Abstract
About 7% of 7,252 nonduplicated clinical Escherichia coli strains from a Spanish hospital showed reduced susceptibility to amoxicillin-clavulanate. Of these, 0.37% produced the IRTs TEM-30, TEM-31, TEM-33, TEM-34, TEM-37, TEM-40, TEM-51, and TEM-54; 5.3% were probable class C β-lactamase overproducers; 0.8% were probable TEM-1 hyperproducers; 0.18% produced OXA-30; 0.15% overexpressed SHV-1; and 0.03% produced a PSE-1 enzyme.
Resistance to β-lactam-β-lactamase inhibitor combinations in Escherichia coli isolates has been reported to be due to hyperproduction of class A β-lactamases (such as TEM-1 or SHV-1), class D plasmid-mediated enzyme, or chromosomal or plasmidic class C β-lactamase and/or to modified outer membrane permeability (4, 15, 17, 27). Another mechanism, first recognized in 1992, is the production of inhibitor-resistant derivatives of TEM-1, TEM-2, SHV-1, and OXY-2-derived β-lactamases (1-4, 9, 16, 22). The most common inhibitor-resistant enzymes were derivatives of TEM-1 (IRTs) and have been detected in several members of the Enterobacteriaceae from different European countries (1, 3-11, 23, 26, 28). However, only a few French reports have documented their prevalence (6, 10, 11, 26, 28). The aim of this study was to evaluate the prevalence of IRTs in our hospital from a total of 7,252 clinically significant E. coli strains isolated between 1996 and 1998. Based on β-lactam resistance patterns by a disk diffusion technique, we detected 499 strains with a zone diameter for amoxicillin-clavulanate of <17 mm. The susceptibility pattern was confirmed by an agar dilution method according to NCCLS guidelines (13). We characterized the β-lactamases implicated in amoxicillin-clavulanate resistance by analytical isoelectric focusing and determination of kinetic constants, as previously described (12, 19, 20). Finally, according to the isoelectric point (pI), a PCR using primers for blaSHV-, blaTEM-, and blaOXA-1-related enzymes was done as published elsewhere (12, 20). The PCR products of 907, 1,017, and 921 bp, respectively, were further sequenced by the dideoxy method using fluorescent primers and an automatic laser fluorescent DNA sequencer (Pharmacia Biotech). The blaPSE genes were amplified with the primers PSE-1UP (5′-TAG CCA TAT TAT GGA GCC TC-3′) and PSE-1DN (5′-CCT TAT CAG CGC GAC TGT-3′). The PCR product of 940 bp was further sequenced by using in addition the internal primers PSE-1F (5′-CGC TTC CCG TTA ACA AGT AC-3′) and PSE-1B (5′-CTG GTT CAT TTC AGA TAG CG-3′).
Among the 7,252 strains evaluated, 368 were assumed to be AmpC overproducers (due to cefoxitin resistance and decreased susceptibility to broad-spectrum cephalosporins, without synergy between them and clavulanate) and were disregarded. In 1996, we studied isolates that were highly resistant to amoxicillin-clavulanate, cefoxitin, and broad-spectrum cephalosporins, and they have already been reported as AmpC overproducers (20). However, 16 strains susceptible to cefoxitin (MICs were between 8 and 32 μg/ml) were also assumed to be AmpC overproducers because they had a β-lactamase pI of >9 and all were PCR negative, including those for plasmidic AmpC β-lactamase (data not shown). In our hospital the first plasmid-mediated AmpC enzyme (CMY-2 β-lactamase) was detected in 1999 (14). Although other mechanisms could be implicated in this phenotype, such as porin protein alteration, we assume that about 5% of E. coli strains are AmpC overproducers, as in the studies done in France (10, 11, 26).
