Interaction of penicillin with the bacterial cell: penicillin-binding proteins and penicillin-sensitive enzymes

PBP-A, a cyanobacterial DD-peptidase with high specificity for amidated muropeptides, exhibits pH-dependent promiscuous activity harmful to Escherichia coli

Penicillin binding proteins (PBPs) are involved in biosynthesis, remodeling and recycling of peptidoglycan (PG) in bacteria. PBP-A from Thermosynechococcus elongatus belongs to a cyanobacterial family of enzymes sharing close structural and phylogenetic proximity to class A β-lactamases. With the long-term aim of converting PBP-A into a β-lactamase by directed evolution, we simulated what may happen when an organism like Escherichia coli acquires such a new PBP and observed growth defect associated with the enzyme activity. To further explore the molecular origins of this harmful effect, we decided to characterize deeper the activity of PBP-A both in vitro and in vivo. We found that PBP-A is an enzyme endowed with DD-carboxypeptidase and DD-endopeptidase activities, featuring high specificity towards muropeptides amidated on the D-iso-glutamyl residue. We also show that a low promiscuous activity on non-amidated peptidoglycan deteriorates E. coli's envelope, which is much higher under acidic conditions where substrate discrimination is mitigated. Besides expanding our knowledge of the biochemical activity of PBP-A, this work also highlights that promiscuity may depend on environmental conditions and how it may hinder rather than promote enzyme evolution in nature or in the laboratory.

Mapping the Evolution of Activity-Based Protein Profiling: A Bibliometric Review

Activity-based protein profiling (ABPP) is a chemoproteomic approach that employs small-molecule probes to directly evaluate protein functionality within complex proteomes. This technology has proven to be a potent strategy for mapping ligandable sites in organisms and has significantly impacted drug discovery processes by enabling the development of highly selective small-molecule inhibitors and the identification of new therapeutic molecular targets. Despite being nearly a quarter of a century old as a chemoproteomic tool, ABPP has yet to undergo a bibliometric analysis. In order to gauge its scholarly impact and evolution, a bibliometric analysis was performed, comparing all 1919 reported articles with the articles published in the last five years. Through a comprehensive data analysis, including a 5-step workflow, the most influential articles were identified, and their bibliometric parameters were determined. The 1919 analyzed articles span from 1999 to 2022, providing a comprehensive overview of the historical and current state of ABPP research. This analysis presents, for the first time, the characteristics of the most influential ABPP articles, offering valuable insight into the research conducted in this field and its potential future directions. The findings underscore the crucial role of ABPP in drug discovery and novel therapeutic target identification, as well as the need for continued advancements in the development of novel chemical probes and proteomic technologies to further expand the utility of ABPP.

Activity-based protein profiling: A graphical review

Activity-based protein profiling (ABPP) is a chemoproteomic technology that employs small chemical probes to directly interrogate protein function within complex proteomes. Since its initial application almost 25 years ago, ABPP has proven to be a powerful and versatile tool for addressing numerous challenges in drug discovery, including the development of highly selective small-molecule inhibitors, the discovery of new therapeutic targets, and the illumination of target proteins in tissues and organisms. This graphical review provides an overview of the rapid evolution of ABPP strategies, highlighting the versatility of the approach with selected examples of its successful application.

Novel insights into molecular and cellular aspects of delayed drug hypersensitivity reactions

INTRODUCTION

Delayed drug hypersensitivity reactions (DDHRs) represent a major health problem. They are unpredictable and can cause life-long disability or even death. The pathophysiology of DDHRs is complicated, multifactorial, and not well understood mainly due to the lack of validated animal models or in vitro systems. The role of the immune system is well demonstrated but its exact pathophysiology still a matter of debate.

AREA COVERED

This review summarizes the current understanding of DDHRs pathophysiology and abridges the available new evidence supporting each hypothesis. A comprehensive literature search for relevant publications was performed using PubMed, Google Scholar, and Medline databases with no date restrictions and focusing on the most recent 10 years.

EXPERT OPINION

Although multiple milestones have been achieved in our understanding of DDHRs pathophysiology as a result of the development of useful experimental models, many questions are yet to be fully answered. A deeper understanding of the mechanistic basis of DDHRs would not only facilitate the development of robust and reliable diagnostic assays for diagnosis, but would also inform therapy by providing specific target(s) for immunomodulation and potentially permit pre-therapeutic risk assessment to pursue the common goal of safe and effective drug therapy.

Structure-Activity Relationship of Penem Antibiotic Side Chains Used against Mycobacteria Reveals Highly Active Compounds

The rise of antibiotic-resistant Mycobacterium tuberculosis and non-tuberculous mycobacterial infections has placed ever-increasing importance on discovering new antibiotics to treat these diseases. Recently, a new penem, T405, was discovered to have strong antimicrobial activity against M. tuberculosis and Mycobacteroides abscessus. Here, a penem library of C2 side-chain variants was synthesized, and their antimicrobial activities were evaluated against M. tuberculosis H₃₇Rv and M. abscessus ATCC 19977. Several new penems with antimicrobial activity stronger than the standard-of-care carbapenem antibiotics were identified with some candidates improving on the activity of the lead compound, T405. Moreover, many candidates showed little or no increase in the minimum inhibitory concentration in the presence of serum compared to the highly protein-bound T405. The penems with the strongest activity identified in this study were then biochemically characterized by reaction with the representative l,d-transpeptidase Ldt_Mt2 and the representative penicillin-binding protein d,d-carboxypeptidase DacB2.

T405, a New Penem, Exhibits In Vivo Efficacy against M. abscessus and Synergy with β-Lactams Imipenem and Cefditoren

Mycobacteroides abscessus (Mab) is an emerging environmental microbe that causes chronic lung disease in patients with compromised lung function such as cystic fibrosis and bronchiectasis. It is intrinsically resistant to most antibiotics, therefore there are only few antibiotics that can be repurposed to treat Mab disease. Although current recommendations require daily intake of multiple antibiotics for more than a year, cure rate is low and often associated with significant adverse events. Here, we describe in vivo efficacy of T405, a recently discovered β-lactam antibiotic of the penem subclass, in a mouse model of pulmonary Mab infection. Imipenem, one of the standard-of-care drugs to treat Mab disease, and also a β-lactam antibiotic from a chemical class similar to T405, was included as a comparator. Probenecid was included with both T405 and imipenem to reduce the rate of their renal clearance. T405 exhibited bactericidal activity against Mab from the onset of treatment and reduced Mab lung burden at a rate similar to that exhibited by imipenem. The MIC of T405 against Mab was unaltered after 4 weeks of exposure to T405 in the lungs of mice. Using an in vitro assay, we also demonstrate that T405 in combination with imipenem, cefditoren or avibactam exhibits synergism against Mab. Additionally, we describe a scheme for synthesis and purification of T405 on an industrial scale. These attributes make T405 a promising candidate for further preclinical assessment to treat Mab disease.

