigecycline plus imipenem, tigecycline plus amikacin, and tigecycline plus ciprofloxacin. Imipenem (MIC90 = 1 µg/ ml for both K. pneumoniae and E. coli) and tigecycline (MIC90 = 2 µg/ml for K. pneumoniae and 1 µg/ml for E. coli) were the most potent agents. Combination studies with tigecycline plus imipenem resulted in synergy against 18 K. pneumoniae and 3 E. coli isolates; tigecycline plus amikacin yielded synergy against 8 K. pneumoniae and 3 E. coli isolates; tigecycline plus ciprofloxacin yielded synergy against 7K. pneumoniae and 2 E. coli isolates. No antagonism was observed with any combination. In the present study, imipenem, amikacin, and ciprofloxacin led to indifferent and some synergistic effects in combination with tigecycline, and none of them demonstrated antagonistic effects. Introduction AmpCs, and isolates with reduced but may not always be effective as susceptibility to carbapenems con- monotherapy [9]. Therefore, a study β-Lactamase production is the most tinue to spread worldwide, new was designed to investigate the in common resistance mechanism to therapeutic options are needed [2-4]. vitro activities of combinations of β-lactams in Klebsiella pneumoniae Tigecycline, the first class of tigecycline and representative bac- and Escherichia coli. K. pneumoniae glycyclines, exhibits expanded- tericidal agents including imipenem, and E. coli isolates producing exten- spectrum of activity against a wide amikacin, and ciprofloxacin. Anti- ded-spectrum β-lactamases (ESBLs) variety of bacteria, including microbial combination therapy may and/or acquired AmpC β-lactamases multidrug-resistant strains such as provide clinicians an option in have been prevalent worldwide. ESBL-producing Enterobacteria- addition to imipenem for difficult ESBL-producing isolates are consid- ceae, multidrug-resistant Acineto- infections due to ESBL- or AmpC- ered resistant to all penicillins, bacter baumannii, methicillin- producing Enterobacteriaceae. cephalosporins, and aztreonam; resistant Staphylococcus aureus, and Although ESBL- and/or acquired carbapenems are the only β-lactams vancomycin-resistant enterococci AmpC-producing K. pneumoniae that remain consistently active for [5,6]. Tigecycline acts by binding to and E. coli routinely express cross- treatment in severe cases. The the 30S ribosomal subunit and resistance to other drug classes, therapeutic options for severe prevents the binding of aminoacyl synergy from combination therapy infections caused by acquired transfer RNA to the acceptor site on may achieve successful outcomes. AmpC-producing isolates are the messenger RNA-ribosome The purpose of this study was to almost restricted to carbapenems complex. Protein synthesis is evaluate the in vitro antimicrobial and cefepime [1]. Since K. pneu- inhibited, thereby exhibiting a activity of tigecycline combined moniae- and E. coli-resistant pheno- bacteriostatic effect [7]. Tigecycline with other antimicrobials against types, such as ESBLs, acquired has potentially useful activity [8] clinical isolates of multidrug- Address correspondence to Wonkeun Song, M.D., Ph.D., Department of Laboratory Medicine, Kangnam Sacred Heart Hospital, 948-1 Daelim-dong, Youngdeungpo-gu, Seoul 150-950, Republic of Korea; e-mail [email protected]. 0091-7370/11/0039-0043. $2.50. © 2011 by the Association of Clinical Scientists, Inc. 40 Annals of Clinical & Laboratory Science, vol. 41, no. 1, 2011 resistant K. pneumoniae and E. coli 5 E. coli) with tigecycline MICs of each isolates, and killing was co-producing ESBLs and acquired ≥0.125 µg/ml were chosen for time- assessed at 0, 4, 8, and 24 hr. AmpC β-lactamases. kill experiments. The combination Aliquots (0.1 ml) were removed activities of tigecycline and either from 2-ml cultures at 0, 4, 8, and 24 Materials and Methods imipenem, amikacin, or ciproflox- hr and serially diluted in 0.85% Bacterial isolates. This study acin were further evaluated in time- sterile saline. Bacterial counts were included 35 clinical isolates of K. kill experiments with concentrations determined by plating 0.1 ml of pneumoniae and 8 clinical isolates of at equal and one-fourth the MICs. appropriate dilutions to enumerate