Mode of action of penicillin

Molecular Mechanism of Drug Resistance

The treatment of microbial infections has suffered greatly in this present century of pathogen dominance. Inspite of extensive research efforts and scientific advancements, the worldwide emergence of microbial tolerance continues to plague survivability. The innate property of microbe to resist any antibiotic due to evolution is the virtue of intrinsic resistance. However, the classical genetic mutations and extrachromosomal segments causing gene exchange attribute to acquired tolerance development. Rampant use of antimicrobials causes certain selection pressure which increases the resistance frequency. Genomic duplication, enzymatic site modification, target alteration, modulation in membrane permeability, and the efflux pump mechanism are the major contributors of multidrug resistance (MDR), specifically antibiotic tolerance development. MDRs will lead to clinical failures for treatment and pose health crisis. The molecular mechanisms of antimicrobial resistance are diverse as well as complex and still are exploited for new discoveries in order to prevent the surfacing of “superbugs.” Antimicrobial chemotherapy has diminished the threat of infectious diseases to some extent. To avoid the indiscriminate use of antibiotics, the new ones licensed for use have decreased with time. Additionally, in vitro assays and genomics for anti-infectives are novel approaches used in resolving the issues of microbial resistance. Proper use of drugs can keep it under check and minimize the risk of MDR spread.

Highly improved acarbose production of Actinomyces through the combination of ARTP and penicillin susceptible mutant screening

Atmospheric and room temperature plasma (ARTP) was first employed to generate mutants of Actinomyces JN537 for improving acarbose production. To obtain higher acarbose producing strains, the method of screening the strains for susceptibility to penicillin was used after treatment with ARTP. The rationale for the strategy was that mutants showing penicillin susceptibility were likely to be high acarbose producers, as their ability to synthesize cell walls was weak which might enhance metabolic flux to the pathway of acarbose biosynthesis. Acarbose yield of the mutant strain M37 increased by 62.5 % than that of the original strain. The contents of monosaccharides and amino acids of the cell wall of M37 were lower than that of the original strain. The acarbose production ability in mutant strain remained relatively stable after 10 generations. This work provides a promising strategy for obtaining high acarbose-yield strains by combination of ARTP mutation method and efficient screening technique.

Synthesis and biological evaluation of N-naphthoyl-phenylglyoxamide-based small molecular antimicrobial peptide mimics as novel antimicrobial agents and biofilm inhibitors

Antimicrobial peptides (AMPs) are a key component of the human immune system. Synthetic AMP mimics represent a novel strategy to counteract the increasing incidence of antimicrobial resistance. Here, we describe the synthesis of novel glyoxamide derivatives via ring-opening reactions of N-hexanoyl, N-benzoyl and N-naphthoylisatins with N,N-dimethylethane-1,2-diamine and N,N-dimethylpropane-1,3-diamine. These were converted to both the hydrochloric acid (HCl) or quaternary ammonium iodide (MeI) salts and their antibacterial activity against Staphylococcus aureus was investigated by their zone-of-inhibition and minimum inhibitory concentration (MIC). The HCl salt 22b exhibited the lowest MIC of 16 μg mL(-1), whereas the corresponding MeI salt 22c had a MIC of 39 μg mL(-1). We also investigated the in vitro toxicity of active compounds against the MRC-5 normal human lung fibroblasts and their activity against established biofilm in S. aureus.

Living biointerfaces based on non-pathogenic bacteria support stem cell differentiation

Lactococcus lactis, a non-pathogenic bacteria, has been genetically engineered to express the III_7–10 fragment of human fibronectin as a membrane protein. The engineered L. lactis is able to develop biofilms on different surfaces (such as glass and synthetic polymers) and serves as a long-term substrate for mammalian cell culture, specifically human mesenchymal stem cells (hMSC). This system constitutes a living interface between biomaterials and stem cells. The engineered biofilms remain stable and viable for up to 28 days while the expressed fibronectin fragment induces hMSC adhesion. We have optimised conditions to allow long-term mammalian cell culture, and found that the biofilm is functionally equivalent to a fibronectin-coated surface in terms of osteoblastic differentiation using bone morphogenetic protein 2 (BMP-2) added to the medium. This living bacteria interface holds promise as a dynamic substrate for stem cell differentiation that can be further engineered to express other biochemical cues to control hMSC differentiation.