Penicillin Binding Proteins and β-Lactamases of Mycobacterium tuberculosis: Reexamination of the Historical Paradigm

Penicillin binding proteins (PBPs) have been extensively studied due to their importance to the physiology of bacterial cell wall peptidoglycan and as targets of the most widely used class of antibiotics, the β-lactams. The existing paradigm asserts that PBPs catalyze the final step of peptidoglycan biosynthesis, and β-lactams inhibit their activities. According to this paradigm, a distinct enzyme class, β-lactamases, exists to inactivate β-lactams. This paradigm has been the basis for how bacterial diseases are treated with β-lactams. We tested whether this historical view accurately reflects the relationship between β-lactams and the PBPs and the β-lactamase, BlaC, of Mycobacterium tuberculosis. BlaC was the major inactivator of the cephalosporin subclass of β-lactams. However, the PBPs PonA1 and PonA2 inactivated penicillins and carbapenems more effectively than BlaC. These findings demonstrate that select M. tuberculosis PBPs are effective at inactivating several β-lactams. Lesser-known PBPs, DacB, DacB1, DacB2, and Rv2864c, a putative PBP, were comparably more resistant to inhibition by all β-lactam subclasses. Additionally, Rv1730c exhibited low affinity to most β-lactams. Based on these findings, we conclude that in M. tuberculosis, BlaC is not the only source of inactivation of β-lactams. Therefore, the historical paradigm does not accurately describe the relationship between β-lactams and M. tuberculosis. IMPORTANCE M. tuberculosis, the causative agent of tuberculosis, kills more humans than any other bacterium. β-lactams are the most widely used class of antibiotics to treat bacterial infections. Unlike in the historical model that describes the relationship between β-lactams and M. tuberculosis, we find that M. tuberculosis penicillin binding proteins are able to inactivate select β-lactams with high efficiency.

β-Lactam Combinations That Exhibit Synergy against Mycobacteroides abscessus Clinical Isolates

Mycobacteroides abscessus (Mab) is an opportunistic environmental pathogen that can cause chronic pulmonary disease in the setting of structural lung conditions such as bronchiectasis, chronic obstructive pulmonary disease, and cystic fibrosis. These infections are often incurable and associated with rapid lung function decline. Mab is naturally resistant to most of the antibiotics available today, and current treatment guidelines require at least 1 year of daily multidrug therapy, which is often ineffective and is associated with significant toxicities. β-Lactams are the most widely used class of antibiotics and have a demonstrated record of safety and tolerability. Here, using a panel of recent clinical isolates of Mab, we evaluated the in vitro activities of dual-β-lactam combinations to identify new treatments with the potential to treat infections arising from a wide range of Mab strains. The Mab clinical isolates were heterogeneous, as reflected by the diversity of their genomes and differences in their susceptibilities to various drugs. Cefoxitin and imipenem are currently the only two β-lactams included in the guidelines for treating Mab disease, yet they are not used concurrently in clinical practice. However, this dual-β-lactam combination exhibited synergy against 100% of the isolates examined (n = 21). Equally surprising is the finding that the combination of two carbapenems, doripenem and imipenem, exhibited synergy against the majority of Mab isolates. In the setting of multidrug-resistant Mab disease with few therapeutic options, these combinations may offer viable immediate treatment options with efficacy against the broad spectrum of Mab strains infecting patients today.

Activation of the Two-Component System LisRK Promotes Cell Adhesion and High Ampicillin Tolerance in Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen which can survive in harsh environmental conditions. It responds to external stimuli through an array of two-component systems (TCS) that sense external cues. Several TCS, including LisRK, have been linked to Listeria’s ability to grow at slightly elevated antibiotic levels. The aim of this study was to determine if the TCS LisRK is also involved in acquiring the high antibiotic tolerance that is characteristic of persister cells. LisRK activates a response that leads to remodeling of the cell envelope, and we therefore hypothesized that activation of LisRK could also increase in the cells’ adhesiveness and initiate the first step in biofilm formation. We used a ΔlisR mutant to study antibiotic tolerance in the presence and absence of LisRK, and a GFP reporter strain to visualize the activation of LisRK in L. monocytogenes LO28 at a single-cell level. LisRK was activated in most cells in stationary phase cultures. Antimicrobial susceptibility tests showed that LisRK was required for the generation of ampicillin tolerance under these conditions. The wildtype strain tolerated exposure to ampicillin at 1,000 × inhibitory levels for 24 h, and the fraction of surviving cells was 20,000-fold higher in the wildtype strain compared to the ΔlisR mutant. The same protection was not offered to other antibiotics (vancomycin, gentamicin, tetracycline), and the mechanism for antibiotic tolerance is thus highly specific. Furthermore, quantification of bacterial attachment rates and attachment force also revealed that the absence of a functional LisRK rendered the cells less adhesive. Hence, LisRK TCS promotes multiple protective mechanisms simultaneously.

Recent advances in electrochemical sensors for amoxicillin detection in biological and environmental samples

Amoxicillin (AMX) is among the most successful antibiotics used for human therapy. It is used extensively to prevent or treat bacterial infections in humans and animals. However, the widespread distribution and excess utilization of AMX can be an environmental and health risk due to the hazardous potential associated to its pharmaceutical industries effluents. Besides, their extensive use in food animal production may result in some undesirable residues in food, e.g. meat, eggs and milk. Consequently, at high enough concentrations in biological fluids, AMX may be responsible of various diseases such as nausea, vomiting, rashes, and antibiotic-associated colitis. For this reason, the detection and quantification of amoxicillin in pharmaceuticals, biological fluids, environmental samples and foodstuffs require new electroanalytical techniques with sensitive and rapid measurement abilities. This review discusses recent advances in the development of electrochemical sensors and bio-sensors for AMX analysis in complex matrices such as pharmaceuticals, biological fluids, environmental water and foodstuffs. The main electrochemical sensors used are based on chemically modified electrodes involving carbon materials and nanomaterials, nanoparticles, polymers and biological recognition molecules.

Chemical tools to characterize peptidoglycan synthases

The peptidoglycan cell wall is a unique macromolecular structure in bacteria that defines their shape and confers protection from the surrounding environment. Decades of research has focused on understanding the peptidoglycan synthesis pathway and exploiting its essentiality for antibiotic development. Recently, a new class of peptidoglycan polymerases known as the SEDS (shape, elongation, division and sporulation) proteins were identified; these polytopic membrane proteins function together with the better-known penicillin-binding proteins (PBPs) to build the cell wall. In this review, we will highlight recent developments in chemical tools and methods to label the bacterial cell wall and discuss how these developments are leading to a better understanding of peptidoglycan synthases and their cellular roles.

