Structure of the extended-spectrum class C β-lactamase ADC-1 from Acinetobacter baumannii

Whole genome sequencing characterization and comparative genome analysis of Acinetobacter baumannii JJAB01: A comprehensive insights on antimicrobial resistance and virulence genotype

The emergence of antibiotic resistance has significantly elevated the threat posed by Acinetobacter baumannii as an opportunistic pathogen. A.baumannii, a notorious bacterium, poses a serious threat to health care, leading to severe nosocomial infections, particularly in immunocompromised individuals. Whole-Genome Sequencing studies are efficient in providing accurate genetic information, aiding in detecting outbreaks, surveillance of resistance, and controlling infection transmission. In this study, we investigated the whole genome of a clinical isolate A. baumannii JJAB01 which sourced from a urine sample of an Intensive Care Unit (ICU) patient. This strain showed resistance to 24 available antibiotics, signifying Extremely Drug Resistant (XDR) and high potential for pathogenicity. Whole Genome Sequencing was performed using Illumina, and the raw reads were evaluated using the FastQC tool. Genome assembly and annotation were performed with Unicycler and the RAST server. The JJAB01 genome is 4.07 Mb with a GC content of 38.9 %. A total of 51 and 31 virulence factors and antimicrobial-resistant (AMR) genes were predicted using the VFDB and CARD databases. Comparative genome studies were carried out on virulence factors, resistance genes, prophages, and Multi-Locus Sequence Typing (MLST) across twelve closely related A. baumannii genomes, including JJAB01, X4-584, X4-705, 2023CK-00423, 2023CK-00890, 2023CK-00127, 2022CK-00066, B20AB01, B20AB10, F20AB03, G20AB08, and X4-65. These computational investigations in this study emphasis the multidimensional nature of the ICU strain JJAB01 and its genetic similarity to other strains, thereby enhancing our understanding of drug resistance and the pathogenicity associated with A. baumannii infections.

A new type of Class C β-lactamases defined by PIB-1. A metal-dependent carbapenem-hydrolyzing β-lactamase, from Pseudomonas aeruginosa: Structural and functional analysis

Antibiotic resistance is one of most important health concerns nowadays, and β-lactamases are the most important resistance determinants. These enzymes, based on their structural and functional characteristics, are grouped in four categories (A, B, C and D). We have solved the structure of PIB-1, a Pseudomonas aeruginosa chromosomally-encoded β-lactamase, in its apo form and in complex with meropenem and zinc. These crystal structures show that it belongs to the Class C β-lactamase group, although it shows notable differences, especially in the Ω- and P2-loops, which are important for the enzymatic activity. Functional analysis showed that PIB-1 is able to degrade carbapenems but not cephalosporins, the typical substrate of Class C β-lactamases, and that its catalytic activity increases in the presence of metal ions, especially zinc. They do not bind to the active-site but they induce the formation of trimers that show an increased capacity for the degradation of the antibiotics, suggesting that this oligomer is more active than the other oligomeric species. While PIB-1 is structurally a Class C β-lactamase, the low sequence conservation, substrate profile and its metal-dependence, prompts us to position this enzyme as the founder of a new group inside the Class C β-lactamases. Consequently, its diversity might be wider than expected.

Insights into dynamic changes in ADC-7 and P99 cephalosporinases using small angle x-ray scattering (SAXS)

To counter the emergence of β-lactamase (BL) mediated resistance, design of new β-lactamase inhibitors (BLIs) is critical. Many high-resolution crystallographic structures of BL complexed with BLIs are available. However, their impact on BLI design is struggling to keep pace with novel and emerging variants. Small angle x-ray scattering (SAXS) in combination with molecular modeling is a useful tool to determine dynamic structures of macromolecules in solution. An important application of SAXS is to determine the conformational changes that occur when BLI bind to BL. To probe if conformational dynamics occur in class C cephalosporinases, we studied SAXS profiles of two clinically relevant class C β-lactamases, Acinetobacter baumannii ADC-7 and Enterobacter cloacae P99 in apo format complexed with BLIs. Importantly, SAXS data analysis demonstrated that in solution, these representative class C enzymes remain monomeric and did not show the associated assemblies that were seen in various crystal structures. SAXS data acquired for ADC-7 and P99, in apo and inhibitor bound states, clearly showed that these enzymes undergo detectable conformational changes, and these class C β-lactamases also close upon binding inhibitors as does BlaC. Further analysis revealed that addition of inhibitor led to the compacting of a range of residues around the active site, indicating that the conformational changes that both P99 and ADC-7 undergo are central to inhibitor recognition and efficacy. Our findings support the importance of exploring conformational changes using SAXS analysis in the design of future BLIs.Communicated by Ramaswamy H. Sarma.

