ppGpp concentration, growth without PBP2 activity, and growth-rate control in Escherichia coli

Glyphosate affects persistence and tolerance but not antibiotic resistance

Glyphosate is a herbicide widely used in food production that blocks the synthesis of aromatic amino acids in plants and in microorganisms and also induces the accumulation of the alarmone (p)ppGpp. The purpose of this study was to investigate whether glyphosate affects the resistance, tolerance or persistence of bacteria towards three different classes of antibiotics and the possible role of (p)ppGpp in this activity. Glyphosate did not affect the minimum inhibitory concentration of the tested antibiotics, but enhanced bacterial tolerance and/or persistence towards them. The upshift in ciprofloxacin and kanamycin tolerance was partially dependent on the presence of relA that promotes (p)ppGpp accumulation in response to glyphosate. Conversely, the strong increase in ampicillin tolerance caused by glyphosate was independent of relA. We conclude that by inducing aromatic amino acid starvation glyphosate contributes to the temporary increase in E. coli tolerance or persistence, but does not affect antibiotic resistance.

Increased energy demand from anabolic-catabolic processes drives β-lactam antibiotic lethality

β-Lactam antibiotics disrupt the assembly of peptidoglycan (PG) within the bacterial cell wall by inhibiting the enzymatic activity of penicillin-binding proteins (PBPs). It was recently shown that β-lactam treatment initializes a futile cycle of PG synthesis and degradation, highlighting major gaps in our understanding of the lethal effects of PBP inhibition by β-lactam antibiotics. Here, we assess the downstream metabolic consequences of treatment of Escherichia coli with the β-lactam mecillinam and show that lethality from PBP2 inhibition is a specific consequence of toxic metabolic shifts induced by energy demand from multiple catabolic and anabolic processes, including accelerated protein synthesis downstream of PG futile cycling. Resource allocation into these processes is coincident with alterations in ATP synthesis and utilization, as well as a broadly dysregulated cellular redox environment. These results indicate that the disruption of normal anabolic-catabolic homeostasis by PBP inhibition is an essential factor for β-lactam antibiotic lethality.

Stringent Response in Mycobacteria: From Biology to Therapeutic Potential

Mycobacterium tuberculosis is a human pathogen that can thrive inside the host immune cells for several years and cause tuberculosis. This is due to the propensity of M. tuberculosis to synthesize a sturdy cell wall, shift metabolism and growth, secrete virulence factors to manipulate host immunity, and exhibit stringent response. These attributes help M. tuberculosis to manage the host response, and successfully establish and maintain an infection even under nutrient-deprived stress conditions for years. In this review, we will discuss the importance of mycobacterial stringent response under different stress conditions. The stringent response is mediated through small signaling molecules called alarmones “(pp)pGpp”. The synthesis and degradation of these alarmones in mycobacteria are mediated by Rel protein, which is both (p)ppGpp synthetase and hydrolase. Rel is important for all central dogma processes—DNA replication, transcription, and translation—in addition to regulating virulence, drug resistance, and biofilm formation. Rel also plays an important role in the latent infection of M. tuberculosis. Here, we have discussed the literature on alarmones and Rel proteins in mycobacteria and highlight that (p)ppGpp-analogs and Rel inhibitors could be designed and used as antimycobacterial compounds against M. tuberculosis and non-tuberculous mycobacterial infections.

Cell Size Is Coordinated with Cell Cycle by Regulating Initiator Protein DnaA in E. coli

Sixty years ago, bacterial cell size was found to be an exponential function of growth rate. Fifty years ago, a more general relationship was proposed, in which cell mass was equal to the initiation mass multiplied by 2 to the power of the ratio of the total time of C and D periods to the doubling time. This relationship has recently been experimentally confirmed by perturbing doubling time, C period, D period, or initiation mass. However, the underlying molecular mechanism remains unclear. Here, we developed a theoretical model for initiator protein DnaA mediating DNA replication initiation in Escherichia coli. We introduced an initiation probability function for competitive binding of DnaA-ATP and DnaA-ADP at oriC. We established a kinetic description of regulatory processes (e.g., expression regulation, titration, inactivation, and reactivation) of DnaA. Cell size as a spatial constraint also participates in the regulation of DnaA. By simulating DnaA kinetics, we obtained a regular DnaA oscillation coordinated with cell cycle and a converged cell size that matches replication initiation frequency to the growth rate. The relationship between the simulated cell size and growth rate, C period, D period, or initiation mass reproduces experimental results. The model also predicts how DnaA number and initiation mass vary with perturbation parameters, comparable with experimental data. The results suggest that 1) when growth rate, C period, or D period changes, the regulation of DnaA determines the invariance of initiation mass; 2) ppGpp inhibition of replication initiation may be important for the growth rate independence of initiation mass because three possible mechanisms therein produce different DnaA dynamics, which is experimentally verifiable; and 3) perturbation of some DnaA regulatory process causes a changing initiation mass or even an abnormal cell cycle. This study may provide clues for concerted control of cell size and cell cycle in synthetic biology.

(p)ppGpp and Its Role in Bacterial Persistence: New Challenges

Antibiotic failure not only is due to the development of resistance by pathogens but can also often be explained by persistence and tolerance. Persistence and tolerance can be included in the "persistent phenotype," with high relevance for clinics. Two of the most important molecular mechanisms involved in tolerance and persistence are toxin-antitoxin (TA) modules and signaling via guanosine pentaphosphate/tetraphosphate [(p)ppGpp], also known as "magic spot." (p)ppGpp is a very important stress alarmone which orchestrates the stringent response in bacteria; hence, (p)ppGpp is produced during amino acid or fatty acid starvation by proteins belonging to the RelA/SpoT homolog family (RSH). However, (p)ppGpp levels can also accumulate in response to a wide range of signals, including oxygen variation, pH downshift, osmotic shock, temperature shift, or even exposure to darkness. Furthermore, the stringent response is not only involved in responses to environmental stresses (starvation for carbon sources, fatty acids, and phosphates or heat shock), but it is also used in bacterial pathogenesis, host invasion, and antibiotic tolerance and persistence. Given the exhaustive and contradictory literature surrounding the role of (p)ppGpp in bacterial persistence, and with the aim of summarizing what is known so far about the magic spot in this bacterial stage, this review provides new insights into the link between the stringent response and persistence. Moreover, we review some of the innovative treatments that have (p)ppGpp as a target, which are in the spotlight of the scientific community as candidates for effective antipersistence agents.