Accordingly, a total of 114 amoxicillin-clavulanate-resistant, cefoxitin-susceptible isolates were evaluated for the presence of IRT (Table 1). An additional strain was unavailable for study. Eighty-eight of them were positive in PCRs using specific primers for blaTEM. Fourteen showed β-lactamases with a pI of 5.2; 68 strains had a pI of 5.4, and six showed two enzymes with pIs of 5.2 and 5.4. The production of IRT in the strains with only one β-lactamase was detected by determining the 50% inhibitory concentration (IC50) of clavulanic acid. For 19 strains, IC50s were between 0.5 and 4 μM, and 42 had values between 0.01 and 0.05 μM, which were similar to that of the TEM-1 control strain (0.03 μM). For all 61 strains, the nucleotide sequence of the PCR fragments showed a TEM-1 enzyme. Kinetic studies of two TEM-1 producer strains showed a hyperproduction of the enzyme (from 76 mmol mg−1 min−1 of the E. coli control strain to more than 1,300 mmol mg−1 min−1). Therefore, the hyperproduction of TEM-1 was observed in 61 of 499 clavulanic acid-resistant strains (12.2%); this rate was similar to that described by other authors, between 14.7 and 18.2% (10, 11), but different from the 67% rate reported by Stapleton et al. (26). It was previously found that this hyperproduction is also effective in causing resistance to the combinations amoxicillin-sulbactam and piperacillin-tazobactam (17); however, in our study MICs of ampicillin-sulbactam were ≤64/32 μg/ml for 32 of 61 TEM-1-producing strains, and 37 were susceptible to piperacillin-tazobactam (MICs of ≤4/4 to 16/4 μg/ml) (data not shown).
TABLE 1.
β-Lactamases implicated in the reduced susceptibility of amoxicillin-clavulanate and susceptibility to cefoxitin in E. coli strains isolated between 1996 and 1998
| β-Lactamase | No. (%a) of isolates with enzyme
|
% of total (n = 7,252) | |||
|---|---|---|---|---|---|
| In 1996 | In 1997 | In 1998 | Total | ||
| TEM-1 | 16 | 27 | 18 | 61 (53.5) | 0.84 |
| IRTs | 8 | 8 | 5 | 21 (18.4) | 0.30 |
| IRT + TEM-1 | 3 | 1 | 2 | 6 (5.3) | 0.07 |
| OXA-1 | 0 | 7 | 6 | 13 (11.4) | 0.18 |
| SHV-1 | 1 | 7 | 3 | 11 (9.6) | 0.15 |
| PSE-1 | 0 | 2 | 0 | 2 (1.7) | 0.03 |
From 114 amoxicillin-clavulanate resistant E. coli strains with a cefoxitin inhibition zone diameter of >18 mm.
For 21 of the 88 strains, the IC50 was between 1 and 45.6 μM (Table 2). The nucleotide sequence results showed that five strains produced TEM-30 (IRT-2), two produced TEM-31 (IRT-1), two produced TEM-33 (IRT-5), one produced TEM-34 (IRT-6), three produced TEM-37 (IRT-8), four produced TEM-40 (IRT-11), two produced TEM-51 (IRT-15), and two produced TEM-54.
TABLE 2.
Phenotypical characteristics of IRT-producing strainsa
| IRT (pI) | Strain | IC50 (μM) | MIC of β-lactam (μg/ml)
|
Biotype | PFGE type | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| AMP | SAM | PIP | TZP | AMC | CFZ | CEF | FOX | CAZ | |||||
