Bistability in bacteria

Gene expression in bacteria is traditionally studied from the average behaviour of cells in a population, which has led to the assumption that under a particular set of conditions all cells express genes in an approximately uniform manner. The advent of methods for visualizing gene expression in individual cells reveals, however, that populations of genetically identical bacteria are sometimes heterogeneous, with certain genes being expressed in a non-uniform manner across the population. In some cases, heterogeneity is manifested by the bifurcation into distinct subpopulations, and we adopt the common usage, referring to this phenomenon as bistability. Here we consider four cases of bistability, three from Bacillus subtilis and one from Escherichia coli, with an emphasis on random switching mechanisms that generate alternative cell states and the biological significance of phenotypic heterogeneity. A review describing additional examples of bistability in bacteria has been published recently.     

Allolysis in bacteria

The review deals with the phenomenon of allolysis, i.e., lysis of a part of a bacterial population induced by a group of epigenetically differentiated cells of the same species or phylotype. Allolysis is best studied in two species of gram-positive bacteria, Streptococcus pneumoniae and Bacillus subtilis. In S. pneumoniae, allolysis is associated with the onset of the competence stage, while in B. subtilis it is associated with transition to the stage of spore formation. The mechanisms of allolysis are considered, as well as its possible role in the populational and symbiotic relationships of bacterial cells. The relation between allolysis ant the programmed death of a part of the cells within a bacterial population (apoptosis) is discussed.     

Lipoproteins in bacteria

Covalent modification of membrane proteins with lipids appears to be ubiquitous in all living cells. The major outer membrane (Braun's) lipoprotein ofE. coli, the prototype of bacterial lipoproteins, is first synthesized as a precursor protein. Analysis of signal sequences of 26 distinct lipoprotein precursors has revealed a consensus sequence of lipoprotein modification/processing site of Leu-(Ala, Ser)-(Gly, Ala)-Cys at – 3 to + 1 positions which would represent the cleavage region of about three-fourth of all lipoprotein signal sequences in bacteria. Unmodified prolipoprotein with the putative consensus sequence undergoes sequential modification and processing reactions catalyzed by glyceryl transferase, O-acyl transferase(s), prolipoprotein signal peptidase (signal peptidase II), and N-acyl transferase to form mature lipoprotein. Like all exported proteins, the export of lipoprotein requires functional SecA, SecY, and SecD proteins. Thus all precursor proteins are exported through a common pathway accessible to both signal peptidase I and signal peptidase II. The rapidly increasing list of lipid-modified proteins in both prokaryotic as well as eukaryotic cells indicates that lipoproteins comprise a diverse group of structurally and functionally distinct proteins. They share a common structural feature which is derived from a common biosynthetic pathway.     

Poly-gamma-glutamate in bacteria

Poly-gamma-glutamate (PGA), a natural polymer, is synthesized by several bacteria (all Gram-positive), one archaea and one eukaryote. PGA has diverse biochemical properties, enabling it to play different roles, depending on the organism and its environment. Indeed, PGA allows bacteria to survive at high salt concentrations and may also be involved in virulence. The minimal gene sets required for PGA synthesis were recently defined. There are currently two nomenclatures depending on the PGA final status: cap, for 'capsule', when PGA is surface associated or pgs, for 'polyglutamate synthase', when PGA is released. The minimal gene sets contain four genes termed cap or pgs B, C, A and E. The PGA synthesis complex is membrane-anchored and uses glutamate and ATP as substrates. Schematically, the reaction may be divided into two steps, PGA synthesis and PGA transport through the membrane. PGA synthesis depends primarily on CapB-CapC (or PgsB-PgsC), whereas PGA transport requires the presence, or the addition, of CapA-CapE (or PgsAA-PgsE). The synthesis complex is probably responsible for the stereochemical specificity of PGA composition. Finally, PGA may be anchored to the bacterial surface or released. An additional enzyme is involved in this reaction: either CapD, a gamma-glutamyl-transpeptidase that catalyses anchorage of the PGA, or PgsS, a hydrolase that facilitates release. The anchoring of PGA to the bacterial surface is important for virulence. All cap genes are therefore potential targets for inhibitors specifically blocking PGA synthesis or anchorage.     