Cell Shape and Antibiotic Resistance Are Maintained by the Activity of Multiple FtsW and RodA Enzymes in Listeria monocytogenes

Rod-shaped bacteria have two modes of peptidoglycan synthesis: lateral synthesis and synthesis at the cell division site. These two processes are controlled by two macromolecular protein complexes, the elongasome and divisome. Recently, it has been shown that the Bacillus subtilis RodA protein, which forms part of the elongasome, has peptidoglycan glycosyltransferase activity. The cell division-specific RodA homolog FtsW fulfils a similar role at the divisome. The human pathogen Listeria monocytogenes carries genes that encode up to six FtsW/RodA homologs; however, their functions have not yet been investigated. Analysis of deletion and depletion strains led to the identification of the essential cell division-specific FtsW protein, FtsW1. Interestingly, L. monocytogenes carries a gene that encodes a second FtsW protein, FtsW2, which can compensate for the lack of FtsW1, when expressed from an inducible promoter. L. monocytogenes also possesses three RodA homologs, RodA1, RodA2, and RodA3, and their combined absence is lethal. Cells of a rodA1 rodA3 double mutant are shorter and have increased antibiotic and lysozyme sensitivity, probably due to a weakened cell wall. Results from promoter activity assays revealed that expression of rodA3 and ftsW2 is induced in the presence of antibiotics targeting penicillin binding proteins. Consistent with this, a rodA3 mutant was more susceptible to the β-lactam antibiotic cefuroxime. Interestingly, overexpression of RodA3 also led to increased cefuroxime sensitivity. Our study highlights that L. monocytogenes genes encode a multitude of functional FtsW and RodA enzymes to produce its rigid cell wall and that their expression needs to be tightly regulated to maintain growth, cell division, and antibiotic resistance.IMPORTANCE The human pathogen Listeria monocytogenes is usually treated with high doses of β-lactam antibiotics, often combined with gentamicin. However, these antibiotics only act bacteriostatically on L. monocytogenes, and the immune system is needed to clear the infection. Therefore, individuals with a compromised immune system are at risk to develop a severe form of Listeria infection, which can be fatal in up to 30% of cases. The development of new strategies to treat Listeria infections is necessary. Here we show that the expression of some of the FtsW and RodA enzymes of L. monocytogenes is induced by the presence of β-lactam antibiotics, and the combined absence of these enzymes makes bacteria more susceptible to this class of antibiotics. The development of antimicrobial agents that inhibit the activity or production of FtsW and RodA enzymes might therefore help to improve the treatment of Listeria infections and thereby lead to a reduction in mortality.

Application of a Dual Internally Quenched Fluorogenic Substrate in Screening for D-Arginine Specific Proteases

The application of D-stereospecific proteases (DSPs) in resolution of racemic amino acids and in the semisynthesis of proteins has been a successful strategy. The main limitation for a broader application is, however, the accessibility of suitable DSPs covering multiple substrate specificities. To identify DSPs with novel primary substrate preferences, a fast specificity screening method using the easily accessible internally quenched fluorogenic substrate aminobenzoyl-D-arginyl-D-alanyl-p-nitroanilide was developed. By monitoring both UV/vis-absorbance and fluorescence signals at the same time it allows to detect two distinct D-amino acid substrate specificities simultaneously and separately with respect to the individual specificities. In order to identify novel DSP specificities for synthesis applications, DSPs specific for D-arginine were of special interest due to their potential ability as catalysts for substrate mimetics-mediated peptide and protein ligations. D-alanine in the substrate served as positive control and reference based on its known acceptance by numerous DSPs. In silico analysis suggested that DSPs are predominantly present in gram-positive microorganisms, therefore this study focused on the bacilli strains Bacillus thuringiensis and Bacillus subtilis as potential hosts of D-Arg-specific DSPs. While protease activities toward D-alanine were found in both organisms, a novel and so far unknown D-arginine specific DSP was detected within the culture supernatant of B. thuringiensis. Enrichment of this activity via cation exchange and size exclusion chromatography allowed isolation and further characterization of this novel enzyme consisting of a molecular mass of 37.7 kDa and an enzymatic activity of 8.3 U mg^-1 for cleaving the D-Arg|D-Ala bond in the detecting substrate. Independent experiments also showed that the identified enzyme shows similarities to the class of penicillin binding proteins. In future applications this enzyme will be a promising starting point for the development of novel strategies for the semisynthesis of all-L-proteins.

Characterization of Hypermutator Pseudomonas aeruginosa Isolates from Patients with Cystic Fibrosis in Australia

Hypermutable Pseudomonas aeruginosa isolates (hypermutators) have been identified in patients with cystic fibrosis (CF) and are associated with reduced lung function. Hypermutators display a greatly increased mutation rate and an enhanced ability to become resistant to antibiotics during treatment. Their prevalence has been established among patients with CF, but it has not been determined for patients with CF in Australia. This study aimed to determine the prevalence of hypermutable P. aeruginosa isolates from adult patients with CF from a health care institution in Australia and to characterize the genetic diversity and antibiotic susceptibility of these isolates. A total of 59 P. aeruginosa clinical isolates from patients with CF were characterized. For all isolates, rifampin (RIF) mutation frequencies and susceptibility to a range of antibiotics were determined. Of the 59 isolates, 13 (22%) were hypermutable. Whole-genome sequences were determined for all hypermutable isolates. Core genome polymorphisms were used to assess genetic relatedness of the isolates, both to each other and to a sample of previously characterized P. aeruginosa strains. Phylogenetic analyses showed that the hypermutators were from divergent lineages and that hypermutator phenotype was mostly the result of mutations in mutL or, less commonly, in mutS Hypermutable isolates also contained a range of mutations that are likely associated with adaptation of P. aeruginosa to the CF lung environment. Multidrug resistance was more prevalent in hypermutable than nonhypermutable isolates (38% versus 22%). This study revealed that hypermutable P. aeruginosa strains are common among isolates from patients with CF in Australia and are implicated in the emergence of antibiotic resistance.

Effect of C-2 substitution on the stability of non-traditional cephalosporins in mouse plasma

A systematic study of the stability of a set of cephalosporins in mouse plasma reveals that cephalosporins lacking an acidic moiety at C-2 may be vulnerable to β-lactam cleavage in mouse plasma.

Antibiotic sensitivity reveals that wall teichoic acids mediate DNA binding during competence in Bacillus subtilis

Despite decades of investigation of genetic transformation in the model Gram-positive bacterium Bacillus subtilis, the factors responsible for exogenous DNA binding at the surface of competent cells remain to be identified. Here, we report that wall teichoic acids (WTAs), cell wall-anchored anionic glycopolymers associated to numerous critical functions in Gram-positive bacteria, are involved in this initial step of transformation. Using a combination of cell wall-targeting antibiotics and fluorescence microscopy, we show that competence-specific WTAs are produced and specifically localized in the competent cells to mediate DNA binding at the proximity of the transformation apparatus. Furthermore, we propose that TuaH, a putative glycosyl transferase induced during competence, modifies competence-induced WTAs in order to promote (directly or indirectly) DNA binding. On the basis of our results and previous knowledge in the field, we propose a model for DNA binding and transport during genetic transformation in B. subtilis.

Natural genetic transformation in bacteria requires DNA binding at the surface of competent cells. Here, Mirouze et al. show that wall teichoic acids are specifically produced or modified during competence in Bacillus subtilis and promote (directly or indirectly) DNA binding at the cell surface.

