Suppression of lytic effect of beta lactams on Escherichia coli and other bacteria

Urinary pH and antibiotics, choose carefully. A systematic review

Uncomplicated urinary tract infection (UTI) is the most common bacterial infection in women. Since 1948, the relationship between urinary pH and antibiotics (ABs) has been established. We aimed to search for the best urinary pH for each family of antibiotics and to assess whether pH changes bacterial susceptibility to them. We included in vitro research and in vivo studies including one or more bacterial species and tested the effect of one or more ABs at different pH values. We also included randomized controlled clinical trials (RCTs) in uncomplicated UTI (EAU guidelines 2019 definition), choosing the ABs based on urinary pH or using an antibiotic plus urinary pH modifiers (L-methionine, vitamin C…) vs. an antibiotic and a placebo. Quadas-2 tool was used as a quality assessment of the studies and PRISMA set of items for systematic reviews. Two authors independently screened and evaluated the papers, while two additional authors individually repeated the search. A fifth researcher acted as an arbiter, and another author collaborated as a hospital pharmaceutical consultant. Alkaline-friendly antibiotics are most fluoroquinolones, aminoglycosides, trimethoprim. Acidic-friendly antibiotics are fosfomycin, tetracycline, nitrofurantoin and some β-lactams. We suggest performing urine cultures with antibiogram tests, in both acidic and alkaline media, to define the bacterial susceptibility profile. There is insufficient in vivo evidence to support whether choosing an antibiotic based on a patient's urinary pH or adding urinary pH modifiers will lead to a higher cure rate.

Divergent Effects of Peptidoglycan Carboxypeptidase DacA on Intrinsic β-Lactam and Vancomycin Resistance

Vancomycin and β-lactams are clinically important antibiotics that inhibit the formation of peptidoglycan cross-links, but their binding targets are different. The binding target of vancomycin is d-alanine-d-alanine (d-Ala-d-Ala), whereas that of β-lactam is penicillin-binding proteins (PBPs). In this study, we revealed the divergent effects of peptidoglycan (PG) carboxypeptidase DacA on vancomycin and β-lactam resistance in Escherichia coli and Bacillus subtilis. The deletion of DacA induced sensitivity to most β-lactams, whereas it induced strong resistance toward vancomycin. Notably, both phenotypes did not have a strong association with ld-transpeptidases, which are necessary for the formation of PG 3-3 cross-links and covalent bonds between PG and an Lpp outer membrane (OM) lipoprotein. Vancomycin resistance was induced by an increased amount of decoy d-Ala-d-Ala residues within PG, whereas β-lactam sensitivity was associated with physical interactions between DacA and PBPs. The presence of an OM permeability barrier strongly strengthened vancomycin resistance, but it significantly weakened β-lactam sensitivity. Collectively, our results revealed two distinct functions of DacA, which involved inverse modulation of bacterial resistance to clinically important antibiotics, β-lactams and vancomycin, and presented evidence for a link between DacA and PBPs.

IMPORTANCE Bacterial PG hydrolases play important roles in various aspects of bacterial physiology, including cytokinesis, PG synthesis, quality control of PG, PG recycling, and stress adaptation. Of all the PG hydrolases, the role of PG carboxypeptidases is poorly understood, especially regarding their impacts on antibiotic resistance. We have revealed two distinct functions of PG carboxypeptidase DacA with respect to antibiotic resistance. The deletion of DacA led to sensitivity to most β-lactams, while it caused strong resistance to vancomycin. Our study provides novel insights into the roles of PG carboxypeptidases in the regulation of antibiotic resistance and a potential clue for the development of a drug to improve the clinical efficacy of β-lactam antibiotics.

Physical properties of the bacterial outer membrane

It has long been appreciated that the Gram-negative outer membrane acts as a permeability barrier, but recent studies have uncovered a more expansive and versatile role for the outer membrane in cellular physiology and viability. Owing to recent developments in microfluidics and microscopy, the structural, rheological and mechanical properties of the outer membrane are becoming apparent across multiple scales. In this Review, we discuss experimental and computational studies that have revealed key molecular factors and interactions that give rise to the spatial organization, limited diffusivity and stress-bearing capacity of the outer membrane. These physical properties suggest broad connections between cellular structure and physiology, and we explore future prospects for further elucidation of the implications of outer membrane construction for cellular fitness and survival.

Understanding tolerance to cell wall-active antibiotics

Antibiotic tolerance-the ability of bacteria to survive for an extended time in the presence of bactericidal antibiotics-is an understudied contributor to antibiotic treatment failure. Herein, I review the manifestations, mechanisms, and clinical relevance of tolerance to cell wall-active (CWA) antibiotics, one of the most important groups of antibiotics at the forefront of clinical use. I discuss definitions of tolerance and assays for tolerance detection, comprehensively discuss the mechanism of action of β-lactams and other CWA antibiotics, and then provide an overview of how cells mitigate the potentially lethal effects of CWA antibiotic-induced cell damage to become tolerant. Lastly, I discuss evidence for a role of CWA antibiotic tolerance in clinical antibiotic treatment failure.

Image-Based Dynamic Phenotyping Reveals Genetic Determinants of Filamentation-Mediated β-Lactam Tolerance

Antibiotic tolerance characterized by slow killing of bacteria in response to a drug can lead to treatment failure and promote the emergence of resistance. β-lactam antibiotics inhibit cell wall growth in bacteria and many of them cause filamentation followed by cell lysis. Hence delayed cell lysis can lead to β-lactam tolerance. Systematic discovery of genetic factors that affect β-lactam killing kinetics has not been performed before due to challenges in high-throughput, dynamic analysis of viability of filamented cells during bactericidal action. We implemented a high-throughput time-resolved microscopy approach in a gene deletion library of Escherichia coli to monitor the response of mutants to the β-lactam cephalexin. Changes in frequency of lysed and intact cells due to the antibiotic action uncovered several strains with atypical lysis kinetics. Filamentation confers tolerance because antibiotic removal before lysis leads to recovery through numerous concurrent divisions of filamented cells. Filamentation-mediated tolerance was not associated with resistance, and therefore this phenotype is not discernible through most antibiotic susceptibility methods. We find that deletion of Tol-Pal proteins TolQ, TolR, or Pal but not TolA, TolB, or CpoB leads to rapid killing by β-lactams. We also show that the timing of cell wall degradation determines the lysis and killing kinetics after β-lactam treatment. Altogether, this study uncovers numerous genetic determinants of hitherto unappreciated filamentation-mediated β-lactam tolerance and support the growing call for considering antibiotic tolerance in clinical evaluation of pathogens. More generally, the microscopy screening methodology described here can easily be adapted to study lysis in large numbers of strains.