The Bacterial Cell Wall in the Antibiotic Era: An Ontology in Transit Between Morphology and Metabolism, 1940s-1960s

This essay details a historical crossroad in biochemistry and microbiology in which penicillin was a co-agent. I narrate the trajectory of the bacterial cell wall as the precise target for antibiotic action. As a strategic object of research, the bacterial cell wall remained at the core of experimental practices, scientific narratives and research funding appeals throughout the antibiotic era. The research laboratory was dedicated to the search for new antibiotics while remaining the site at which the mode of action of this new substance was investigated. This combination of circumstances made the bacterial wall an ontology in transit. As invisible as the bacterial wall was for clinical purposes, in the biological laboratory, cellular meaning in regard to the action of penicillin made the bacterial wall visible within both microbiology and biochemistry. As a border to be crossed, some components of the bacterial cell wall and the biochemical destruction produced by penicillin became known during the 1950s and 1960s. The cell wall was constructed piece by piece in a transatlantic circulation of methods, names, and images of the shape of the wall itself. From 1955 onwards, microbiologists and biochemists mobilized new names and associated conceptual meanings. The composition of this thin and rigid layer would account for its shape, growth and destruction. This paper presents a history of biochemical morphology: a chemistry of shape - the shape of bacteria, as provided by its wall - that accounted for biology, for life itself. While penicillin was being established as an industrially-manufactured object, it remained a scientific tool within the research laboratory, contributing to the circulation of further scientific objects.

Activities and regulation of peptidoglycan synthases

Peptidoglycan (PG) is an essential component in the cell wall of nearly all bacteria, forming a continuous, mesh-like structure, called the sacculus, around the cytoplasmic membrane to protect the cell from bursting by its turgor. Although PG synthases, the penicillin-binding proteins (PBPs), have been studied for 70 years, useful in vitro assays for measuring their activities were established only recently, and these provided the first insights into the regulation of these enzymes. Here, we review the current knowledge on the glycosyltransferase and transpeptidase activities of PG synthases. We provide new data showing that the bifunctional PBP1A and PBP1B from Escherichia coli are active upon reconstitution into the membrane environment of proteoliposomes, and that these enzymes also exhibit DD-carboxypeptidase activity in certain conditions. Both novel features are relevant for their functioning within the cell. We also review recent data on the impact of protein-protein interactions and other factors on the activities of PBPs. As an example, we demonstrate a synergistic effect of multiple protein-protein interactions on the glycosyltransferase activity of PBP1B, by its cognate lipoprotein activator LpoB and the essential cell division protein FtsN.

Beta-lactam antibiotics induce a lethal malfunctioning of the bacterial cell wall synthesis machinery

Penicillin and related beta-lactams comprise one of our oldest and most widely used antibiotic therapies. These drugs have long been known to target enzymes called penicillin-binding proteins (PBPs) that build the bacterial cell wall. Investigating the downstream consequences of target inhibition and how they contribute to the lethal action of these important drugs, we demonstrate that beta-lactams do more than just inhibit the PBPs as is commonly believed. Rather, they induce a toxic malfunctioning of their target biosynthetic machinery involving a futile cycle of cell wall synthesis and degradation, thereby depleting cellular resources and bolstering their killing activity. Characterization of this mode of action additionally revealed a quality control function for enzymes that cleave bonds in the cell wall matrix. The results thus provide insight into the mechanism of cell wall assembly and suggest how best to interfere with the process for future antibiotic development.