| TEM-30/IRT-2 (5.2) | 843-D | 2.5 | 1,024 | 16/8 | 512 | 8/4 | 32/16 | ≤4 | 8 | 4 | 0.5 | 0775 | 3 |
| 851-D | 2.5 | >1,024 | 16/8 | 1,024 | 32/4 | 32/16 | ≤4 | 16 | 2 | 0.12 | 1775 | 4 | |
| 852-D | 2.5 | 1,024 | 16/8 | 256 | 8/4 | 32/16 | ≤4 | ≤4 | 2 | 0.12 | 0573 | 5 | |
| 904-D | 2.1 | 1,024 | 16/8 | 64 | ≤4/4 | 32/16 | ≤4 | 16 | 8 | 0.12 | 7775 | 6 | |
| 1323-D | 1.6 | 512 | 8/4 | ≤32 | ≤4/4 | 16/8 | ≤4 | ≤4 | 2 | 0.12 | 1575 | 11 | |
| TEM-30+TEM-1 (5.2 + 5.4) | 758-D | ND | 1,024 | 32/16 | 256 | ≤4/4 | 64/32 | ≤4 | 16 | 4 | 0.12 | 7775 | 5 |
| 763-D | ND | 1,024 | 32/16 | 512 | 16/8 | 32/16 | 8 | 8 | 2 | 0.12 | 0573 | 15 | |
| 841-D | ND | >1,024 | 32/16 | 1,024 | 128/4 | 32/16 | 8 | 16 | 8 | 0.5 | 6573 | 16 | |
| 883-D | ND | 1,024 | 32/16 | 512 | 8/4 | 32/16 | ≤4 | 16 | 4 | 0.5 | 7775 | 17 | |
| 1143-D | ND | 512 | 32/16 | 128 | ≤4/4 | 32/16 | ≤4 | 8 | 4 | ≤0.06 | 7571 | 1 | |
| TEM-31/IRT-1 (5.2) | 913-D | 2.5 | 512 | 32/16 | ≤32 | 8/4 | 32/16 | ≤4 | 16 | 16 | 0.5 | 7775 | 10 |
| 1220-D | 24 | 512 | 16/8 | ≤32 | ≤4/4 | 32/16 | ≤4 | 8 | 4 | 0.25 | 6775 | 8 | |
| TEM-31+TEM-1 (5.2 + 5.4) | 1258-D | ND | 1,024 | 32/16 | 64 | 8/4 | 32/16 | ≤4 | 16 | 8 | 0.25 | 7775 | 18 |
| TEM-33/IRT-5 (5.4) | 1041-D | 2.5 | >1,024 | 256/128 | 1,024 | 128/4 | 64/32 | ≤4 | 8 | 4 | 0.25 | 1575 | 11 |
| 1279-D | 19.1 | >1,024 | 256/128 | 1,024 | 64/4 | 32/16 | ≤4 | 8 | 4 | 0.12 | 7775 | 9 | |
| TEM-34/IRT-6 (5.4) | 781-D | 1 | >1,024 | 16/8 | 1,024 | 8/4 | 32/16 | 8 | 16 | 4 | 0.12 | 7775 | 12 |
| TEM-37/IRT-8 (5.2) | 659-D | 1 | 512 | 32/16 | 128 | 8/4 | 32/16 | ≤4 | ≤4 | 4 | 0.25 | 7775 | 1 |
| 1111-D | 2.5 | 512 | 32/16 | 64 | ≤4/4 | 64/32 | ≤4 | 8 | 8 | 0.12 | 7571 | 1 | |
| 1238-D | 1.6 | 512 | 16/8 | 128 | 8/4 | 32/16 | ≤4 | ≤4 | 4 | 0.25 | 7575 | 1 | |
| TEM-40/IRT-11 (5.4) | 746-D | NACT | 1,024 | 16/8 | 512 | 8/4 | 32/16 | ≤4 | 8 | 2 | 0.12 | 6771 | 13 |
| 819-D | NACT | >1,024 | 32/16 | 1,024 | 8/4 | 32/16 | ≤4 | 4 | 2 | 0.12 | 1775 | 14 | |
| 1009-D | NACT | 1,024 | 32/16 | 256 | 8/4 | 32/16 | ≤4 | 16 | 8 | 0.25 | 6775 | 14 | |
| 1259-D | NACT | 1,024 | 32/16 | 2,256 | 8/4 | 64/32 | ≤4 | 16 | 8 | 0.25 | 6775 | 14 | |
| TEM-51/IRT-15 (5.2) | 756-Db | 4.2 | 1,024 | 32/16 | ≤32 | 8/4 | 32/16 | ≤4 | 8 | 4 | 0.25 | 1775 | 2 |
| 1020-D | 45 | 1,024 | 32/16 | 128 | 16/8 | 32/16 | ≤4 | 32 | 16 | 0.25 | 1175 | 15 | |
| TEM-54 (5.2) | 875-Db | 45.6 | 1,024 | 32/16 | 64 | 8/4 | 64/32 | ≤4 | 8 | 4 | 0.25 | 1775 | 2 |
| 911-D | 45.6 | 512 | 16/8 | ≤32 | 8/4 | 32/16 | ≤4 | 8 | 8 | 0.25 | 1775 | 2 | |
AMP, ampicillin; SAM, ampicillin-sulbactam; PIP, piperacillin; TZP, piperacillin-tazobactam; AMC, amoxicillin-clavulanate; CFZ, cefazolin; CEF, cephalothin; FOX, cefoxitin; CAZ, ceftazidime; PFGE, pulsed field gel electrophoresis; ND, not determined; NACT, not able to be determined.