Plasticity of Escherichia coli cell wall metabolism promotes fitness and antibiotic resistance across environmental conditions

Although the peptidoglycan cell wall is an essential structural and morphological feature of most bacterial cells, the extracytoplasmic enzymes involved in its synthesis are frequently dispensable under standard culture conditions. By modulating a single growth parameter-extracellular pH-we discovered a subset of these so-called 'redundant' enzymes in Escherichia coli are required for maximal fitness across pH environments. Among these pH specialists are the class A penicillin binding proteins PBP1a and PBP1b; defects in these enzymes attenuate growth in alkaline and acidic conditions, respectively. Genetic, biochemical, and cytological studies demonstrate that synthase activity is required for cell wall integrity across a wide pH range and influences pH-dependent changes in resistance to cell wall active antibiotics. Altogether, our findings reveal previously thought to be redundant enzymes are instead specialized for distinct environmental niches. This specialization may ensure robust growth and cell wall integrity in a wide range of conditions.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Hydrolysis of peptidoglycan is modulated by amidation of meso-diaminopimelic acid and Mg2+ in Bacillus subtilis

The ability of excess Mg²⁺ to compensate the absence of cell wall related genes in Bacillus subtilis has been known for a long time, but the mechanism has remained obscure. Here, we show that the rigidity of wild‐type cells remains unaffected with excess Mg²⁺, but the proportion of amidated meso‐diaminopimelic (mDAP) acid in their peptidoglycan (PG) is significantly reduced. We identify the amidotransferase AsnB as responsible for mDAP amidation and show that the gene encoding it is essential without added Mg²⁺. Growth without excess Mg²⁺ causes ΔasnB mutant cells to deform and ultimately lyse. In cell regions with deformations, PG insertion is orderly and indistinguishable from the wild‐type. However, PG degradation is unevenly distributed along the sidewalls. Furthermore, ΔasnB mutant cells exhibit increased sensitivity to antibiotics targeting the cell wall. These results suggest that absence of amidated mDAP causes a lethal deregulation of PG hydrolysis that can be inhibited by increased levels of Mg²⁺. Consistently, we find that Mg²⁺ inhibits autolysis of wild‐type cells. We suggest that Mg²⁺ helps to maintain the balance between PG synthesis and hydrolysis in cell wall mutants where this balance is perturbed in favor of increased degradation.

De novo morphogenesis in L-forms via geometric control of cell growth

In virtually all bacteria, the cell wall is crucial for mechanical integrity and for determining cell shape. Escherichia coli's rod-like shape is maintained via the spatiotemporal patterning of cell-wall synthesis by the actin homologue MreB. Here, we transiently inhibited cell-wall synthesis in E. coli to generate cell-wall-deficient, spherical L-forms, and found that they robustly reverted to a rod-like shape within several generations after inhibition cessation. The chemical composition of the cell wall remained essentially unchanged during this process, as indicated by liquid chromatography. Throughout reversion, MreB localized to inwardly curved regions of the cell, and fluorescent cell wall labelling revealed that MreB targets synthesis to those regions. When exposed to the MreB inhibitor A22, reverting cells regrew a cell wall but failed to recover a rod-like shape. Our results suggest that MreB provides the geometric measure that allows E. coli to actively establish and regulate its morphology.

Mechanisms of β-lactam killing and resistance in the context of Mycobacterium tuberculosis

β-Lactams are one of the most useful classes of antibiotics against many common bacterial pathogens. One exception is Mycobacterium tuberculosis. However, with increasing incidence of multidrug-resistant tuberculosis and a need for new agents to treat it, the use of β-lactams, specifically the combination of carbapenem and clavulanate, is now being revisited. With this attention, comes the need to better understand both the mechanisms of action of β-lactams against M. tuberculosis as well as possible mechanisms of resistance, within the context of what is known about the β-lactam action in other bacteria. M. tuberculosis has two major mechanisms of intrinsic resistance: a highly active β-lactamase and a poorly permeable outer membrane. Within the cell wall, β-lactams bind several enzymes with differing peptidoglycan-synthetic and -lytic functions. The inhibition of these enzymes may lead to cell death through several mechanisms, involving disruption of the balance of synthetic and lethal activities. Currently, all known means of resistance to the β-lactams rely on diminishing the proportion of peptidoglycan-synthetic proteins bound and inhibited by β-lactams, through either exclusion or destruction of the antibiotic, or through replacement or supplementation of target enzymes. In this review, we discuss possible mechanisms for β-lactam activity in M. tuberculosis and the means by which it may acquire resistance, within the context of what is known in other bacterial species.

The role of hydrolases in bacterial cell-wall growth

Although hydrolysis is known to be as important as synthesis in the growth and development of the bacterial cell wall, the coupling between these processes is not well understood. Bond cleavage can generate deleterious pores, but may also be required for the incorporation of new material and for the expansion of the wall, highlighting the importance of mechanical forces in interpreting the consequences of hydrolysis in models of growth. Critically, minimal essential subsets of hydrolases have now been identified in several model organisms, enabling the reduction of genetic complexity. Recent studies in Bacillus subtilis have provided evidence for both the presence and absence of coupling between synthesis and hydrolysis during sporulation and elongation, respectively. In this review, we discuss strategies for dissecting the relationship between synthesis and hydrolysis using time-lapse imaging, biophysical measurements of cell-wall architecture, and computational modeling.

Cell death of Streptococcus mutans induced by a quorum-sensing peptide occurs via a conserved streptococcal autolysin

Streptococcus mutans, a member of the human indigenous oral microbiome, produces a quorum-sensing peptide called the competence-stimulating peptide (CSP) pheromone. We previously demonstrated that S. mutans expresses its CSP pheromone under specific stresses and responds to high levels of CSP by inducing cell death in a fraction of the bacterial population. Streptococci lack the classical SOS response, and the induction of the SigX regulon has been proposed to act as a general stress response in Gram-positive bacteria. We show here that inactivation of SigX abolished the CSP-induced cell death phenotype. Among SigX-regulated genes, SMU.836 (now named lytF(Sm)), encoding a conserved streptococcal protein, is a functional peptidoglycan hydrolase involved in CSP-induced cell lysis. We also demonstrated that LytF(Sm) is most likely a self-acting autolysin, since LytF(Sm) produced by attacker cells cannot trigger CSP-induced lysis of LytF(Sm)-deficient target cells present in the same environment. Electron microscopy revealed important morphological changes accompanying autolysis of CSP-induced wild-type cultures that were absent in the LytF(Sm)-deficient mutant. The LytF(Sm) promoter was activated in the physiological context of elevated concentrations of the CSP pheromone under stress conditions, such as exposure to heat, hydrogen peroxide, and acid. In a long-term survival assay, the viability of a mutant deficient in LytF(Sm) autolysin was significantly lower than that observed for the wild-type strain. The results of this study suggest that cell death of S. mutans induced by its quorum-sensing CSP pheromone may represent a kind of altruistic act that provides a way for the species to survive environmental stresses at the expense of some of its cells.