Synthesis and Kinetic Analysis of Two Conformationally Restricted Peptide Substrates of Escherichia coli Penicillin-Binding Protein 5

Escherichia coli PBP5 (penicillin-binding protein 5) is a dd-carboxypeptidase involved in bacterial cell wall maturation. Beyond the C-terminal d-alanyl-d-alanine moiety, PBP5, like the essential high-molecular mass PBPs, has little specificity for other elements of peptidoglycan structure, at least as elicited in vitro by small peptidoglycan fragments. On the basis of the crystal structure of a stem pentapeptide derivative noncovalently bound to E. coli PBP6 (Protein Data Bank entry 3ITB ), closely similar in structure to PBP5, we have modeled a pentapeptide structure at the active site of PBP5. Because the two termini of the pentapeptide are directed into solution in the PBP6 crystal structure, we then modeled a 19-membered cyclic peptide analogue by cross-linking the terminal amines by succinylation. An analogous smaller, 17-membered cyclic peptide, in which the l-lysine of the original was replaced by l-diaminobutyric acid, could also be modeled into the active site. We anticipated that, just as the reactivity of stem peptide fragments of peptidoglycan with PBPs in vivo may be entropically enhanced by immobilization in the polymer, so too would that of our cyclic peptides with respect to their acyclic analogues in vitro. This paper describes the synthesis of the peptides described above that were required to examine this hypothesis and presents an analysis of their structures and reaction kinetics with PBP5.

Contact Antimicrobial Surface Obtained by Chemical Grafting of Microfibrillated Cellulose in Aqueous Solution Limiting Antibiotic Release

Contact active surfaces are an innovative tool for developing antibacterial products. Here, the microfibrillated cellulose (MFC) surface was modified with the β-lactam antibiotic benzyl penicillin in aqueous medium to prepare antimicrobial films. Penicillin was grafted on the MFC surface using a suspension of these nanofilaments or directly on films. Films prepared from the penicillin-modified MFC were characterized by Fourier transform infrared spectroscopy, contact angle measurements, elemental analysis, and X-ray photoelectron spectroscopy and tested for antibacterial activity against the Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli. Penicillin-grafted MFC films exhibited successful killing effect on Gram-positive bacteria with 3.5-log reduction whereas bacteriostatic efficiency was found in penicillin-grafted MFC suspension. The zone of inhibition test and leaching dynamic assay demonstrated that penicillin was not diffused into the surrounding media, thus proving that the films were indeed contact active. Thus, penicillin can be chemically bound to the modified substrate surface to produce promising nonleaching antimicrobial systems.

Sequence-Specific Peptide Nucleic Acid-Based Antisense Inhibitors of TEM-1 β-Lactamase and Mechanism of Adaptive Resistance

The recent surge of drug-resistant superbugs and shrinking antibiotic pipeline are serious challenges to global health. In particular, the emergence of β-lactamases has caused extensive resistance against the most frequently prescribed class of β-lactam antibiotics. Here, we develop novel synthetic peptide nucleic acid-based antisense inhibitors that target the start codon and ribosomal binding site of the TEM-1 β-lactamase transcript and act via translation inhibition mechanism. We show that these antisense inhibitors are capable of resensitizing drug-resistant Escherichia coli to β-lactam antibiotics exhibiting 10-fold reduction in the minimum inhibitory concentration (MIC). To study the mechanism of resistance, we adapted E. coli at MIC levels of the β-lactam/antisense inhibitor combination and observed a nonmutational, bet-hedging based adaptive antibiotic resistance response as evidenced by phenotypic heterogeneity as well as heterogeneous expression of key stress response genes. Our data show that both the development of new antimicrobials and an understanding of cellular response during the development of tolerance could aid in mitigating the impending antibiotic crisis.

Envelope Structures of Gram-Positive Bacteria

Gram-positive organisms, including the pathogens Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis, have dynamic cell envelopes that mediate interactions with the environment and serve as the first line of defense against toxic molecules. Major components of the cell envelope include peptidoglycan (PG), which is a well-established target for antibiotics, teichoic acids (TAs), capsular polysaccharides (CPS), surface proteins, and phospholipids. These components can undergo modification to promote pathogenesis, decrease susceptibility to antibiotics and host immune defenses, and enhance survival in hostile environments. This chapter will cover the structure, biosynthesis, and important functions of major cell envelope components in gram-positive bacteria. Possible targets for new antimicrobials will be noted.

Role of PBPD1 in stimulation of Listeria monocytogenes biofilm formation by subminimal inhibitory β-lactam concentrations

Disinfectant-tolerant Listeria monocytogenes biofilms can colonize surfaces that come into contact with food, leading to contamination and, potentially, food-borne illnesses. To better understand the process of L. monocytogenes biofilm formation and dispersal, we screened 1,120 off-patent FDA-approved drugs and identified several that modulate Listeria biofilm development. Among the hits were more than 30 β-lactam antibiotics, with effects ranging from inhibiting (≤50%) to stimulating (≥200%) biofilm formation compared to control. Most β-lactams also dispersed a substantial proportion of established biofilms. This phenotype did not necessarily involve killing, as >50% dispersal could be achieved with concentrations as low as 1/20 of the MIC of some cephalosporins. Penicillin-binding protein (PBP) profiling using a fluorescent penicillin analogue showed similar inhibition patterns for most β-lactams, except that biofilm-stimulatory drugs did not bind PBPD1, a low-molecular-weight d,d-carboxypeptidase. Compared to the wild type, a pbpD1 mutant had an attenuated biofilm response to stimulatory β-lactams. The cephalosporin-responsive CesRK two-component regulatory system, whose regulon includes PBPs, was not required for the response. The requirement for PBPD1 activity for β-lactam stimulation of L. monocytogenes biofilms shows that the specific set of PBPs that are inactivated by a particular drug dictates whether a protective biofilm response is provoked.

The molecular basis for the mode of action of Beta-lactam antibiotics and mechanisms of resistance

This review on the molecular basis for the mode of action of β-lactam antibiotics and mechanisms of resistance is divided into three main sections. Firstly, a brief introduction to the β-lactam antibiotic family is presented from the standpoint of their natural product origins. The second section is concerned with bacterial cell wall structure and biosynthesis, and the mode of action of β-lactam antibiotics. It includes an attempted rationalization of the multiple enzyme targets of penicillin, the so-called "penicillin binding proteins", into one or two lethal sites of action and the interaction of these enzymes with β-lactams in terms of their analogy to the natural substrate and to the substrate-enzyme transition state. The final part covers the phenomenon of bacterial resistance to β-lactam antibiotic therapy and deals with the two most important manifestations of resistance; permeability and the production of β-lactamases. This latter more crucial factor is then expanded with particular reference to the irreversible inhibition of these enzymes by suicide inactivators; a general theory for irreversible β-lactamase inhibition is discussed and the future prospects within this whole area are briefly overviewed.