756-D and 875-D were isolated from the same patient.
Of the six strains that produced two enzymes of pI 5.2 and 5.4, five produced TEM-30 (IRT-2) and one produced TEM-31 (IRT-1), all associated with TEM-1 (Table 2).
Accordingly, 5.4% of the studied strains (27 of 499) were IRT-producing strains. This percentage differs from those reported by Henquell et al. (10) and Leflon-Guibout et al. (11), who found 21.6 and 30.5%, respectively, of IRTs producing strains among the amoxicillin-clavulanate resistant strains, whereas they were similar to those reported by Stapleton et al. (26). As in other reports, TEM-30 was the main enzyme detected, followed by TEM-40 (9, 11, 26, 28). Twenty-five strains carrying IRTs were isolated from urine, and two were from abdominal exudates.
The remaining 26 of 114 amoxicillin-clavulanate-resistant strains give different results (Table 1). Thirteen (2.6% of 499) produced an OXA-30 β-lactamase, according to the nucleotide sequence of the PCR product, which showed an intrinsic resistance to amoxicillin-clavulanate (24). Nine strains carrying OXA-30 were isolated from urine, three were from blood, and one was from abdominal exudate.
Eleven strains (2.2% of 499), all them isolated from urine, showed an overproduction of an SHV-1 enzyme (according to PCR, sequence, and kinetic studies). The observed phenotype of these strains was similar to that published previously (12), with MICs of ceftazidime between 2 and 8 μg/ml. Kinetic studies of two strains showed 6- to 10-fold more activity to penicillin G than the control strain (42 to more than 270 mmol mg−1 min−1). The presence of SHV-type enzymes resistant to inhibitors has been communicated only by Prinarakis et al. (16), who described the SHV-10 β-lactamase.
Finally, we found that two strains (0.4% of 499) produce a PSE-1 enzyme according to the nucleotide sequence of the PCR product. Both strains were isolated from urine. Different kinetics studies have suggested that the amoxicillin (ticarcillin)-clavulanate resistance in these strains is due to the rapid hydrolysis of substrate by the enzyme rather than to any lack of susceptibility of the enzyme for the inhibitor (21). However, the presence of additional mechanisms such as efflux or altered permeability in this and the other strains has not been evaluated, but the detected enzymes have been documented as being responsible for the phenotype.
The clonality of all strains carrying an IRT was disregarded by biotyping (18) and pulsed-field gel electrophoresis studies as previously described (12), except for the strains carrying TEM-37, TEM-40, and TEM-54 (there was no known epidemiological connection between them). The diversity of mutant enzymes, which appeared over a short time in nonrepetitive and nonlinked strains, cannot be explained by an epidemic phenomenon. These results suggest the independent emergence and selection of these TEM variants under antibiotic-selective pressure (3, 9). We found two clonally related strains producing two different IRTs (TEM-51 and TEM-54). Both strains were isolated from a patient with a myelomeningocele, who suffered repeated urinary tract infections (UTIs). This patient was treated with amoxicillin-clavulanate for almost 6 years before isolation of these strains. Sotto et al. (25) found in their logistic-regression model that a UTI during the previous year of hospitalization was a common risk factor for resistance to several antibiotics, one being amoxicillin-clavulanate. Although a prior UTI was probably associated with prior antibiotic exposure, the latter factor was significantly associated with resistance only to amoxicillin-clavulanate.
Acknowledgments
We thank the “Fundación Ma Francisca de Roviralta” for financial support. This work was supported by governmental grant FIS 98/1522.
