Characterization and nucleotide sequence of a Klebsiella oxytoca cryptic plasmid encoding a CMY-type beta-lactamase: confirmation that the plasmid-mediated cephamycinase originated from the Citrobacter freundii AmpC beta-lactamase.
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Plasmid pTKH11, originally obtained by electroporation of a Klebsiella oxytoca plasmid preparation into Escherichia coli XAC, expressed a high level of an AmpC-like beta-lactamase. The enzyme, designated CMY-5, conferred resistance to extended-spectrum beta-lactams in E. coli; nevertheless, the phenotype was cryptic in the K. oxytoca donor. Determination of the complete nucleotide sequence of pTKH11 revealed that the 8,193-bp plasmid encoded seven open reading frames, including that for the CMY-5 beta-lactamase (blaCMY-5). The blaCMY-5 product was similar to the plasmidic CMY-2 beta-lactamase of K. pneumoniae and the chromosomal AmpC of Citrobacter freundii, with 99.7 and 97.0% identities, respectively; there was a substitution of phenylalanine in CMY-5 for isoleucine 105 in CMY-2. blaCMY-5 was followed by the Blc and SugE genes of C. freundii, and this cluster exhibited a genetic organization identical to that of the ampC region on the chromosome of C. freundii; these results confirmed that C. freundii AmpC was the evolutionary origin of the plasmidic cephamycinases. In the K. oxytoca host, the copy number of pTKH11 was very low and the plasmid coexisted with plasmid pNBL63. Analysis of the replication regions of the two plasmids revealed 97% sequence similarity in the RNA I transcripts; this result implied that the two plasmids might be incompatible. Incompatibility of the two plasmids might explain the cryptic phenotype of blaCMY-5 in K. oxytoca through an exclusion effect on pTKH11 by resident plasmid pNBL63. (+info)
Use of microdilution panels with and without beta-lactamase inhibitors as a phenotypic test for beta-lactamase production among Escherichia coli, Klebsiella spp., Enterobacter spp., Citrobacter freundii, and Serratia marcescens.
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Over the past decade, a number of new beta-lactamases have appeared in clinical isolates of Enterobacteriaceae that, unlike their predecessors, do not confer beta-lactam resistance that is readily detected in routine antibiotic susceptibility tests. Because optimal methodologies are needed to detect these important new beta-lactamases, a study was designed to evaluate the ability of a panel of various beta-lactam antibiotics tested alone and in combination with beta-lactamase inhibitors to discriminate between the production of extended-spectrum beta-lactamases, AmpC beta-lactamases, high levels of K1 beta-lactamase, and other beta-lactamases in 141 isolates of Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, Enterobacter cloacae, Enterobacter aerogenes, Citrobacter freundii, and Serratia marcescens possessing well-characterized beta-lactamases. The microdilution panels studied contained aztreonam, cefpodoxime, ceftazidime, cefotaxime, and ceftriaxone, with and without 1, 2, and 4 microg of clavulanate per ml or 8 microg of sulbactam per ml and cefoxitin and cefotetan with and without 8 microg of sulbactam per ml. The results indicated that a minimum panel of five tests would provide maximum separation of extended-spectrum beta-lactamase high AmpC, high K1, and other beta-lactamase production in Enterobacteriaceae. These included cefpodoxime, cefpodoxime plus 4 microg of clavulanate per ml, ceftazidime, ceftriaxone, and ceftriaxone plus 8 microg of sulbactam per ml. Ceftriaxone plus 2 microg of clavulanate per ml could be substituted for cefpodoxime plus 4 microg of clavulanate per ml without altering the accuracy of the tests. This study indicated that tests with key beta-lactam drugs, alone and in combination with beta-lactamase inhibitors, could provide a convenient approach to the detection of a variety of beta-lactamases in members of the family Enterobacteriaceae. (+info)
Citrobacter freundii invades and replicates in human brain microvascular endothelial cells.