Genetic determinants of reutericyclin biosynthesis in Lactobacillus reuteri

Reutericyclin is a unique antimicrobial tetramic acid produced by some strains of Lactobacillus reuteri. This study aimed to identify the genetic determinants of reutericyclin biosynthesis. Comparisons of the genomes of reutericyclin-producing L. reuteri strains with those of non-reutericyclin-producing strains identified a genomic island of 14 open reading frames (ORFs) including genes coding for a nonribosomal peptide synthetase (NRPS), a polyketide synthase (PKS), homologues of PhlA, PhlB, and PhlC, and putative transport and regulatory proteins. The protein encoded by rtcN is composed of a condensation domain, an adenylation domain likely specific for d-leucine, and a thiolation domain. rtcK codes for a PKS that is composed of a ketosynthase domain, an acyl-carrier protein domain, and a thioesterase domain. The products of rtcA, rtcB, and rtcC are homologous to the diacetylphloroglucinol-biosynthetic proteins PhlABC and may acetylate the tetramic acid moiety produced by RtcN and RtcK, forming reutericyclin. Deletion of rtcN or rtcABC in L. reuteri TMW1.656 abrogated reutericyclin production but did not affect resistance to reutericyclin. Genes coding for transport and regulatory proteins could be deleted only in the reutericyclin-negative L. reuteri strain TMW1.656ΔrtcN, and these deletions eliminated reutericyclin resistance. The genomic analyses suggest that the reutericyclin genomic island was horizontally acquired from an unknown source during a unique event. The combination of PhlABC homologues with both an NRPS and a PKS has also been identified in the lactic acid bacteria Streptococcus mutans and Lactobacillus plantarum, suggesting that the genes in these organisms and those in L. reuteri share an evolutionary origin.

Glycans in pathogenic bacteria--potential for targeted covalent therapeutics and imaging agents

A substantial obstacle to the existing treatment of bacterial diseases is the lack of specific probes that can be used to diagnose and treat pathogenic bacteria in a selective manner while leaving the microbiome largely intact. To tackle this problem, there is an urgent need to develop pathogen-specific therapeutics and diagnostics. Here, we describe recent evidence that indicates distinctive glycans found exclusively on pathogenic bacteria could form the basis of targeted therapeutic and diagnostic strategies. In particular, we highlight the use of metabolic oligosaccharide engineering to covalently deliver therapeutics and imaging agents to bacterial glycans.

Peptidoglycan remodeling by the coordinated action of multispecific enzymes

The peptidoglycan (PG) cell wall constitutes the main defense barrier of bacteria against environmental insults and acts as communication interface. The biochemistry of this macromolecule has been well characterized throughout the years but recent discoveries have unveiled its chemical plasticity under environmental stresses. Non-canonical D-amino acids (NCDAA) are produced and released to the extracellular media by diverse bacteria. Such molecules govern cell wall adaptation to challenging environments through their incorporation into the polymer, a widespread capability among bacteria that reveals the inherent catalytic plasticity of the enzymes involved in the cell wall metabolism. Here, we analyze the recent structural and biochemical characterization of Bsr, a new family of broad spectrum racemases able to generate a wide range of NCDAA. We also discuss the necessity of a coordinated action of PG multispecific enzymes to generate adequate levels of modification in the murein sacculus. Finally, we also highlight how this catalytic plasticity of NCDAA-incorporating enzymes has allowed the development of new revolutionary methodologies for the study of PG modes of growth and in vivo dynamics.

The cell wall architecture of Enterococcus faecium: from resistance to pathogenesis

The cell wall of Gram-positive bacteria functions as a surface organelle that continuously interacts with its environment through a plethora of cell wall-associated molecules. Enterococcus faecium is a normal inhabitant of the GI tract of mammals, but has recently become an important etiological agent of hospital-acquired infections in debilitated patients. Insights into the assembly and function of enterococcal cell wall components and their interactions with the host during colonization and infection are essential to explain the worldwide emergence of E. faecium as an important multiantibiotic-resistant nosocomial pathogen. Understanding the biochemistry of cell wall biogenesis and principles of antibiotic resistance at the molecular level may open up new frontiers in research on enterococci, particularly for the development of novel antimicrobial strategies. In this article, we outline the current knowledge on the most important antimicrobial resistance mechanisms that involve peptidoglycan synthesis and the role of cell wall constituents, including lipoteichoic acid, wall teichoic acid, capsular polysaccharides, LPxTG cell wall-anchored surface proteins, WxL-type surface proteins and pili, in the pathogenesis of E. faecium.