REFERENCES
- 1.Blázquez, J., M. R. Baquero, R. Cantón, I. Alós, and F. Baquero. 1993. Characterisation of a new TEM-type β-lactamase resistant to clavulanate, sulbactam, and tazobactam in a clinical isolate of Escherichia coli. Antimicrob. Agents Chemother. 37:2059-2063. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Bonomo, R. A., C. Currie-McCumber, and D. M. Shales. 1992. OHIO-1 β-lactamase resistant to mechanism-based inactivators. FEMS Microbiol. Lett. 71:79-82. [DOI] [PubMed] [Google Scholar]
- 3.Caniça, M. M., C. Y. Lu, R. Krishnamoorthy, and G. C. Paul. 1997. Molecular diversity and evolution of blaTEM genes encoding β-lactamases resistant to clavulanic acid in clinical E. coli. J. Mol. Evol. 44:57-65. [DOI] [PubMed] [Google Scholar]
- 4.Chaïbi, E. B., D. Sirot, G. Paul, and R. Labia. 1999. Inhibitor-resistant TEM β-lactamases: phenotypic, genetic and biochemical characteristics. J. Antimicrob. Chemother. 43:447-458. [DOI] [PubMed] [Google Scholar]
- 5.Chaïbi, E. B., S. Farzaneh, A. Morand, J. Peduzzi, M. Barthélémy, D. Sirot, and R. Labia. 1996. Problems encountered in the characterization of IRT β-lactamase-producing clinical Escherichia coli isolates intermediate-resistant to cephalothin. J. Antimicrob. Chemother. 37:190-191. [DOI] [PubMed] [Google Scholar]
- 6.Chanal, C., R. Bonnet, C. De Champs, D. Sirot, R. Labia, and J. Sirot. 2000. Prevalence of β-lactamases among 1,072 clinical strains of Proteus mirabilis: a 2-year survey in a French hospital. Antimicrob. Agents Chemother. 44:1930-1935. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Espinasse, F., R. Gheorghiu, A. Poiata, R. Labia, and M.-H. Nicolas-Chanoine. 1997. Reduced susceptibility to co-amoxiclav in Escherichia coli, Salmonella typhimurium and Klebsiella pneumoniae isolated in Romania between 1985 and 1993. J. Antimicrob. Chemother. 39:103-106. [DOI] [PubMed] [Google Scholar]
- 8.Granier, S. A., J.-C. N. Van, M.-D. Kitzis, F. W. Goldstein, V. Leflon-Guibout, and M-H. Nicolas-Chanoine. 2002. First description of a TEM-30 (IRT-2)-producing Klebsiella oxytoca isolate. Antimicrob. Agents Chemother. 46:1158-1159. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Henquell, C., C. Chanal, D. Sirot, R. Labia, and J. Sirot. 1995. Molecular characterization of nine different types of mutants among 107 inhibitor-resistant TEM β-lactamases from clinical isolates of Escherichia coli. Antimicrob. Agents Chemother. 39:427-430. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Henquell, C., D. Sirot, C. Chanal, C. De Champs, P. Chatron, B. Lafeuille, P. Texier, J. Sirot, and R. Cluzel. 1994. Frequency of inhibitor-resistant TEM β-lactamases in Escherichia coli isolates from urinary tract infections in France. J. Antimicrob. Chemother. 34:707-714. [DOI] [PubMed] [Google Scholar]
- 11.Leflon-Guibout, V., V. Speldooren, B. Heym, and M.-H. Nicolas-Chanoine. 2000. Epidemiological survey of amoxicillin-clavulanate resistance and corresponding molecular mechanisms in Escherichia coli isolates in France: new genetic features of blaTEM genes. Antimicrob. Agents Chemother. 44:2709-2714. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Miró, E., M. Del Cuerpo, F. Navarro, M. Sabaté, B. Mirelis, and G. Prats. 1998. Emergence of clinical Escherichia coli isolates with decreased susceptibility to ceftazidime and synergic effect with co-amoxiclav due to SHV-1 hyperproduction. J. Antimicrob. Chemother. 42:535-538. [DOI] [PubMed] [Google Scholar]
- 13.National Committee for Clinical Laboratory Standards. 1995. Performance standards for antimicrobial susceptibility testing. Sixth informational supplement: approved standard M100-S6. NCCLS, Villanova, Pa.