Diversity in E. coli (p)ppGpp Levels and Its Consequences

(p)ppGpp is at the core of global bacterial regulation as it controls growth, the most important aspect of life. It would therefore be expected that at least across a species the intrinsic (basal) levels of (p)ppGpp would be reasonably constant. On the other hand, the historical contingency driven by the selective pressures on bacterial populations vary widely resulting in broad genetic polymorphism. Given that (p)ppGpp controls the expression of many genes including those involved in the bacterial response to environmental challenges, it is not surprising that the intrinsic levels of (p)ppGpp would also vary considerably. In fact, null mutations or less severe genetic polymorphisms in genes associated with (p)ppGpp synthesis and hydrolysis are common. Such variation can be observed in laboratory strains, in natural isolates as well as in evolution experiments. High (p)ppGpp levels result in low growth rate and high tolerance to environmental stresses. Other aspects such as virulence and antimicrobial resistance are also influenced by the intrinsic levels of (p)ppGpp. A case in point is the production of Shiga toxin by certain E. coli strains which is inversely correlated to (p)ppGpp basal level. Conversely, (p)ppGpp concentration is positively correlated to increased tolerance to different antibiotics such as β-lactams, vancomycin, and others. Here we review the variations in intrinsic (p)ppGpp levels and its consequences across the E. coli species.

(p)ppGpp and the Stringent Response: An Emerging Threat to Antibiotic Therapy

In 1969, Cashel and Gallant first observed the presence of (p)ppGpp-the signaling molecule of the stringent response-in starved bacterial cells. Fifty years later, (p)ppGpp and the stringent response have emerged as essential master regulators of not only the bacterial response to stress but also almost all aspects of bacterial physiology, virulence, and immune evasion. More worryingly, a wealth of data now indicate that (p)ppGpp and stringent response activation pose a serious threat to the efficacy and clinical success of antimicrobial therapy. Here, we focus on the central role that (p)ppGpp and the stringent response play in the phenomenon of antibiotic tolerance, as well as the acquisition, development, and expression of antibiotic resistance. We review these consequences of stringent response activation in relation to the main proteins involved in (p)ppGpp production and control, in particular the complex interplay between monofunctional and bifunctional long RelA/SpoT homologues (RSHs) and small alarmone synthetases (SASs). We also review the growing evidence to suggest that there are multiple other indirect pathways of stringent response induction that can affect antibiotic efficacy. Finally, we summarize recent studies that indicate the in vivo and clinical impact of (p)ppGpp production on antibiotic treatment outcomes. We conclude by reviewing the progress to date in the search for novel therapeutics that target the stringent response.

Metabolism Shapes the Cell

More than 5 decades of work support the idea that cell envelope synthesis, including the inward growth of cell division, is tightly coordinated with DNA replication and protein synthesis through central metabolism. Remarkably, no unifying model exists to account for how these fundamentally disparate processes are functionally coupled. Recent studies demonstrate that proteins involved in carbohydrate and nitrogen metabolism can moonlight as direct regulators of cell division, coordinate cell division and DNA replication, and even suppress defects in DNA replication. In this minireview, we focus on studies illustrating the intimate link between metabolism and regulation of peptidoglycan (PG) synthesis during growth and division, and we identify the following three recurring themes. (i) Nutrient availability, not growth rate, is the primary determinant of cell size. (ii) The degree of gluconeogenic flux is likely to have a profound impact on the metabolites available for cell envelope synthesis, so growth medium selection is a critical consideration when designing and interpreting experiments related to morphogenesis. (iii) Perturbations in pathways relying on commonly shared and limiting metabolites, like undecaprenyl phosphate (Und-P), can lead to pleotropic phenotypes in unrelated pathways.

(p)ppGpp and the bacterial cell cycle

Genes of the Rel/Spo homolog (RSH) superfamily synthesize and/or hydrolyse the modified nucleotides pppGpp/ ppGpp (collectively referred to as (p)ppGpp) and are prevalent across diverse bacteria and in plant chloroplasts. Bacteria accumulate (p)ppGpp in response to nutrient deprivation (generically called the stringent response) and elicit appropriate adaptive responses mainly through the regulation of transcription. Although at different concentrations (p)ppGpp affect the expression of distinct set of genes, the two well-characterized responses are reduction in expression of the protein synthesis machinery and increase in the expression of genes coding for amino acid biosynthesis. In Escherichia coli, the cellular (p)ppGpp level inversely correlates with the growth rate and increasing its concentration decreases the steady state growth rate in a defined growth medium. Since change in growth rate must be accompanied by changes in cell cycle parameters set through the activities of the DNA replication and cell division apparatus, (p)ppGpp could coordinate protein synthesis (cell mass increase) with these processes. Here we review the role of (p)ppGpp in bacterial cell cycle regulation.

Inactivation of Cell Division Protein FtsZ by SulA Makes Lon Indispensable for the Viability of a ppGpp0 Strain of Escherichia coli

The modified nucleotides (p)ppGpp play an important role in bacterial physiology. While the accumulation of the nucleotides is vital for adaptation to various kinds of stress, changes in the basal level modulates growth rate and vice versa. Studying the phenotypes unique to the strain lacking (p)ppGpp (ppGpp(0)) under overtly unstressed growth conditions may be useful to understand functions regulated by basal levels of (p)ppGpp and its physiological significance. In this study, we show that the ppGpp(0) strain, unlike the wild type, requires the Lon protease for cell division and viability in LB. Our results indicate the decrease in FtsZ concentration in the ppGpp(0) strain makes cell division vulnerable to SulA inhibition. We did not find evidence for SOS induction contributing to the cell division defect in the ppGpp(0) Δlon strain. Based on the results, we propose that basal levels of (p)ppGpp are required to sustain normal cell division in Escherichia coli during growth in rich medium and that the basal SulA level set by Lon protease is important for insulating cell division against a decrease in FtsZ concentration and conditions that can increase the susceptibility of FtsZ to SulA.

IMPORTANCE

The physiology of the stringent response has been the subject of investigation for more than 4 decades, with the majority of the work carried out using the bacterial model organism Escherichia coli. These studies have revealed that the accumulation of (p)ppGpp, the effector of the stringent response, is associated with growth retardation and changes in gene expression that vary with the intracellular concentration of (p)ppGpp. By studying a synthetic lethal phenotype, we have uncovered a function modulated by the basal levels of (p)ppGpp and studied its physiological significance. Our results show that (p)ppGpp and Lon protease contribute to the robustness of the cell division machinery in E. coli during growth in rich medium.