Heterogeneous bacterial persisters and engineering approaches to eliminate them

Bacterial persistence is a state in which a subpopulation of cells (persisters) survives antibiotic treatment, and has been implicated in the tolerance of clinical infections and the recalcitrance of biofilms. There has been a renewed interest in the role of bacterial persisters in treatment failure in light of a wealth of recent findings. Here we review recent laboratory studies of bacterial persistence. Further, we pose the hypothesis that each bacterial population may contain a diverse collection of persisters and discuss engineering strategies for persister eradication.

Comparative in vitro activities of the novel antibacterial finafloxacin against selected Gram-positive and Gram-negative bacteria tested in Mueller-Hinton broth and synthetic urine

Kill kinetics and MICs of finafloxacin and ciprofloxacin against 34 strains with defined resistance mechanisms grown in cation-adjusted Mueller-Hinton broth (CAMHB) at pH values of 7.2 and 5.8 and in synthetic urine at pH 5.8 were determined. In general, finafloxacin gained activity at low pH values in CAMHB and remained almost unchanged in artificial urine. Ciprofloxacin MICs increased and bactericidal activity decreased strain dependently in acidic CAMHB and particularly in artificial urine.

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.

LytM-domain factors are required for daughter cell separation and rapid ampicillin-induced lysis in Escherichia coli

Bacterial cytokinesis is coupled to the localized synthesis of new peptidoglycan (PG) at the division site. This newly generated septal PG is initially shared by the daughter cells. In Escherichia coli and other gram-negative bacteria, it is split shortly after it is made to promote daughter cell separation and allow outer membrane constriction to closely follow that of the inner membrane. We have discovered that the LytM (lysostaphin)-domain containing factors of E. coli (EnvC, NlpD, YgeR, and YebA) are absolutely required for septal PG splitting and daughter cell separation. Mutants lacking all LytM factors form long cell chains with septa containing a layer of unsplit PG. Consistent with these factors playing a direct role in septal PG splitting, both EnvC-mCherry and NlpD-mCherry fusions were found to be specifically recruited to the division site. We also uncovered a role for the LytM-domain factors in the process of beta-lactam-induced cell lysis. Compared to wild-type cells, mutants lacking LytM-domain factors were delayed in the onset of cell lysis after treatment with ampicillin. Moreover, rather than lysing from midcell lesions like wild-type cells, LytM(-) cells appeared to lyse through a gradual loss of cell shape and integrity. Overall, the phenotypes of mutants lacking LytM-domain factors bear a striking resemblance to those of mutants defective for the N-acetylmuramyl-l-alanine amidases: AmiA, AmiB, and AmiC. E. coli thus appears to rely on two distinct sets of putative PG hydrolases to promote proper cell division.

Cell shape and cell-wall organization in Gram-negative bacteria

In bacterial cells, the peptidoglycan cell wall is the stress-bearing structure that dictates cell shape. Although many molecular details of the composition and assembly of cell-wall components are known, how the network of peptidoglycan subunits is organized to give the cell shape during normal growth and how it is reorganized in response to damage or environmental forces have been relatively unexplored. In this work, we introduce a quantitative physical model of the bacterial cell wall that predicts the mechanical response of cell shape to peptidoglycan damage and perturbation in the rod-shaped Gram-negative bacterium Escherichia coli. To test these predictions, we use time-lapse imaging experiments to show that damage often manifests as a bulge on the sidewall, coupled to large-scale bending of the cylindrical cell wall around the bulge. Our physical model also suggests a surprising robustness of cell shape to peptidoglycan defects, helping explain the observed porosity of the cell wall and the ability of cells to grow and maintain their shape even under conditions that limit peptide crosslinking. Finally, we show that many common bacterial cell shapes can be realized within the same model via simple spatial patterning of peptidoglycan defects, suggesting that minor patterning changes could underlie the great diversity of shapes observed in the bacterial kingdom.

Molecular control of bacterial death and lysis

Although the phenomenon of bacterial cell death and lysis has been studied for over 100 years, the contribution of these important processes to bacterial physiology and development has only recently been recognized. Contemporary study of cell death and lysis in a number of different bacteria has revealed that these processes, once thought of as being passive and unregulated, are actually governed by highly complex regulatory systems. An emerging paradigm in this field suggests that, analogous to programmed cell death in eukaryotes, regulated cell death and lysis in bacteria play an important role in both developmental processes, such as competence and biofilm development, and the elimination of damaged cells, such as those irreversibly injured by environmental or antibiotic stress. Further study in this exciting field of bacterial research may provide new insight into the potential evolutionary link between control of cell death in bacteria and programmed cell death (apoptosis) in eukaryotes.

Bacterial peptidoglycan (murein) hydrolases

Most bacteria have multiple peptidoglycan hydrolases capable of cleaving covalent bonds in peptidoglycan sacculi or its fragments. An overview of the different classes of peptidoglycan hydrolases and their cleavage sites is provided. The physiological functions of these enzymes include the regulation of cell wall growth, the turnover of peptidoglycan during growth, the separation of daughter cells during cell division and autolysis. Specialized hydrolases enlarge the pores in the peptidoglycan for the assembly of large trans-envelope complexes (pili, flagella, secretion systems), or they specifically cleave peptidoglycan during sporulation or spore germination. Moreover, peptidoglycan hydrolases are involved in lysis phenomena such as fratricide or developmental lysis occurring in bacterial populations. We will also review the current view on the regulation of autolysins and on the role of cytoplasm hydrolases in peptidoglycan recycling and induction of beta-lactamase.

Antimicrobial mechanisms of ortho-phthalaldehyde action

Biocides generally have multiple biochemical targets. Such a feature easily entangles the analysis of the mechanisms of antimicrobial action. In this study, the action of the dialdehyde biocide ortho-phtalaldehyde (OPA), on bacteria, was investigated using the Gram-negative Pseudomonas fluorescens. The targets of the biocide action were studied using different bacterial physiological indices. The respiratory activity, membrane permeabilization, physico-chemical characterization of the bacterial surfaces, outer membrane proteins (OMP) expression, concomitant influence of pH, contact time and presence of bovine serum albumin (BSA) on respiratory activity, morphological changes and OPA-DNA interactions were assessed for different OPA concentrations. With the process conditions used, the minimum inhibitory concentration was 1500 mg/l, the concentration to promote total loss of bacterial culturability was 65 mg/l and the concentration needed to inactivate respiratory activity was 80 mg/l. These data are evidence that culturability and respiratory activity were markedly affected by the biocide. OPA lead, moreover, to a significant change in cell surface hydrophobicity and induced propidium iodide uptake. Such results suggest cytoplasmic membrane damage, although no release of ATP was detected. At pH 5, the bactericidal action of OPA was stronger, though not influenced by BSA presence. Nevertheless, at pH 9, BSA noticeably (p < 0.05) impaired biocide action. A time-dependent effect in OPA action was evident when contemplating respiratory activity variation, mainly for the lower exposure times. Scanning electron microscopy allowed to detect bacterial morphological changes, translated on cellular elongation, for OPA concentrations higher than 100 mg/l. Interferences at DNA level were, however, restricted to extreme biocide concentrations. The overall bactericidal events occurred without detectable OMP expression changes. In conclusion, the results indicated a sequence of events responsible for the antimicrobial action of OPA: it binds to membrane receptors due to cross-linkage; impairs the membrane functions allowing the biocide to enter through the permeabilized membrane; it interacts with intracellular reactive molecules, such as RNA, compromising the growth cycle of the cells and, at last, with DNA.