Aging in bacteria

Analysis of senescent Escherichia coli cells reveals a link between protein oxidation and the fidelity of the translational apparatus. This model system has also provided a mechanistic molecular explanation for a trade-off between reproduction and survival activities, which may inspire proponents of the disposable soma theory and antagonistic pleiotropy hypothesis of aging.     

Ultrastructure of bacteria and the proportion of Gram-negative bacteria in marine sediments

Bacteria in sediments from the surface aerobic layer (0–1 cm) and a deeper anaerobic layer (20–21 cm) of a seagrass bed were examined in section by transmission electron microscopy. Bacteria with a Gram-negative ultrastructure made up 90% of bacteria in the surface layer, and Gram-positive bacteria comprised 10%. In the anaerobic zone, Gram-negative bacteria comprised 70% and Gram-positive bacteria 30% of the bacterial population. These differences were highly significant and support predictions of these proportions made from muramic acid measurements and direct counting with fluorescence microscopy. Most cells were enveloped in extracellular slime layers or envelopes, some with considerable structural complexity. The trophic value to animals of these envelopes is discussed. A unique organism with spines was observed.     

Interactions between amino-acid-degrading bacteria and methanogenic bacteria in anaerobic digestion

The degradation of amino acids in anaerobic digestion was examined in terms of the interactions between amino-acid-degrading bacteria and methanogenic bacteria. Certain amino acids were degraded oxidatively by dehydrogenation, with methanogenic bacteria acting as H(2) acceptors. The inhibition of methanogenesis by chloroform also inhibited the degradation of these amino acids and/or caused variations in the composition of volatile acids produced from them. The presence of glycine reduced the inhibitory effect caused by chloroform, probably because glycine acted as an H(2) acceptor in place of methanogenic bacteria. This fact suggested that the coupled oxidation-reduction reactions between two amino acids-one acting as the H(2) donor and the other acting as the H(2) acceptor-may occur in the anaerobic digestion of proteins or amino-acid mixtures. The conversion of some proteins to volatile acids was not affected when methanogenesis was inhibited by chloroform. This suggested that the component amino acids of proteins may be degraded by the coupled oxidation-reduction reactions and that the degradation of proteins may not be dependent on the activity of methanogenic bacteria as H(2) acceptors.     

Competitive reaction kinetics of sulfate-reducing bacteria and methanogenic bacteria in anaerobic filters

A kinetic model for the anaerobic filter (AF) that takes into account the mass fractions of sulfate-reducing bacteria (SRB) (f(SRB)) and methanogenic bacteria (MB) (f(MB)) and an inhibiting effect of H(2)S on bacterial groups is proposed. When the acetate-fed AFs were maintained at the low organic loading rate of 2.5kg COD/m(3)d, variations of the influent COD/SO(4)(2-) ratio (0.5-3.0) does not materially affect the acetate removal efficiency (all varying between 98.1% and 99.7%). With an increase in influent COD/SO(4)(2-) ratio, both the biofilm thickness and the specific substrate utilization rate decreased slightly but f(SRB) decreased markedly. The estimated results of f(SRB) and f(MB) showed that SRB out-competed MB for bacterial growth if the influent COD/SO(4)(2-) ratio was maintained at less than 1.3, whereas MB out-competed SRB for bacterial growth if the influent COD/SO(4)(2-) ratio was maintained at greater than 2.0. The specific substrate utilization rate of SRB (0.19-0.24mg acetate/mg VSSd) was lower than that of MB (0.31-0.59mg acetate/mg VSSd). The estimated kinetic parameters disclosed that the affinity of acetate to MB was higher and unionized H(2)S imposed a greater inhibiting effect on MB. The model simulation results (acetate and sulfate removal) agreed well with the experimental results.     