Interactions of OP0595, a Novel Triple-Action Diazabicyclooctane, with β-Lactams against OP0595-Resistant Enterobacteriaceae Mutants

OP0595 is a novel diazabicyclooctane which, like avibactam, inhibits class A and C β-lactamases. In addition, unlike avibactam, it has antibacterial activity, with MICs of 0.5 to 4 μg/ml for most members of the family Enterobacteriaceae, owing to inhibition of PBP2; moreover, it acts synergistically with PBP3-active β-lactams independently of β-lactamase inhibition, via an "enhancer effect." Enterobacteriaceae mutants stably resistant to 16 μg/ml OP0595 were selected on agar at frequencies of approximately 10(-7). Unsurprisingly, OP0595 continued to potentiate substrate β-lactams against mutants derived from Enterobacteriaceae with OP0595-inhibited class A and C β-lactamases. Weaker potentiation of partners, especially aztreonam, cefepime, and piperacillin--less so meropenem--remained frequent for OP0595-resistant Enterobacteriaceae mutants lacking β-lactamases or with OP0595-resistant metallo-β-lactamases (MBLs), indicating that the enhancer effect is substantially retained even when antibiotic activity is lost.

OP0595, a new diazabicyclooctane: mode of action as a serine β-lactamase inhibitor, antibiotic and β-lactam 'enhancer'

OBJECTIVES

The production of a growing diversity of β-lactamases by Gram-negative bacteria challenges antimicrobial chemotherapy. OP0595, discovered separately by each of Meiji Seika Pharma and Fedora Pharmaceuticals, is a new diazabicyclooctane serine β-lactamase inhibitor that also acts as an antibiotic and as a β-lactamase-independent β-lactam 'enhancer'.

METHODS

Inhibitory activity against serine β-lactamases and affinity for PBPs were determined using nitrocefin and Bocillin FL, respectively. MICs alone and in combination with β-lactam agents were measured according to CLSI recommendations. Morphological changes in Escherichia coli were examined by phase-contrast microscopy.

RESULTS

IC50s of OP0595 for class A and C β-lactamases were <1000 nM, with covalent binding demonstrated to the active-site serine of CTX-M-44 and AmpC enzymes. OP0595 also had direct antibiotic activity against many Enterobacteriaceae, associated with inhibition of PBP2 and conversion of the bacteria into spherical forms. Synergy between OP0595 and β-lactam agents was seen against strains producing class A and C β-lactamases vulnerable to inhibition. Lastly, OP0595 lowered the MICs of PBP3-targeted partner β-lactam agents for a non-β-lactamase-producing E. coli mutant that was resistant to OP0595 itself, indicating β-lactamase-independent 'enhancer'-based synergy.

CONCLUSIONS

OP0595 acts in three ways: (i) as an inhibitor of class A and C β-lactamases, covalently binding at their active sites; (ii) as an antibacterial, by inhibiting PBP2 of several Enterobacteriaceae; and (iii) as an 'enhancer' of β-lactam agents that bind to other PBPs besides PBP2 for several Enterobacteriaceae. OP0595 has considerable potential to overcome resistance when it is combined with various β-lactam agents.

Many means to a common end: the intricacies of (p)ppGpp metabolism and its control of bacterial homeostasis

In nearly all bacterial species examined so far, amino acid starvation triggers the rapid accumulation of the nucleotide second messenger (p)ppGpp, the effector of the stringent response. While for years the enzymes involved in (p)ppGpp metabolism and the significance of (p)ppGpp accumulation to stress survival were considered well defined, a recent surge of interest in the field has uncovered an unanticipated level of diversity in how bacteria metabolize and utilize (p)ppGpp to rapidly synchronize a variety of biological processes important for growth and stress survival. In addition to the classic activation of the stringent response, it has become evident that (p)ppGpp exerts differential effects on cell physiology in an incremental manner rather than simply acting as a biphasic switch that controls growth or stasis. Of particular interest is the intimate relationship of (p)ppGpp with persister cell formation and virulence, which has spurred the pursuit of (p)ppGpp inhibitors as a means to control recalcitrant infections. Here, we present an overview of the enzymes responsible for (p)ppGpp metabolism, elaborate on the intricacies that link basal production of (p)ppGpp to bacterial homeostasis, and discuss the implications of targeting (p)ppGpp synthesis as a means to disrupt long-term bacterial survival strategies.

Amdinocillin (Mecillinam) resistance mutations in clinical isolates and laboratory-selected mutants of Escherichia coli

Amdinocillin (mecillinam) is a β-lactam antibiotic that is used mainly for the treatment of uncomplicated urinary tract infections. The objectives of this study were to identify mutations that confer amdinocillin resistance on laboratory-isolated mutants and clinical isolates of Escherichia coli and to determine why amdinocillin resistance remains rare clinically even though resistance is easily selected in the laboratory. Under laboratory selection, frequencies of mutation to amdinocillin resistance varied from 8 × 10(-8) to 2 × 10(-5) per cell, depending on the concentration of amdinocillin used during selection. Several genes have been demonstrated to give amdinocillin resistance, but here eight novel genes previously unknown to be involved in amdinocillin resistance were identified. These genes encode functions involved in the respiratory chain, the ribosome, cysteine biosynthesis, tRNA synthesis, and pyrophosphate metabolism. The clinical isolates exhibited significantly greater fitness than the laboratory-isolated mutants and a different mutation spectrum. The cysB gene was mutated (inactivated) in all of the clinical isolates, in contrast to the laboratory-isolated mutants, where mainly other types of more costly mutations were found. Our results suggest that the frequency of mutation to amdinocillin resistance is high because of the large mutational target (at least 38 genes). However, the majority of these resistant mutants have a low growth rate, reducing the probability that they are stably maintained in the bladder. Inactivation of the cysB gene and a resulting loss of cysteine biosynthesis are the major mechanism of amdinocillin resistance in clinical isolates of E. coli.

Antimicrobial resistance and virulence: a successful or deleterious association in the bacterial world?

Hosts and bacteria have coevolved over millions of years, during which pathogenic bacteria have modified their virulence mechanisms to adapt to host defense systems. Although the spread of pathogens has been hindered by the discovery and widespread use of antimicrobial agents, antimicrobial resistance has increased globally. The emergence of resistant bacteria has accelerated in recent years, mainly as a result of increased selective pressure. However, although antimicrobial resistance and bacterial virulence have developed on different timescales, they share some common characteristics. This review considers how bacterial virulence and fitness are affected by antibiotic resistance and also how the relationship between virulence and resistance is affected by different genetic mechanisms (e.g., coselection and compensatory mutations) and by the most prevalent global responses. The interplay between these factors and the associated biological costs depend on four main factors: the bacterial species involved, virulence and resistance mechanisms, the ecological niche, and the host. The development of new strategies involving new antimicrobials or nonantimicrobial compounds and of novel diagnostic methods that focus on high-risk clones and rapid tests to detect virulence markers may help to resolve the increasing problem of the association between virulence and resistance, which is becoming more beneficial for pathogenic bacteria.