Deciphering the bacterial microbiome of citrus plants in response to 'Candidatus Liberibacter asiaticus'-infection and antibiotic treatments

The bacterial microbiomes of citrus plants were characterized in response to 'Candidatus Liberibacter asiaticus' (Las)-infection and treatments with ampicillin (Amp) and gentamicin (Gm) by Phylochip-based metagenomics. The results revealed that 7,407 of over 50,000 known Operational Taxonomic Units (OTUs) in 53 phyla were detected in citrus leaf midribs using the PhyloChip™ G3 array, of which five phyla were dominant, Proteobacteria (38.7%), Firmicutes (29.0%), Actinobacteria (16.1%), Bacteroidetes (6.2%) and Cyanobacteria (2.3%). The OTU62806, representing 'Candidatus Liberibacter', was present with a high titer in the plants graft-inoculated with Las-infected scions treated with Gm at 100 mg/L and in the water-treated control (CK1). However, the Las bacterium was not detected in the plants graft-inoculated with Las-infected scions treated with Amp at 1.0 g/L or in plants graft-inoculated with Las-free scions (CK2). The PhyloChip array demonstrated that more OTUs, at a higher abundance, were detected in the Gm-treated plants than in the other treatment and the controls. Pairwise comparisons indicated that 23 OTUs from the Achromobacter spp. and 12 OTUs from the Methylobacterium spp. were more abundant in CK2 and CK1, respectively. Ten abundant OTUs from the Stenotrophomonas spp. were detected only in the Amp-treatment. These results provide new insights into microbial communities that may be associated with the progression of citrus huanglongbing (HLB) and the potential effects of antibiotics on the disease and microbial ecology.

A Simple Method for the Efficient Isolation of Genomic DNA from Lactobacilli Isolated from Traditional Indian Fermented Milk (dahi)

A simple, inexpensive and effective genomic DNA isolation procedure for Lactobacillus isolates from traditional Indian fermented milk (dahi) is described. A total of 269 Lactobacillus isolates from fermented milk collected from four places in North and west India were tested for lysis by an initial weakening of the Gram positive cell wall with Ampicillin followed by Lysozyme treatment. The average genomic DNA yield was ~50 μg/ml log phase culture. Quality and repeatability of the method was found to be adequate for subsequent molecular applications. The quality of the genomic DNA isolated by this method was verified by restriction digestion and polymerase chain reaction (PCR). No inhibition was observed in subsequent PCR amplification and restriction digestion. The presented method is rapid, cheap and useful for routine DNA isolation from gram positive bacteria such as Lactobacillus.

Safety and Efficacy of Cefazolin Sodium in the Management of Bacterial Infection and in Surgical Prophylaxis

Cefazolin sodium is a first-generation cephalosporin antibiotic and has been used worldwide since the early 1970s. It is used for the treatment of bacterial infections in various organs, such as the respiratory tract, skin and skin structure, genital tract, urinary tract, biliary tract, and bone and joint infections. It has also been used for septicemia due to susceptible gram-positive cocci (except Enterococcus), some gram-negative bacilli including E. coli, Proteus, and Klebsiella may be susceptible, and for perioperative prophylaxis. After the introduction of penicillins and other cephalosporins, occasional outbreaks of methicillin-resistant Staphylococcus aureus were noted. As a result, vancomycin use was increased; however, very recently and most alarmingly, vancomycin-resistant strains have been described. In this setting, to avoid the risk of the development of vancomycin-resistant strains further, vancomycin use should be curtailed. In consideration of this historical background, the appropriate use of antibiotics, such as dosage, dosage intervals, and the duration of administration is required not only for the protection of patients’ health but also for the prevention of the development of drug resistance. Cefazolin has been used in clinical practice for about 40 years, and a large body of evidence has been accumulated, and its efficacy and safety are well established compared with other antibiotics. Therefore, cefazolin has been chosen as a first-line anti-microbial for prophylaxis after various surgical procedures, including cardiovascular surgery, hysterectomy, arthroplasty and so on. Based on these facts, especially for the prophylaxis of surgical site infections, the first-generation cephalosporin, cefazolin, is now being “re-visited”.

Who's Winning the War? Molecular Mechanisms of Antibiotic Resistance in Helicobacter pylori

The ability of clinicians to wage an effective war against many bacterial infections is increasingly being hampered by skyrocketing rates of antibiotic resistance. Indeed, antibiotic resistance is a significant problem for treatment of diseases caused by virtually all known infectious bacteria. The gastric pathogen Helicobacter pylori is no exception to this rule. With more than 50% of the world's population infected, H. pylori exacts a tremendous medical burden and represents an interesting paradigm for cancer development; it is the only bacterium that is currently recognized as a carcinogen. It is now firmly established that H. pylori infection is associated with diseases such as gastritis, peptic and duodenal ulceration and two forms of gastric cancer, gastric adenocarcinoma and mucosa-associated lymphoid tissue (MALT) lymphoma. With such a large percentage of the population infected, increasing rates of antibiotic resistance are particularly vexing for a treatment regime that is already fairly complicated; treatment consists of two antibiotics and a proton pump inhibitor. To date, resistance has been found to all primary and secondary lines of antibiotic treatment as well as to drugs used for rescue therapy.

The Rcs phosphorelay is a cell envelope stress response activated by peptidoglycan stress and contributes to intrinsic antibiotic resistance

Gram-negative bacteria possess stress responses to maintain the integrity of the cell envelope. Stress sensors monitor outer membrane permeability, envelope protein folding, and energization of the inner membrane. The systems used by gram-negative bacteria to sense and combat stress resulting from disruption of the peptidoglycan layer are not well characterized. The peptidoglycan layer is a single molecule that completely surrounds the cell and ensures its structural integrity. During cell growth, new peptidoglycan subunits are incorporated into the peptidoglycan layer by a series of enzymes called the penicillin-binding proteins (PBPs). To explore how gram-negative bacteria respond to peptidoglycan stress, global gene expression analysis was used to identify Escherichia coli stress responses activated following inhibition of specific PBPs by the beta-lactam antibiotics amdinocillin (mecillinam) and cefsulodin. Inhibition of PBPs with different roles in peptidoglycan synthesis has different consequences for cell morphology and viability, suggesting that not all perturbations to the peptidoglycan layer generate equivalent stresses. We demonstrate that inhibition of different PBPs resulted in both shared and unique stress responses. The regulation of capsular synthesis (Rcs) phosphorelay was activated by inhibition of all PBPs tested. Furthermore, we show that activation of the Rcs phosphorelay increased survival in the presence of these antibiotics, independently of capsule synthesis. Both activation of the phosphorelay and survival required signal transduction via the outer membrane lipoprotein RcsF and the response regulator RcsB. We propose that the Rcs pathway responds to peptidoglycan damage and contributes to the intrinsic resistance of E. coli to beta-lactam antibiotics.