Overproduction of inactive variants of the murein synthase PBP1B causes lysis in Escherichia coli

Penicillin-binding protein 1B (PBP1B) of Escherichia coli is a bifunctional murein synthase containing both a transpeptidase domain and a transglycosylase domain. The protein is present in three forms (alpha, beta, and gamma) which differ in the length of their N-terminal cytoplasmic region. Expression plasmids allowing the production of native PBP1B or of PBP1B variants with an inactive transpeptidase or transglycosylase domain or both were constructed. The inactive domains contained a single amino acid exchange in an essential active-site residue. Overproduction of the inactive PBP1B variants, but not of the active proteins, caused lysis of wild-type cells. The cells became tolerant to lysis by inactive PBP1B at a pH of 5.0, which is similar to the known tolerance for penicillin-induced lysis under acid pH conditions. Lysis was also reduced in mutant strains lacking several murein hydrolases. In particular, a strain devoid of activity of all known lytic transglycosylases was virtually tolerant, indicating that mainly the lytic transglycosylases are responsible for the observed lysis effect. A possible structural interaction between PBP1B and murein hydrolases in vivo by the formation of a multienzyme complex is discussed.

Identification and molecular analysis of PcsB, a protein required for cell wall separation of group B streptococcus

Group B streptococcus (GBS) is the leading cause of bacterial sepsis and meningitis in neonates. N-terminal sequencing of major proteins in the culture supernatant of a clinical isolate of GBS identified a protein of about 50 kDa which could be detected in all of 27 clinical isolates tested. The corresponding gene, designated pcsB, was isolated from a GBS cosmid library and subsequently sequenced. The deduced PcsB polypeptide consists of 447 amino acid residues (M(r), 46,754), carries a potential N-terminal signal peptide sequence of 25 amino acids, and shows significant similarity to open reading frames of unknown function from different organisms and to the murein hydrolase P45 from Listeria monocytogenes. Northern blot analysis revealed a monocistronic transcriptional organization for pcsB in GBS. Insertional inactivation of pcsB in the genome of GBS resulted in mutant strain Sep1 exhibiting a drastically reduced growth rate compared to the parental GBS strain and showing an increased susceptibility to osmotic pressure and to various antibiotics. Electron microscopic analysis of GBS mutant Sep1 revealed growth in clumps, cell separation in several planes, and multiple division septa within single cells. These data suggest a pivotal role of PcsB for cell division and antibiotic tolerance of GBS.

From growth to autolysis: the murein hydrolases in Escherichia coli

Murein hydrolases cleave bonds in the bacterial exoskeleton, the murein (peptidoglycan) sacculus, a covalently closed bag-shaped polymer made of glycan strands that are crosslinked by peptides. During growth and division of a bacterial cell, these enzymes are involved in the controlled metabolism of the murein sacculus. Murein hydrolases are believed to function as pacemaker enzymes for the enlargement of the murein sacculus since opening of bonds in the murein net is needed to allow the insertion of new subunits into the sacculus. Furthermore, they are responsible for splitting the septum during cell division. The murein turnover products that are released during growth are further degraded by these (1 --> 6)-anhydromuramic acid derivatives by an intramolecular transglycosylation reaction.

Bacteriophage lysis: mechanism and regulation

Bacteriophage lysis involves at least two fundamentally different strategies. Most phages elaborate at least two proteins, one of which is a murein hydrolase, or lysin, and the other is a membrane protein, which is given the designation holin in this review. The function of the holin is to create a lesion in the cytoplasmic membrane through which the murein hydrolase passes to gain access to the murein layer. This is necessary because phage-encoded lysins never have secretory signal sequences and are thus incapable of unassisted escape from the cytoplasm. The holins, whose prototype is the lambda S protein, share a common organization in terms of the arrangement of charged and hydrophobic residues, and they may all contain at least two transmembrane helical domains. The available evidence suggests that holins oligomerize to form nonspecific holes and that this hole-forming step is the regulated step in phage lysis. The correct scheduling of the lysis event is as much an essential feature of holin function as is the hole formation itself. In the second strategy of lysis, used by the small single-stranded DNA phage phi X174 and the single-stranded RNA phage MS2, no murein hydrolase activity is synthesized. Instead, there is a single species of small membrane protein, unlike the holins in primary structure, which somehow causes disruption of the envelope. These lysis proteins function by activation of cellular autolysins. A host locus is required for the lytic function of the phi X174 lysis gene E.

The antibacterial peptide seminal plasmin alters permeability of the inner membrane of E. coli

Seminal plasmin (SPLN) a 47-residue peptide, isolated from bovine seminal plasma, exhibits antibacterial activity against Gram-positive and Gram-negative bacteria. Although SPLN strongly inhibits the transcription of various natural and synthetic templates by E. coli RNA polymerase in vitro, it also associates with model membranes of phosphatidylcholine and phosphatidic acid. We have undertaken experiments to ascertain whether SPLN permeabilizes the bacterial inner membrane and thereby exerts its antibacterial activity, as in the case of recently isolated antibacterial peptides from mammalian sources. Our results show that SPLN affects the permeability properties of the bacterial inner membrane which is reflected by increased uptake of ortho-nitrophenylgalactoside (ONPG), which can normally be translocated only by protein transporters. SPLN has also been shown to act on the outer membrane, since divalent cations inhibit antibacterial activity.

Specific localization of the lysis protein of bacteriophage MS2 in membrane adhesion sites of Escherichia coli

Specific localization of the lysis (L) protein of bacteriophage MS2 in the cell wall of Escherichia coli was determined by immunoelectron microscopy. After induction of the cloned lysis gene, the cells were plasmolyzed, fixed, and embedded in either Epon or Lowicryl K4M. Polyclonal L-protein-specific antiserum was purified by preabsorption to membranes from cells harboring a control plasmid. Protein A-gold was used to label the protein-antibody complexes. Between 42.8% (Lowicryl) and 33.8% (Epon) of the label was found in inner and outer membranes, but 30.3% (Lowicryl) and 32.8% (Epon) was present mostly in clusters in the adhesion sites visible after plasmolysis. The remaining label (26.9 and 33.4%, respectively) appeared to be present in the periplasmic space but may also have been part of membrane junctions not visible because of poor contrast of the specimen. In contrast, a quite different distribution of the L protein was found in cells grown under conditions of penicillin tolerance, i.e., at pH 5, a condition that had previously been shown to protect cells from L-protein-induced lysis. At tolerant conditions, only 21.0% of the L protein was in the adhesion sites; most of the protein (68.2%) was found in inner and outer membranes. It is concluded that lysis of the host, E. coli, was a result of the formation of specific L-protein-mediated membrane adhesion sites.

