a Institute of Medical Microbiology b Institute of Hygiene and Environmental Medicine c Department for Anaesthesiology and Intensive Medicine, University of Greifswald, Germany
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Abstract |
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Introduction |
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Materials and methods |
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Between December 1996 and May 1998, multiply-resistant strains of P. aeruginosa were isolated from routine clinical specimens (tracheal secretions, blood, venous catheter and urine) from 80 ICU patients.
Strains and antimicrobial testing
Primary isolation was performed on Columbia agar containing 5% sheep blood. Antibiotic susceptibility was determined using Mueller-Hinton agar by a disc diffusion method with the following discs (Becton Dickinson GmbH, Heidelberg, Germany): ceftazidime (30 µg), cefsulodine (30 µg), cefepime (30 µg), azlocillin (30 µg), piperacillin/ tazobactam (30 µg/10 µg), imipenem (10 µg), meropenem (10 µg), gentamicin (10 µg), tobramycin (10 µg), amikacin (30 µg), ciprofloxacin (5 µg), ofloxacin (5 µg), aztreonam (30 µg) and polymyxin B (300 µg).
Susceptibility, resistance and moderate susceptibility were defined by the criteria of Deutsches Institut für Normung e.V. (DIN). 1 For selected strains (isolates from the beginning of the outbreak and 15 months later) MIC values were determined by the Etest (AB Biodisk, Solna, Sweden).
Screening tests for synergy were performed by a simplified disc method. When positive results were obtained, further tests were performed using a combination of agar dilution and agar diffusion test. 2
Serotyping
The O serotype was determined using a slide agglutination test with a set of four pools (OMA, OMC, OME, OMF) and 16 monovalent antisera numbered O1 to O16 (Sanofi Diagnostics Pasteur GmbH, Freiburg, Germany).
Genotyping
The genetic relationship of strains with identical multiresistance patterns was investigated by pulsed-field gel electrophoresis (PFGE) using a Gene Path Group 3 Kit (Bio-Rad Laboratories GmbH, Munich, Germany) in a CHEF DR III System (Bio-Rad).
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Results |
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All strains were resistant to ß-lactams (ceftazidime, cefepime, cefsulodine, azlocillin and piperacillin/tazobactam), including carbapenems (imipenem, meropenem) and aztreonam, and to aminoglycosides (amikacin, tobramycin, gentamicin) and quinolones (ciprofloxacin, ofloxacin). Only polymyxin B demonstrated in-vitro activity. Agar diffusion tests were confirmed by Etest. Multiple resistance was usually associated with high-level resistance (Table I).
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The results of serotyping were ambigious. Weak agglutination was obtained with the serum pool OME. After several passages over cetrimide agar this agglutination became stronger, but examination of the monovalent anti-O sera contained in this pool (O2, O5, O15 and O16) did not confirm a serotype. PFGE showed that isolates from patients and those from the environment were genetically identical or closely related (Figure).
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Discussion |
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Our results concerning the synergic in-vitro interactions of ceftazidime and amikacin correspond to the studies of Giamarellos-Bourboulis et al. 7
The detection of a new group of ß-lactamases that hydrolyse carbapenems and other broad-spectrum ß-lactams has led to a new therapeutic strategy in our hospital. Restricted clinical use of broad- spectrum ß-lactams, including carbapenems, and a rigorous hygienic regime are the most important measures to prevent the spread of the metallo-ß-lactamase gene in P. aeruginosa and other Gram-negative bacilli.
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Acknowledgments |
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Notes |
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References |
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2 . Ravizzola, G., Cabibbo, E., Peroni, L., Longo, M., Pollara, P. C., Corulli, M. et al. (1997). In-vitro study of the synergy between ß-lactam antibiotics and glycopeptides against enterococci. Journal of Antimicrobial Chemotherapy 39, 461-70.[Abstract]
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7 . Giamarellos-Bourboulis, E. J., Grecka, P. & Giamarellou, H. (1997). Comparative in vitro interactions of ceftazidime, meropenem, and imipenem with amikacin on multiresistant Pseudomonas aeruginosa. Diagnostic Microbiology and Infectious Diseases 29,81 -6.[ISI][Medline]
8 . Minami, S., Akama, M., Araki, H., Watanabe, Y., Narita, H., Iyobe, S. et al. (1996). Imipenem and cephem resistant Pseudomonas aeruginosa carrying plasmids coding for class B ß-lactamases. Journal of Antimicrobial Chemotherapy 37, 433-44.[Abstract]
9 . Senda, K., Arakawa, Y., Nakashima, K., Ito, H., Ichiyama, S., Shimokata, K. et al. (1996). Multifocal outbreaks of metallo-ß-lactamase-producing Pseudomonas aeruginosa resistant to broad-spectrum ß-lactams, including carbapenems. Antimicrobial Agents and Chemotherapy 40, 349-53.
10 . Troillet, K., Samore, M. H. & Carmeli, Y. (1997). Imipenem-resistant Pseudomona aeruginosa: risk factors and antibiotic susceptibility patterns. Clinical Infectious Diseases 25,1094 -8.[ISI][Medline]
Received 23 June 1998; returned 13 July 1998; revised 22 July 1998; accepted 8 October 1998