Bacterial sporulation as a modified procaryotic cell division

Bacterial spore structures and their protective role in biocide resistance

The structure and chemical composition of bacterial spores differ considerably from those of vegetative cells. These differences largely account for the unique resistance properties of the spore to environmental stresses, including disinfectants and sterilants, resulting in the emergence of spore-forming bacteria such as Clostridium difficile as major hospital pathogens. Although there has been considerable work investigating the mechanisms of action of many sporicidal biocides against Bacillus subtilis spores, there is far less information available for other species and particularly for various Clostridia. This paucity of information represents a major gap in our knowledge given the importance of Clostridia as human pathogens. This review considers the main spore structures, highlighting their relevance to spore resistance properties and detailing their chemical composition, with a particular emphasis on the differences between various spore formers. Such information will be vital for the rational design and development of novel sporicidal chemistries with enhanced activity in the future.

Regulation of growth of the mother cell and chromosome replication during sporulation of Bacillus subtilis

During spore formation, Bacillus subtilis divides asymmetrically, resulting in two cells with different fates. Immediately after division, the transcription factor σ(F) becomes active in the smaller prespore, followed by activation of σ(E) in the larger mother cell. We recently showed that a delay in σ(E) activation resulted in the novel phenotype of two spores (twins) forming within the same mother cell. Mother cells bearing twins are substantially longer than mother cells with single spores. Here we explore the regulation of the growth and DNA replication of the mother cell. We find that length correlates with chromosome number in the mother cell. We show that replication and growth could occur after asymmetric division in mother cells with no active σ(E). In contrast, when σ(E) was active, replication and growth ceased. In growing mother cells, with no active σ(E), Spo0A-directed transcription levels remained low. In the presence of active σ(E), Spo0A-directed gene expression was enhanced in the mother cells. Artificial Spo0A activation blocked mother cell growth in the absence of σ(E). Spo0A activation blocked growth even in the absence of SirA, the Spo0A-directed inhibitor of the initiation of replication. Together, the results indicate that the burst of Spo0A-directed expression along with the activation of σ(E) provides mechanisms to block the DNA replication and growth of the mother cell.

Compartmentalization of gene expression during Bacillus subtilis spore formation

Gene expression in members of the family Bacillaceae becomes compartmentalized after the distinctive, asymmetrically located sporulation division. It involves complete compartmentalization of the activities of sporulation-specific sigma factors, sigma(F) in the prespore and then sigma(E) in the mother cell, and then later, following engulfment, sigma(G) in the prespore and then sigma(K) in the mother cell. The coupling of the activation of sigma(F) to septation and sigma(G) to engulfment is clear; the mechanisms are not. The sigma factors provide the bare framework of compartment-specific gene expression. Within each sigma regulon are several temporal classes of genes, and for key regulators, timing is critical. There are also complex intercompartmental regulatory signals. The determinants for sigma(F) regulation are assembled before septation, but activation follows septation. Reversal of the anti-sigma(F) activity of SpoIIAB is critical. Only the origin-proximal 30% of a chromosome is present in the prespore when first formed; it takes approximately 15 min for the rest to be transferred. This transient genetic asymmetry is important for prespore-specific sigma(F) activation. Activation of sigma(E) requires sigma(F) activity and occurs by cleavage of a prosequence. It must occur rapidly to prevent the formation of a second septum. sigma(G) is formed only in the prespore. SpoIIAB can block sigma(G) activity, but SpoIIAB control does not explain why sigma(G) is activated only after engulfment. There is mother cell-specific excision of an insertion element in sigK and sigma(E)-directed transcription of sigK, which encodes pro-sigma(K). Activation requires removal of the prosequence following a sigma(G)-directed signal from the prespore.

