β-Lactam antibiotics, β-lactamases and bacterial resistance

Impact of eight widely consumed antibiotics on the growth and physiological profile of natural soil microbial communities

Antibiotics' (ATBs) occurrence in soil ecosystems has a relevant effect in the structure and functionality of edaphic microbial communities, mainly because of their amendment with manure and biosolids that alter their key ecological functions. In this study, the impact of eight widely consumed ATBs on a natural soil microbial community, characterized through 16 S rRNA gene sequencing, was evaluated. Changes induced by the ATBs in the growth of the soil microbiota and in the community-level physiological profiling (CLPP), using Biolog EcoPlates™, were measured as endpoint. The eight assayed ATBs lead to a significant decrease in the growth of soil microbial communities in a dose-dependent way, ordered by its effect as follows: chloramphenicol > gentamycin > erythromycin > ampicillin > penicillin > amoxicillin > tetracycline > streptomycin. Chloramphenicol, gentamycin, and erythromycin adversely affected the physiological profile of the soil community, especially its ability to metabolize amino acids, carboxylic and ketonic acids and polymers. The analysis of the relationship between the physico-chemical properties of ATBs, as well as their mechanism of action, revealed that, except for the aminoglycosides, each ATB is influenced by a different physico-chemical parameters, even for ATBs of the same family. Significant effects were detected from 100 μg mL to 1, concentrations that can be found in digested sludge, biosolids and even in fertilized soils after repeated application of manure, so cumulative and long-term effects of these antibiotics on soil environment cannot be ruled out.

Broad-spectrum cyclic boronate β-lactamase inhibitors featuring an intramolecular prodrug for oral bioavailability

Early efforts to broaden the spectrum and potency of cyclic boronic acid β-lactamase inhibitor vaborbactam included a series of 7-membered ring boronates. Exploration of stereoisomers and incorporation of heteroatoms allowed identification of the all-carbon cyclic boronate with substituents trans as the preferred core structure, showing inhibition of Class A and C enzymes. Crystal structures of one analog bound to important β-lactamase enzymes were obtained. When isolated under acidic conditions, these compounds spontaneously formed a neutral cyclic anhydride (intramolecular prodrug) which was shown to have much-improved oral bioavailability (52-69%) compared to the ring-opened carboxylate salt (9%).

Evaluation of Antioxidant, Antimicrobial and Tyrosinase Inhibitory Activities of Extracts from Tricholosporum goniospermum, an Edible Wild Mushroom

Tricholosporum goniospermum (Bres.) Guzmán ex T.J. Baroni is an excellent edible mushroom whose compounds and biological properties are still unknown. In this study, n-hexane, ethyl acetate and methanol extracts from fruiting bodies and liquid-cultured mycelia were compared for the analysis of phenolic compounds, the evaluation of scavenger (DPPH, ABTS) and reducing (CUPRAC, FRAP) activities, and the enzyme inhibition of α-amylase, acetylcholinesterase (AChE), butyrylcholinesterase (BChE) and tyrosinase. Additionally, T. goniospermum extracts were evaluated for antibacterial and antimycotic activities against Gram+ and Gram- bacteria, and clinical yeast and fungal dermatophytes. Finally, based on the extract content in phenolic compounds, in silico studies, including the docking approach, were conducted to predict the putative targets (namely tyrosinase, lanosterol-14-α-demethylase, the multidrug efflux system transporters of E. coli (mdtK) and P. aeruginosa (pmpM), and S. aureus β-lactamase (ORF259)) underlying the observed bio-pharmacological and microbiological effects. The methanolic extract from mycelia was the richest in gallic acid, whereas the ethyl acetate extract from fruiting bodies was the sole extract to show levels of catechin. Specifically, docking runs demonstrated an affinity of catechin towards all docked proteins, in the micromolar range. These in silico data are consistent, at least in part, with the highest activity of ethyl acetate extract as an antimicrobial and anti-tyrosinase (554.30 mg KAE/g for fruiting bodies and 412.81 mg KAE/g for mycelia) agent. The ethyl acetate extracts were also noted as being the most active (2.97 mmol ACAE/g for fruiting bodies and 2.25 mmol ACAE/g for mycelia) on α-amylase. BChE inhibitory activities varied from 2.61 to 26.78 mg GALAE/g, while the tested extracts were not active on AChE. In conclusion, all mushroom extracts tested in this study had potent antimicrobial activities. Particularly, among the tested extracts, the ethyl acetate extract showed the highest efficacy as both an antimicrobial and anti-tyrosinase agent. This could be related, albeit partially, to its content of catechin. In this regard, the bioinformatics analyses showed interactions of catechin with tyrosinase and specific microbial proteins involved in the resistance to chemotherapeutic drugs, thus suggesting innovative pharmacological applications of T. goniospermum extracts.