Characterizing the effects of inorganic acid and alkaline shock on the Staphylococcus aureus transcriptome and messenger RNA turnover

Staphylococcus aureus pathogenesis can be attributed partially to its ability to adapt to otherwise deleterious host-associated stresses. Here, Affymetrix GeneChips® were used to examine the S. aureus responses to inorganic acid and alkaline shock and to assess whether stress-dependent changes in mRNA turnover are likely to facilitate the organism's ability to tolerate a pH challenge. The results indicate that S. aureus adapts to pH shock by eliciting responses expected of cells coping with pH alteration, including neutralizing cellular pH, DNA repair, amino acid biosynthesis, and virulence factor expression. Further, the S. aureus response to alkaline conditions is strikingly similar to that of stringent response-induced cells. Indeed, we show that alkaline shock stimulates the accumulation of the stringent response activator (p)ppGpp. The results also revealed that pH shock significantly alters the mRNA properties of the cell. A comparison of the mRNA degradation properties of transcripts whose titers either increased or decreased in response to a sudden pH change revealed that alterations in mRNA degradation may, in part, account for the changes in the mRNA levels of factors predicted to mediate pH tolerance. A set of small stable RNA molecules were induced in response to acid- or alkaline-shock conditions and may mediate adaptation to pH stress.

Genome sequence of the Fleming strain of Micrococcus luteus, a simple free-living actinobacterium

Micrococcus luteus (NCTC2665, "Fleming strain") has one of the smallest genomes of free-living actinobacteria sequenced to date, comprising a single circular chromosome of 2,501,097 bp (G+C content, 73%) predicted to encode 2,403 proteins. The genome shows extensive synteny with that of the closely related organism, Kocuria rhizophila, from which it was taxonomically separated relatively recently. Despite its small size, the genome harbors 73 insertion sequence (IS) elements, almost all of which are closely related to elements found in other actinobacteria. An IS element is inserted into the rrs gene of one of only two rrn operons found in M. luteus. The genome encodes only four sigma factors and 14 response regulators, a finding indicative of adaptation to a rather strict ecological niche (mammalian skin). The high sensitivity of M. luteus to beta-lactam antibiotics may result from the presence of a reduced set of penicillin-binding proteins and the absence of a wblC gene, which plays an important role in the antibiotic resistance in other actinobacteria. Consistent with the restricted range of compounds it can use as a sole source of carbon for energy and growth, M. luteus has a minimal complement of genes concerned with carbohydrate transport and metabolism and its inability to utilize glucose as a sole carbon source may be due to the apparent absence of a gene encoding glucokinase. Uniquely among characterized bacteria, M. luteus appears to be able to metabolize glycogen only via trehalose and to make trehalose only via glycogen. It has very few genes associated with secondary metabolism. In contrast to most other actinobacteria, M. luteus encodes only one resuscitation-promoting factor (Rpf) required for emergence from dormancy, and its complement of other dormancy-related proteins is also much reduced. M. luteus is capable of long-chain alkene biosynthesis, which is of interest for advanced biofuel production; a three-gene cluster essential for this metabolism has been identified in the genome.

The molecular alarmone (p)ppGpp mediates stress responses, vancomycin tolerance, and virulence in Enterococcus faecalis

The stringent response is a global bacterial response to stress that is mediated by accumulation of the alarmone (p)ppGpp. In this study, treatment with mupirocin was shown to induce high levels of (p)ppGpp production in Enterococcus faecalis, indicating that this nosocomial pathogen can mount a classic stringent response. In addition, (p)ppGpp was found to accumulate in cells subjected to heat shock, alkaline shock, and inhibitory concentrations of vancomycin. Sequence analysis of the E. faecalis genome indicated that (p)ppGpp synthesis is catalyzed by the bifunctional synthetase/hydrolase RelA and the RelQ small synthase. The (p)ppGpp profiles of DeltarelA, DeltarelQ, and DeltarelAQ strains revealed that RelA is the major enzyme responsible for the accumulation of (p)ppGpp during antibiotic or physical stresses, while RelQ appears to be responsible for maintaining basal levels of alarmone during homeostatic growth. Compared to its parent, the DeltarelA strain was more susceptible to several stress conditions, whereas complete elimination of (p)ppGpp in a DeltarelAQ double mutant restored many of the stress-sensitive phenotypes of DeltarelA. Interestingly, growth curves and time-kill studies indicated that tolerance to vancomycin is enhanced in the DeltarelA strain but diminished in the DeltarelQ and DeltarelAQ strains. Finally, virulence of the DeltarelAQ strain but not of the DeltarelA or DeltarelQ strain was significantly attenuated in the Caenorhabditis elegans model. Taken together, these results indicate that (p)ppGpp pools modulate environmental stress responses, vancomycin tolerance, and virulence in this important nosocomial pathogen.

Pleiotropic effects of a rel mutation on stress survival of Rhizobium etli CNPAF512

Background

The rel gene of Rhizobium etli (rel_Ret), the nodulating endosymbiont of the common bean plant, determines the cellular level of the alarmone (p)ppGpp and was previously shown to affect free-living growth and symbiosis. Here, we demonstrate its role in cellular adaptation and survival in response to various stresses.

Results

Growth of the R. etli rel_Ret mutant was strongly reduced or abolished in the presence of elevated NaCl levels or at 37°C, compared to the wild type. In addition, depending on the cell density, decreased survival of exponentially growing or stationary phase rel_Ret mutant cells was obtained after H₂O₂, heat or NaCl shock compared to the wild-type strain. Survival of unstressed stationary phase cultures was differentially affected depending on the growth medium used. Colony forming units (CFU) of rel_Ret mutant cultures continuously decreased in minimal medium supplemented with succinate, whereas wild-type cultures stabilised at higher CFU levels. Microscopic examination of stationary phase cells indicated that the rel_Ret mutant was unable to reach the typical coccoid morphology of the wild type in stationary phase cultures. Assessment of stress resistance of re-isolated bacteroids showed increased sensitivity of the rel_Ret mutant to H₂O₂ and a slightly increased resistance to elevated temperature (45°C) or NaCl shock, compared to wild-type bacteroids.

Conclusion

The rel_Ret gene is an important factor in regulating rhizobial physiology, during free-living growth as well as in symbiotic conditions. Additionally, differential responses to several stresses applied to bacteroids and free-living exponential or stationary phase cells point to essential physiological differences between the different states.

Protective action of ppGpp in microcin J25-sensitive strains

As Escherichia coli strains enter the stationary phase of growth they become more resistant to the peptide antibiotic microcin J25. It is known that starvation for nutrients such as amino acids or glucose leads to increases in guanosine 3',5'-bispyrophosphate (ppGpp) levels and that the intracellular concentration of this nucleotide increases as cells enter the stationary phase of growth. Therefore, we examined the effects of artificially manipulating the ppGpp levels on sensitivity to microcin J25. A direct correlation was found between ppGpp accumulation and microcin resistance. Our results indicate that the nucleotide is required to induce production of YojI, a chromosomally encoded efflux pump which, in turn, expels microcin from cells. This would maintain the intracellular level of the antibiotic below a toxic level.