Release of protein A from the cell wall of Staphylococcus aureus

Staphylococcal protein A (SpA) is anchored to the cell wall envelope of Staphylococcus aureus by sortase A, which links the threonyl (T) of its C-terminal LPXTG motif to peptidoglycan cross-bridges (i.e., Gly5). SpA binds the Fcγ domains of IgG and protects staphylococci from opsonophagocytic clearance. Moreover, SpA cross-links B-cell receptors to modify host adaptive immune responses. The mechanisms whereby SpA is released from the bacterial surface to access the host's immune system are not known. Here we demonstrate that SpA is released with murein tetrapeptide-tetraglycyl [L-Ala-D-iGln-(SpA-Gly5)L-Lys-D-Ala-Gly4] linked to its C-terminal threonyl. LytN, a cross-wall murein hydrolase, contributes to the release of SpA by removing amino sugars [i.e., N-acetylmuramic acid-N-acetylglucosamine (MurNAc-GlcNAc)] from attached peptidoglycan, whereas LytM, a pentaglycyl-endopeptidase, triggers polypeptide release from the bacterial envelope. A model is proposed whereby murein hydrolases cleave the anchor structure of released SpA to modify host immune responses.

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.

Protein secretion and surface display in Gram-positive bacteria

The cell wall peptidoglycan of Gram-positive bacteria functions as a surface organelle for the transport and assembly of proteins that interact with the environment, in particular, the tissues of an infected host. Signal peptide-bearing precursor proteins are secreted across the plasma membrane of Gram-positive bacteria. Some precursors carry C-terminal sorting signals with unique sequence motifs that are cleaved by sortase enzymes and linked to the cell wall peptidoglycan of vegetative forms or spores. The sorting signals of pilin precursors are cleaved by pilus-specific sortases, which generate covalent bonds between proteins leading to the assembly of fimbrial structures. Other precursors harbour surface (S)-layer homology domains (SLH), which fold into a three-pronged spindle structure and bind secondary cell wall polysaccharides, thereby associating with the surface of specific Gram-positive microbes. Type VII secretion is a non-canonical secretion pathway for WXG100 family proteins in mycobacteria. Gram-positive bacteria also secrete WXG100 proteins and carry unique genes that either contribute to discrete steps in secretion or represent distinctive substrates for protein transport reactions.

Peptidoglycan plasticity in bacteria: stress-induced peptidoglycan editing by noncanonical D-amino acids

It has been generally assumed that the role of D-amino acids in bacterial physiology is rather limited. However, recent new evidence demonstrated that millimolar concentrations of noncanonical D-amino acids are synthesized and released to the environment by bacteria from diverse phyla. These D-amino acids help bacteria adapt to environmental challenges by modulating the structure and composition of the peptidoglycan (PG). This regulation, which appears to be well conserved among bacterial species, occurs principally through the incorporation of the D-amino acids into the terminus of the peptide moiety of muropeptides. These findings revived interest in studies investigating D-amino acids as an exciting and trendy topic in current microbiology, which considers them as fundamental players in different aspects of bacterial physiology. In this article, we provide an overview of the origins of research on the effects of D-amino acids in the biology of bacterial cell walls, including their recent implication as key factors for stress-associated PG remodeling.

Determinants of murein hydrolase targeting to cross-wall of Staphylococcus aureus peptidoglycan

Cells of eukaryotic or prokaryotic origin express proteins with LysM domains that associate with the cell wall envelope of bacteria. The molecular properties that enable LysM domains to interact with microbial cell walls are not yet established. Staphylococcus aureus, a spherical microbe, secretes two murein hydrolases with LysM domains, Sle1 and LytN. We show here that the LysM domains of Sle1 and LytN direct murein hydrolases to the staphylococcal envelope in the vicinity of the cross-wall, the mid-cell compartment for peptidoglycan synthesis. LysM domains associate with the repeating disaccharide β-N-acetylmuramic acid, (1→4)-β-N-acetylglucosamine of staphylococcal peptidoglycan. Modification of N-acetylmuramic acid with wall teichoic acid, a ribitol-phosphate polymer tethered to murein linkage units, prevents the LysM domain from binding to peptidoglycan. The localization of LytN and Sle1 to the cross-wall is abolished in staphylococcal tagO mutants, which are defective for wall teichoic acid synthesis. We propose a model whereby the LysM domain ensures septal localization of LytN and Sle1 followed by processive cleavage of peptidoglycan, thereby exposing new LysM binding sites in the cross-wall and separating bacterial cells.