Polymer-supported triphenylphosphine: application in organic synthesis and organometallic reactions

This comprehensive review highlights the diverse chemistry and applications of polymer-supported triphenylphosphine (PS-TPP) in organic synthesis since its inception. Specifically, the review describes applications of the preceding reagent in functional group interconversions, heterocycle synthesis, metal complexes and their application in synthesis, and total synthesis of natural products. Many examples are provided from the literature to show the scope and selectivity (regio, stereo, and chemo) in these transformations.

Novel β-lactam/β-lactamase inhibitors versus alternative antibiotics for the treatment of complicated intra-abdominal infection and complicated urinary tract infection: a meta-analysis of randomized controlled trials

The aim of this study is to compare the efficacy and safety of novelBL/BLIs with alternative antibiotics for the treatment of cIAI and cUTI. Area covered: We performed a systematic review and meta-analysis of all randomized controlled trials comparing novel BL/BLIs with other antibiotics for the treatment of cIAI and cUTI. The primary outcome included clinical and microbiological treatment success. Expert commentary: We found that novel BL/BLIs obtained a similar clinical outcome with other antibiotics in CE population (OR = 1.07, 95%CI = (0.80, 1.44), P = 0.64). However, novel BL/BLIs had better clinical treatment success in the cUTI subgroup (OR = 2.14, 95%CI = (1.06, 4.31), P = 0.03). Furthermore, novel BL/BLIs achieved significant microbiological treatment success in patie nts with cUTI (OR = 1.70, 95%CI = (1.29, 2.25), P  =  0.0002) and had higher eradication rates for Gram-negative pathogens (OR = 1.82, 95%CI = (1.26, 2.64), P = 0.001) including E.coli and K.pneumoniae. No difference was observed concerning the incidence of mortality and adverse events between the two groups. Therefore, we concluded that novel BL/BLIs are not inferior to other available antibiotics for the treatment of cIAI, and they have advantages in patients with cUTI. Simultaneously, they are sensitive to Gram-negative pathogens, especially for E.coli and K.pneumoniae.

Characterization of a highly efficient antibiotic-degrading metallo-β-lactamase obtained from an uncultured member of a permafrost community

Microorganisms in the permafrost contain a potent mechanism to inactivate antibiotics.

Genome-Wide Identification of Antimicrobial Intrinsic Resistance Determinants in Staphylococcus aureus

The emergence of antimicrobial resistance severely threatens our ability to treat bacterial infections. While acquired resistance has received considerable attention, relatively little is known of intrinsic resistance that allows bacteria to naturally withstand antimicrobials. Gene products that confer intrinsic resistance to antimicrobial agents may be explored for alternative antimicrobial therapies, by potentiating the efficacy of existing antimicrobials. In this study, we identified the intrinsic resistome to a broad spectrum of antimicrobials in the human pathogen, Staphylococcus aureus. We screened the Nebraska Transposon Mutant Library of 1920 single-gene inactivations in S. aureus strain JE2, for increased susceptibility to the anti-staphylococcal antimicrobials (ciprofloxacin, oxacillin, linezolid, fosfomycin, daptomycin, mupirocin, vancomycin, and gentamicin). Sixty-eight mutants were confirmed by E-test to display at least twofold increased susceptibility to one or more antimicrobial agents. The majority of the identified genes have not previously been associated with antimicrobial susceptibility in S. aureus. For example, inactivation of genes encoding for subunits of the ATP synthase, atpA, atpB, atpG and atpH, reduced the minimum inhibitory concentration (MIC) of gentamicin 16-fold. To elucidate the potential of the screen, we examined treatment efficacy in the Galleria mellonella infection model. Gentamicin efficacy was significantly improved, when treating larvae infected with the atpA mutant compared to wild type cells with gentamicin at a clinically relevant concentration. Our results demonstrate that many gene products contribute to the intrinsic antimicrobial resistance of S. aureus. Knowledge of these intrinsic resistance determinants provides alternative targets for compounds that may potentiate the efficacy of existing antimicrobial agents against this important pathogen.