Influence of antibiotic dose, dosing interval, and duration of therapy on outcome in experimental pneumococcal meningitis in rabbits

We examined the influence of several pharmacokinetic parameters on cure rates in rabbits with experimental pneumococcal meningitis. When the duration of treatment was kept constant, cure rates improved as the individual dose of ampicillin was increased. On the other hand, when four doses of ampicillin at 60 mg/kg of body weight, producing peak concentrations in cerebrospinal fluid (CSF) of approximately 40 times the MBC, were administered at intervals of 24 instead of 4 h and the duration of therapy was thus prolonged from 12 to 72 h, cure rates also increased (85 versus 25%; P less than 0.01). These high cure rates were achieved even though bacterial titers in CSF 24 h after the first dose had reached levels similar to those present at the beginning of therapy. Cure in these animals was explained by the fact that the second ampicillin dose reduced bacterial titers in CSF significantly more than did the first dose (5.2 versus 2.5 log10 CFU/ml; P less than 0.02). The clinical relevance of these observations remains to be determined.

Induction of the autolytic system of Escherichia coli by specific insertion of bacteriophage MS2 lysis protein into the bacterial cell envelope

Bacterial lysis induced by the expression of the cloned lysis gene of the RNA bacteriophage MS2 in Escherichia coli was shown to be under the same regulatory control mechanisms as penicillin-induced lysis. It was controlled by the stringent response and showed the phenomenon of tolerance when E. coli was grown at pH 5. Changes in the fine structure of the murein were found to be the earliest physiological changes in the cell, taking place 10 min before the onset of cellular lysis and inhibition of murein synthesis. Both the average length of the glycan strands and, with a time lag, the degree of cross-linkage were altered, indicating that a lytic transglycosylase and a DD-endopeptidase had been triggered. After extensive separation of the membranes by isopycnic sucrose gradient centrifugation, the lysis protein was present predominantly in the cytoplasmic membrane and in a fraction of intermediate density and, to a lesser degree, in the outer membrane, irrespective of the conditions of growth. However, only under lysis-permissive conditions could a 17% increase in the number of adhesion sites between the inner and outer membranes be observed. Thus, a casual relationship between lysis and the formation of lysis protein-induced adhesion sites seems to exist.

Influence of inflammation on the efficacy of antibiotic treatment of experimental pyelonephritis

An acute exudative Escherichia coli pyelonephritis rat model was used to study the influence of progressive pyelonephritis on the efficacy of antibiotic treatment. In this model, transient ureteral obstruction after E. coli bladder inoculation induces early bacterial multiplication in the kidney parenchyma, and the bacterial counts peak by 48 h. The inflammatory response (assessed by the increase in kidney weight) is somewhat delayed, starting 36 h after inoculation and peaking by 72 h. Groups of rats received 4 doses over 48 h of saline, ceftriaxone (100 mg/kg), or ceftriaxone (100 mg/kg) plus gentamicin (4 mg/kg). These treatments were initiated 24, 36, 48, or 72 h after bladder inoculation. Antibiotic treatment started at 24 h was significantly more effective in reducing bacterial counts in the kidney parenchyma than at any later therapy onset. Only when started 24 h after inoculation was the synergistic combination of ceftriaxone plus gentamicin more effective in reducing bacterial counts than ceftriaxone alone. Ceftriaxone and ceftriaxone plus gentamicin regimens started at 24 h reduced significantly (by 42 and 55%, respectively) the incidence of acute exudative pyelonephritis when compared with the incidence in saline-treated controls. Early therapy onset (24 h) strikingly reduced the development of the inflammatory response. This reduction was less marked when antibiotic therapy was started at 36 h and no longer apparent when therapy onset was delayed up to 48 or 72 h. In conclusion, the efficacy of antibiotics in eradicating bacteria from the kidney parenchyma and in preventing acute exudative pyelonephritis was markedly hampered by the development of pyelonephritis.

Correlation between growth curve and killing curve of Escherichia coli after a brief exposure to suprainhibitory concentrations of ampicillin and piperacillin

Escherichia coli strains that were susceptible to multiple antibiotics were exposed to suprainhibitory concentrations of ampicillin and piperacillin. As with the majority of beta-lactam antibiotics, the growth curves showed an increase in optical density (OD) before lysis during the first hours. This increase in OD depended on the concentration of ampicillin and was independent of the concentration of piperacillin. A good correlation was found between the prelytic increase in OD and the killing curve. During the prelytic increase in OD, the number of CFU per milliliter remained constant. The decrease in the number of CFU per milliliter depended on the concentration of ampicillin and was independent of the concentration of piperacillin. pH variations gave rise to similar effects on growth curves and killing curves.

Growth-inhibitory and bactericidal effects of human parotid salivary histidine-rich polypeptides on Streptococcus mutans

Growth inhibition and cell viability assays demonstrate that the histidine-rich polypeptides isolated from human parotid saliva are bacteriostatic and bactericidal for strains of Streptococcus mutans belonging to the serotype b and c classifications. Both inhibition of growth and cell division are enhanced by preincubation of bacteria with these polypeptides in low-ionic-strength buffers of acidic and neutral pH before dilution into enriched growth media. With prior exposure at pH 6.8, inhibition by these polypeptides of the serotype c strains, S. mutans GS5 and SB, as well as the serotype b strain, S. mutans BHT, is reversible over time under the experimental conditions selected. With similar exposure at pH 5.2, however, irreversible damage is manifested by complete inhibition of both growth and cell viability. At concentrations of 250 micrograms of the mixture of histidine-rich polypeptides per 5 X 10(5) bacterial cells per ml in the acidic preincubation buffer, bacterial lethality is maintained for a period of 48 h in the enriched growth media. At a 50-micrograms/ml concentration of these salivary agents, approximately 80% killing of S. mutans SB is noted after a 24-h incubation; however, surviving bacteria multiply and reach turbidities of untreated control cells when examined at the 48-h growth point. Similarly, hen egg white lysozyme is also found to be bactericidal for these microorganisms when preincubation is carried out under acidic conditions. However, in contrast to the histidine-rich polypeptides, lysozyme under these experimental conditions does not inhibit growth of S. mutans SB at neutral pH, although it does inhibit growth of both S. mutans BHT and S. mutans GS5 at this pH. Preexposure of S. mutans SB to the peptides in buffer at ionic strengths of 0.025 to 0.125, followed by either viability assays under nongrowing conditions or growth inhibition studies, suggests that there is very little effect of ionic strength on the antibacterial function of these peptides. In contrast to the inhibition of viability noted under growing conditions, lower concentrations of the histidine-rich polypeptides were required to elicit immediate cell death under nongrowing conditions.