The chromosomal location of the Bacillus subtilis sporulation gene spoIIR is important for its function

Formation of the asymmetrically located septum during sporulation of Bacillus subtilis results in enclosure of the origin-proximal 30% of the chromosome in the prespore compartment. The rest of the chromosome is then translocated into the prespore from the mother cell. Transcription of spoIIR is initiated in the prespore by RNA polymerase containing sigma(F) soon after the septum is formed. The SpoIIR protein is required for the activation of the transcription program directed by sigma(E) in the mother cell. The spoIIR locus is located at 324 degrees, near the origin of replication (0/360 degrees ). We show here that movement of spoIIR to 28 degrees had little effect on sporulation. However, movement to regions not in the origin-proximal part of the chromosome substantially reduced sporulation efficiency. At 283 degrees sporulation was reduced to less than 20% of the level obtained when spoIIR was at its natural location, and movement to 190 degrees reduced sporulation to about 6% of that level. These positional effects were also seen in the transcription of a spoIIR-lacZ fusion. In contrast, movement of other spo-lacZ fusions from 28 degrees to 190 degrees had little effect on their expression. These results suggest that spoIIR is the subject of "positional regulation," in the sense that the chromosomal position of spoIIR is important for its expression and function.

The Bacillus SpoIIGA protein is targeted to sites of spore septum formation in a SpoIIE-independent manner

The process of bacterial cell division involves the assembly of a complex of proteins at the site of septation that probably provides both the structural and the cytokinetic functions required for elaboration and closure of the septal annulus. During sporulation in Bacillus subtilis, this complex of proteins is modified by the inclusion of a sporulation-specific protein, SpoIIE, which plays a direct role in gene regulation and also has a genetically separable role in determining the gross structural properties of the specialized sporulation septum. We demonstrate by both green fluorescent protein (GFP) fusions and indirect immunofluorescence microscopy that SpoIIGA, a protein required for proteolytic cleavage of pro-sigmaE, is also targeted to the sporulation septum. Septal localization of SpoIIGA-GFP occurred even in the structurally abnormal septum formed by a SpoIIE null mutant. We also report the isolation of a spoIIGA homologue from Bacillus megaterium, a species in which the cells are significantly larger than those of B. subtilis. We have exploited the physical dimensions of the B. megaterium sporangium, in conjunction with wide-field deconvolution microscopy, to construct three-dimensional projections of sporulating cells. These projections indicate that SpoIIGA-GFP is initially localized in an annulus at the septal periphery and is only later localized uniformly throughout the septa. Localization was also detected in a B. subtilis spo0H null strain that fails to construct a spore septum. We propose that SpoIIGA is sequestered in the septum by an interaction with components of the septation machinery and that this interaction begins before the construction of the asymmetric septum.

Purification of human recombinant interleukin 1 receptor antagonist proteins upon Bacillus subtilis sporulation

Human interleukin 1 receptor antagonist (IL-1ra) and IL-1ra mutants were constitutively expressed in recombinant Bacillus subtilis in endocellular and active form. In order to optimize the purification of the recombinant proteins, a new method has been developed. After bacterial growth in fermenter, release of recombinant protein was achieved by starvation-induced sporulation. The sporulation supernatant was recovered by centrifugation, filtered, and subjected sequentially to cation- and anion-exchange chromatography. Alternatively, the fermenter's contents were directly subjected to expanded bed adsorption on a Streamline cation-exchange column, thus avoiding the centrifugation and filtration steps. Up to 88 mg of biological active purified recombinant protein per liter of culture was obtained, with a 72-79% recovery and 98% purity, depending on the molecule. By using the method described here, it is possible to achieve a spontaneous release of recombinant proteins expressed endocellularly at high levels in B. subtilis without need of a cell breakage step. Thus, this method could allow purification of the endocellular recombinant protein as if it were secreted. Furthermore, when using the expanded bed adsorption, highly purified protein was obtained in only two steps after sporulation. Among the advantages of the method, one of the most relevant is the possibility of keeping the system closed up to completion of the first purification step.

Determination of cell fate in Bacillus subtilis

On starvation, the soil bacterium Bacillus subtilis stops dividing and initiates sporulation, a simple developmental process involving the differentiation of two cell types. Sporulation begins with a reorganization of the cell cycle, to produce cells with the size and chromosome content appropriate for the developmental process. The central division that would normally occur, to produce a pair of identical daughter cells, is blocked and the cell divides asymmetrically to produce a small, polar prespore cell and a much larger mother cell. The developmental fates of the two cells are dictated by the localized activation of cell-specific transcription factors, which are controlled by mechanisms that respond to the cellular asymmetry.

