Search

Objectives Two natural variants of ADC-type β-lactamases of Acinetobacter spp., ADC-1 and ADC-5, differ by nine mutations in their protein sequence. ADC-5 hydrolyses cefoxitin better than ADC-1 and the opposite is true for ceftazidime. We produced single and combined mutations in ADC-5 and characterized the variants microbiologically and biochemically to determine which amino acid residues are involved in the hydrolysis of β-lactam antibiotics in this family of β-lactamases. Methods Site-directed mutagenesis, with blaADC-5 as a source of DNA, was used to generate nine single mutated and three combined mutated enzymes. The proteins (wild-type and derivatives) were then expressed in isogenic conditions in Escherichia coli. MICs of β-lactams were determined using Etest strips. ADC-1, ADC-5, ADC-5-P167S and ADC-5-P167S/D242G/Q163K/G342R were also purified and the kinetic parameters determined for ceftazidime, cefoxitin, cefalotin and ampicillin. Results Single mutations did not significantly convert the hydrolysis spectrum of the ADC-5 enzyme into that of the ADC-1 enzyme, although among all studied mutants only the quadruple mutant (ADC-5-P167S/D242G/Q163K/G342R) displayed microbiological and biochemical properties consistent with those of ADC-1. Conclusions Although some single mutations are known to affect cefepime hydrolysis in ADC-type β-lactamases, little is known about ceftazidime and cefoxitin hydrolysis in this family of β-lactamases. Hydrolysis of these antibiotics appears to be positively and negatively affected, respectively, by the Q163K, P167S, D242G and G342R amino acid replacements.

We investigated the mechanism of carbapenem resistance in 10 Acinetobacter baumannii strains isolated from the United States and Mexico between 2005 and 2009. The detection of known metallo-β-lactamase or carbapenem-hydrolyzing oxacillinase (OXA) genes by PCR was negative. The presence of plasmid-encoded carbapenem resistance genes was investigated by transformation of A. baumannii ATCC 17978. Shotgun cloning experiments and sequencing were performed, followed by the expression of a novel β-lactamase in A. baumannii . Three novel OXA enzymes were identified, OXA-235 in 8 isolates and the amino acid variants OXA-236 (Glu173-Val) and OXA-237 (Asp208-Gly) in 1 isolate each. The deduced amino acid sequences shared 85% identity with OXA-134, 54% to 57% identities with the acquired OXA-23, OXA-24, OXA-58, and OXA-143, and 56% identity with the intrinsic OXA-51 and, thus, represent a novel subclass of OXA. The expression of OXA-235 in A. baumannii led to reduced carbapenem susceptibility, while cephalosporin MICs were unaffected. Genetic analysis revealed that bla OXA-235 , bla OXA-236 , and bla OXA-237 were bracketed between two IS Aba1 insertion sequences. In addition, the presence of these acquired β-lactamase genes might result from a transposition-mediated mechanism. This highlights the propensity of A. baumannii to acquire multiple carbapenem resistance determinants.

Proteomic studies have improved our understanding of the microbial world. The most recent advances in this field have helped us to explore aspects beyond genomics. For example, by studying proteins and their regulation, researchers now understand how some pathogenic bacteria have adapted to the lethal actions of antibiotics. Proteomics has also advanced our knowledge of mechanisms of bacterial virulence and some important aspects of how bacteria interact with human cells and, thus, of the pathogenesis of infectious diseases. This review article addresses these issues in some of the most important human pathogens. It also reports some applications of Matrix-Assisted Laser Desorption/Ionization-Time-Of-Flight (MALDI-TOF) mass spectrometry that may be important for the diagnosis of bacterial resistance in clinical laboratories in the future. The reported advances will enable new diagnostic and therapeutic strategies to be developed in the fight against some of the most lethal bacteria affecting humans.

The CTX-M β-lactamases are an increasingly prevalent group of extended-spectrum β-lactamases (ESBL). Point mutations in CTX-M β-lactamases are considered critical for enhanced hydrolysis of cefotaxime. In order to clarify the structural determinants of the activity against cefotaxime in CTX-M β-lactamases, screening for random mutations was carried out to search for decreased activity against cefotaxime, with the CTX-M-1 gene as a model. Thirteen single mutants with a considerable reduction in cefotaxime MICs were selected for biochemical and stability studies. The 13 mutated genes of the CTX-M-1 β-lactamase were expressed, and the proteins were purified for kinetic studies against cephalothin and cefotaxime (as the main antibiotics). Some of the positions, such as Val103Asp, Asn104Asp, Asn106Lys, and Pro107Ser, are located in the 103 VNYN 106 loop, which had been described as important in cefotaxime hydrolysis, although this has not been experimentally confirmed. There are four mutations located close to catalytic residues—Thr71Ile, Met135Ile, Arg164His, and Asn244Asp—that may affect the positioning of these residues. We show here that some distant mutations, such as Ala219Val, are critical for cefotaxime hydrolysis and highlight the role of this loop at the top of the active site. Other distant substitutions, such as Val80Ala, Arg191, Ala247Ser, and Val260Leu, are in hydrophobic cores and may affect the dynamics and flexibility of the enzyme. We describe here, in conclusion, new residues involved in cefotaxime hydrolysis in CTX-M β-lactamases, five of which are in positions distant from the catalytic center.