Super-resolution microscopy reveals cell wall dynamics and peptidoglycan architecture in ovococcal bacteria

Cell morphology and viability in Eubacteria is dictated by the architecture of peptidoglycan, the major and essential structural component of the cell wall. Although the biochemical composition of peptidoglycan is well understood, how the peptidoglycan architecture can accommodate the dynamics of growth and division while maintaining cell shape remains largely unknown. Here, we elucidate the peptidoglycan architecture and dynamics of bacteria with ovoid cell shape (ovococci), which includes a number of important pathogens, by combining biochemical analyses with atomic force and super-resolution microscopies. Atomic force microscopy analysis showed preferential orientation of the peptidoglycan network parallel to the short axis of the cell, with distinct architectural features associated with septal and peripheral wall synthesis. Super-resolution three-dimensional structured illumination fluorescence microscopy was applied for the first time in bacteria to unravel the dynamics of peptidoglycan assembly in ovococci. The ovococci have a unique peptidoglycan architecture and growth mode not observed in other model organisms.

PBP5, PBP6 and DacD play different roles in intrinsic β-lactam resistance of Escherichia coli

Escherichia coli PBP5, PBP6 and DacD, encoded by dacA, dacC and dacD, respectively, share substantial amino acid identity and together constitute ~50 % of the total penicillin-binding proteins of E. coli. PBP5 helps maintain intrinsic β-lactam resistance within the cell. To test if PBP6 and DacD play simlar roles, we deleted dacC and dacD individually, and dacC in combination with dacA, from E. coli 2443 and compared β-lactam sensitivity of the mutants and the parent strain. β-Lactam resistance was complemented by wild-type, but not dd-carboxypeptidase-deficient PBP5, confirming that enzymic activity of PBP5 is essential for β-lactam resistance. Deletion of dacC and expression of PBP6 during exponential or stationary phase did not alter β-lactam resistance of a dacA mutant. Expression of DacD during mid-exponential phase partially restored β-lactam resistance of the dacA mutant. Therefore, PBP5 dd-carboxypeptidase activity is essential for intrinsic β-lactam resistance of E. coli and DacD can partially compensate for PBP5 in this capacity, whereas PBP6 cannot.

Beta-lactam antibiotics: from antibiosis to resistance and bacteriology

This review focuses on the era of antibiosis that led to a better understanding of bacterial morphology, in particular the cell wall component peptidoglycan. This is an effort to take readers on a tour de force from the concept of antibiosis, to the serendipity of antibiotics, evolution of beta-lactam development, and the molecular biology of antibiotic resistance. These areas of research have culminated in a deeper understanding of microbiology, particularly in the area of bacterial cell wall synthesis and recycling. In spite of this knowledge, which has enabled design of new even more effective therapeutics to combat bacterial infection and has provided new research tools, antibiotic resistance remains a worldwide health care problem.

A weak DD-carboxypeptidase activity explains the inability of PBP 6 to substitute for PBP 5 in maintaining normal cell shape in Escherichia coli

Penicillin-binding protein (PBP) 5 plays a critical role in maintaining normal cellular morphology in mutants of Escherichia coli lacking multiple PBPs. The most closely related homologue, PBP 6, is 65% identical to PBP 5, but is unable to substitute for PBP 5 in returning these mutants to their wild-type shape. The relevant differences between PBPs 5 and 6 are localized in a 20-amino acid stretch of domain I in these proteins, which includes the canonical KTG motif at the active site. We determined how these differences affected the enzymatic properties of PBPs 5 and 6 toward beta-lactam binding and the binding and hydrolysis of two peptide substrates. We also investigated the enzymatic properties of recombinant fusion proteins in which active site segments were swapped between PBPs 5 and 6. The results suggest that the in vivo physiological role of PBP 5 is distinguished from PBP 6 by the higher degree of DD-carboxypeptidase activity of the former.