Genetic profile and characterization of antimicrobial resistance in Acinetobacter baumannii post-COVID-19 pandemic: a study in a tertiary hospital in Recife, Brazil

AIMS

To investigate the genetic profile and characterize antimicrobial resistance, including the main β-lactam antibiotic resistance genes, in Acinetobacterbaumannii isolates from a tertiary hospital in Recife-PE, Brazil, in the post-COVID-19 pandemic period.

METHODS AND RESULTS

Acinetobacter baumannii isolates were collected between 2023 and 2024 from diverse clinical samples. Antimicrobial resistance testing followed standardized protocols, with β-lactamase-encoding genes detected via PCR and sequencing. Investigation into ISAba1 upstream of blaOXA-carbapenemase and blaADC genes was also conducted. Genetic diversity was assessed through ERIC-PCR. Among the 78 A. baumannii, widespread resistance to multiple antimicrobials was evident. Various acquired β-lactamase-encoding genes (blaOXA-23,-24,-58,-143, blaVIM, and blaNDM) were detected. Furthermore, this is the first report of blaVIM-2 in A. baumannii isolates harboring either the blaOXA-23-like or the blaOXA-143 gene in Brazil. Molecular typing revealed a high genetic heterogeneity among the isolates, and multi-clonal dissemination.

CONCLUSION

The accumulation of genetic resistance determinants underscores the necessity for stringent infection control measures and robust antimicrobial stewardship programs to curb multidrug-resistant strains.

Drug Discovery in the Field of β-Lactams: An Academic Perspective

β-Lactams are the most widely prescribed class of antibiotics that inhibit penicillin-binding proteins (PBPs), particularly transpeptidases that function in peptidoglycan synthesis. A major mechanism of antibiotic resistance is the production of β-lactamase enzymes, which are capable of hydrolyzing β-lactam antibiotics. There have been many efforts to counter increasing bacterial resistance against β-lactams. These studies have mainly focused on three areas: discovering novel inhibitors against β-lactamases, developing new β-lactams less susceptible to existing resistance mechanisms, and identifying non-β-lactam inhibitors against cell wall transpeptidases. Drug discovery in the β-lactam field has afforded a range of research opportunities for academia. In this review, we summarize the recent new findings on both β-lactamases and cell wall transpeptidases because these two groups of enzymes are evolutionarily and functionally connected. Many efforts to develop new β-lactams have aimed to inhibit both transpeptidases and β-lactamases, while several promising novel β-lactamase inhibitors have shown the potential to be further developed into transpeptidase inhibitors. In addition, the drug discovery progress against each group of enzymes is presented in three aspects: understanding the targets, screening methodology, and new inhibitor chemotypes. This is to offer insights into not only the advancement in this field but also the challenges, opportunities, and resources for future research. In particular, cyclic boronate compounds are now capable of inhibiting all classes of β-lactamases, while the diazabicyclooctane (DBO) series of small molecules has led to not only new β-lactamase inhibitors but potentially a new class of antibiotics by directly targeting PBPs. With the cautiously optimistic successes of a number of new β-lactamase inhibitor chemotypes and many questions remaining to be answered about the structure and function of cell wall transpeptidases, non-β-lactam transpeptidase inhibitors may usher in the next exciting phase of drug discovery in this field.