OBJECTIVES The new metallo-beta-lactamase VIM-13 has been recently characterized. In comparison with the VIM-1 enzyme, VIM-13 showed 19 amino acid differences, 2 of which were located in the active site centre. The main objective of the present study was to assess whether differences between VIM-1 and VIM-13 beta-lactamases in the active site, at His224Leu and Ser228Arg, are necessary and sufficient to explain the microbiological and biochemical differences between the two enzymes. METHODS Single mutants VIM-13 (Leu224His) and VIM-13 (Arg228Ser) and double mutant VIM-13 (Leu224His, Arg228Ser) were created by site-directed mutagenesis with the bla(VIM-13) gene as template. VIM-1, VIM-13 and VIM-13 (Leu224His, Arg228Ser) were purified by affinity chromatography, and kinetic parameters for these enzymes were obtained with ceftazidime, cefepime and ampicillin. RESULTS Ceftazidime and cefepime MICs (mg/L) for Escherichia coli TG1 expressing VIM-1, VIM-13, VIM-13 (Leu224His), VIM-13 (Arg228Ser) and VIM-13 (Leu224His, Arg228Ser) were >256 and 64, 6 and 4, 8 and 1, >256 and 8, and >256 and 48, respectively. VIM-1, VIM-13 and VIM-13 (Leu224His, Arg228Ser) revealed k(cat)/K(m) values (M(-1)s(-1)) for ceftazidime of 3.7 E(4), 1.9 E(4) and 10 E(4), respectively, and revealed k(cat)/K(m) values for cefepime of 3.5 E(5), 3 E(4) and 1.5 E(5), respectively. CONCLUSIONS Overall, the results showed that the two residues located in the L3 loop are sufficient to confer the substrate specificity of each enzyme, thus highlighting the importance of the L3 loop of the active site in the evolution of VIM-type metallo-beta-lactamases.

Antimicrobial resistance constitutes one of the major threats regarding pathogenic microorganisms. Gram-negative pathogens, such as Enterobacteriaceae (specially those producing extended-spectrum beta-lactamases), Pseudomonas aeruginosa, and Acinetobacter baumannii have acquired an important role in hospital infections, which is of particular concern because of the associated broad spectrum of antibiotic resistance. beta-Lactam antibiotics are considered the most successful antimicrobial agents since the beginning of the antibiotic era. Soon after the introduction of penicillin, microorganisms able to destroy this beta-lactam antibiotic were reported, thus emphasizing the facility of pathogenic microorganisms to develop beta-lactam resistance. In Gram-negative pathogens, beta-lactamase production is the main mechanism involved in acquired beta-lactam resistance. Four classes of beta-lactamases have been described: A, B, C, and D. Classes A, C, and D are enzymes with a serine moiety in the active centre that catalyzes hydrolysis of the beta -lactam ring through an acyl-intermediate of serine, whereas the class B enzymes require a metal cofactor (e.g. zinc in the natural form) to function, and for this reason, they are also referred to as metallo- beta-lactamases (MBLs). To overcome beta-lactamase-mediated resistance, a combination of beta-lactam and a beta-lactamase inhibitor, which protects the beta-lactam antibiotic from the activity of the beta-lactamase, has been widely used in the treatment of human infections. Although there are some very successful combinations of beta-lactams and beta-lactamase inhibitors, most of the inhibitors act against class A beta-lactamases and remain ineffective against class B, C, and D beta-lactamases. This review constitutes an update of the current status and knowledge regarding class A to D beta-lactamase inhibitors, as well as a summary of the drug discovery strategy currently used to identify new beta-lactamase inhibitors, mainly based on the knowledge of crystal structure of beta-lactamase enzymes.