Conditional lethality, division defects, membrane involution, and endocytosis in mre and mrd shape mutants of Escherichia coli

Maintenance of rod shape in Escherichia coli requires the shape proteins MreB, MreC, MreD, MrdA (PBP2), and MrdB (RodA). How loss of the Mre proteins affects E. coli viability has been unclear. We generated Mre and Mrd depletion strains under conditions that minimize selective pressure for undefined suppressors and found their phenotypes to be very similar. Cells lacking one or more of the five proteins were fully viable and propagated as small spheres under conditions of slow mass increase but formed large nondividing spheroids with noncanonical FtsZ assembly patterns at higher mass doubling rates. Extra FtsZ was sufficient to suppress lethality in each case, allowing cells to propagate as small spheres under any condition. The failure of each unsuppressed mutant to divide under nonpermissive conditions correlated with the presence of elaborate intracytoplasmic membrane-bound compartments, including vesicles/vacuoles and more-complex systems. Many, if not all, of these compartments formed by FtsZ-independent involution of the cytoplasmic membrane (CM) rather than de novo. Remarkably, while some of the compartments were still continuous with the CM and the periplasm, many were topologically separate, indicating they had been released into the cytoplasm by an endocytic-like membrane fission event. Notably, cells failed to adjust the rate of phospholipid synthesis to their new surface requirements upon depletion of MreBCD, providing a rationale for the "excess" membrane in the resulting spheroids. Both FtsZ and MinD readily assembled on intracytoplasmic membrane surfaces, and we propose that this contributes significantly to the lethal division block seen in all shape mutants under nonpermissive conditions.

Of the morphogenes that make a ring, a rod and a sphere in Escherichia coli

In 1993, William Donachie wrote "The success of molecular genetics in the study of bacterial cell division has been so great that we find ourselves, armed with much greater knowledge of detail, confronted once again with the same naive questions that we set to answer in the first place". Indeed, attempts to answer the apparently simple question of how a bacterial cell divides have led to a wealth of new knowledge, in particular over the past decade and a half. And while some questions have been answered to a great extent since the early reports of isolation of division mutants of Escherichia coli, some key pieces of the puzzle remain elusive. In addition to it being a fundamental process in bacteria that merits investigation in its own right, studying the process of cell division offers an abundance of new targets for the development of new antibacterial compounds that act directly against key division proteins and other components of the cytoskeleton, which are encoded by the morphogenes of E. coli. This review aims to present the reader with a snapshot summary of the key players in E. coli morphogenesis with emphasis on cell division and the rod to sphere transition.

Iron limitation induces SpoT-dependent accumulation of ppGpp in Escherichia coli

In Escherichia coli the beta-lactam mecillinam specifically inhibits penicillin-binding protein 2 (PBP2), a peptidoglycan transpeptidase essential for maintaining rod shape. We have previously shown that PBP2 inactivation results in a cell division block and that an increased concentration of the nucleotide ppGpp, effector of the RelA-dependent stringent response, confers mecillinam resistance and allows cells to divide as spheres in the absence of PBP2 activity. In this study we have characterized an insertion mutation which confers mecillinam resistance in wild-type and DeltarelA strains but not in DeltarelADeltaspoT strains, devoid of ppGpp. The mutant has an insertion in the fes gene, coding for enterochelin esterase. This cytoplasmic enzyme hydrolyses enterochelin-Fe(3+) complexes, making the scavenged iron available to the cells. We show that inactivation of the fes gene causes iron limitation on rich medium plates and a parallel SpoT-dependent increase of the ppGpp pool, as judged by the induction of the iron-regulated fiu::lacZ fusion and the repression of the stringently controlled P1(rrnB)::lacZ fusion respectively. We further show, by direct ppGpp assays, that iron starvation in liquid medium produces a SpoT-dependent increase of the ppGpp pool, strongly suggesting a role for iron in the balance of the two activities of SpoT, synthesis and hydrolysis of (p)ppGpp. Finally, we present evidence that ppGpp exerts direct or indirect positive control on iron uptake, suggesting a simple homeostatic regulatory circuit: iron limitation leads to an increased ppGpp pool, which increases the expression of iron uptake genes, thereby alleviating the limitation.

Biochemistry and comparative genomics of SxxK superfamily acyltransferases offer a clue to the mycobacterial paradox: presence of penicillin-susceptible target proteins versus lack of efficiency of penicillin as therapeutic agent

The bacterial acyltransferases of the SxxK superfamily vary enormously in sequence and function, with conservation of particular amino acid groups and all-alpha and alpha/beta folds. They occur as independent entities (free-standing polypeptides) and as modules linked to other polypeptides (protein fusions). They can be classified into three groups. The group I SxxK D,D-acyltransferases are ubiquitous in the bacterial world. They invariably bear the motifs SxxK, SxN(D), and KT(S)G. Anchored in the plasma membrane with the bulk of the polypeptide chain exposed on the outer face of it, they are implicated in the synthesis of wall peptidoglycans of the most frequently encountered (4-->3) type. They are inactivated by penicillin and other beta-lactam antibiotics acting as suicide carbonyl donors in the form of penicillin-binding proteins (PBPs). They are components of a morphogenetic apparatus which, as a whole, controls multiple parameters such as shape and size and allows the bacterial cells to enlarge and duplicate their particular pattern. Class A PBP fusions comprise a glycosyltransferase module fused to an SxxK acyltransferase of class A. Class B PBP fusions comprise a linker, i.e., protein recognition, module fused to an SxxK acyltransferase of class B. They ensure the remodeling of the (4-->3) peptidoglycans in a cell cycle-dependent manner. The free-standing PBPs hydrolyze D,D peptide bonds. The group II SxxK acyltransferases frequently have a partially modified bar code, but the SxxK motif is invariant. They react with penicillin in various ways and illustrate the great plasticity of the catalytic centers. The secreted free-standing PBPs, the serine beta-lactamases, and the penicillin sensors of several penicillin sensory transducers help the D,D-acyltransferases of group I escape penicillin action. The group III SxxK acyltransferases are indistinguishable from the PBP fusion proteins of group I in motifs and membrane topology, but they resist penicillin. They are referred to as Pen(r) protein fusions. Plausible hypotheses are put forward on the roles that the Pen(r) protein fusions, acting as L,D-acyltransferases, may play in the (3-->3) peptidoglycan-synthesizing molecular machines. Shifting the wall peptidoglycan from the (4-->3) type to the (3-->3) type could help Mycobacterium tuberculosis and Mycobacterium leprae survive by making them penicillin resistant.