Emergence of ADC-5 Cephalosporinase in environmental Acinetobacter baumannii from a German tank milk with a novel Sequence Type

Bacteria resistant to antibiotics arguably pose the greatest threat to human health in the twenty-first century. One such bacterium that typifies antibiotic resistance is Acinetobacter baumannii . Frequently, hospital strains of A. baumannii display multidrug resistant (MDR) or extensively drug resistant (XDR) phenotypes, often requiring the use of last resort antibiotics for treatment. In addition to hospital settings, A. baumannii has been isolated from many highly divergent sources including wastewater treatment plant effluent, soil, and agricultural run-off with global distribution. However, such isolates remain poorly characterized. In this study, we characterized a strain of A. baumannii, AB341-IK15, isolated from bulk tank milk in Germany that demonstrated resistance to ceftazidime and intermediate resistance to ceftriaxone and piperacillin/tazobactam. Further genetic characterization identified an ADC-5 cephalosporinase, first incidence in an environmental isolate; and an OXA-408 oxacillinase that may contribute to this phenotype. Interestingly, AB341-IK15 is of a novel sequence type. This research underscores the importance of studying isolates of A. baumannii of non-clinical origin to understand the antibiotic resistance and virulence potential of environmental isolates of A. baumannii as well to understand the diversity of this species.

Class C β-Lactamases: Molecular Characteristics

Class C β-lactamases or cephalosporinases can be classified into two functional groups (1, 1e) with considerable molecular variability (≤20% sequence identity). These enzymes are mostly encoded by chromosomal and inducible genes and are widespread among bacteria, including Proteobacteria in particular. Molecular identification is based principally on three catalytic motifs (⁶⁴SXSK, ¹⁵⁰YXN, ³¹⁵KTG), but more than 70 conserved amino-acid residues (≥90%) have been identified, many close to these catalytic motifs. Nevertheless, the identification of a tiny, phylogenetically distant cluster (including enzymes from the genera Legionella, Bradyrhizobium, and Parachlamydia) has raised questions about the possible existence of a C2 subclass of β-lactamases, previously identified as serine hydrolases. In a context of the clinical emergence of extended-spectrum AmpC β-lactamases (ESACs), the genetic modifications observed in vivo and in vitro (point mutations, insertions, or deletions) during the evolution of these enzymes have mostly involved the Ω- and H-10/R2-loops, which vary considerably between genera, and, in some cases, the conserved triplet ¹⁵⁰YXN. Furthermore, the conserved deletion of several amino-acid residues in opportunistic pathogenic species of Acinetobacter, such as A. baumannii, A. calcoaceticus, A. pittii and A. nosocomialis (deletion of residues 304-306), and in Hafnia alvei and H. paralvei (deletion of residues 289-290), provides support for the notion of natural ESACs. The emergence of higher levels of resistance to β-lactams, including carbapenems, and to inhibitors such as avibactam is a reality, as the enzymes responsible are subject to complex regulation encompassing several other genes (ampR, ampD, ampG, etc.). Combinations of resistance mechanisms may therefore be at work, including overproduction or change in permeability, with the loss of porins and/or activation of efflux systems.

Enzyme-catalyzed biodegradation of penicillin fermentation residues by β-lactamase OtLac from Ochrobactrum tritici

BACKGROUND

Biodegradation of antibiotics is a promising method for the large-scale removal of antibiotic residues in the environment. However, the enzyme that is involved in the biodegradation process is the key information to be revealed.

RESULTS

In this study, the beta-lactamase from Ochrobactrum tritici that mediates the biodegradation of penicillin V was identified and characterized. When searching the proteins of Ochrobactrum tritici, the β-lactamase (OtLac) was identified. OtLac consists of 347 amino acids, and predicted isoelectric point is 7.0. It is a class C β-lactamase according to BLAST analysis. The coding gene of OtLac was amplified from the genomic DNA of Ochrobactrum tritici. The OtLac was overexpressed in E. coli BL21 (DE3) and purified with Ni2+ column affinity chromatography. The biodegradation ability of penicillin V by OtLac was identified in an in vitro study and analyzed by HPLC. The optimal temperature for OtLac is 32 ℃ and the optimal pH is 7.0. Steady-state kinetics showed that OtLac was highly active against penicillin V with a Km value of 17.86 μM and a kcat value of 25.28 s-1 respectively.

CONCLUSIONS

OtLac demonstrated biodegradation activity towards penicillin V potassium, indicating that OtLac is expected to degrade penicillin V in the future.