Mechanisms of resistance among respiratory tract pathogens

Antimicrobial resistance among respiratory tract pathogens represents a significant health care threat. Identifying the antimicrobial agents that remain effective in the presence of resistance, and knowing why, requires a thorough understanding of the mechanisms of action of the various agents as well as the mechanisms of resistance demonstrated among respiratory tract pathogens. The primary goal of antimicrobial therapy is to eradicate the pathogen, via killing or inhibiting bacteria, from the site of infection; the defenses of the body are required for killing any remaining bacteria. Targeting a cellular process or function specific to bacteria and not to the host limits the toxicity to patients. Currently, there are four general cellular targets to which antimicrobials are targeted: cell wall formation and maintenance, protein synthesis, DNA replication, and folic acid metabolism. Resistance mechanisms among respiratory tract pathogens have been demonstrated for all four targets. In general, the mechanisms of resistance used by these pathogens fall into one of three categories: enzymatic inactivation of the antimicrobial, prevention of intracellular accumulation, and modification of the target site to which agents bind to exert an antimicrobial effect. Resistance to some agents can be overcome by modifying the dosage regimens (e.g., using high-dose therapy) or inhibiting the resistance mechanism (e.g., b-lactamase inhibitors), whereas other mechanisms of resistance can only be overcome by using an agent from a different class. Understanding the mechanisms of action of the various agents and the mechanisms of resistance used by respiratory tract pathogens can help clinicians identify the agents that will increase the likelihood of achieving optimal outcomes.

Mechanism of action of the mannopeptimycins, a novel class of glycopeptide antibiotics active against vancomycin-resistant gram-positive bacteria

The naturally occurring mannopeptimycins (formerly AC98-1 through AC98-5) are a novel class of glycopeptide antibiotics that are active against a wide variety of gram-positive bacteria. The structures of the mannopeptimycins suggested that they might act by targeting cell wall biosynthesis, similar to other known glycopeptide antibiotics; but the fact that the mannopeptimycins retain activity against vancomycin-resistant organisms suggested that they might have a unique mode of action. By using a radioactive mannopeptimycin derivative bearing a photoactivation ligand, it was shown that mannopeptimycins interact with the membrane-bound cell wall precursor lipid II [C(55)-MurNAc-(peptide)-GlcNAc] and that this interaction is different from the binding of other lipid II-binding antibiotics such as vancomycin and mersacidin. The antimicrobial activities of several mannopeptimycin derivatives correlated with their affinities toward lipid II, suggesting that the inhibition of cell wall biosynthesis was primarily through lipid II binding. In addition, it was shown that mannopeptimycins bind to lipoteichoic acid in a rather nonspecific interaction, which might facilitate the accumulation of antibiotic on the bacterial cell surface.

Characterization of an In vitro-selected amoxicillin-resistant strain of Helicobacter pylori

An amoxicillin-resistant (Amox(r)) strain of Helicobacter pylori was selected for by culturing an amoxicillin-sensitive (Amox(s)) strain in increasingly higher concentrations of amoxicillin, resulting in a 133-fold increase in MIC, from 0.03 to 0.06 microg/ml to 4 to 8 microg/ml. This resistance was stable upon freezing for at least 6 months and conferred cross-resistance to seven other beta-lactam antibiotics. beta-Lactamase activity was not detected in this Amox(r) strain; however, analysis of the penicillin-binding protein (PBP) profiles generated from isolated bacterial membranes of the Amox(s) parental strain and the Amox(r) strain revealed a significant decrease in labeling of PBP 1 by biotinylated amoxicillin (bio-Amox) in the Amox(r) strain. Comparative binding studies of PBP 1 for several beta-lactams demonstrated that PBP 1 in the Amox(r) strain had decreased affinity for mezlocillin but not significantly decreased affinity for penicillin G. In addition, PBP profiles prepared from whole bacterial cells showed decreased labeling of PBP 1 and PBP 2 in the Amox(r) strain at all bio-Amox concentrations tested, suggesting a diffusional barrier to bio-Amox or a possible antibiotic efflux mechanism. Uptake analysis of (14)C-labeled penicillin G showed a significant decrease in uptake of the labeled antibiotic by the Amox(r) strain compared to the Amox(s) strain, which was not affected by pretreatment with carbonyl cyanide m-chlorophenylhydrazone, eliminating the possibility of an efflux mechanism in the resistant strain. These results demonstrate that alterations in PBP 1 and in the uptake of beta-lactam antibiotics in H. pylori can be selected for by prolonged exposure to amoxicillin, resulting in increased resistance to this antibiotic.

Competition of various beta-lactam antibiotics for the major penicillin-binding proteins of Helicobacter pylori: antibacterial activity and effects on bacterial morphology

The penicillin-binding proteins (PBPs) of helical (log-phase) Helicobacter pylori ATCC 43579 were identified by using biotinylated ampicillin. The major PBPs had apparent molecular masses of 47, 60, 63, and 66 kDa; an additional minor PBP of 95 to 100 kDa was also detected. The relative affinities of various beta-lactams for these PBPs were tested by competitive-binding assays. Only PBP63 appeared to be significantly bound to each of the competing antibiotics, whereas PBP66 strongly bound mezlocillin, oxacillin, amoxicillin, and ceftriaxone. Whereas most of the beta-lactams significantly bound two or more PBPs, aztreonam specifically targeted PBP63. The influence of sub-MICs of these beta-lactams on the morphologies of log-phase H. pylori was observed at both the phase-contrast and transmission electron microscopy levels. Each of the eight beta-lactams examined induced blebbing and sphere formation, whereas aztreonam was the only antibiotic studied which induced pronounced filamentation in H. pylori. Finally, studies comparing the PBPs of helical (log-phase) cultures with those of coccoid (7-, 14-, and 21-day-old) cultures of H. pylori revealed that the major PBPs at 60 and 63 kDa seen in the helical form were almost undetectable in the coccoid forms, whereas PBP66 remained the major PBP in the coccoid forms, although somewhat reduced in level compared to the helical form. PBP47 was present in both forms at approximately equal concentrations. These studies thus identified the major PBPs in both helical and coccoid forms of H. pylori and compared the relative affinities of seven different beta-lactams for the PBPs in the helical forms and their effects on bacterial morphology.

Identification of two penicillin-binding multienzyme complexes in Haemophilus influenzae

Dansyl-labeled penicillin, reversed-phase chromatography, and peptide mapping have been used to detect, separate, and study penicillin-binding proteins (PBPs) and PBP multienzyme complexes of H. influenzae. The cross-linking of proteins in the multienzyme complex was accomplished with the aid of cyanogen, a salt-bridge specific cross-linking agent. The chromatographic profile of the PBPs clearly showed a dramatic change in the number and identity of peaks after treatment of the bacterial cells with cyanogen. The disappearance of all seven peaks corresponding to the PBPs was accompanied by the emergence of two new peaks with molecular weights between 400 kDa and 600 kDa. The results hint at the existence of two penicillin-binding multienzyme complexes, each containing subunits that interact via salt-bridges. Chromatographic active site peptide mapping of PBPs and PBP complexes was used to determine the identity of PBPs involved in each complex. It is postulated that one multienzyme complex containing PBP 2 may be involved in cell elongation while the other complex containing PBP 3 may be responsible for cell division.