Characterization of cell cycle events during the onset of sporulation in Bacillus subtilis

To elucidate the process of asymmetric division during sporulation of Bacillus subtilis, we have measured changes in cell cycle parameters during the transition from vegetative growth to sporulation. Because the propensity of B. subtilis to grow in chains of cells precludes the use of automated cell-scanning devices, we have developed a fluorescence microscopic method for analyzing cell cycle parameters in individual cells. From the results obtained, and measurements of DNA replication fork elongation rates and the escape time of sporulation from the inhibition of DNA replication, we have derived a detailed time scale for the early morphological events of sporulation which is mainly consistent with the cell cycle changes expected following nutritional downshift. The previously postulated sensitive stage in the DNA replication cycle, beyond which the cell is unable to sporulate without a new cell cycle, could represent a point in the division cycle at which the starved cell cannot avoid attaining the initiation mass for DNA replication and thus embarking on another round of the cell cycle. The final cell cycle event, formation of the asymmetric spore septum, occurs at about the time in the cell cycle at which the uninduced cell would have divided centrally, in keeping with the view that spore septation is a modified version of vegetative division.

A conjugation-like mechanism for prespore chromosome partitioning during sporulation in Bacillus subtilis

Spore formation in Bacillus subtilis begins with an asymmetric cell division that superficially resembles the division of vegetative cells. Mutations in the spoIIIE gene of B. subtilis partially block partitioning of one chromosome into the smaller (prespore) compartment of the sporulating cell. Point mutations that specifically block prespore chromosome partitioning affect a carboxy-terminal domain of SpoIIIE that shows significant sequence similarity to the DNA transfer (Tra) proteins of several conjugative plasmids of Streptomyces. In wild-type sporulating cells, the prespore chromosome passes through an intermediate stage resembling the state in which spoIIIE mutant cells are blocked. The prespore chromosome is then transferred progressively through the newly formed spore septum. We propose that translocation of the prespore chromosome occurs by a mechanism that is functionally related to the conjugative transfer of plasmid DNA.

The importance of morphological events and intercellular interactions in the regulation of prespore-specific gene expression during sporulation in Bacillus subtilis

We have established a time course for the early morphological events of sporulation in Bacillus subtilis and related this to changes in gene expression, particularly those occurring in the prespore compartment. We have also systematically studied the effects of mutations in various regulatory (spo) genes on prespore-specific gene expression. On the basis of these results, and those of other laboratories, at least four distinct temporal classes of prespore-specific gene expression can now be distinguished. The first class begins within 15 min of the formation of the spore septum, and requires the sigma F form of RNA polymerase. The second class, also directed by RNA polymerase containing sigma F, begins soon after the completion of prespore engulfment, and depends on an intercellular signal from the mother cell. This transcription results in synthesis of sigma G. However, sigma G activity, directing the third class of gene expression, appears only about 30 min later and is dependent on the completion of prespore engulfment and on further interactions with the mother cell. The fourth class of gene expression has been described. The results demonstrate that the prespore programme of gene expression incorporates a series of control points modulated by information from the mother cell and on progress through the morphogenetic process.

Identification of Bacillus subtilis genes for septum placement and shape determination

The Bacillus subtilis divIVB1 mutation causes aberrant positioning of the septum during cell division, resulting in the formation of small, anucleate cells known as minicells. We report the cloning of the wild-type allele of divIVB1 and show that the mutation lies within a stretch of DNA containing two open reading frames whose predicted products are in part homologous to the products of the Escherichia coli minicell genes minC and minD. Just upstream of minC and minD, and in the same orientation, are three genes whose products are homologous to the products of the E. coli shape-determining genes mreB, mreC, and mreD. The B. subtilis mreB, mreC, and mreD genes are the site of a conditional mutation (rodB1) that causes the production of aberrantly shaped cells under restrictive conditions. Northern (RNA) hybridization experiments and disruption experiments based on the use of integrational plasmids indicate that the mre and min genes constitute a five-cistron operon. The possible involvement of min gene products in the switch from medial to polar placement of the septum during sporulation is discussed.