pH-dependent oxacillin tolerance of Staphylococcus aureus

A proportion of clinical isolates of Staphylococcus aureus exhibit resistance to bactericidal activity of certain antibiotics, despite normal susceptibility to inhibition. This phenomenon is termed "tolerance." The methodology used to determine tolerance varies greatly. To clarify the relationship between laboratory methodology and tolerance, we determined the minimal bactericidal concentrations and minimal inhibitory concentrations for 20 clinical isolates by two methods. Inocula were prepared by either 3-h growth of the organism in Mueller-Hinton broth or overnight (22 to 24 h) cultures in Trypticase soy broth (BBL Microbiology Systems, Cockeysville, Md.). All inocula were plated for colony counts and tested for pH. An American Type Culture Collection (Rockville, Md.) reference strain was included in all tests for standardization. Tolerance was defined by the strictest criterion, i.e., a minimal bactericidal concentration/minimal inhibitory concentration ratio of greater than or equal to 100. With the first method, none of the 20 isolates displayed tolerance (mean inoculum pH, 7.15). When inocula were grown in Trypticase soy broth with overnight incubation, 35% (7 of 20) showed tolerance (mean inoculum pH, 6.22). There was a significant association between the decreased bactericidal capacity at high oxacillin concentrations and overnight incubation in Trypticase soy broth (P less than 0.01). We suggest that tolerance in staphylococci is in some way related to the pH value of the inoculating culture. Such pH-induced tolerance may have a clinical corollary in sequestered infections where the pH is acidic.

Bacteriolysis of Streptococcus mutans GS5 by lysozyme, proteases, and sodium thiocyanate

Streptococcus mutans GS5 was grown in a synthetic medium containing radioactive thymidine to monitor cell lysis by assay of the release of DNA. Bacteriolysis was achieved by sequential treatment of the cells with either hen egg white lysozyme and sodium thiocyanate or a combination of hen egg white lysozyme and a proteolytic enzyme followed by addition of the thiocyanate. In the absence of sodium thiocyanate, a small percentage of the total macromolecular thymidine was released in control reaction mixtures during incubation. This amount of released DNA more than doubled in trypsin-treated cells, but the inclusion of lysozyme in reaction mixtures prevented assay of the DNA. Lysis was found to be optimal in the late log phase of growth and was dependent on the concentrations of both lysozyme and protease. Concentrations of trypsin or chymotrypsin as low as 0.01 microgram/ml were found to be effective in enhancing the lytic process. The addition of protease to lysozyme-inorganic salt reaction mixtures altered both the pH and ionic strength profiles of cell lysis. At pHs of 5.5 or lower, both the lysozyme-NaSCN and the lysozyme-trypsin-NaSCN systems were inactive in mediating lysis. The loss of insoluble cell wall peptidoglycan by lysozyme treatment was pH independent and did not appear to be affected by the addition of protease. Either diluted whole saliva or neutrophil extracts could replace trypsin to enhance cell lysis further.

Colicin M is an inhibitor of murein biosynthesis

Colicin M inhibited the incorporation of DL + meso-2,6-diamino[3,4,5-3H]pimelic acid into the murein (peptidoglycan) of growing cells of Escherichia coli W7 dap lys. The inhibition of the UDP-N-acetylmuramyl pentapeptide-dependent incorporation of UDP-N-acetyl-D-[U-14C]glucosamine into isolated cell envelopes indicated interference with a late step of murein biosynthesis. After the inhibition of murein biosynthesis, cells lysed, and they released lysis products of murein. In vitro, the murein biosynthesis of colicin M-tolerant mutants (tolM) was inhibited by colicin M. Therefore, tolerance is probably conferred by an impaired uptake of an altered fixation close to the target site and not by a mutation of the target itself. Preliminary studies with beta-lactam antibiotics and with mutants in penicillin-binding proteins did not reveal a specific enzymatic step inhibited by colicin M. The unique action among the colicins renders colicin M a potentially useful tool for studying murein biosynthesis.

Induction and control of the autolytic system of Escherichia coli

Various methods of inducing autolysis of Escherichia coli cells were investigated, some being described here for the first time. For the autolysis of growing cells only induction methods interfering with the biosynthesis of peptidoglycan were taken into consideration, whereas with harvested cells autolysis was induced by rapid osmotic or EDTA shock treatments. The highest rates of autolysis were observed after induction by moenomycin, EDTA, or cephaloridine. The different autolyses examined shared certain common properties. In particular, regardless of the induction method used, more or less extensive peptidoglycan degradation was observed, and 10(-2) M Mg2+ efficiently inhibited the autolytic process. However, for other properties a distinction was made between methods used for growing cells and those used for harvested cells. Autolysis of growing cells required RNA, protein, and fatty acid synthesis. No such requirements were observed with shock-induced autolysis performed with harvested cells. Thus, the effects of Mg2+, rifampicin, chloramphenicol, and cerulenin clearly suggest that distinct factors are involved in the control of the autolytic system of E. Coli. Uncoupling agents such as sodium azide, 2,4-dinitrophenol, and carbonyl-cyanide-m-chlorophenyl hydrazone used at their usual inhibiting concentration had no effect on the cephaloridine or shock-induced autolysis.

Extracellular proteases increase tolerance of Bacillus subtilis to nafcillin

Mutants of Bacillus subtilis capable of secreting high amounts of protease were highly tolerant to the lethal and lytic effects of nafcillin. Protease-deficient mutants were more susceptible. However, when subtilisin was added to exogenously to a protease-deficient strain, the organism assumed the characteristics of nafcillin tolerance. Similarly, when phenylmethylsulfonyl fluoride, a serine protease inhibitor, was added to the tolerant strains, they became susceptible to nafcillin-induced lysis. The effects of nafcillin on B. subtilis were studied with both viability determinations and assay of cellular lysis. The minimum inhibitory concentrations of nafcillin tended to be higher for the protease hyperproducing strains, but these values could be reduced by the protease inhibitor. No loss of antibiotic activity was observed when nafcillin was incubated with either subtilisin or trypsin. Furthermore, protease and autolysin from B. subtilis were not modified by nafcillin. The results showed that extracellular proteases could render B. subtilis relatively tolerant to the killing and lytic effects of a cell wall antibiotic. The proteases were probably acting on the autolysins of the organism, thereby increasing tolerance to nafcillin.

pH-dependent penicillin tolerance may protect intraleukocytic Staphylococcus aureus from killing by cloxacillin

Exposure of Staphylococcus aureus to cloxacillin at neutral pH rendered the bacteria more susceptible to subsequent lysis by lysostaphin. This sensitization effect also occurred at lower pH levels which were nonpermissive for the bactericidal action of cloxacillin, but the effect disappeared at pH 5.0. These findings elucidate previous observations on the protection of intraleukocytic S. aureus from killing by cloxacillin and indicate that low pH in the phagolysosomes may be involved.