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Neonatal bacterial meningitis remains a disease with unacceptable rates of morbidity and mortality despite the availability of effective antimicrobial therapy. Citrobacter spp. cause neonatal meningitis but are unique in their frequent association with brain abscess formation. The pathogenesis of Citrobacter spp. causing meningitis and brain abscess is not well characterized; however, as with other meningitis-causing bacteria (e.g., Escherichia coli K1 and group B streptococci), penetration of the blood-brain barrier must occur. In an effort to understand the pathogenesis of Citrobacter spp. causing meningitis, we have used the in vitro blood-brain barrier model of human brain microvascular endothelial cells (HBMEC) to study the interaction between C. freundii and HBMEC. In this study, we show that C. freundii is capable of invading and trancytosing HBMEC in vitro. Invasion of HBMEC by C. freundii was determined to be dependent on microfilaments, microtubules, endosome acidification, and de novo protein synthesis. Immunofluorescence microscopy studies revealed that microtubules aggregated after HBMEC came in contact with C. freundii; furthermore, the microtubule aggregation was time dependent and seen with C. freundii but not with noninvasive E. coli HB101 and meningitic E. coli K1. Also in contrast to other meningitis-causing bacteria, C. freundii is able to replicate within HBMEC. This is the first demonstration of a meningitis-causing bacterium capable of intracellular replication within BMEC. The important determinants of the pathogenesis of C. freundii causing meningitis and brain abscess may relate to invasion of and intracellular replication in HBMEC. (+info)
In vitro and in vivo activities of Syn2190, a novel beta-lactamase inhibitor.
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Syn2190, a monobactam derivative containing 1,5-dihydroxy-4-pyridone as the C-3 side chain, is a potent inhibitor of group 1 beta-lactamase. The concentrations of inhibitor needed to reduce the initial rate of hydrolysis of substrate by 50% for Syn2190 against these enzymes were in the range of 0.002 to 0.01 microM. These values were 220- to 850-fold lower than those of tazobactam. Syn2190 showed in vitro synergy with ceftazidime and cefpirome. This synergy was dependent on the concentration of the inhibitor against group 1 beta-lactamase-producing strains, such as Pseudomonas aeruginosa, Enterobacter cloacae, Citrobacter freundii, and Morganella morganii. However, against beta-lactamase-derepressed mutants of P. aeruginosa, the MICs of ceftazidime plus Syn2190 were not affected by the amount of beta-lactamase, and the values were the same for the parent strains. The MICs at which 50% of isolates are inhibited (MIC(50)s) of ceftazidime plus Syn2190 were 2- to 16-fold lower than those of ceftazidime alone for ceftazidime-resistant, clinically isolated gram-negative bacteria. Similarly, the MIC(50)s of cefpirome plus Syn2190 were two- to eightfold lower for cefpirome-resistant clinical isolates. The synergies of Syn2190 plus ceftazidime or cefpirome observed in vitro were also reflected in vivo. Syn2190 improved the efficacies of both cephalosporins in both a murine systemic infection model with cephalosporin-resistant rods and urinary tract infection models with cephalosporin-resistant P. aeruginosa. (+info)
Role of permeability in the activities of beta-lactams against gram-negative bacteria which produce a group 3 beta-lactamase.
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The production of a group 3 beta-lactamase permitted Escherichia coli to raise the MICs of ceftazidime, cefpirome, and meropenem greatly but those of imipenem and piperacillin only slightly. The ratios of maximum rate of hydrolysis to K(m) of ceftazidime, cefpirome, and piperacillin were lower than those of meropenem and imipenem for the group 3 beta-lactamase. The permeability coefficients for piperacillin and meropenem were higher than those for ceftazidime and cefpirome. Imipenem had the highest permeability coefficient. (+info)
Analysis of the mechanisms of quinolone resistance in clinical isolates of Citrobacter freundii.