Achievement of complete Bacillus subtilis microcycle sporulation by the addition of S-adenosylmethionine and phospholipids

In an attempt to find factors that may be responsible for the initiation of sporulation, a system in which the germination and outgrowth phases were separate was applied to Bacillus subtilis. Outgrowth of the germinated spores to only the primary singlet cells was followed in chemically defined medium. Addition of specific metabolites induced the primary singlet cells to sporulate via microcycle sporulation. Experiments are described that led to complete sporulation by the addition of diaminopimelic acid, S-adenosyl-L-methionine and phosphatidylethanolamine.

A model for asymmetric septum formation during sporulation in Bacillus subtilis

Many differentiation processes in both prokaryotes and eukaryotes begin with an asymmetric division, producing 'daughter' cells that differ in size and developmental fate. This is particularly obvious in the well-studied prokaryotic life cycles of Caulobacter and Bacillus. In no system, however, is the mechanism of asymmetric division understood. Here I propose a model for the mechanism of asymmetric division during sporulation in Bacillus subtilis. The model explains both the timing and asymmetric localization of spore-septum formation. It also explains the morphological phenotypes of various asporogenous (spo) mutants.

Transcriptional regulation of the spo0F gene of Bacillus subtilis

We have cloned the early sporulation gene spo0F, which encodes an open reading frame of 124 codons. The putative Spo0F protein derived from this open reading frame, which has been shown to share homology with the Spo0A protein as well as several other regulatory proteins from Escherichia coli, Salmonella typhimurium, and Klebsiella pneumoniae, also shares homology with the E. coli EcoRI methyltransferase. We have shown by S1 nuclease mapping of in vivo transcripts that spo0F is regulated from dual promoters: RNA II was transcribed from an upstream promoter, and RNA I was initated 30 base pairs downstream from RNA II. The promoter sequences for RNA II, but not those for RNA I, conformed to the -10 region consensus sequence for sigma 43 promoters. RNA II was found in low amounts in exponentially growing cells but was not observed in stationary-phase cells, and the presence of RNA II was glucose insensitive. RNA I was found in low amounts in exponentially growing cells, increased three- to fivefold at the end of exponential growth, and remained at this higher level for at least 3 h into stationary phase. RNA I was repressed by glucose during exponential growth but not during stationary phase.

A model of bacterial DNA segregation based upon helical geometry

A new mechanism to segregate daughter genomes in bacterial cells is suggested that is based upon the rules of geometry governing the helix clock (Mendelson, 1982a). The reorientation of cell surface string arrays used as a timing reference in the helix clock is capable of drawing apart the initial products of DNA replication. Physically linking the sister DNA replication origins to the ends of the initial cell surface string inserted into the cell surface at the start of a helix clock cycle, and linking the DNA terminus to a point along the length of the same string provides a means to mark the locations to which the genomes will segregate as well as the place where cell division will occur. The parallel packing of additional cell surface strings into an array which includes the string to which DNA is attached provides the necessary spatial rearrangements. The helical segregation model can account for the precise registration of cell divisions with the completion of replication forks in a multifork replication system, provides a basis for determining the relationship of sister cell sizes at division, and can also accommodate the asymmetrical divisions associated with minicell production and sporulation. Examination of the helical segregation theory under multifork DNA replication conditions moreover reveals that adjacent helical clocks are physically linked to one another although totally independent in terms of their progression through the clock cycle. A relationship between the initiation of DNA replication forks and the insertion of the first cell surface string associated with the start of a helix clock cycle is predicted by the model.