Antibiotic-induced lysis of enterococci

Enterococci are resistant to penicillin killing in vivo and in vitro. Because some bacteria resistant to penicillin killing have reduced autolytic activity, we examined the lysis of clinical enterococcal isolates suspended in buffer (spontaneous lysis), and compared it with their susceptibility to antibiotic-induced lysis and killing. We found significant correlations between spontaneous and antibiotic-induced lysis, using five antibiotics that inhibit cell wall synthesis (penicillin, cephalothin, bacitracin, cycloserine, and vancomycin). Among isolates, strains more rapidly lysed by one antibiotic were more rapidly lysed by the other antibiotics, and more susceptible to spontaneous lysis. In studies involving a single strain grown in different media, spontaneous lysis also correlated closely with antibiotic-induced lysis. These results are consistent with a common mechanism for spontaneous and antibiotic-induced lysis, such as the autolytic enzyme system. Human serum was one of the least permissive media tested for enterococcal growth and antibiotic-induced lysis and killing. We suggest that the inhibitory effect of human serum on growth and the activation of the enterococcal autolytic enzyme system may be a critical factor in the resistance of enterococcal endocarditis to treatment with penicillin alone.

pH-dependent penicillin tolerance of group B streptococci

Group B streptococci lose viability without apparent lysis during treatment with beta-lactam antibiotics and vancomycin. Rapid loss of viability was observed in early-exponential-phase cultures. Cultures in the mid-exponential growth phase exhibited various degrees of resistance to the bactericidal effect of the antibiotics, whereas their susceptibilities to the growth-inhibitory effect remained unchanged. This growth-phase-dependent tolerance was caused by the gradual increase in acidity of the cultures as the cell concentration increased. Retitration of the pH to neutrality made the formerly tolerant bacteria again fully susceptible to the killing effect of penicillin. Conversely, lowering the pH value of the medium resulted in antibiotic tolerance throughout culture growth. The penicillin-binding proteins of whole bacteria and their labeling pattern were found to be independent of culture pH. It is suggested that the mechanism of Ph-dependent tolerance is indirect and may be mediated by an autolysin. The tolerance of group B streptococci for penicillin could be clinically relevant in view of the relatively low pH values known to prevail in the natural host environments colonized by these bacteria.

Bacteriolysis of Veillonella alcalescens by lysozyme and inorganic anions present in saliva

Veillonella alcalescens subsp. dispar was grown in a synthetic medium containing either radiolabeled thymidine or uridine to monitor cell lysis by assay of the release of deoxyribonucleic acid or ribonucleic acid (RNA), respectively. Biochemical analyses demonstrated that, although human or hen egg white lysozymes alone did not release deoxyribonucleic acid or RNA, the nucleic acids were liberated in equal amounts from lysozyme-treated cells by the addition of low concentrations of the sodium salts of HCO-3, SCN-, Cl-, and F-, RNA release was dependent on enzyme and anion concentration. Human lysozyme was more potent than hen egg white lysozyme, and bicarbonate was the most effective anion in promoting bacteriolysis. Surprisingly, ultrastructural analyses differed from biochemical results. Lysozyme alone caused lysis in approximately 40% of the cell population. Detailed ultrastructural examination revealed aggregated cytoplasmic components which appeared as small clumps, explaining why nucleic acids were not measurable in the biochemical assays. In reaction mixtures containing lysozyme plus inorganic salts, electron microscopy results were compatible with biochemical data. Ultrastructural studies demonstrated that the addition of inorganic salts to lysozyme-treated cells resulted in the solubilization of the protoplasmic aggregates of lysed cells, presumably freeing the complexed RNA, and in the rapid lysis of the remaining cells (approximately 60%). These data suggest that electron microscopy must be used in conjunction with biochemical assays to assess lytic damage of bacterial cells.

Lysis of Streptococcus mutans by hen egg white lysozyme and inorganic sodium salts

Streptococcus mutans BHT was grown in a synthetic medium containing radioactive thymidine to monitor deoxyribonucleic acid release. Kinetic experiments demonstrated that although lysozyme alone could not liberate deoxyribonucleic acid, cellular deoxyribonucleic acid was liberated from lysozyme-treated cells by addition of low concentrations of inorganic sodium salts. When the salts were tested for their ability to dislodge cell-bound tritiated lysozyme, the extent of the initial release of enzyme by individual anions correlated with the anion potency for deoxyribonucleic acid liberation (SCN- greater than ClO4- greater than I- greater than Br- greater than NO3- greater than Cl- greater than F-), although the total amount of lysozyme dislodged did not correspond directly with cell lysis. Differences in the effectiveness of anions (SCN-, HCO3-, Cl- and F-) in potentiating cell lysis could be enhanced or minimized by varying the lysozyme, anion, and bacterial cell concentrations. As the anion concentration was increased for each enzyme concentration and cell concentration, the lysis increased, in some cases markedly, until maximum levels of released deoxyribonucleic acid were attained. The maximum levels of lysis of SCN- and HCO3- were similar and were greater than those for Cl- and F-. In addition, the maximum levels were observed to increase for each of the anions as the concentration of lysozyme increased.

Alteration of Escherichia coli murein during amino acid starvation

We have studied the mechanisms by which amino acid starvation of Escherichia coli induces resistance against the lytic and bactericidal effects of penicillin. Starvation of E. coli strain W7 of the amino acids lysine or methionine resulted in the rapid development of resistance to autolytic cell wall degradation, which may be effectively triggered in growing bacteria by a number of chemical or physical treatments. The mechanism of this effect in the amino acid-starved cells involved the production of a murein relatively resistant to the hydrolytic action of crude murein hydrolase extracts prepared from normally growing E. coli. Resistance to the autolysins was not due to the covalently linked lipoprotein. Resistance to murein hydrolase developed most rapidly and most extensively in the portion of cell wall synthesized after the onset of amino acid starvation. Lysozymes digests of the autolysin-resistant murein synthesized during the first 10 min of lysine starvation yielded (in addition to the characteristic degradation products) a high-molecular-weight material that was absent from the lysozyme-digests of control cell wall preparations. It is proposed that inhibition of protein synthesis causes a rapid modification of murein structure at the cell wall growth zone in such a manner that attachment of murein hydrolase molecules is inhibited. The mechanism may involve some aspects of the relaxed control system since protection against penicillin-induced lysis developed much slower in amino acid-starved relaxed controlled (relA) cells than in isogenic stringently controlled (relA+) bacteria.