Structural Insights into Inhibition of the Acinetobacter-Derived Cephalosporinase ADC-7 by Ceftazidime and Its Boronic Acid Transition State Analog

Extended-spectrum class C β-lactamases have evolved to rapidly inactivate expanded-spectrum cephalosporins, a class of antibiotics designed to be resistant to hydrolysis by β-lactamase enzymes. To better understand the mechanism by which Acinetobacter-derived cephalosporinase-7 (ADC-7), a chromosomal AmpC enzyme, hydrolyzes these molecules, we determined the X-ray crystal structure of ADC-7 in an acyl-enzyme complex with the cephalosporin ceftazidime (2.40 Å) as well as in complex with a boronic acid transition state analog inhibitor that contains the R1 side chain of ceftazidime (1.67 Å). In the acyl-enzyme complex, the carbonyl oxygen is situated in the oxyanion hole where it makes key stabilizing interactions with the main chain nitrogens of Ser64 and Ser315. The boronic acid O1 hydroxyl group is similarly positioned in this area. Conserved residues Gln120 and Asn152 form hydrogen bonds with the amide group of the R1 side chain in both complexes. These complexes represent two steps in the hydrolysis of expanded-spectrum cephalosporins by ADC-7 and offer insight into the inhibition of ADC-7 by ceftazidime through displacement of the deacylating water molecule as well as blocking its trajectory to the acyl carbonyl carbon. In addition, the transition state analog inhibitor, LP06, was shown to bind with high affinity to ADC-7 (K_i , 50 nM) and was able to restore ceftazidime susceptibility, offering the potential for optimization efforts of this type of inhibitor.

Integrative analysis of outer membrane vesicles proteomics and whole-cell transcriptome analysis of eravacycline induced Acinetobacter baumannii strains

BACKGROUND

Acinetobacter baumannii is a multidrug-resistant (MDR) hazardous bacterium with very high antimicrobial resistance profiles. Outer membrane vesicles (OMVs) help directly and/or indirectly towards antibiotic resistance in these organisms. The present study aims to look on the proteomic profile of OMV as well as on the bacterial transcriptome upon exposure and induction with eravacycline, a new synthetic fluorocycline. RNA sequencing analysis of whole-cell and LC-MS/MS proteomic profiling of OMV proteome abundance were done to identify the differential expression among the eravacycline-induced A. baumannii ATCC 19606 and A. baumannii clinical strain JU0126.

RESULTS

The differentially expressed genes from the RNA sequencing were analysed using R package and bioinformatics software and tools. Genes encoding drug efflux and membrane transport were upregulated among the DEGs from both ATCC 19606 and JU0126 strains. As evident with the induction of eravacycline resistance, ribosomal proteins were upregulated in both the strains in the transcriptome profiles and also resistance pumps, such as MFS, RND, MATE and ABC transporters. High expression of stress and survival proteins were predominant in the OMVs proteome with ribosomal proteins, chaperons, OMPs OmpA, Omp38 upregulated in ATCC 19606 strain and ribosomal proteins, toluene tolerance protein, siderophore receptor and peptidases in the JU0126 strain. The induction of resistance to eravacycline was supported by the presence of upregulation of ribosomal proteins, resistance-conferring factors and stress proteins in both the strains of A. baumannii ATCC 19606 and JU0126, with the whole-cell gene transcriptome towards both resistance and stress genes while the OMVs proteome enriched more with survival proteins.

CONCLUSION

The induction of resistance to eravacycline in the strains were evident with the increased expression of ribosomal and transcription related genes/proteins. Apart from this resistance-conferring efflux pumps, outer membrane proteins and stress-related proteins were also an essential part of the upregulated DEGs. However, the expression profiles of OMVs proteome in the study was independent with respect to the whole-cell RNA expression profiles with low to no correlation. This indicates the possible role of OMVs to be more of back-up additional protection to the existing bacterial cell defence during the antibacterial stress.

Molecular Characterization of a Novel Family VIII Esterase with β-Lactamase Activity (PsEstA) from Paenibacillus sp

Molecular information about family VIII esterases, which have similarities with class C β-lactamases and penicillin-binding proteins, remains largely unknown. In this study, a novel family VIII esterase with β-lactamase activity (PsEstA) from Paenibacillus sp. was characterized using several biochemical and biophysical methods. PsEstA was effective on a broad range of substrates including tertiary butyl acetate, glyceryl tributyrate, glucose pentaacetate, olive oil, and p-nitrophenyl esters. Additionally, PsEstA hydrolyzed nitrocefin, cefotaxime, and 7-aminocephalosporanic acid. Interestingly, two forms of immobilized PsEstA (CLEAs-PsEstA and mCLEAs-PsEstA) showed high recycling property and enhanced stability, but hybrid nanoflowers (hNFs) of PsEstA require improvement. This study provides a molecular understanding of substrate specificities, catalytic regulation, and immobilization of PsEstA, which can be efficiently used in biotechnological applications.