Antimicrobial action of N-(n-dodecyl)diethanolamine on Escherichia coli: effects on enzymes and growing cultures

AIMS

This study investigates the effects of N-(n-dodecyl)diethanolamine (DDA) on enzymes and growing cells of Escherichia coli NCIMB 8277.

METHODS AND RESULTS

Enzyme activities in the presence of DDA were determined by measuring substrate-dependent oxygen consumption by whole cells, or of NADH formation or oxidation by cell extracts. Lysis of growing cells was followed by measuring changes in turbidity and cell count. DDA promptly arrested oxygen uptake on pyruvate and acetate, due to cofactor loss rather than to enzyme denaturation, since cell-free glyceraldehyde-3-phosphate and NADH dehydrogenases remained active. Formate and succinate oxidation by membrane-bound enzyme systems independent of cofactors was likewise unaffected. DDA lysed growing cells at rates related to drug concentration, pH, and the previous growth rate.

CONCLUSIONS

Loss of cellular enzyme activity following addition of DDA is due to cofactor leakage and not to enzyme denaturation. Whereas nongrowing cells remain intact in the presence of DDA, actively-growing organisms undergo lysis, consistent with autolysin action.

SIGNIFICANCE AND IMPACT OF THE STUDY

Cell lysis, not normally observed with membrane-active antimicrobials, also occurs with cetrimide, and may be dependent on the alkyl chain length in these compounds. The action on growing cells parallels that of penicillin and daptomycin, which bears a decanoyl residue that penetrates the cell membrane, causing leakage and membrane depolarization.

Specific labeling of peptidoglycan precursors as a tool for bacterial cell wall studies

Because of its importance for bacterial cell survival, the bacterial cell wall is an attractive target for new antibiotics in a time of great demand for new antibiotic compounds. Therefore, more knowledge about the diverse processes related to bacterial cell wall synthesis is needed. The cell wall is located on the exterior of the cell and consists mainly of peptidoglycan, a large macromolecule built up from a three-dimensional network of aminosugar strands interlinked with peptide bridges. The subunits of peptidoglycan are synthesized inside the cell before they are transported to the exterior in order to be incorporated into the growing peptidoglycan. The high flexibility of the cell wall synthesis machinery towards unnatural derivatives of these subunits enables research on the bacterial cell wall using labeled compounds. This review highlights the high potential of labeled cell wall precursors in various areas of cell wall research. Labeled precursors can be used in investigating direct cell wall-antibiotic interactions and in cell wall synthesis and localization studies. Moreover, these compounds can provide a powerful tool in the elucidation of the cell wall proteome, the "wallosome," and thus, might provide new targets for antibiotics.

Auxotrophy in rhizobia revisited

Among the various types of mutations studied in rhizobia, the auxotrophic mutations (which confer on the mutants the inability to synthesize certain essential substances such as amino acids, vitamins and nucleic acids), are the most favoured ones as these can be used as suitable markers for genetic analysis. An important property of rhizobia is their effectiveness i.e. their ability to fix atmospheric nitrogen into ammonia within the nodule. Special interest in this category of mutations by rhizobial geneticists is due to the fact that there is a strong correlation between the metabolic defects and the ineffectiveness (Nod(-) and/or Fix(-)) of the rhizobial strains. Auxotrophic mutants of various species of rhizobia with defects in the synthesis of nucleic bases, vitamins and amino acids have been obtained by mutagenising with physical, chemical and Tn5 mutagens. These mutants have been used in mapping studies as well as in establishing a correlation between its metabolic requirement and symbiotic relationship with the host plant. The present review deals with the isolation of auxotrophs, and their genetic, biochemical and symbiotic characterization. The review also encompasses the studies on the elucidation of biosynthetic pathways of nutritional substances in rhizobia.