Staphylococcal cell wall: morphogenesis and fatal variations in the presence of penicillin

The primary goal of this review is to provide a compilation of the complex architectural features of staphylococcal cell walls and of some of their unusual morphogenetic traits including the utilization of murosomes and two different mechanisms of cell separation. Knowledge of these electron microscopic findings may serve as a prerequisite for a better understanding of the sophisticated events which lead to penicillin-induced death. For more than 50 years there have been controversial disputes about the mechanisms by which penicillin kills bacteria. Many hypotheses have tried to explain this fatal event biochemically and mainly via bacteriolysis. However, indications that penicillin-induced death of staphylococci results from overall biochemical defects or from a fatal attack of bacterial cell walls by bacteriolytic murein hydrolases were not been found. Rather, penicillin, claimed to trigger the activity of murein hydrolases, impaired autolytic wall enzymes of staphylococci. Electron microscopic investigations have meanwhile shown that penicillin-mediated induction of seemingly minute cross wall mistakes is the very reason for this killing. Such "morphogenetic death" taking place at predictable cross wall sites and at a predictable time is based on the initiation of normal cell separations in those staphylococci in which the completion of cross walls had been prevented by local penicillin-mediated impairment of the distribution of newly synthesized peptidoglycan; this death occurs because the high internal pressure of the protoplast abruptly kills such cells via ejection of some cytoplasm during attempted cell separation. An analogous fatal onset of cell partition is considered to take place without involvement of a detectable quantity of autolytic wall enzymes ("mechanical cell separation"). The most prominent feature of penicillin, the disintegration of bacterial cells via bacteriolysis, is shown to represent only a postmortem process resulting from shrinkage of dead cells and perturbation of the cytoplasmic membrane. Several schematic drawings have been included in this review to facilitate an understanding of the complex morphogenetic events.

The lantibiotic mersacidin inhibits peptidoglycan synthesis by targeting lipid II

The lantibiotic mersacidin exerts its bactericidal action by inhibition of peptidoglycan biosynthesis. It interferes with the membrane-associated transglycosylation reaction; during this step the ultimate monomeric peptidoglycan precursor, undecaprenyl-pyrophosphoryl-MurNAc-(pentapeptide)-GlcNAc (lipid II) is converted into polymeric nascent peptidoglycan. In the present study we demonstrate that the molecular basis of this inhibition is the interaction of mersacidin with lipid II. The adsorption of [14C]mersacidin to growing cells, as well as to isolated membranes capable of in vitro peptidoglycan synthesis, was strictly dependent on the availability of lipid II, and antibiotic inhibitors of lipid II formation strongly interfered with this binding. Direct evidence for the interaction was provided by studies with isolated lipid II. [14C]mersacidin associated tightly with [14C]lipid II micelles; the complex was stable even in the presence of 1% sodium dodecyl sulfate. Furthermore, the addition of isolated lipid II to the culture broth efficiently antagonized the bactericidal activity of mersacidin. In contrast to the glycopeptide antibiotics, complex formation does not involve the C-terminal D-alanyl-D-alanine moiety of the lipid intermediate. Thus, the interaction of mersacidin with lipid II apparently occurs via a binding site which is not targeted by any antibiotic currently in use.

Identification of a penicillin-binding protein 3 homolog, PBP3x, in Pseudomonas aeruginosa: gene cloning and growth phase-dependent expression

A homolog of Pseudomonas aeruginosa penicillin-binding protein 3 (PBP3), named PBP3x in this study, was identified by using degenerate primers based on conserved amino acid motifs in the high-molecular-weight PBPs. Analysis of the translated sequence of the pbpC gene encoding this PBP3x revealed that 41 and 48% of its amino acids were identical to those of Escherichia coli and P. aeruginosa PBP3s, respectively. The downstream sequence of pbpC encoded convergently transcribed homologs of the E. coli soxR gene and the Mycobacterium bovis adh gene. The pbpC gene product was expressed from the T7 promoter in E. coli and was exported to the cytoplasmic membrane of E. coli cells and could bind [3H] penicillin. By using a broad-host-range vector, pUCP27, the pbpC gene was expressed in P. aeruginosa PAO4089. [3H]penicillin-binding competition assays indicated that the pbpC gene product had lower affinities for several PBP3-targeted beta-lactam antibiotics than P. aeruginosa PBP3 did, and overexpression of the pbpC gene product had no effect on the susceptibility to the PBP3-targeted antibiotics tested. By gene replacement, a PBP3x-defective interposon mutant (strain HC132) was obtained and confirmed by Southern blot analysis. Inactivation of PBP3x caused no changes in the cell morphology or growth rate of exponentially growing cells, suggesting that pbpC was not required for cell viability under normal laboratory growth conditions. However, the upstream sequence of pbpC contained a potential sigma(s) recognition site, and pbpC gene expression appeared to be growth rate regulated. [3H]penicillin-binding assays indicated that PBP3 was mainly produced during exponential growth whereas PBP3x was produced in the stationary phase of growth.

Different patterns of bacterial DNA synthesis during postantibiotic effect

Studies on bacterial metabolism during the postantibiotic effect (PAE) period are limited but might provide insight into the nature of the PAE. We evaluated the rate of DNA synthesis in bacteria during the PAE period after a 1-h exposure of organisms in the logarithmic growth phase to various antibiotics. Staphylococcus aureus ATCC 25923 was exposed to vancomycin, dicloxacillin, rifampin, and ciprofloxacin; Escherichia coli ATCC 25922 was exposed to gentamicin, tobramycin, rifampin, imipenem, and ciprofloxacin; and Pseudomonas aeruginosa ATCC 25783 was exposed to imipenem, tobramycin, and ciprofloxacin. DNA synthesis was determined by measuring the rate of [3H]thymidine incorporation in S. aureus and E. coli and [3H]adenine incorporation in P. aeruginosa. DNA synthesis in S. aureus was suppressed during the PAE phase with vancomycin, dicloxacillin, and rifampin, it was suppressed in E. coli with rifampin, and it was suppressed in P. aeruginosa after exposure to tobramycin. Conversely, DNA synthesis was relatively enhanced in the gram-negative bacilli after exposure to imipenem and in all three species after exposure to ciprofloxacin. However, DNA synthesis in E. coli was only minimally affected after exposure to tobramycin and gentamicin. The differences in DNA synthesis observed after exposure to various antimicrobial agents suggest multiple mechanisms for the PAE.

Penicillin-binding protein 7/8 of Escherichia coli is a DD-endopeptidase

Penicillin-binding protein 7 (PBP7) and its proteolytic degradation product PBP8 are shown to be soluble proteins, which can be set free from whole cells of Escherichia coli by an osmotic shock. The proteins are loosely associated with the membranes and are totally released into the supernatant in the presence of 1 M NaCl. Partial purification of PBP8 was accomplished by hydroxyapatite, heparin-Sepharose and MonoS chromatography. Murein meso-diaminopimelate-D-alanine DD-endopeptidase activity was demonstrated for both PBP7 and PBP8, which specifically hydrolyse the DD-diaminopimelate-alanine bonds in high-molecular-mass murein sacculi but fail to cleave these bonds in isolated dimeric muropeptides. The enzyme is inhibited by the 'penem' beta-lactam antibiotic CGP31608 at a concentration of 0.25 micrograms/ml by 50%. Thus besides PBP4 and the mepA gene product, a third endopeptidase exists in E. coli.