Methane production from thermophilic co-digestion of dairy manure and waste milk obtained from therapeutically treated cows

Methane production from co‐digestion of dairy manure and waste milk, milk from cows treated with antibiotics for mastitis, was tested in a 2 × 4 factorial design. Four different waste milk percentages (w/w): 0% (SM), 10% (SMWM10), 20% (SMWM20) and 30% (SMWM30), were tested with two slurry percentages (w/w): 50% (A) and 25% (B) and the rest being manure at 55°C for 12 days in batch digesters. The results analyzed using a Gompertz model showed SMWM10 produced the highest methane production potential (P_m)/g volatile solids added followed by SM in both A and B. This P_m of SMWM10 in A and B was statistically non‐significant (P > 0.05). More than 96% of cefazolin‐resistant bacteria and 100% of multi‐drug‐resistant bacteria reductions were observed in all the treatments. Inclusion of waste milk at 10% in single stage digester enhances the methane production from dairy manure and could offer added benefit of waste milk treatment and disposal.

Syntheses and studies of new forms of N-sulfonyloxy β-lactams as potential antibacterial agents and β-lactamase inhibitors

The synthesis of a small library of N-sulfonyloxy-2-azetidinones is reported and the preliminary results of the investigation of the biological activity of these molecules are discussed. These new multi-electrophilic β-lactams ('electrophilic bombs') display unexpected selectivity in their antibacterial activity and β-lactamase inhibitory activity.

A degenerate PCR-based strategy as a means of identifying homologues of aminoglycoside and β-lactam resistance genes in the gut microbiota

Background

The potential for the human gut microbiota to serve as a reservoir for antibiotic resistance genes has been the subject of recent discussion. However, this has yet to be investigated using a rapid PCR-based approach. In light of this, here we aim to determine if degenerate PCR primers can detect aminoglycoside and β-lactam resistance genes in the gut microbiota of healthy adults, without the need for an initial culture-based screen for resistant isolates. In doing so, we would determine if the gut microbiota of healthy adults, lacking recent antibiotic exposure, is a reservoir for resistance genes.

Results

The strategy employed resulted in the identification of numerous aminoglycoside (acetylation, adenylation and phosphorylation) and β-lactam (including bla_OXA, bla_TEM, bla_SHV and bla_CTX-M) resistance gene homologues. On the basis of homology, it would appear that these genes originated from different bacterial taxa, with members of the Enterobacteriaceae being a particularly rich source. The results demonstrate that, even in the absence of recent antibiotic exposure, the human gut microbiota is a considerable reservoir for antibiotic resistance genes.

Conclusions

This study has demonstrated that the gut can be a significant source of aminoglycoside and β-lactam resistance genes, even in the absence of recent antibiotic exposure. The results also demonstrate that PCR-based approaches can be successfully applied to detect antibiotic resistance genes in the human gut microbiota, without the need to isolate resistant strains. This approach could also be used to rapidly screen other complex environments for target genes.

In silico assigned resistance genes confer Bifidobacterium with partial resistance to aminoglycosides but not to β-lactams

Bifidobacteria have received significant attention due to their contribution to human gut health and the use of specific strains as probiotics. It is thus not surprising that there has also been significant interest with respect to their antibiotic resistance profile. Numerous culture-based studies have demonstrated that bifidobacteria are resistant to the majority of aminoglycosides, but are sensitive to β-lactams. However, limited research exists with respect to the genetic basis for the resistance of bifidobacteria to aminoglycosides. Here we performed an in-depth in silico analysis of putative Bifidobacterium-encoded aminoglycoside resistance proteins and β-lactamases and assess the contribution of these proteins to antibiotic resistance. The in silico-based screen detected putative aminoglycoside and β-lactam resistance proteins across the Bifidobacterium genus. Laboratory-based investigations of a number of representative bifidobacteria strains confirmed that despite containing putative β-lactamases, these strains were sensitive to β-lactams. In contrast, all strains were resistant to the aminoglycosides tested. To assess the contribution of genes encoding putative aminoglycoside resistance proteins in Bifidobacterium sp. two genes, namely Bbr_0651 and Bbr_1586, were targeted for insertional inactivation in B. breve UCC2003. As compared to the wild-type, the UCC2003 insertion mutant strains exhibited decreased resistance to gentamycin, kanamycin and streptomycin. This study highlights the associated risks of relying on the in silico assignment of gene function. Although several putative β-lactam resistance proteins are located in bifidobacteria, their presence does not coincide with resistance to these antibiotics. In contrast however, this approach has resulted in the identification of two loci that contribute to the aminoglycoside resistance of B. breve UCC2003 and, potentially, many other bifidobacteria.