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The presence of gyrA, gyrB and/or parC mutations, quinolone uptake, outer membrane protein profiles and epidemiological relationship were studied in 12 clinical isolates of Citrobacter freundii. No alterations were observed in the gyrB gene of any of the strains, or gyrA or parC of the four quinolone-susceptible strains (nalidixic acid MIC of 2-4 mg/L, and a ciprofloxacin MIC of 0.006-0.06 mg/L). The quinolone-resistant strains were classified into two groups: one group (group A) composed of strains resistant to nalidixic acid but not to ciprofloxacin and another (group B) including those resistant to both antibiotics with a mutation at codon 83 of the gyrA gene (Thr-->Ile), but no alteration in either parC or gyrB genes. In group B, three of the four resistant isolates, with a nalidixic acid MIC > 1024 mg/L and ciprofloxacin MIC of 8-32 mg/L, showed concomitant mutations at codons 83 and 87 of the gyrA gene (Thr-->Ile and Asp-->Tyr, respectively) as well as a single mutation in codon 80 of the parC gene (Ser-->Ile). The fourth isolate did not possess the mutation at codon 87 of gyrA. Two strains belong to the same clone and, although they had the same type of mutations in the gyrA and parC genes, showed different MICs of ciprofloxacin. This difference was related to an efflux pump mechanism. Mutations in the gyrA and parC genes play the main role in quinolone resistance development in Citrobacter freundii, although other factors such as overexpression of efflux pumps can play a complementary role and thus modulate the final quinolone MIC. (+info)
The complexed structure and antimicrobial activity of a non-beta-lactam inhibitor of AmpC beta-lactamase.
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Beta-lactamases are the major resistance mechanism to beta-lactam antibiotics and pose a growing threat to public health. Recently, bacteria have become resistant to beta-lactamase inhibitors, making this problem pressing. In an effort to overcome this resistance, non-beta-lactam inhibitors of beta-lactamases were investigated for complementarity to the structure of AmpC beta-lactamase from Escherichia coli. This led to the discovery of an inhibitor, benzo(b)thiophene-2-boronic acid (BZBTH2B), which inhibited AmpC with a Ki of 27 nM. This inhibitor is chemically dissimilar to beta-lactams, raising the question of what specific interactions are responsible for its activity. To answer this question, the X-ray crystallographic structure of BZBTH2B in complex with AmpC was determined to 2.25 A resolution. The structure reveals several unexpected interactions. The inhibitor appears to complement the conserved, R1-amide binding region of AmpC, despite lacking an amide group. Interactions between one of the boronic acid oxygen atoms, Tyr150, and an ordered water molecule suggest a mechanism for acid/base catalysis and a direction for hydrolytic attack in the enzyme catalyzed reaction. To investigate how a non-beta-lactam inhibitor would perform against resistant bacteria, BZBTH2B was tested in antimicrobial assays. BZBTH2B significantly potentiated the activity of a third-generation cephalosporin against AmpC-producing resistant bacteria. This inhibitor was unaffected by two common resistance mechanisms that often arise against beta-lactams in conjunction with beta-lactamases. Porin channel mutations did not decrease the efficacy of BZBTH2B against cells expressing AmpC. Also, this inhibitor did not induce expression of AmpC, a problem with many beta-lactams. The structure of the BZBTH2B/AmpC complex provides a starting point for the structure-based elaboration of this class of non-beta-lactam inhibitors. (+info)
Pyridoxal 5'-phoshate schiff base in Citrobacter freundii tyrosinephenol-lyase. Ionic and tautomeric equilibria.
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Spectral properties of the internal Schiff base in tyrosine phenol-lyase have been investigated in the presence of an activating cation K+ and a cation-inhibitor Na+. The holoenzyme absorption spectra in the pH range 6.5-8.7 were recorded in the presence of K+. No apparent pKa value of the coenzyme chromophore was found in this pH range, indicating that the internal Schiff base does not change its ionic form on going from pH 6.5 to 8.7. To determine the ionic state and tautomeric composition of the Schiff base in tyrosine phenol-lyase, the absorption and circular dichroism spectra were analyzed using lognormal distribution curves. The predominant form of the internal Schiff base is that with protonated pyridinium and aldimine nitrogen atoms and deprotonated 3'-hydroxy group, i.e. the ketoenamine. This form is in prototropic equilibrium with its enolimine tautomer. The internal aldimine ionic form is changed upon replacement of K+ with Na+. This replacement leads to a significant decrease in the pKa value of pyridinium nitrogen of the pyridoxal-P. (+info)