Unbound and acylated structures of the MecR1 extracellular antibiotic-sensor domain provide insights into the signal-transduction system that triggers methicillin resistance

Methicillin-resistant Staphylococcus aureus (MRSA) strains are responsible for most hospital-onset bacterial infections. Lately, they have become a major threat to the community through infections of skin, soft tissue and respiratory tract, and subsequent septicaemia or septic shock. MRSA strains are resistant to most beta-lactam antibiotics (BLAs) as a result of the biosynthesis of a penicillin-binding protein with low affinity for BLAs, called PBP2a, PBP2' or MecA. This response is regulated by the chromosomal mec-divergon, which encodes a signal-transduction system including a transcriptional repressor, MecI, and a sensor/transducer, MecR1, as well as the structural mecA gene. This system is similar to those encoded by bla divergons in S. aureus and Bacillus licheniformis. MecR1 comprises an integral-membrane latent metalloprotease domain facing the cytosol and an extracellular sensor domain. The latter binds BLAs and transmits a signal through the membrane that eventually triggers activation of the metalloprotease moiety, which in turn switches off MecI-induced repression of mecA transcription. The MecR1 sensor domain, MecR1-PBD, reveals a two-domain structure of alpha/beta-type fold reminiscent of penicillin-binding proteins and beta-lactamases, and a catalytic serine residue as the ultimate cause for BLA-binding. Covalent complexes with benzylpenicillin and oxacillin provide evidence that serine acylation does not entail significant structural changes, thus supporting the hypothesis that additional extracellular segments of MecR1 are involved in signal transmission. The chemical nature of the residues shaping the active-site cleft favours stabilisation of the acyl enzyme complexes in MecR1-PBD, in contrast to the closely related OXA beta-lactamases, where the cleft is more likely to promote subsequent hydrolysis. The present structural data provide insights into the mec-encoded BLA-response mechanism and an explanation for kinetic differences in signal transmission with the related bla-encoded systems.

STREPTOCOCCAL L FORMS V. : Acid-Soluble Nucleotides of a Group A Streptococcus and Derived L Form

Edwards, John (University of Illinois College of Medicine, Chicago, and Albert Einstein Medical Center, Philadelphia, Pa.) and Charles Panos. Streptococcal L-forms. V. Acid-soluble nucleotides of a group A Streptococcus and derived L form. J. Bacteriol. 84:1202-1208. 1962.-This report deals with a comparison of the acid-soluble nucleotides from a group A, type 12, beta-hemolytic streptococcus and a derived stable L form. This is the first report of the presence of a cell-wall precursor (uridine diphosphate-muramic acid-peptide) in a stable L form. Cells of each organism were obtained during their logarithmic phases of growth, harvested by centrifugation, and washed with osmotically protective NaCl solutions. The analytical procedures were essentially those of Franzen and Binkley. Calculated values indicated that these results could not be accounted for by dry-weight differences due to loss of the streptococcal cell wall. It was found that both organisms contained the same amount of total nucleotide material. The L form contained no uridine monophosphate (UMP), a large concentration of uridine diphosphate (UDP)-muramic acid-peptide, and a significant increase of UDP-N-acetylglucosamine. A similar nucleotide containing muramic acid-peptide was not demonstrable in the parent coccus. Instead, UMP and an unidentified uridine nucleotide were resolved in this region. Analyses of extracts from this streptococcal L form indicate the probable presence of two of the three nucleotides originally isolated by Park from penicillin-treated Staphylococcus aureus. The presence of the UDP-muramic acid-peptide cell-wall precursor in the L form cultured in the continual absence of penicillin points to an inability of this form to resynthesize the rigid cell wall and indicates that this synthetic mechanism has been permanently impaired.

STABILIZATION OF PROTOPLASTS AND SPHEROPLASTS BY SPERMINE AND OTHER POLYAMINES

Tabor, Celia W. (National Institute of Arthritis and Metabolic Diseases, Bethesda, Md.). Stabilization of protoplasts and spheroplasts by spermine and other polyamines. J. Bacteriol. 83:1101-1111. 1962.-Spermine (10(-3)m) or spermidine prevents lysis of lysozyme-produced protoplasts of Escherichia coli W, E. coli B, and Micrococcus lysodeikticus in hypotonic media. Spheroplasts prepared by the action of penicillin are also stabilized by these concentrations of spermine and spermidine, but the protection is not as complete. Streptomycin, polylysine, and Ca(++) are also effective or partially effective stabilizers, but 1,4-diaminobutane, 1,5-diaminopentane, ornithine, Mg(++), and monovalent cations have no protective action at 10(-3)m concentration, and only a slight effect at higher concentrations. The osmotic stability conferred on protoplasts by spermine is irreversible. However, the protective effect of polyamines against lysis is not accompanied by restoration of viability to lysozyme protoplasts. There is a marked reduction in the loss of ultra-violet-absorbing material from the protoplasts to the medium when 10(-3)m spermine is present.