Antimicrobial activities of some mushrooms in Turkey

CONTEXT

Terfezia boudieri Chatin (Pezizaceae), Agaricus brunnescens Peck (Agaricaceae) and Lactarius vellereus (Fr.) Fr. (Russulaceae) are well-known species in Turkey, and are used both for food and traditional medicine.

OBJECTIVE

The powdered fruit bodies of T. boudieri, A. brunnescens and L. vellereus were used to evaluate the antimicrobial activities.

MATERIALS AND METHODS

Chloroform, acetone and methanol extracts of T. boudieri, A. brunnescens and L. vellereus were tested for their antimicrobial activities against four Gram-positive bacteria, five Gram-negative bacteria and yeast using a micro-dilution method.

RESULTS

The strongest minimum inhibitory concentration (MIC) value observed against the test microorganisms was with the chloroform extract of T. boudieri (MIC 2.4 µg/mL) against Streptococcus pyogenes. Maximum antimicrobial effects were observed with the acetone extracts of T. boudieri and L. vellereus (MIC 4.8 µg/mL) against Bacillus subtilis. The strongest antifungal activity was observed with the acetone extracts of T. boudieri (MIC 2.4 µg/mL) and A. brunnescens (MIC 19.5 µg/mL) against Candida albicans. The strongest MIC values for all fungal extracts were observed between 78 and 2.4 µg/mL.

DISCUSSION AND CONCLUSION

Present results demonstrated that these three mushroom species have excellent antimicrobial and antifungal activities, and thus have great potential as a source for natural health products.

Survival of multidrug-resistant bacteria in thermophilic and mesophilic anaerobic co-digestion of dairy manure and waste milk

Anaerobic digestion is considered as a promising method to manage animal waste with antibiotic-resistant bacteria. Current research was conducted to investigate the survival of multidrug-resistant bacteria (MDRB) resistant to three groups of antibiotics: (i) cefazolin, neomycin, vancomycin, kanamycin (group 1); (ii) penicillin, oxytetracycline, ampicillin, streptomycin (group 2); and (iii) cefazolin, neomycin, vancomycin, kanamycin, penicillin, oxytetracycline, ampicillin, streptomycin (group 3), in anaerobic digestion of dairy manure and co-digestion of dairy manure and waste milk at 37°C and 55°C for 22 days, respectively. The population densities of three groups of MDRB on peptone, tryptone, yeast and glucose agar plates incubated at 30°C for 7 days before and after digestion showed 100% destruction in both digestates at thermophilic temperature. Overall reduction of more than 90% of three groups of MDRB was observed in mesophilic digestion with no significant differences (P > 0.05) between manure and milk mixture. Co-digestion of dairy manure and waste milk always produced significantly (P < 0.05) higher total gas and methane gas than digestion of manure alone at both temperatures. Gas production in each case was significantly (P < 0.05) higher in thermophilic digestion than in mesophilic digestion. The results demonstrate that thermophilic co-digestion of dairy manure and waste milk offers more benefits in terms of the environment and economy.

Biohydrogen production from co-digestion of cow manure and waste milk under thermophilic temperature

Biohydrogen production from co-digestion of cow manure (M) and waste milk (WM), milk from mastitis cows treated with cefazolin, was evaluated in a 3×5 factorial design. Organic loading of 20, 40 and 60g volatile solid (VS)L(-1) were tested at temperature of 55°C using M:WM (VS/VS) 70:30, 50:50, 30:70, 10:90 and 0:100. Hydrogen production increased with organic loading and M:WM to a maximum of 59.5mLg(-1) VS fed at 40g VSL(-1) in M:WM 70:30. Butyrate was the main volatile fatty acid (VFA) accumulated in M:WM 50:50, 30:70 and 10:90. Overall reduction of more than 90% of cefazolin resistant bacteria was observed in all the treatments. The reduction was higher at 40 and 60 than 20g VSL(-1) (P<0.05). Inclusion of waste milk enhances hydrogen production from cow manure and could offer added benefit of waste milk treatment and disposal.