Selected amplification of the cell division genes ftsQ-ftsA-ftsZ in Escherichia coli

Rapidly growing Escherichia coli is unable to divide in the presence of the antibiotic mecillinam, whose direct target is penicillin-binding protein 2 (PBP2), responsible for the elongation of the cylindrical portion of the cell wall. Division can be restored in the absence of PBP2 activity by increasing the concentration of the cell division proteins FtsQ, FtsA, and FtsZ. We tried to identify regulators of the ftsQ-ftsA-ftsZ operon among mecillinam-resistant mutants, which include strains overexpressing these genes. By insertional mutagenesis with mini-Tn10 elements, we selected for insertions that conferred mecillinam resistance. Among 15 such mutants, 7 suppressed the thermosensitivity of the ftsZ84(Ts) mutant, strongly suggesting that they had increased FtsZ activity. In all 7 cases, however, the mutants resulted from a duplication of the ftsQAZ region. These duplications seemed to result from multiple events, suggesting that no simple insertional inactivation can result in a mutant with sufficiently amplified ftsQAZ expression to confer mecillinam resistance. The structure of the duplications suggests a general method for constructing directed duplications of precise sequences.

Penicillin and beyond: evolution, protein fold, multimodular polypeptides, and multiprotein complexes

As the protein sequence and structure databases expand, the relationships between proteins, the notion of protein superfamily, and the driving forces of evolution are better understood. Key steps of the synthesis of the bacterial cell wall peptidoglycan are revisited in light of these advances. The reactions through which the D-alanyl-D-alanine depeptide is formed, utilized, and hydrolyzed and the sites of action of the glycopeptide and beta-lactam antibiotics illustrate the concept according to which new enzyme functions evolve as a result of tinkering of existing proteins. This occurs by the acquisition of local structural changes, the fusion into multimodular polypeptides, and the association into multiprotein complexes.

ppGpp accumulation in Pseudomonas aeruginosa and Pseudomonas fluorescens subjected to nutrient limitation and biocide exposure

The effect of a commonly used biocide, 1,2-benzisothiazolin-3-one (BIT) on ppGpp accumulation in the pathogen, Pseudomonas aeruginosa PAO1, and an environmental isolate, Ps. fluorescens, was examined. It is concluded that BIT is able to induce a stringent response in Ps. aeruginosa and Ps. fluorescens, determined by the rapid accumulation of ppGpp following addition of BIT to exponentially-growing cells. Western blot analysis of whole-cell extracts with anti-RelA antibody demonstrated that both species contain a RelA homologue. This is the first report of a RelA-like protein in pseudomonads.

The intrinsic resistance of Escherichia coli to various antimicrobial agents requires ppGpp and sigma s

We have examined the effect of a wide range of antimicrobial compounds (antibiotics and biocides) on the growth of various strains of Escherichia coli which vary in their ability to produce ppGpp and sigma s. We conclude that strains able to synthesize ppGpp, either in a RelA- or SpoT-dependent manner, possess a greater resistance to antimicrobial compounds compared with strains that cannot produce ppGpp. Investigation of an E. coli strain, unable to produce sigma s, and an isogenic parent strain, suggests that there is a requirement for this sigma factor in increased expression of intrinsic resistance. We propose that ppGpp is required to induce production of sigma s, which in turn directs gene expression of intrinsic resistance determinants.

Multimodular penicillin-binding proteins: an enigmatic family of orthologs and paralogs

The monofunctional penicillin-binding DD-peptidases and penicillin-hydrolyzing serine beta-lactamases diverged from a common ancestor by the acquisition of structural changes in the polypeptide chain while retaining the same folding, three-motif amino acid sequence signature, serine-assisted catalytic mechanism, and active-site topology. Fusion events gave rise to multimodular penicillin-binding proteins (PBPs). The acyl serine transferase penicillin-binding (PB) module possesses the three active-site defining motifs of the superfamily; it is linked to the carboxy end of a non-penicillin-binding (n-PB) module through a conserved fusion site; the two modules form a single polypeptide chain which folds on the exterior of the plasma membrane and is anchored by a transmembrane spanner; and the full-size PBPs cluster into two classes, A and B. In the class A PBPs, the n-PB modules are a continuum of diverging sequences; they possess a five-motif amino acid sequence signature, and conserved dicarboxylic amino acid residues are probably elements of the glycosyl transferase catalytic center. The PB modules fall into five subclasses: A1 and A2 in gram-negative bacteria and A3, A4, and A5 in gram-positive bacteria. The full-size class A PBPs combine the required enzymatic activities for peptidoglycan assembly from lipid-transported disaccharide-peptide units and almost certainly prescribe different, PB-module specific traits in peptidoglycan cross-linking. In the class B PBPs, the PB and n-PB modules cluster in a concerted manner. A PB module of subclass B2 or B3 is linked to an n-PB module of subclass B2 or B3 in gram-negative bacteria, and a PB module of subclass B1, B4, or B5 is linked to an n-PB module of subclass B1, B4, or B5 in gram-positive bacteria. Class B PBPs are involved in cell morphogenesis. The three motifs borne by the n-PB modules are probably sites for module-module interaction and the polypeptide stretches which extend between motifs 1 and 2 are sites for protein-protein interaction. The full-size class B PBPs are an assortment of orthologs and paralogs, which prescribe traits as complex as wall expansion and septum formation. PBPs of subclass B1 are unique to gram-positive bacteria. They are not essential, but they represent an important mechanism of resistance to penicillin among the enterococci and staphylococci. Natural evolution and PBP- and beta-lactamase-mediated resistance show that the ability of the catalytic centers to adapt their properties to new situations is limitless. Studies of the reaction pathways by using the methods of quantum chemistry suggest that resistance to penicillin is a road of no return.

Genetic analysis of mecillinam-resistant mutants of Caulobacter crescentus deficient in stalk biosynthesis

Stalk synthesis in Caulobacter crescentus is a developmentally controlled and spatially restricted event that requires the synthesis of peptidoglycan at the stalk-cell body junction. We show that the beta-lactam antibiotic mecillinam prevents stalk synthesis by inhibiting stalk elongation. In addition, mecillinam causes an increase in the diameter of the stalk at the stalk-cell body junction. We describe two mutations that confer resistance to mecillinam and that prevent stalk elongation. These mutations are probably allelic, and they map to a locus previously not associated with stalk synthesis.