A Synthetic Dual Drug Sideromycin Induces Gram-Negative Bacteria To Commit Suicide with a Gram-Positive Antibiotic

Many antibiotics lack activity against Gram-negative bacteria because they cannot permeate the outer membrane or suffer from efflux and, in the case of β-lactams, are degraded by β-lactamases. Herein, we describe the synthesis and studies of a dual drug conjugate (1) of a siderophore linked to a cephalosporin with an attached oxazolidinone. The cephalosporin component of 1 is rapidly hydrolyzed by purified ADC-1 β-lactamase to release the oxazolidinone. Conjugate 1 is active against clinical isolates of Acinetobacter baumannii as well as strains producing large amounts of ADC-1 β-lactamase. Overall, the results are consistent with siderophore-mediated active uptake, inherent activity of the delivered dual drug, and in the presence of β-lactamases, intracellular release of the oxazolidinone upon cleavage of the cephalosporin to allow the freed oxazolidinone to inactivate its target. The ultimate result demonstrates that Gram-positive oxazolidinone antibiotics can be made to be effective against Gram-negative bacteria by β-lactamase triggered release.

Comparative Phosphoproteomics Reveals the Role of AmpC β-lactamase Phosphorylation in the Clinical Imipenem-resistant Strain Acinetobacter baumannii SK17

Nosocomial infectious outbreaks caused by multidrug-resistant Acinetobacter baumannii have emerged as a serious threat to human health. Phosphoproteomics of pathogenic bacteria has been used to identify the mechanisms of bacterial virulence and antimicrobial resistance. In this study, we used a shotgun strategy combined with high-accuracy mass spectrometry to analyze the phosphoproteomics of the imipenem-susceptible strain SK17-S and -resistant strain SK17-R. We identified 410 phosphosites on 248 unique phosphoproteins in SK17-S and 285 phosphosites on 211 unique phosphoproteins in SK17-R. The distributions of the Ser/Thr/Tyr/Asp/His phosphosites in SK17-S and SK17-R were 47.0%/27.6%/12.4%/8.0%/4.9% versus 41.4%/29.5%/17.5%/6.7%/4.9%, respectively. The Ser-90 phosphosite, located on the catalytic motif S(88)VS(90)K of the AmpC β-lactamase, was first identified in SK17-S. Based on site-directed mutagenesis, the nonphosphorylatable mutant S90A was found to be more resistant to imipenem, whereas the phosphorylation-simulated mutant S90D was sensitive to imipenem. Additionally, the S90A mutant protein exhibited higher β-lactamase activity and conferred greater bacterial protection against imipenem in SK17-S compared with the wild-type. In sum, our results revealed that in A. baumannii, Ser-90 phosphorylation of AmpC negatively regulates both β-lactamase activity and the ability to counteract the antibiotic effects of imipenem. These findings highlight the impact of phosphorylation-mediated regulation in antibiotic-resistant bacteria on future drug design and new therapies.

Structural basis for carbapenem-hydrolyzing mechanisms of carbapenemases conferring antibiotic resistance

Carbapenems (imipenem, meropenem, biapenem, ertapenem, and doripenem) are β-lactam antimicrobial agents. Because carbapenems have the broadest spectra among all β-lactams and are primarily used to treat infections by multi-resistant Gram-negative bacteria, the emergence and spread of carbapenemases became a major public health concern. Carbapenemases are the most versatile family of β-lactamases that are able to hydrolyze carbapenems and many other β-lactams. According to the dependency of divalent cations for enzyme activation, carbapenemases can be divided into metallo-carbapenemases (zinc-dependent class B) and non-metallo-carbapenemases (zinc-independent classes A, C, and D). Many studies have provided various carbapenemase structures. Here we present a comprehensive and systematic review of three-dimensional structures of carbapenemase-carbapenem complexes as well as those of carbapenemases. We update recent studies in understanding the enzymatic mechanism of each class of carbapenemase, and summarize structural insights about regions and residues that are important in acquiring the carbapenemase activity.