A theoretical analysis of the prodrug delivery system for treating antibiotic-resistant bacteria

Simulations were carried out to analyze a promising new antimicrobial treatment strategy for targeting antibiotic-resistant bacteria called the β-lactamase-dependent prodrug delivery system. In this system, the antibacterial drugs are delivered as inactive precursors that only become activated after contact with an enzyme characteristic of many species of antibiotic-resistant bacteria (β-lactamase enzyme). The addition of an activation step contributes an extra layer of complexity to the system that can lead to unexpected emergent behavior. In order to optimize for treatment success and minimize the risk of resistance development, there must be a clear understanding of the system dynamics taking place and how they impact on the overall response. It makes sense to use a systems biology approach to analyze this method because it can facilitate a better understanding of the complex emergent dynamics arising from diverse interactions in populations. This paper contains an initial theoretical examination of the dynamics of this system of activation and an assessment of its therapeutic potential from a theoretical standpoint using an agent-based modeling approach. It also contains a case study comparison with real-world results from an experimental study carried out on two prodrug candidate compounds in the literature.

Antibacterial activity of Ecklonia cava against methicillin-resistant Staphylococcus aureus and Salmonella spp

Ecklonia cava is a brown alga (Laminariales, Phaeophyta) growing on the subtidal rocky shores of Korea. It has antioxidant, antidiarrhea, and anticoagulant effects. In this study, the antimicrobial activity of E. cava EtOH extract and its fractions (n-hexane, CH2Cl2, EtOAc, n-BuOH, and H2O) were investigated against methicillin-resistant Staphylococcus aureus and Salmonella spp. The E. cava EtOAc fraction showed good antibacterial activity against all bacteria. Eckol isolated from E. cava EtOAc fraction showed antimicrobial activity against all the tested strains. The minimum inhibitory concentration of eckol against S. aureus strains ranged from 125 to 250 microg/mL and 125 to 250 microg/mL for Salmonella strains. The fraction inhibitory concentration index of eckol in combination with ampicillin ranged from 0.31 to 0.5, indicating remarkable synergism against S. aureus. However, against Salmonella gallinarum ATCC 9184 and Salmonella typhimurium, it ranges from 0.75 to 1.0. The combinations of eckol + ampicillin exhibited improved inhibition of S. aureus and Salmonella with synergy or additive effect. We suggest that eckol ingredients of the E. cava against S. aureus and Salmonella have antibacterial activity.

Occurrence and diversity of integrons and beta-lactamase genes among ampicillin-resistant isolates from estuarine waters

The aim of the present study was to assess the occurrence and molecular diversity of beta-lactamase genes and integrons among Gram-negative ampicillin-resistant bacteria from Ria de Aveiro. Ampicillin-resistant isolates were selected and subjected to genotyping using REP-PCR. Representatives from each REP pattern were affiliated with the following taxa by sequencing the 16S rRNA gene: Aeromonas caviae, A. hydrophila, A. media, A. molluscorum, A. veronii, A. salmonicida, Aeromonas sp., Pseudomonas putida, Pseudomonas sp., Escherichia coli, Escherichia sp., Shigella sonnei, Shigella sp., Klebsiella pneumoniae, K. oxytoca, Raoultella ornithinolytica, R. terrigena, R. planticola, Citrobacter freundii, Morganella morganii and Enterobacter sp. Isolates affiliated with genera Escherichia or Shigella were identified as Escherichia coli using phenotypic-based tests. PCR was used to assess beta-lactamase encoding sequences (bla(TEM), bla(SHV), bla(CARB), bla(CTX-M), bla(IMP), bla(VIM), bla(CphA/IMIS), bla(OXA-A), bla(OXA-B), bla(OXA-C)), class 1 and class 2 integrases, and integron variable regions. Sequence analysis of PCR products was performed. beta-Lactamase genes were detected in 77.8% of the Enterobacteriaceae and in 10.5% of the Aeromonas. The most frequently detected gene was bla(TEM), followed by bla(SHV,)bla(OXA-B), bla(CphA/IMIS) and bla(CARB). Retrieved sequences shared high homology with previously described beta-lactamases. The intI1 gene was present in 29.6% of the Enterobacteriaceae and in 21% of the Aeromonas isolates. The intI2 gene was present in 4 isolates. A total of 13 cassettes included in 12 different cassette arrays were identified. The most frequently found resistance gene cassettes were aadA variants. Previous investigations based on cultivation-independent approaches revealed higher molecular diversity among beta-lactamase-encoding sequences in this estuary. This fact reinforces the hypothesis that cultivation-dependent approaches may underestimate the prevalence of antibiotic resistance genes in environmental samples and may introduce bias in the recovery of their molecular variants.