Analysis of the effect of ppGpp on the ftsQAZ operon in Escherichia coli

Escherichia coli loses its rod shape by inactivation of PBP2 (penicillin-binding protein 2), target of the beta-lactam mecillinam. Under these conditions, cell division is blocked in rich medium. Division in the absence of PBP2 activity is restored (and resistance to mecillinam is conferred) when the three cell division proteins FtsQ, FtsA and FtsZ are overproduced, but not when only one or two of them are overproduced. Division in the absence of PBP2 activity is also restored by a doubling in the ppGpp pool, as in the argS201 mutant. However, the nucleotide ppGpp, a transcriptional regulator of many operons, does not govern any of the five promoters of the ftsQAZoperon, as shown by S1 mapping of ftsQAZ mRNA 5' ends in exponentially growing wild-type cells in the mecillinam-resistant argS201 mutant (intermediate ppGpp level) or during the stringent response elicited by isoleucine starvation (high ppGpp level). Furthermore, the concentration of FtsZ protein is not increased in exponentially growing mecillinam-resistant argS201 cells. These results show that the ftsQAZ operon is not the ppGpp target responsible for mecillinam resistance. We are currently trying to identify those targets that, at intermediate ppGpp levels, allow cells to divide as spheres in the absence of PBP2.

Morphogenesis of Escherichia coli

The shape of Escherichia coli is strikingly simple compared to those of higher eukaryotes. In fact, the end result of E. coli morphogenesis is a cylindrical tube with hemispherical caps. It is argued that physical principles affect biological forms. In this view, genes code for products that contribute to the production of suitable structures for physical factors to act upon. After introduction of a physical model, the discussion is focused on the shape-maintaining (peptidoglycan) layer of E. coli. This is followed by a detailed analysis of the structural relationship of the cellular interior to the cytoplasmic membrane. A basic theme of this review is that the transcriptionally active nucleoid and the cytoplasmic translation machinery form a structural continuity with the growing cellular envelope. An attempt has been made to show how this dynamic relationship during the cell cycle affects cell polarity and how it leads to cell division.

Two polypeptide products of the Escherichia coli cell division gene ftsW and a possible role for FtsW in FtsZ function

Two new mutations in the cell division gene ftsW have been isolated and characterized. The ftsW263(Ts) mutation results in a block to division at the initiation stage, similar to that previously observed with the ftsW201(Ts) mutation. The ftsW1640(Ts) mutation, however, causes a block to division at a later stage. The ftsW201 and ftsW263 mutants were shown to be phenotypically sensitive to the genetic background and growth conditions and are possibly relA dependent. Immunofluorescence microscopy showed that the FtsZ protein can localize to presumptive division sites in strains carrying ftsW(Ts) mutations at the nonpermissive temperature, suggesting that FtsW is unlikely to be specifically required for the localization of FtsZ to the division site. Examination of the localization of FtsZ in an ftsW rodA double mutant (lemon-shaped cells) revealed several classes of cells ranging from a common class where an FtsZ ring structure is absent to a class where FtsZ forms a complete ring at the midpoint of a lemon-shaped cell, suggesting a role for FtsW in the establishment of a stable FtsZ-based septal structure. We further demonstrate that two FtsW peptides, FtsWL (large) and FtsWS (small), can be identified and that the expression of ftsWS is sufficient for complementation of ftsW(Ts) mutations.

The Escherichia coli cell cycle, cell division and ppGpp: regulation and mechanisms

The literature demonstrating tight regulation of the Escherichia coli cell cycle is reviewed. Recent evidence is presented indicating that the normal rod cell shape can be abandoned, allowing growth as a coccus, either by increasing the amount of the division proteins FtsZ, FtsA and FtsQ, or by increasing the pool of the nucleotide ppGpp. It is argued that ppGpp may be a cell cycle signal in E. coli.

rRNA transcription and growth rate-dependent regulation of ribosome synthesis in Escherichia coli

The synthesis of ribosomal RNA is the rate-limiting step in ribosome synthesis in bacteria. There are multiple mechanisms that determine the rate of rRNA synthesis. Ribosomal RNA promoter sequences have evolved for exceptional strength and for regulation in response to nutritional conditions and amino acid availability. Strength derives in part from an extended RNA polymerase (RNAP) recognition region involving at least two RNAP subunits, in part from activation by a transcription factor and in part from modification of the transcript by a system that prevents premature termination. Regulation derives from at least two mechanistically distinct systems, growth rate-dependent control and stringent control. The mechanisms contributing to rRNA transcription work together and compensate for one another when individual systems are rendered inoperative.

Roles of SpoT and FNR in NH4+ assimilation and osmoregulation in GOGAT (glutamate synthase)-deficient mutants of Escherichia coli

An osmosensitive mutant of Escherichia coli was isolated and shown to harbor two mutations that were together necessary for osmosensitivity. One (ossB) was an insertion mutation in the gltBD operon, which encodes the enzyme glutamate synthase (GOGAT), involved in ammonia assimilation and L-glutamate biosynthesis. The other (ossA) was in the fnr gene, encoding the regulator protein FNR for anaerobic gene expression. Several missense or deletion mutations in fnr and gltBD behaved like ossA and ossB, respectively, in conferring osmosensitivity. A mutation affecting the DNA-binding domain of FNR was recessive to fnr+ with respect to the osmotolerance phenotype but was dominant-negative for its effect on expression of genes in anaerobic respiration. Our results may most simply be interpreted as suggesting the requirement for monomeric FNR during aerobic growth of E. coli in high-osmolarity media, presumably for L-glutamate accumulation via the GOGAT-independent pathway (catalyzed by glutamate dehydrogenase [GDH]), but the mechanism of FNR action is not known. We also found that the spoT gene (encoding guanosine 3',5'-bispyrophosphate [ppGpp] synthetase II/ppGpp-3' pyrophosphohydrolase), in multiple copies, overcomes the defect in NH4+ assimilation associated with GOGAT deficiency and thereby suppresses osmosensitivity in gltBD fnr strains. Enhancement of GDH activity in these derivatives appears to be responsible for the observed suppression. Its likely physiological relevance was established by the demonstration that growth of gltBD mutants (that are haploid for spoT+) on moderately low [NH4+] was restored with the use of C sources poorer than glucose in the medium. Our results raise the possibility that SpoT-mediated accumulation of ppGpp during C-limited growth leads to GDH activation and that the latter enzyme plays an important role in N assimilation in situ hitherto unrecognized from studies on laboratory-grown cultures.