THE TORTUOUS JOURNEY OF A BIOCHEMIST TO IMMUNOLAND AND WHAT HE FOUND THERE

After starting out to become a physician, by a series of accidents I found myself at NIH in 1951 during its most productive growth phase. At age 26, I had a fully funded, independent laboratory and did not know what to work on. With advice from colleagues, I initiated a study of how penicillin kills bacteria. Twenty years later, my lab had outlined the structure and biosynthesis of the peptidoglycan of bacterial cell walls and had discovered that penicillin inhibited the terminal step in its biosynthesis catalyzed by transpeptidases. I then switched fields, moving to Harvard in 1968 and beginning the study of human HLA proteins. Twenty-five years later, the last half of which was spent in a stimulating collaboration with the late Don Wiley, our labs had isolated, crystallized, and elucidated the three-dimensional structures of these molecules and shown that their principal function was to present peptides to the immune system in initiating an immune response. More recently, the laboratory has focused on natural killer cells and their roles in peripheral blood and in the pregnant uterine decidua. It has been a wonderful scientific journey.

Extraction and identification by mass spectrometry of undecaprenyl diphosphate-MurNAc-pentapeptide-GlcNAc from Escherichia coli

Undecaprenyl diphosphate-MurNAc-pentapeptide-GlcNAc (lipid II) is extracted from Escherichia coli cells by utilizing its unusual pH-dependent solubility property in a Bligh-Dyer system, and identified by electrospray ionization mass spectrometry in conjunction with a novel 15N mass shift analysis. The described approach will facilitate the structural characterization of lipid II variants from diverse bacteria, including antibiotic-resistant mutants, as well as the numerous minor uncharacterized lipids present in all biological systems.

Genomic filtering: an approach to discovering novel antiparasitics

Genomic filtering is a rapid approach to identifying and prioritizing molecular targets for drug discovery. For infectious disease applications, comparative genomics filters allow the selection of pathogen-specific gene products, whereas functional genomics filters, such as RNA interference (RNAi), allow the selection of gene products essential for pathogen survival. The approach is especially applicable to antiparasitic drug discovery where the phylogenetic distance between parasite and host make the likelihood of drug cross-toxicity due to conservation of molecular targets greater than for more distantly related pathogens such as prokaryotes. This article discusses some of the inherent challenges of applying genomics to the early steps of drug discovery and describes one successful comparative and functional genomics filtering strategy that has been implemented to prioritize molecular targets and identify chemical leads for nematode control.

HPLC method for simultaneous determination of cefprozil diastereomers in human plasma

A high-performance liquid chromatography method was developed for the determination of cefprozil diastereomers in human plasma. Cefprozil exists as cis and trans isomer at the ratio of 90:10. Plasma samples were prepared by protein precipitation using acetonitrile, trichloroacetic acid and methylene chloride. After the mixtures were vortexed and centrifuged, the aqueous supernatant was injected into a reversed-phase C8 column. The mobile phase consisted of acetonitrile, glacial acetic acid and distilled water at the volume ratio of 5.5:1.75:92.75 (pH 2.7). The signals were monitored with UV detection at 280 nm. The calibration curves of cis and trans isomer were linear in concentration ranges of 0.1-25 and 0.02-2.5 microg/mL with the correlation coefficient of 0.9999 and 0.9989, respectively. After oral administration of cefprozil in humans, Cmax and Tmax of total cefprozil were 18.80 +/- 2.14 microg/mL and 2.06 +/- 0.62 h. This method was sensitive with excellent selectivity and reproducibility, and successfully applied to a bioavailability study of cefprozil in healthy subjects.