Factors affecting survival of test bacteria in sea water: marine bacteria,test bacteria and solid surfaces

Summary 1. The antibacterial activity of raw sea water varied considerably during incubation of successive inocula ofEscherichia coli, Staphylococcus aureus, andSerratia marinorubra. In most cases inactivation of second inocula was stronger than that of first ones. However, withS. aureus, contradictory results were obtained also.2. The bactericidal effect of filter-sterilized sea water was strengthened by inactivated cells ofEscherichia coli andStaphylococcus aureus. Contradictory findings were obtained from autoclaved sea water.3. Inactivation of test bacteria was greatly influenced by solid surfaces. In most cases, the kill ofEscherichia coli andStaphylococcus aureus in raw and sterile-filtered sea water was stronger at increased surface/volume ratios than under standard conditions. More rapid inactivation of these test strains in sterile-filtered, than in raw, sea water occurred more often at enlarged ratios of solid surface per unit volume. The survival ofSerratia marinorubra was positively affected by solid surfaces.4. It is concluded that changes in nutritive conditions occurring during the experiments are more important in regard to antibacterial activity of sea water than production of harmful matter by marine bacteria.

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Faktoren, welche das Überleben von Testbakterien in Meerwasser beeinflussen: Meeresbakterien, Testbakterien und feste Oberflächen

Kurzfassung Der Einfluß der vorstehend genannten Faktoren wurde auf die Überlebensfähigkeit vonEscherichia coli, Staphylococcus aureus undSerratia marinorubra in Meerwasser untersucht. Aktivitäten mariner Bakterien führten nicht generell zu verstärkter antibakterieller Wirkung rohen Meerwassers. Häufig waren sie für das Überleben vonE. coli undS. aureus förderlich. Inaktivierte Zellen vonE. coli undS. aureus erhöhten die bakterizide Wirkung rohen und filtersterilisierten Meerwassers gegenüber sekundär inokulierten, gleichartigen Testbakterien, während sie die inaktivierende Potenz autoklavierten Meerwassers verminderten. Durch erhöhtes Angebot an Glasoberfläche/Volumeneinheit, welches die adsorptive Anreicherung organischer Substanzen verstärkt, wurde die Inaktivation vonE. coli undS. aureus meistens beschleunigt, während sich diejenige vonS. marinorubra um so stärker verminderte, je größer das Verhältnis Oberfläche/Volumen war. Raschere Abtötung vonE. coli undS. aureus in Sterilfiltraten als in rohem Meerwasser trat bei erhöhtem Oberfläche/Volumen-Verhältnis häufiger auf als unter Standardbedingungen. Aus den Ergebnissen wird geschlossen, daß die während der Versuche eintretenden Veränderungen des Nährstoffangebotes, hervorgerufen durch Nährstoffverbrauch sowie durch Lysis inaktivierter Testbakterien, bezüglich der bakteriziden Wirkung von Meerwasser generell von größerer Bedeutung sind als bakterizide Stoffwechselprodukte mariner Bakterien.

     

Biosynthesis of phosphatidylcholine in bacteria

Phosphatidylcholine (PC) is the major membrane-forming phospholipid in eukaryotes and can be synthesized by either of two pathways, the methylation pathway or the CDP-choline pathway. Many prokaryotes lack PC, but it can be found in significant amounts in membranes of rather diverse bacteria and based on genomic data, we estimate that more than 10% of all bacteria possess PC. Enzymatic methylation of phosphatidylethanolamine via the methylation pathway was thought to be the only biosynthetic pathway to yield PC in bacteria. However, a choline-dependent pathway for PC biosynthesis has been discovered in Sinorhizobium meliloti. In this pathway, PC synthase, condenses choline directly with CDP-diacylglyceride to form PC in one step. A number of symbiotic (Rhizobium leguminosarum, Mesorhizobium loti) and pathogenic (Agrobacterium tumefaciens, Brucella melitensis, Pseudomonas aeruginosa, Borrelia burgdorferi and Legionella pneumophila) bacteria seem to possess the PC synthase pathway and we suggest that the respective eukaryotic host functions as the provider of choline for this pathway. Pathogens entering their hosts through epithelia (Streptococcus pneumoniae, Haemophilus influenzae) require phosphocholine substitutions on their cell surface components that are biosynthetically also derived from choline supplied by the host. However, the incorporation of choline in these latter cases proceeds via choline phosphate and CDP-choline as intermediates. The occurrence of two intermediates in prokaryotes usually found as intermediates in the eukaryotic CDP-choline pathway for PC biosynthesis raises the question whether some bacteria might form PC via a CDP-choline pathway.