Analysing diversity among beta-lactamase encoding genes in aquatic environments

The most common mechanism of resistance to beta-lactam antibiotics is the production of beta-lactamases. These enzymes are encoded by genes that evolve rapidly, thus constituting a group characterized by high levels of molecular diversity. Most of the genetic determinants of resistance to beta-lactam antibiotics characterized until now were obtained from clinical isolates. This study was designed in order to exploit the presence of beta-lactamase gene sequences in an aquatic environment, and to get information on the distinctive features of those sequences when compared to others available on databases. DNA sequences potentially encoding proteins of three different families of clinically relevant beta-lactamases were assessed: TEM, IMP and OXA-2 derivatives. The presence of bla sequences in DNA extracted from water samples from the lagoon Ria de Aveiro was checked by PCR and hybridization. Sequences representing the three families of beta-lactamases studied were detected. The molecular diversity of the amplicons was assessed by cloning and sequence analysis, and denaturing gradient gel electrophoresis (PCR-DGGE) separation. Most of the retrieved sequences (particularly sequences representing bla(TEM)and bla(OXA-2)) were identical or very similar to beta-lactamase gene sequences previously characterized from clinical isolates. Phylogenetic analysis suggests that this aquatic ecosystem is a reservoir of molecular diverse putative bla sequences. The patterns of molecular diversity found within the beta-lactamase gene families studied do not correspond to those reported in studies focussing on clinical isolates.

Screening of Brazilian basidiomycetes for antimicrobial activity

A total of 103 isolates of basidiomycetes, representing 84 species from different Brazilian ecosystems, were evaluated for their antifungal and antibacterial activity in a panel of pathogenic and non-pathogenic microorganisms. Tissue plugs of the fruiting bodies were cultivated in liquid media and the whole culture extracted with ethyl acetate. Crude extracts from Agaricus cf. nigrecentulus, Agrocybe perfecta, Climacodon pulcherrimus, Gloeoporus thelephoroides, Hexagonia hydnoides, Irpex lacteus, Leucoagaricus cf. cinereus, Marasmius cf. bellus, Marasmius sp., Nothopanus hygrophanus, Oudemansiella canarii, Pycnoporus sanguineus, Phellinus sp., and Tyromyces duracinus presented significant activity against one or more of the target microorganisms. Eight isolates were active only against bacteria while three inhibited exclusively the growth of fungi. Two extracts presented wide antimicrobial spectrum and were active against both fungi and bacteria. Differences in the bioactivity of extracts obtained from isolates from the same species were observed.

Evaluation of inhibition of the carbenicillin-hydrolyzing beta-lactamase PSE-4 by the clinically used mechanism-based inhibitors

Characterization of the biochemical steps in the inactivation chemistry of clavulanic acid, sulbactam and tazobactam with the carbenicillin-hydrolyzing beta-lactamase PSE-4 from Pseudomonas aeruginosa is described. Although tazobactam showed the highest affinity to the enzyme, all three inactivators were excellent inhibitors for this enzyme. Transient inhibition was observed for the three inactivators before the onset of irreversible inactivation of the enzyme. Partition ratios (k(cat)/k(inact)) of 11, 41 and 131 were obtained with clavulanic acid, tazobactam and sulbactam, respectively. Furthermore, these values were found to be 14-fold, 3-fold and 80-fold lower, respectively, than the values obtained for the clinically important TEM-1 beta-lactamase. The kinetic findings were put in perspective by determining the computational models for the pre-acylation complexes and the immediate acyl-enzyme intermediates for all three inactivators. A discussion of the pertinent structural factors is presented, with PSE-4 showing subtle differences in interactions with the three inhibitors compared to the TEM-1 enzyme.