E. coli known to co-produce ESBLs Mueller-Hinton broth cultures were CFU/ml. Plating was performed in 5 and AmpCs, which were obtained inoculated with ≥5 x 10 CFU/ml of duplicate and the blood agar plates from two teaching hospitals in Korea during 2009 (Table 1). The Table 1. Bacterial isolates co-producing extended-spectrum β-lactamases and isolates were identified with the acquired AmpC β-lactamases used in this study Vitek 2 system (bioMerieux Vitek, Hazelwood, MO). Searches for Organism No. of isolates β-Lactamase genes coding for the class A ESBLs K. pneumoniae (n = 35) 23 SHV-12 plus DHA-1 were performed by PCR amplifi- 1 SHV-12 plus CMY-2 cation as described previousely [10]. 1 SHV-12 and CTX-M-12 plus DHA-1 To identify isolates with ampC 1 SHV-12 and CTX-M-15 plus DHA-1 genes, AmpC multiplex PCR was 2 CTX-M-14 plus DHA-1 performed by the method of Perez- 7 CTX-M-15 plus DHA-1 Perez and Hanson [11]. Sequencing of ampC genes with primers DHA- E. coli (n = 8) 4 SHV-12 plus DHA-1 1, CMY-1, and CMY-2 was perform- 1 CTX-M-14 plus DHA-1 ed as described previously [12]. The 1 CTX-M-15 plus CMY-1 1 CTX-M-15 plus CMY-2 PCR products were subjected to 1 CTX-M-24 plus CMY-2 direct sequencing. Both strands of each PCR product were sequenced twice with an automatic sequencer (model 3730xl; Applied Biosystems, Table 2. MICs for 43 isolates of E. coli and K. pneumoniae co-producing extended- Weiterstadt, Germany). Sequence spectrum β-lactamases and acquired AmpC β-lactamases. alignment and analysis was performed online using the BLAST Organism and Agent MIC (µg/ml) of: program (National Center for Bio- Range 50% 90% technology Information (Bethesda, K. pneumoniae (n = 35) MD; www.ncbi.nlm.nih.gov). Ceftazidime 8 – ≥ 512 ≥ 512 ≥ 512 Antimicrobial susceptibility testing. Cefotaxime 8 – ≥ 512 64 ≥ 512 Broth microdilution MIC tests were Cefepime 0.25 – ≥ 512 8 256 performed for ceftazidime (Sigma- Imipenem 0.125 – 64 0.5 1 Aldrich, St. Louis, MO), cefotaxime Amikacin ≤ 0.125 – ≥ 512 ≥ 512 ≥ 512 (Sigma-Aldrich), cefepime (Bor- Ciprofloxacin ≤ 0.125 – ≥ 512 32 ≥ 512 yung, Seoul, Korea), imipenem Tigecycline ≤ 0.125 – 4 0.5 2 (LKT Laboratories, St. Paul, MN), amikacin (Sigma-Aldrich), cipro- E. coli (n = 8) floxacin (LKT Laboratories), and tigecycline (Wyeth, Pearl River, Ceftazidime 8 – ≥ 512 ≥ 512 ≥ 512 Cefotaxime 16 – ≥ 512 256 ≥ 512 NY), according to the CLSI M7-A7 Cefepime 1 – ≥ 512 16 ≥ 512 methods [13]. For quality control, Imipenem ≤ 0.125 – 1 0.25 1 E. coli ATCC 25922 and K. Amikacin 1 – ≥ 512 4 ≥ 512 pneumoniae ATCC 700603, were Ciprofloxacin 0.25 – ≥ 512 128 ≥ 512 included in each set of tests. Tigecycline ≤ 0.125 – 1 ≤ 0.125 1 Time-kill synergy study. Thirty- three strains (28 K. pneumoniae and Tigecycline against Enterobactericeae 41 Table 3. Results of time-kill synergy for 33 isolates of E. coli and K. pneumoniae co-producing extended-spectrum β-lactamases and acquired AmpC β-lactamases at equal to one-fourth the MICs for tigecycline, imipenem, amikacin, and ciprofloxacin. Isolate MIC (µg/ml) Time-kill synergy (µg/ml) TIG IPM AMK CIP TIG+IPM TIG+AMK TIG+CIP K. pneumoniae K19 0.5 0.5 0.125 2 S(0.5+0.5) I S(0.5+2) K21 0.125 0.5 >256 16 I NT S(0.125+16) K24 0.25 32 >256 64 S(0.25+32) NT I K35 2 <0.125 32 32 NT S(2+32, 2+8, 0.5+32, I 0.5+8) K36 0.25 1 >256 1 I NT 1 K50 0.5 1 >256 2 S(0.5+1, 0.5+0.125) NT S(0.5+2, 0.5+0.5) K55 0.5 1 16 0.5 S(0.5+1, 0.5+0.125) S(0.5+16) I K56 2 0.5 >256 128 S(2+0.5) NT S(2+128) K65 1 0.5 16 >256 S(1+0.5) I NT K72 0.5 0.5 >256 2 S(0.5+0.5) NT I K75 1 64 >256 >256 I NT NT K76 0.25 <0.125 2 64 NT S(0.25+2) I K77 4 1 8 >256 S(4+1, 4+0.125, 1+1) S(4+8, 4+2) NT K86 0.25 1 >256 4 I NT I K87 0.5 0.5 2 0.25 S(0.5+0.5) S(0.5+2, 0.5+0.5) S(0.5+0.25) K89 0.5 1 >256 64 S(0.5+1) NT I K91 0.25 0.25 0.5 32 I S(0.25+0.5) S(0.25+32) K92 1 0.5 16 >256 S(1+0.5) I NT K97 2 0.5 16 >256 I I NT K102 0.25 0.5 >256 2 S(0.25+0.5) NT I K124 0.5 1 >256 128 S(0.5+1) NT I K137 1 1 8 >256 S(1+1, 1+0.125) S(1+8) NT K138 4 1 >256 >256 S(4+1, 4+0.125) NT NT K149 1 0.5 8 >256 I I NT K153 4 1 >256 16 S(4+1) NT S(4+16, 4+4, 1+16) K155 0.25 0.25 1 0.125 I I I K188 0.25 1 >256 64 S(0.25+1, 0.25+0.125) NT I K223 2 0.5 16 4 S(2+0.5) S(2+16) I E.