Inhibition of peptidoglycan, ribonucleic acid, and protein synthesis in tolerant strains of Streptococcus mutans

Exposure of exponentially growing cultures of Streptococcus mutans strains FA-1 and GS-5 to various concentrations of benzylpenicillin (Pen G) resulted in inhibition of turbidity increases at low concentrations (0.02 to 0.04 mug/ml). However, in contrast to some other streptococcal species, growth inhibition was not accompanied by cellular lysis or by a rapid loss of viability. In both strains, synthesis of insoluble cell wall peptidoglycan was very sensitive to Pen G inhibition and responded in a dose-dependent manner to concentrations of about 0.2 and 0.5 mug/ml for strains GS-5 and FA-1, respectively. Higher Pen G concentrations failed to inhibit further either growth or insoluble peptidoglycan assembly. Somewhat surprisingly, Pen G also inhibited both ribonucleic acid (RNA) and protein syntheses, each in a dose-dependent manner. Compared with inhibition of peptidoglycan synthesis, inhibition of RNA and protein syntheses by Pen G was less rapid and less extensive. Maximum amounts of radiolabeled Pen G were specifically bound to intact cells upon exposure to about 0.2 and 0.5 mug/ml of Pen G for strains GS-5 and FA-1, respectively, concentrations consistent with those that resulted in maximum or near-maximum inhibitions of the synthesis of cellular peptidoglycan, RNA, and protein. Five polypeptide bands that had a very high affinity for [(14)C]Pen G were detected in a crude cell envelope preparation of strain FA-1. After exposure of cultures of strain FA-1 to the effects of saturating concentrations of the drug for up to 3 h, addition of penicillinase was followed by recovery of growth after a lag. The length of the lag before regrowth depended on both Pen G concentration and time of exposure. On the basis of these and other observations, it is proposed that the secondary inhibitions of cellular RNA or protein synthesis, or both, are involved in the tolerance of these organisms to lysis and killing by Pen G and other inhibitors of insoluble peptidoglycan assembly.

Lethal effect of a heterologous murein hydrolase on penicillin-treated Streptococcus sanguis

Nine strains of Streptococcus sanguis exhibited tolerance to benzylpenicillin: the growth of each strain was susceptible to penicillin with minimal inhibitory concentrations of 0.1 mug/ml or lower, but the bacteriolytic and bactericidal effects were limited in each case. The tolerance of these bacteria was also reflected in the large discrepancies between the minimal inhibitory and minimal bactericidal concentrations for benzylpenicillin. The hypothesis that a natural deficiency of endogenous murein hydrolase (autolysin) in this species accounts for the penicillin tolerance was tested by using a heterologous murein hydrolase, the C-phage-associated lysin. In seven of the strains, addition of the lysin to the culture together with penicillin or other cell wall inhibitors resulted in lysis and rapid loss of viability. The enzyme alone did not appreciably affect normally growing cultures. The irreversible effects of penicillin plus lysin were drastically reduced in the presence of the bacteriostatic agents chloramphenicol and cerulenin. Speculations based on experiments are presented for the mechanisms by which penicillin treatment sensitizes these bacteria to an exogenous lytic enzyme. Similar phenomena requiring cooperation of host factors and penicillin may occur during infection, since somewhat similar although less pronounced results were obtained by addition of human lysozyme to penicillin-treated S. sanguis.

Triggering of autolytic cell wall degradation in Escherichia coli by beta-lactam antibiotics

A biochemical method was developed to quantitatively compare the effectiveness of beta-lactams in triggering murein degradation (autolysin activity) in Escherichia coli. Bacteria prelabeled in their cell walls with radioactive diaminopimelic acid in growth medium were exposed for 10 min to the antibiotics at the appropriate minimal growth inhibitory concentrations and at multiples of these values, and the rate of cell wall degradation was followed during subsequent penicillin-binding protein (PBP)-1 were the most effective triggers of autolytic wall degradation; beta-lactams selective for PBP-2 were the poorest; and antibiotics preferentially binding to PBP-3 showed intermediate activities. The relative effectiveness of beta-lactams in autolysin triggering was found to parallel the effectiveness of the same drugs in causing rapid loss of viability, culture lysis, and spheroplast formation. Autolysin triggering was suppressed by inhibitors of protein and ribonucleic acid biosynthesis but not by inhibitors of deoxyribonucleic acid synthesis. The beta-lactam-induced cell wall degradation did not seem to involve a direct stimulation of enzyme activity or synthesis of new enzyme molecules, and murein sacculi isolated from cells that had been preexposed to a triggering dose of beta-lactam treatment exhibited the same sensitivity to crude, homologous autolysins as sacculi prepared from untreated control bacteria. On the basis of these observations, mechanisms are considered for the triggering of E. coli autolysins and for the role of autolytic activity in bacterial spheroplast formation, lysis, and death.

Sensitivity of Escherichia coli to cephaloridine at different growth rates

Steady-state populations of Escherichia coli B/r were treated with cephaloridine at minimal inhibitory concentrations. The antibiotic sensitivity of the cells and the localization of spheroplast emergence along the cell surface were examined as a function of cell length and growth rate. In fast-growing populations (greater than 1 division per h) the sites of cephaloridine interaction occurred preferentially at the cell pole in the smaller cells and at the cell center in dividing cells. At decreasing growth rates the cells became more resistant to cephaloridine, and a gradual shift from the cell pole toward the cell center was observed for the sphere position. A similar growth rate-dependent change in localization was found for sucrose-induced plasmolysis vacuoles.

High and selective resistance to mecillinam in adenylate cyclase-deficient or cyclic adenosine 3',5'-monophosphate receptor protein-deficient mutants of Escherichia coli

Adenylate cyclase-deficient (cya) mutants of Escherichia coli K-12 were selectively and highly resistant to mecillinam (FL1060) among several beta-lactam antibiotics in the absence of cyclic adenosine 3',5'-monophosphate (cAMP). They became sensitive to the drug in the presence of cAMP. Also, cAMP receptor protein-negative (crp) mutants, with the exception of strain 5333, were highly resistant to mecillinam in the presence and in the absence of cAMP. Mecillinam exerted two distinct and sequential effects in both cya+ strains and cya strains supplemented with cAMP: (i) rounding of cells and (ii) cessation of cell division. The first effect was accompanied by a decrease in growth rate, whereas the second effect was accompanied by enlargement and lysis of the rounded cells. The second effect of mecillinam was dependent on inoculum size and cAMP. When the cell density was above about 10(6) cells per ml, the rounded cells stopped dividing but did not lyse. In the absence of cAMP, cya strains neither stopped dividing nor lysed; they were resistant to the second, lethal effect of mecillinam.

Effect of benzylpenicillin on the synthesis and structure of the cell envelope of Neisseria gonorrhoeae

The effect of benzylpenicillin on the synthesis and morphology of the cell envelope of Neisseria gonorrhoeae was examined. Penicillin immediately stopped murein synthesis; it also enhanced the rate of turnover of glucosamine, but not diaminopimelic acid, in the murein. In addition, penicillin greatly increased the shedding of lipid and lipopolysaccharide into the medium. In the electron microscope, protrusions of the cell membrane were evident, as well as apparent holes in the murein cell wall. All of these changes occurred while active synthesis was taking place, before the lysis of the cells. Lysis could be prevented by growing the cells at low pH and high concentrations of Mg2+; however, the effects of penicillin on murein synthesis and turnover and on the release of lipid were not affected.