Mecillinam resistance in Escherichia coli is conferred by loss of a second activity of the AroK protein

Mecillinam, a beta-lactam antibiotic specific to penicillin-binding protein 2 (PBP 2) in Escherichia coli, blocks cell wall elongation and, indirectly, cell division, but its lethality can be overcome by increased levels of ppGpp, the nucleotide effector of the stringent response. We have subjected an E. coli K-12 strain to random insertional mutagenesis with a mini-Tn10 element. One insertion, which was found to confer resistance to mecillinam in relA+ and relA strains, was mapped at 75.5 min on the E. coli map and was located between the promoters and the coding sequence of the aroK gene, which codes for shikimate kinase 1, one of two E. coli shikimate kinases, both of which are involved in aromatic amino acid biosynthesis. The mecillinam resistance conferred by the insertion was abolished in a delta relA delta spoT strain completely lacking ppGpp, and it thus depends on the presence of ppGpp. Furthermore, the insertion increased the ppGpp pool approximately twofold in a relA+ strain. However, this increase was not observed in relA strains, although the insertion still conferred mecillinam resistance in these backgrounds, showing that mecillinam resistance is not due to an increased ppGpp pool. The resistance was also abolished in an ftsZ84(Ts) strain under semipermissive conditions, and the aroK::mini-Tn10 allele partially suppressed ftsZ84(Ts); however, it did not increase the concentration of the FtsZ cell division protein. The insertion greatly decreased or abolished the shikimate kinase activity of AroK in vivo and in vitro. The two shikimate kinases of E. coli are not equivalent; the loss of AroK confers mecillinam resistance, whereas the loss of Arol, does not. Furthermore, the ability of the aroK mutation to confer mecillinam resistance is shown to be independent of polar effects on operon expression and of effects on the availability of aromatic amino acids or shikimic acid. Instead, we conclude that the AroK protein has a second activity, possibly related to cell division regulation, which confers mecillinam sensitivity. We were able to separate the AroK activities mutationally with an aroK mutant allele lacking shikimate kinase activity but still able to confer mecillinam sensitivity.

Control of rRNA transcription in Escherichia coli

The control of rRNA synthesis in response to both extra- and intracellular signals has been a subject of interest to microbial physiologists for nearly four decades, beginning with the observations that Salmonella typhimurium cells grown on rich medium are larger and contain more RNA than those grown on poor medium. This was followed shortly by the discovery of the stringent response in Escherichia coli, which has continued to be the organism of choice for the study of rRNA synthesis. In this review, we summarize four general areas of E. coli rRNA transcription control: stringent control, growth rate regulation, upstream activation, and anti-termination. We also cite similar mechanisms in other bacteria and eukaryotes. The separation of growth rate-dependent control of rRNA synthesis from stringent control continues to be a subject of controversy. One model holds that the nucleotide ppGpp is the key effector for both mechanisms, while another school holds that it is unlikely that ppGpp or any other single effector is solely responsible for growth rate-dependent control. Recent studies on activation of rRNA synthesis by cis-acting upstream sequences has led to the discovery of a new class of promoters that make contact with RNA polymerase at a third position, called the UP element, in addition to the well-known -10 and -35 regions. Lastly, clues as to the role of antitermination in rRNA operons have begun to appear. Transcription complexes modified at the antiterminator site appear to elongate faster and are resistant to the inhibitory effects of ppGpp during the stringent response.

Direct correlation between overproduction of guanosine 3',5'-bispyrophosphate (ppGpp) and penicillin tolerance in Escherichia coli

The penicillin tolerance exhibited by amino acid-deprived Escherichia coli has been previously proposed to be a consequence of the stringent response. Evidence indicating that penicillin tolerance is directly attributable to guanosine 3',5'-bispyrophosphate (ppGpp) overproduction and not to some other effect of amino acid deprivation is now presented. Accumulation of ppGpp in the absence of amino acid deprivation was achieved by the controlled overexpression of the cloned relA gene, which encodes ppGpp synthetase I. The overproduction of ppGpp resulted in the inhibition of both peptidoglycan and phospholipid synthesis and in penicillin tolerance. The minimum concentration of ppGpp required to establish these phenomena was determined to be 870 pmol per mg (dry weight) of cells. This represented about 70% of the maximum level of ppGpp accumulated during the stringent response. Penicillin tolerance and the inhibition of peptidoglycan synthesis were both suppressed when ppGpp accumulation was prevented by treatment with chloramphenicol, an inhibitor of ppGpp synthetase I activation. Glycerol-3-phosphate acyltransferase, the product of plsB, was recently identified as the main site of ppGpp inhibition in phospholipid synthesis (R. J. Health, S. Jackowski, and C. O. Rock, J. Biol. Chem. 269:26584-26590, 1994). The overexpression of the cloned plsB gene reversed the penicillin tolerance conferred by ppGpp accumulation. This result supports previous observations indicating that the membrane-associated events in peptidoglycan metabolism were dependent on ongoing phospholipid synthesis. Interestingly, treatment with beta-lactam antibiotics by itself induced ppGpp accumulation, but the maximum levels attained were insufficient to confer penicillin tolerance.

Involvement of cysB and cysE genes in the sensitivity of Salmonella typhimurium to mecillinam

cysB and cysE strains were obtained as spontaneous mecillinam-resistant mutants of Salmonella typhimurium. The resistance to mecillinam was caused by the cys mutations which also conferred tolerance to lethal cell shape mutations. Most, but not all, cysB and cysE mutations from other origins displayed the same behavior. Resistance was abolished by O- and N-acetylserine in cysE mutants; by thiosulfate, sulfite, and sulfide in cysB mutants; and by cysteine in both types of mutants. It is concluded that an event involved in mecillinam action requires the inducer and the activator protein of the cysteine regulon.

Resistance to mecillinam produced by the co-operative action of mutations affecting lipopolysaccharide, spoT, and cya or crp genes of Salmonella typhimurium

Lipopolysaccharide (LPS), spoT, and cya or crp mutations individually do not affect the minimum inhibitory concentration of mecillinam on Salmonella typhimurium. However, when mutations of two of these types were combined in the same strain, high-level resistance appeared, and increased even further when all three types of mutations were present. Most mutations affecting LPS (rfa, rfb, rfc) showed this behaviour, although to different degrees. The highest resistance to mecillinam was caused by galE and rfc mutations whereas almost no effect was noticed with rfaB or rfaK mutations. This phenomenon appears to be specific for mecillinam since none of several other antibiotics elicited it. Reduction of guanosine tetraphosphate (ppGpp) levels by introduction of a relA mutation did not significantly affect the MIC of mecillinam on strains carrying different combinations of spoT, galE, and cya or crp mutations. All the strains produced spherical cells in medium with a low concentration (0.05 microgram ml-1) of the antibiotic. These results suggest that the antibacterial action of mecillinam on S. typhimurium is somehow dependent on the interaction of LPS, cyclic AMP/cyclic AMP receptor protein (cAMP/CRP), and SpoT. The reported resistance to mecillinam of cya and crp mutants of Escherichia coli K-12 is probably due to the natural LPS defectiveness of this strain.