Servei de Microbiologia, Institut Clínic Infeccions i Immunologia (ICII), Hospital Clínic-IDIBAPS, University of Barcelona, Villarroel 170, 08036 Barcelona, Spain
Received 19 September 2001; returned 28 December 2001; revised 28 February 2002; accepted 21 April 2002
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Abstract |
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Introduction |
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A. baumannii is currently resistant to most ß-lactam antibiotics, particularly penicillins and cephalosporins, and lately to carbapenems.1,2 Ceftazidime, piperacillin and imipenem are among the most active ß-lactam antibiotics against A. baumannii. Several authors have studied the presence of ß-lactamases as a mechanism of resistance to these antibiotics.13 Some of these reports were performed using Acinetobacter calcoaceticus, since they were carried out before the studies that defined 13 genospecies, including one species for A. calcoaceticus and another for A. baumannii.1 This study aimed to investigate the distribution of ß-lactamases in a group of epidemiologically well defined A. baumannii clinical isolates. Moreover, the in vitro activity of Syn 2190, a novel AmpC inhibitor,4,5 in combination with various penicillins and cephalosporins was also determined.
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Materials and methods |
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The 20 A. baumannii clinical isolates analysed during this study are part of a collection from the Hospital Clinic, and were isolated in Spanish hospitals during a 10 year period.6,7
Susceptibility testing
All susceptibility testing was by Etest (AB Biobisk, Sölna, Sweden) following the manufacturers recommendations. The MICs of ampicillin, piperacillin, ceftazidime, imipenem and cefepime were determined in both the presence and absence of 4 mg/L Syn 21904,5 (Naeja Pharmaceutical Inc., Edmonton, Canada). In addition, the MICs of ticarcillin, cefotaxime and cefoxitin were determined. The MIC of imipenem was also determined in the presence of 25 mg/L reserpine. Staphylococcus aureus ATCC 29213 and Escherichia coli ATCC 25922 were used as quality controls.
Characterization of ß-lactamase
Determination of isoelectric points was performed as described previously.8 The presence of tem, oxa 2-like, oxa 3-like and shv-type genes was determined by PCR with primers and conditions described previously.3,8 Determination of the presence of oxa 20 and oxa 37 genes was carried out under the same conditions as the other oxa genes, but using the following primers: 5'-CACATCGGTTTATAATGAAT-3' and 5'-TTGGGTGGCAAAGCATTGACG-3'.
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Results and discussion |
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Three out of 20 (15%) A. baumannii strains showed a MIC of imipenem >32 mg/L, indicating the presence of a carbapenemase. Several ß-lactamases affecting imipenem have been described,1,2 some of which have been located in a plasmid that can be transferred, such as ARI-1, which is now OXA-23.1 The combination of imipenem plus Syn 2190 did not modify the activity of this carbapenem, suggesting that AmpC has no effect on this antimicrobial agent. A set of oxacillinases with activity against carbapenems has recently been described.2 The imipenem-susceptible strains presented a range of imipenem MICs between 0.06 and 4 mg/L. All four strains with a MIC of imipenem of 1 mg/L had an MIC of 0.19 (three strains) and 0.38 mg/L (one strain) in the presence of reserpine (25 mg/L). Reserpine is an efflux pump inhibitor, in both Gram-positive and -negative bacteria. These results suggest that an efflux pump, inhibited by reserpine, could be involved in the moderate increase in resistance to imipenem.
In summary, despite the fact that other mechanisms of resistance to ß-lactam antibiotics, such as decreased permeability or increased efflux, may contribute to the final MIC, our results indicate that a high percentage of A. baumannii clinical isolates show increased production of AmpC, and that other mechanisms, such as OXA- and TEM-type ß-lactamases, play an important role in resistance to ß-lactam antibiotics.
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Acknowledgements |
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Footnotes |
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References |
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2
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Bou, G., Oliver, A. & Martinez-Beltrán, J. (2000). OXA-24, a novel class D ß-lactamase with carbapenemase activity in an Acinetobacter baumannii clinical strain. Antimicrobial Agents and Chemotherapy 44, 155661.
3 . Vila, J., Navia, M., Ruiz, J. & Casals, C. (1997). Cloning and nucleotide sequence analysis of a gene encoding an OXA-derived ß-lactamase in Acinetobacter baumannii. Antimicrobial Agents and Chemotherapy 41, 27579.[Abstract]
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Babini, G. S. & Livermore, D. M. (2000). Effect of conalbumin on the activity of Syn 2190, a 1.5 dihydroxy-4-pyridon monobactam inhibitor of AmpC ß-lactamases. Journal of Antimicrobial Chemotherapy 45, 1059.
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Vila, J., Ruiz, J., Navia, M., Becerril, B., Garcia, I., Perea, S. et al. (1999). Spread of amikacin resistance in Acinetobacter baumannii strains isolated in Spain due to an epidemic strain. Journal of Clinical Microbiology 37, 75861.
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Navia, M. M., Capitano, L., Ruiz, J., Vargas, M., Urassa, H., Schellemberg, D. et al. (1999). Typing and characterization of mechanisms of resistance of Shigella spp. isolated from feces of children under 5 years of age from Ifakara, Tanzania. Journal of Clinical Microbiology 37, 31137.
9 . Joly-Guillou, M. L., Vallée, E., Bergogne-Bérézin, E. & Philippon, A. (1988). Distribution of ß-lactamases and phenotype analysis in clinical strains of Acinetobacter calcoaceticus. Journal of Antimicrobial Chemotherapy 22, 597604.[Abstract]
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Bou, G. & Martinez-Beltrán, J. (2000). Cloning, nucleotide sequencing and analysis of the gene encoding an AmpC ß-lactamase in Acinetobacter baumannii. Antimicrobial Agents and Chemotherapy 44, 42832.
11 . Livermore, D. M. (1995). ß-Lactamases in laboratory and clinical resistance. Clinical Microbiology Reviews 8, 55784.[Abstract]