Biochemical and structural analysis of inhibitors targeting the ADC-7 cephalosporinase of Acinetobacter baumannii

β-Lactam resistance in Acinetobacter baumannii presents one of the greatest challenges to contemporary antimicrobial chemotherapy. Much of this resistance to cephalosporins derives from the expression of the class C β-lactamase enzymes, known as Acinetobacter-derived cephalosporinases (ADCs). Currently, β-lactamase inhibitors are structurally similar to β-lactam substrates and are not effective inactivators of this class C cephalosporinase. Herein, two boronic acid transition state inhibitors (BATSIs S02030 and SM23) that are chemically distinct from β-lactams were designed and tested for inhibition of ADC enzymes. BATSIs SM23 and S02030 bind with high affinity to ADC-7, a chromosomal cephalosporinase from Acinetobacter baumannii (K_i = 21.1 ± 1.9 nM and 44.5 ± 2.2 nM, respectively). The X-ray crystal structures of ADC-7 were determined in both the apo form (1.73 Å resolution) and in complex with S02030 (2.0 Å resolution). In the complex, S02030 makes several canonical interactions: the O1 oxygen of S02030 is bound in the oxyanion hole, and the R1 amide group makes key interactions with conserved residues Asn152 and Gln120. In addition, the carboxylate group of the inhibitor is meant to mimic the C₃/C₄ carboxylate found in β-lactams. The C₃/C₄ carboxylate recognition site in class C enzymes is comprised of Asn346 and Arg349 (AmpC numbering), and these residues are conserved in ADC-7. Interestingly, in the ADC-7/S02030 complex, the inhibitor carboxylate group is observed to interact with Arg340, a residue that distinguishes ADC-7 from the related class C enzyme AmpC. A thermodynamic analysis suggests that ΔH driven compounds may be optimized to generate new lead agents. The ADC-7/BATSI complex provides insight into recognition of non-β-lactam inhibitors by ADC enzymes and offers a starting point for the structure-based optimization of this class of novel β-lactamase inhibitors against a key resistance target.

Structure of ADC-68, a novel carbapenem-hydrolyzing class C extended-spectrum β-lactamase isolated from Acinetobacter baumannii

Outbreaks of multidrug-resistant bacterial infections have become more frequent worldwide owing to the emergence of several different classes of β-lactamases. In this study, the molecular, biochemical and structural characteristics of an Acinetobacter-derived cephalosporinase (ADC)-type class C β-lactamase, ADC-68, isolated from the carbapenem-resistant A. baumannii D015 were investigated. The blaADC-68 gene which encodes ADC-68 was confirmed to exist on the chromosome via Southern blot analysis and draft genome sequencing. The catalytic kinetics of β-lactams and their MICs (minimum inhibitory concentrations) for A. baumannii D015 and purified ADC-68 (a carbapenemase obtained from this strain) were assessed: the strain was resistant to penicillins, narrow-spectrum and extended-spectrum cephalosporins, and carbapenems, which were hydrolyzed by ADC-68. The crystal structure of ADC-68 was determined at a resolution of 1.8 Å. The structure of ADC-68 was compared with that of ADC-1 (a non-carbapenemase); differences were found in the central part of the Ω-loop and the C-loop constituting the edge of the R1 and R2 subsites and are close to the catalytic serine residue Ser66. The ADC-68 C-loop was stabilized in the open conformation of the upper R2 subsite and could better accommodate carbapenems with larger R2 side chains. Furthermore, a wide-open conformation of the R2-loop allowed ADC-68 to bind to and hydrolyze extended-spectrum cephalosporins. Therefore, ADC-68 had enhanced catalytic efficiency against these clinically important β-lactams (extended-spectrum cephalosporins and carbapenems). ADC-68 is the first reported enzyme among the chromosomal class C β-lactamases to possess class C extended-spectrum β-lactamase and carbapenemase activities.

New mutations in ADC-type β-lactamases from Acinetobacter spp. affect cefoxitin and ceftazidime hydrolysis

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.