Teichoic acid and lipid metabolism during sporulation of Bacillus megaterium KM

The biochemistry of teichoic acid and lipid metabolism has been studied during sporulation of Bacillus megaterium KM. Measurements of cell-wall and membrane teichoic acid have shown that net synthesis of these polymers ceases at the onset of sporulation. Pulse-labelling studies show that the period of asymmetric septation and forespore engulfment is marked by an initiation of turnover of membrane teichoic acid but not of wall teichoic acid. This is reflected in the presence of inner-membrane teichoic acid and the virtual absence of wall teichoic acid in dormant spores. The total amount of lipid phosphorus in the sporulating cell increases by 70% as a result of asymmetric septation and subsequent engulfment of the forespore. The phosphorus requirement for this synthesis is derived from a pool formed during exponential growth, which is not exchangeable with extracellular Pi during sporulation. These results suggest that during sporulation a proportion of the glycerol 3-phosphate produced by preferential degradation of membrane teichoic acid formed during exponential growth is used for phospholipid synthesis during sporulation.

Comparative macromolecular composition of filaments and rods of a Bacillus megaterium thermoconditional morphological mutant

Filaments of a thermosensitive Bacillus megaterium mutant showed an altered macromolecular composition compared with salt-cured mutant cells and parental cells. Filaments contained more peptidoglycan, polyglucose, poly-beta-hydroxy-butyrate, and deoxyribonucleic acid per unit of protein. The ribonucleic acid-to-protein ratio of filaments was similar to that of rods or salt-cured cells. Filament formation seemed to be due to defective protein or ribonucleic acid metabolism.

Isolation and characterization of fusidic acid-resistant, sporulation-defective mutants of Bacillus subtilis

Fusidic acid-resistant, sporulation-defective mutants were isolated from Bacillus subtilis 168 thy trp. About two-thirds of the fusidic acid-resistant (fusr) mutants were defective in sporulation ability and fell into three classes with respect to sporulation character. The representative mutants FUS426 and FUS429 were characterized in detail. FUS426 [fusr spo (Ts)], a temperature-sensitive sporulation mutant, grew well at 30 and 42 degrees C but did not sporulate at 42 degrees C. FUS429 [fusr spo (Con)], conditional sporulation mutant, grew and sporulated normally in the absence of fusidic acid, but its sporulation and growth rates decreased in the presence of fusidic acid, depending on the concentration of the drug. Although electron microscopic observation showed that both mutants were blocked at stage I of sporulation, the physiological analyses indicate that these mutants belong to the SpoOB class. Both mutants formed a thickened cell wall as compared with that of the parental strain. Genetic and in vitro protein synthesis analyses led to the conclusion that the sporulation-defective character of mutants FUS426 and FUS429 resulted from an alteration in elongation factor G caused by a single lesion in the fus locus. The possible role of elongation factor G in sporulation is discussed.

Order of expression of genes affecting septum location during sporulation of Bacillus subtilis

The mean volumes of stationary-phase cells of wild-type and asporogenous mutants of Bacillus subtilis have been measured. Mutants blocked at stage 0 of sporulation either produced cells that had the same volume as the developing sporangium or they divided to produce cells of one-half this volume. The order of expression of the genes affected by the mutations in these strains was determined by biochemical characterization and by construction of double sporulation mutants. Mutants that produced small cells were blocked at an earlier stage of sporulation than those that produced large cells. It is suggested that the following dependent sequence must occur before the formation of the prespore spetum: (i) the initiation of sporulation, (ii) a signal to block the final central division site, and (iii) a signal to activate a polar septum site.

Encystment and germination in Azotobacter vinelandii

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Accessibility of the (14C)benzylpenicillin binding proteins in membranes of sporulating bacilli

Triton X-100 treatment or freeze-thawing damages the membranes of sporulating or vegetative cells as seen by protein leakage from cells. A 40% increase in the specific [(14)C]benzylpenicillin-binding capacity of detergent-treated or frozen sporulating cells was observed. Neither freezing nor Triton X-100 treatment of vegetative cells produced a detectable effect on their [(14)C]benzylpenicillin-binding capacity. These data indicate the presence of penicillin-binding sites in intact sporulating bacilli not accessible to penicillin in routine binding assays. The chemical specificity of [(14)C]benzylpenicillin binding to detergent-treated sporulating cells is similar to that observed with untreated vegetative or sporulating cells.