A novel, "hidden" penicillin-induced death of staphylococci at high drug concentration, occurring earlier than murosome-mediated killing processes

In log-phase cells of staphylococci, cultivated under high, "non-lytic" concentrations of penicillin G, there occurred a novel killing process hitherto hidden behind seemingly bacteriostatic effects. Two events are essential for the appearance of this "hidden death": (i) the failure of the dividing cell to deposit enough fibrillar cross-wall material to be welded together, and (ii) a premature ripping up of incomplete cross walls along their splitting system. "Hidden death" started as early as 10-15 min after drug addition, already during the first division cycle. It was the consequence of a loss of cytoplasmic constituents which erupted through peripheral slit-like openings in the incomplete cross walls. The loss resulted either in more or less empty cells or in cell shrinkage. These destructions could be prevented by raising the external osmotic pressure. In contrast, the conventional "non-hidden death" occurred only much later and exclusively during the second division cycle and mainly in those dividing cells, whose nascent cross walls of the first division plane had been welded together. These welding processes at nascent cross walls, resulting in tough connecting bridges between presumptive individual cells, were considered as a morphogenetic tool which protects the cells, so that they can resist the otherwise fatal penicillin-induced damages for at least an additional generation time ("morphogenetic resistance system"). Such welded cells, in the virtual absence of underlying cross-wall material, lost cytoplasm and were killed via ejection through pore-like wall openings or via explosions in the second division plane and after liberation of their murosomes, as it was the case in the presence of low, "lytic" concentrations of penicillin. Bacteriolysis did not cause any of the hitherto known penicillin-induced killing processes.

Subcellular distribution of the soluble lytic transglycosylase in Escherichia coli

The localization of the major autolytic enzyme, the soluble lytic transglycosylase, in the different cell compartments of Escherichia coli was investigated by immunoelectron microscopy. Ultrathin sections were labeled with a specific antiserum against purified soluble lytic transglycosylase, and the antibody-enzyme complexes were visualized with colloidal protein A-gold. A preferential localization of the lytic transglycosylase in the envelope was observed, with only 20 to 30% of the enzyme left in the cytoplasm. Most of the enzyme associated with the cell wall was tightly bound to the murein sacculus. Sacculi prepared by boiling of cells in 4% sodium dodecyl sulfate could be immunolabeled with the specific antiserum, indicating a surprisingly strong interaction of the lytic transglycosylase with murein. The enzyme-substrate complex could be reconstituted in vitro by incubating pronase-treated, protein-free murein sacculi with purified lytic transglycosylase at 0 degrees C. Titration of sacculi with increasing amounts of enzyme indicated a limiting number of binding sites for about 1,000 molecules of enzyme per sacculus. Ruptured murein sacculi obtained after penicillin treatment revealed that the enzyme is exclusively bound to the outer surface of the sacculus. This finding is discussed in the light of recent evidence suggesting that the murein of E. coli might be a structure of more than one layer expanding by inside-to-outside growth of patches of murein.

In vitro synergistic activity between meropenem and other beta-lactams against methicillin-resistant Staphylococcus aureus

The in vitro activity of meropenem, a carbapenem antibiotic, combined with eight other beta-lactams against methicillin-resistant Staphylococcus aureus (MRSA) was tested. The MICs of these antibiotics alone ranged from 12.5 to 1,600 micrograms/ml for the 25 clinical isolates of MRSA studied. All combinations with meropenem exhibited marked synergy as determined by the checkerboard method. The MICs in combinations of meropenem with other beta-lactams were reduced to 1/4-1/64 those of the antibiotics alone. No antagonism was observed for any of the combinations of meropenem with other beta-lactams. Synergism between meropenem and cefpiramide was the highest among the combinations tested, the geometric mean of the fractional inhibitory concentration index for this combination being 0.237. This combination also demonstrated strong bactericidal activity, the MBCs decreasing to 1/16-1/64 of those for the agents alone. In terms of the fractional inhibitory concentration index, this was the most effective combination against MRSA highly resistant to meropenem alone with synergism for 85% (81/95) of the strains. In addition, synergism of imipenem with cephalosporins against MRSA was demonstrated. The affinity of meropenem and cefpiramide for MRSA penicillin-binding protein (PBP) 2' was very low, and the combination of both antibiotics showed an additive increase in affinity for this protein, but not a synergistic increase. Thus, the mechanism of synergism did not seem to be related to affinity for PBP2'. It is possible that there is another factor besides PBP2' which increases the resistance that the combinations inhibit the unknown factor.

Affinities of SM-7338 for penicillin-binding proteins and its release from these proteins in Staphylococcus aureus

SM-7338, a carbapenem antibiotic, had high affinities for penicillin-binding proteins (PBPs) 1, 2, and 4 of Staphylococcus aureus but not for PBP 3 when a competition assay with [14C]benzylpenicillin was used. However, binding of [14C]SM-7338 was saturated for PBP 3 at a concentration of 1 microgram/ml. These results were due to the rapid release of SM-7338 from PBP 3-SM-7338 complexes with a half-life of 2 min.

Paradoxical response of Enterococcus faecalis to the bactericidal activity of penicillin is associated with reduced activity of one autolysin

Ten clinical isolates of Enterococcus faecalis were examined for susceptibility to the bactericidal activity of penicillin. Four of these had MBCs of penicillin equal to 2 to 4 x the MIC, and six exhibited a paradoxical response to penicillin, i.e., the bactericidal activity of the antibiotic had a concentration optimum at 2 to 4 x the MIC and decreased significantly at concentrations above this. We found that the paradoxical response to penicillin was an intrinsic and stable property of a strain, but that its phenotypic expression was not homogeneous; only a fraction of the cell population that died at low concentrations was able to survive at high penicillin concentrations. The size of this fraction increased with increasing antibiotic concentration and reached a maximum in the late-log phase of growth. All 10 strains produced a lytic enzyme that was active on Micrococcus luteus heat-killed cells, whereas only some strains lysed E. faecalis heat-killed cells. Strains producing large amounts of the latter enzyme did not show the paradoxical response to penicillin, whereas mutants of these strains that lacked this enzymatic activity paradoxically responded to the antibiotic activity. In addition, from strains that showed paradoxical response to penicillin and produced only the enzyme that was active on M. luteus, it was possible to isolate mutants that were also capable of lysing E. faecalis cells and that were killed with similar efficiency by all concentrations above the MBC. On the basis of these findings, the paradoxical response to penicillin is explained as a property of certain strains of E. faecalis; this property is genetically characterized by alterations in synthesis or activity of one autolysin but phenotypically expressed only by a few cells that are in a particular physiological condition when exposed to high concentrations of antibiotics.