Objectives A natural variant of the AmpC enzyme from Escherichia coli HKY28 with a tripeptide deletion (Gly-286/Ser-287/Asp-288) was recently described. The isolate produced an inhibitor-sensitive AmpC beta-lactamase variant that also conferred higher than usual levels of resistance to ceftazidime in the E. coli host. To demonstrate whether this is true in other class C beta-lactamase enzymes, we deleted the equivalent tripeptide in the FOX-4 plasmid-mediated class C beta-lactamase. Methods By site-directed mutagenesis, we deleted the tripeptide Gly-306/Asn-307/Ser-308 of FOX-4, thus generating FOX-4(DeltaGNS). The enzymes (FOX-4 wild-type and DeltaGNS) were purified and kinetic parameters (kcat, Km, kcat/Km) as well as IC50 values of several beta-lactams were assessed. Modelling studies were also performed. Results FOX-4(DeltaGNS) did not increase the catalytic efficiency towards ceftazidime, although it conferred a slight increase in the susceptibility to beta-lactamase inhibitors. There was also a noteworthy decrease in the cefoxitin MIC with the FOX-4(DeltaGNS) mutant (from 512 to 16 mg/L) as well as a 10-fold decrease in kcat/Km towards imipenem, which revealed specific structural features. Conclusions Although deletions in the R2-loop are able to extend the substrate spectrum of class C enzymes, the present results do not confirm this hypothesis in FOX-4. The FOX-4 R2 site would already be wide enough to accommodate antibiotic molecules, and thus any amino acid replacement or deletion at this location would not affect the hydrolytic efficiency towards beta-lactams and would have a less drastic effect on the susceptibility to beta-lactamase inhibitors.

6 páginas, 2 figuras, 3 tablas -- PAGS nros. 792-797, Objectives In order to assess whether or not the Arg-276 of CTX-M-type enzymes is equivalent to the Arg-244 of IRT-TEM-derivative enzymes, we replaced the former with six different amino acids, some of them previously described as involved in resistance to β-lactamase inhibitors in TEM-IRT derivatives. We also investigated the role of Arg276 in cefotaxime hydrolysis. Methods By site-directed mutagenesis and by use of the blaCTX-M-1 gene as template, Arg-276 was replaced with six different amino acids (Trp, His, Cys, Asn, Gly and Ser). MICs of β-lactams alone and in combination with β-lactamase inhibitors were established. The seven enzymes (CTX-M-1 wild-type and six derived mutants) were purified by affinity chromatography, and kinetic parameters (kcat, Km, kcat/Km) towards cefalotin and cefotaxime were determined. Clavulanic acid IC50 values were also assessed with all enzymes. Results No increase in MICs of β-lactam/β-lactamase inhibitor combination was detected with any of the six CTX-M-1-derived mutants, in agreement with the clavulanic acid IC50 values. The MICs of cefotaxime were clearly lower for the Escherichia coli harbouring the Trp, Cys, Ser and Gly CTX-M-1 mutant enzymes than for CTX-M-1, and these enzymes showed a clearly reduced catalytic efficiency towards cefotaxime. As regards cefalotin, there was a moderate reduction in catalytic efficiency for Cys and His. Conclusions Arg-276 in CTX-M-type β-lactamases is not equivalent to Arg-244 in IRT-type enzymes. Position Arg-276 appears to be important for cefotaxime hydrolysis in CTX-M-type enzymes, although different effects were obtained regarding the replaced amino acid, This work was supported by the Consellería de Innovación, Industria y Comercio, Xunta de Galicia (PGIDIT04BTF916028PR), Fondo de Investigaciones Sanitarias. (PI040514 and PI061368), Ministerio de Educación y Ciencia (BFU2005-05055) and Spanish Network for the Research in Infectious Diseases (REIPI RD06/008). M. C. and A. B. are in receipt of a scholarship from SEIMC. F. J. P.-L. is in receipt of a research support contract from ISCIII

OBJECTIVES The main objective of the present study was to demonstrate the presence of a beta-lactamase ampC gene in the chromosome of the non-pathogenic bacterium Acinetobacter baylyi ADP1. METHODS beta-Lactam MICs were determined by Etest. The ampC gene was amplified by PCR, with specific oligonucleotides, then cloned into pBGS18 and pAT-RA plasmids and transformed into Escherichia coli TG1 and parental A. baylyi as hosts. The gene was sequenced and analysed. The AmpC protein was expressed, purified by affinity chromatography and the kinetic parameters determined. RESULTS An ampC gene was amplified from the ADP1 genome. Sequencing of the gene showed typical SVSK and KTG domains and the typical YXN Class C motif. The amplified gene showed significant identity (48.5% to 49.3%) with the AmpC enzymes of Acinetobacter baumannii and AG3 strains, which have recently been renamed ADC-1 to ADC-7. MIC analysis revealed a cephalosporinase profile for the E. coli TG1 clone as well as for the parental A. baylyi strain that overexpressed the ampC gene cloned under the control of an external promoter. Analysis of kinetic parameters of the purified enzyme showed higher catalytic efficiency for cefalotin than for ampicillin. CONCLUSIONS This study represents the first report of an AmpC beta-lactamase in A. baylyi, which was shown by biochemical and microbiological experiments to have a typical cephalosporinase profile. The presence of the respective gene in the chromosome of A. baylyi ADP1 suggested that this ampC gene is the naturally occurring cephalosporinase in this species, as previously reported for other Acinetobacter spp. We tentatively named the enzyme ADC-8.