- 14.Navarro, F., E. Pérez-Trallero, J. M. Marimon, R. Aliaga, M. Gomariz, and B. Mirelis. 2001. CMY-2-producing Salmonella enterica, Klebsiella pneumoniae, Klebsiella oxytoca, Proteus mirabilis and Escherichia coli strains isolated in Spain (October 1999-December 2000). J. Antimicrob. Chemother. 48:383-389. [DOI] [PubMed] [Google Scholar]
- 15.Oliver, A., M. Pérez-Vázquez, M. Martínez-Ferrer, F. Baquero, L. de Rafael, and R. Cantón. 1999. Ampicillin-sulbactam and amoxicillin-clavulanate susceptibility testing of Escherichia coli isolates with different β-lactam resistance phenotypes. Antimicrob. Agents Chemother. 43:862-867. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Prinarakis, E. E., V. Miriagou, E. Tzelepi, M. Gazouli, and L. S. Tzouvelekis. 1997. Emergence of an inhibitor-resistant β-lactamase (SHV-10) derived from an SHV-5 variant. Antimicrob. Agents Chemother. 41:838-840. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.Reguera, J. A., F. Baquero, J. C. Pérez-Díaz, and J. L. Martínez. 1991. Factors determining resistance to β-lactam combined with β-lactamase inhibitors in Escherichia coli. J. Antimicrob. Chemother. 27:569-575. [DOI] [PubMed] [Google Scholar]
- 18.Richard, C. 1981. Une méthode simple de marquage épidémiologique: la biotypie, application à Enterobacter cloacae et Escherichia coli. Bull. Assoc. Anc. Eleves Inst. Pasteur 87:14-21. [Google Scholar]
- 19.Sabaté, M., C. Vergés, E. Miró, B. Mirelis, F. Navarro, E. Del Rio, and G. Prats. 1999. Incidencia de betalactamasas de espectro ampliado en Escherichia coli en un hospital universitario durante 1994-96. Enferm. Infect. Microbiol. Clin. 17:401-404. [PubMed] [Google Scholar]
- 20.Sabaté, M., E. Miró, F. Navarro, C. Vergés, R. Aliaga, B. Mirelis, and G. Prats. 2002. β-Lactamases involved in resistance to broad-spectrum cephalosporins in Escherichia coli and Klebsiella spp. clinical isolates between 1994-1996, in Barcelona (Spain). J. Antimicrob. Chemother. 49:989-997. [DOI] [PubMed] [Google Scholar]
- 21.Sanders, C. C., J. P. Iaconis, G. P. Bodey, and G. Samonis. 1988. Resistance to ticarcillin-potassium clavulanate among clinical isolates of the family Enterobacteriaceae: role of PSE-1 β-lactamase and high levels of TEM-1 and SHV-1 and problems with false susceptibility in disk diffusion tests. Antimicrob. Agents Chemother. 32:1365-1369. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22.Sirot, D., R. Labia, P. Pouedras, C. Chanal-Claris, C. Cerceau, and J. Sirot. 1998. Inhibitor-resistant OXY-2-derived β-lactamase produced by Klebsiella oxytoca. Antimicrob. Agents Chemother. 42:2184-2187. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 23.Sirot, D., C. Chanal, R. Bonnet, C. De Champs, and J. Sirot. 2001. Inhibitor-resistant TEM-33 β-lactamase in a Shigella sonnei isolate. Antimicrob. Agents Chemother. 45:2179-2180. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 24.Siu, L. K., J. Y. C. Lo, K. Y. Yuen, P. Y. Chau, M. H. Ng, and P. L. Ho. 2000. β-Lactamases in Shigella flexneri isolates from Hong Kong and Shanghai and a novel OXA-1-like β-lactamase, OXA-30. Antimicrob. Agents Chemother. 44:2034-2038. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 25.Sotto, A., C. Merle de Boever, P. Fabbro-Peray, A. Gouby, D. Dirot, and J. Jourdan. 2001. Risk factors for antibiotic-resistant Escherichia coli isolated from hospitalized patients with urinary tract infections: a prospective study. J. Clin. Microbiol. 39:438-444. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 26.Stapleton, P., P. J. Wu, A. King, K. Shanon, G. French, and I. Phillips. 1995. Incidence and mechanisms of resistance to the combination of amoxicillin and clavulanic acid in Escherichia coli. Antimicrob. Agents Chemother. 39:2478-2483. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 27.Thomson, K. S., and E. S. Moland. 2000. Version 2000: the new β-lactamases of gram-negative bacteria at the dawn of the new millennium. Microbes Infect. 2:1225-1235. [DOI] [PubMed] [Google Scholar]
- 28.Zhou, X. Y., F. Bordon, D. Sirot, M. D. Kitzis, and L. Gutmann. 1994. Emergence of clinical isolates of Escherichia coli producing TEM-1 derivatives or an OXA-1 β-lactamase conferring resistance to β-lactamase inhibitors. Antimicrob. Agents Chemother. 38:1085-1089. [DOI] [PMC free article] [PubMed] [Google Scholar]