Antibiotic Resistance Monitoring and Reference Laboratory, Central Public Health Laboratory, 61 Colindale Avenue, London NW9 5HT, UK
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Abstract |
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Introduction |
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Syn 2190 is a 1,5 dihydroxy-4-pyridon monobactam (Figure 1) which inhibits AmpC ß-lactamases but not other types.3 It lacks antibacterial activity of its own. We postulated that the 1,5 dihydroxy-4-pyridon group might allow Syn 2190 to use the TonB pathway to permeate Gramnegative bacteria, as do catecholic ß-lactams. To test this hypothesis, MICs of Syn 2190 combinations were determined with and without conalbumin, which chelates iron and induces TonB-linked uptake.4
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Materials and methods |
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Isogenic ß-lactamase expression mutant series of Enterobacteriaceae and P. aeruginosa were tested. Most series comprised an AmpC ß-lactamase-inducible isolate and its derepressed and basal mutants; but some lacked AmpC-inducible parent strains, being derived from derepressed isolates.5 Also tested were (i) transconjugants of Escherichia coli K-12 and P. aeruginosa PU21 with various class A (TEM-3, TEM-6, TEM-9, TEM-10, SHV-2, SHV-3, SHV-4, SHV-5, PER-1 and NMC-A); B (IMP-1) and D ß-lactamases (OXA-1, OXA-2, OXA-3, OXA-4, OXA-5, OXA-6, OXA-7 and OXA-10);5 (ii) 71 isolates of P. aerugi-nosa with graded levels of AmpC hyperproduction;6 and (iii) clinical Enterobacteriaceae isolates with derepressed AmpC ß-lactamases and Klebsiella spp. with acquired AmpC enzymes.7
Antibiotics and reagents
Iso-Sensitest agar was from Oxoid (Basingstoke, Hants, UK); conalbumin from Fluka (Poole, Dorset, UK); ceftazidime from GlaxoWellcome (Uxbridge, Middlesex, UK); piperacillin and tazobactam from Wyeth (Taplow, Berks, UK) and Syn 2190 from Synphar Inc. (Edmonton, Alberta, Canada).
Susceptibility tests
MICs were determined on Iso-Sensitest agar with or without conalbumin, 3 g/L, added after sterilization. The inocula comprised 104 cfu/spot and MICs were read after 18 h as the lowest concentrations to inhibit growth completely. Inhibitors were used at 4 mg/L.
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Results and discussion |
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The behaviour of Syn 2190 combinations against P. aeruginosa isolates with graded levels of AmpC depression is illustrated in Figure 2. MICs of unprotected piperacillin and ceftazidime rose with the amount of AmpC enzyme produced without induction, and tazobactam only reduced the MICs of piperacillin two-fold or less. Even without conalbumin, Syn 2190 reduced the piperacillin MICs for the derepressed isolates two- to eight-fold and those of ceftazidime two- to four-fold, with the best potentiation seen for the isolates with the least enzyme (<4000 U/mg protein). Synergy was greater in the presence of conalbumin. Thus piperacillinSyn 2190, at <16 + 4 mg/L was active against all the derepressed P. aeruginosa isolates except for one with an exceptional level of AmpC activity (11,000 U/mg protein); ceftazidimeSyn 2190 at 8 + 4 mg/L was also active against all the isolates except this one strain. The presence of tazobactam did not significantly increase the activity of piperacillinSyn 2190 against these strains.
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The effect of conalbumin implies that Syn 2190 uses the TonB-linked iron uptake system to permeate Gram-negative bacteria, as do other 1,5 dihydroxy-4-pyridon and catechol-linked ß-lactams.4 Under conditions of iron limitation, which apply in many infections,8 Syn 2190 apparently reversed resistance caused by hyperproduction of AmpC enzymes in P. aeruginosa and most Enterobacteriaceae except Klebsiella spp. The reasons for this exception are unclear. The Klebsiella spp. with AmpC enzymes were from three different geographical sources7 so it seems unlikely that each had a Syn 2190-resistant enzyme. It may be that the ferric uptake pathway of Klebsiella spp. is more specific for its catecholic substrates than are those of other species.
As with all specific AmpC inhibitors, a major problem is the choice of partner agent.2,6 Aminothiazolyl cephalosporins are obvious candidates, but are inactivated by extended-spectrum class A ß-lactamases, which are resistant to Syn 2190, as well as by AmpC types. The triple Syn 2190piperacillintazobactam overcame class A enzymes as well as AmpC types, giving a very broad spectrum. With the spread of diverse ß-lactamases and the occurrence of strains with increasingly complex ß-lactamase combinations, pharmaceutical developers and clinicians may be pressed towards multi-inhibitor combinations.
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Acknowledgments |
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Notes |
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References |
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2 . Maiti, S. N., Phillips, O. A., Micetich, R. G. & Livermore, D. M. (1998). ß-Lactamase inhibitors: agents to overcome bacterial resistance. Current Medicinal Chemistry 5, 44156.[ISI][Medline]
3 . Atchinson, K. P., Catena, R., Kaleta, J., Atwal, H., Maiti, S. & Micetich, R. G. (1997). Syn 2190: a novel potent ß-lactamase inhibitor: kinetics of enzyme inhibition and penetration of the Pseudomonas aeruginosa outer membrane. In Program and Abstracts of the Thirty-Seventh Interscience Conference on Antimicrobial Agents and Chemotherapy, Toronto, Ontario, Canada, 1997. Abstract F-189, p. 178. American Society for Microbiology, Washington, DC.
4 . Nikaido, H. & Rosenberg, E. Y. (1990). Cir and Fiu proteins in the outer membrane of Escherichia coli catalyze transport of monomeric catechols: study with ß-lactam antibiotics containing catechol and analogous groups. Journal of Bacteriology 172, 13617.[ISI][Medline]
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Babini, G. S., Yuan, M. & Livermore, D. M. (1998). Interactions of ß-lactamases with sanfetrinem (GV 104326) compared to those with imipenem and with oral ß-lactams. Antimicrobial Agents and Chemotherapy 42, 116875.
6 . Livermore, D. M. & Chen, H. Y. (1997). Potentiation of ß-lactams against Pseudomonas aeruginosa strains by Ro 48-1256, a bridged monobactam inhibitor of AmpC ß-lactamases. Journal of Antimicrobial Chemotherapy 40, 33543.[Abstract]
7 . Livermore, D. M. & Yuan, M. (1996). Antibiotic resistance of extended-spectrum ß-lactamases amongst Klebsiella spp. from intensive care units in Europe. Journal of Antimicrobial Chemotherapy 38, 40924.[Abstract]
8 . Bullen, J. J. (1981). The significance of iron in infection. Reviews of Infectious Diseases 6, 112738.
Received 5 March 1999; returned 29 June 1999; revised 20 July 1999; accepted 14 September 1999