Ultrastructural studies of sporulation in Bacillus sphaericus

Spore septum formation in Bacillus sphaericus 9602 occurs 2 h after the end of exponential growth at one end of the vegetative cell, which retains a uniform diameter. The apparently rigid spore septum contains an inner cell wall layer which disappears when the sporulation septum "bulges" into the mother cell cytoplasm. This process occurs simultaneously with terminal swelling at the end of the cell containing the spore septum. It is suggested that the inner cell wall layer is peptidoglycan and that its dissolution and the terminal swelling are consequences of a localized autolysis. Engulfment of the forespore by membrane proliferation results in the production of a forespore surrounded by two flexible, closely apposed membranes. These membranes appear to become more rigid as a peptidoglycan-like layer appears between them, concomitant with the condensation of the forespore nucleoid into a crescent-shaped structure. After nuclear condensation, visible development of distinct cortex, primordial cell wall, and spore coat layers begin, and the forespore cytoplasm assumes an appearance similar to that of a refractile spore. The spore coats consist of an amorphous inner layer, a lamellar midlayer, and a structured outer layer. As cortex synthesis and spore coat assembly continue, exosporium development commences close to that portion of the mother cell plasma membrane which surrounds the forespore. The exosporium is lamellar and in tangential section is seen to have a hexagonal arrangement of subunits. The timing of these morphological events has the expected correlation with the appearance of unique enzyme activites required for cortex synthesis.

Polarized relationship of bacterial spore loci to the "old" and "new" ends of sporangia

The frequency of association of spore loci with the "old" and "new" ends of rod-shaped sporangia in batch cultures of Bacillus megaterium ATCC 19213 was estimated by phase contrast microscopy. The analysis was facilitated by (i) the association of most of the sporangia into chains of two to five sporangia and (ii) the occurrence of two types of cross wall distinguishable by their degree of splitting. It was concluded that a newly formed spore is located at the "old" end of a sporangium. By inference, the sporulation division septum locus is distal to the ultimate normal cell division septum, i.e., proximal to the "old" pole of the B. megaterium sporangium. This result is discussed in relation to deoxyribonucleic acid segregation during sporulation.

Requirement of deoxyribonucleic acid synthesis for microcycle sporulation in Bacillus megaterium

Bacillus megaterium cells have been examined during outgrowth for their macromolecular content, ability to undergo microcycle sporulation, the time of their growth division, the time of deoxyribonucleic acid (DNA) replication initiation, and their ability to synthesize DNA after transfer to sporulation medium. The increase in total DNA content of the cells increased discontinuously beginning at 90 min. Thymidine incorporation became insensitive to chloramphenicol between 90 and 105 min of outgrowth. At 90 min the cells acquired the ability to undergo microcycle sporulation and the degree of sporulation depended on the time spent in outgrowth, with maximal sporulation occurring at 180 min. During outgrowth, cells underwent one synchronous growth division beginning at 225 min and ending at 270 min. Outgrowing cells were not able to continue DNA synthesis after transfer to sporulation medium. The data suggest that DNA replication starts before cells are able to undergo microcycle sporulation; however, the initiation of replication may not be the only requirement for microcycle sporulation.

Penicillin: reversible inhibition of forespore septum development in Bacillus megaterium cells

Benzylpenicillin inhibits the development of the forespore septum in sporulating Bacillus megaterium cells. The inhibitory effect is a function of the duration of exposure to the antibiotic and is completely reversible by penicillinase. Under the incubation conditions employed, less than 20% of the covalently bound antibiotic is released from the cells. The penicillin which remains bound to the cells after treatment with penicillinase may be necessary but is not sufficient for the effect; unbound antibiotic in the sporulation medium is also required.

Properties of a thermosensitive asporogenous filamentous mutant of Bacillus megaterium

Mutant TH14 of Bacillus megaterium ATCC 19213 is thermosensitive and defective in cell-division septation and spore formation at the restrictive temperature (39 C). As a consequence, the mutant forms multinucleate aseptate filaments and is asporogenic. The mutation does not result in any qualitative compositional changes in extractable membrane proteins. At the restrictive temperature, the mutant membrane has a reduced content of a small molecular weight protein(s). A membrane protein(s) with a molecular weight of nearly 80,000 appears to be partially derepressed in the mutant grown at the restrictive temperature. In addition, numerous unidentified spherical inclusions of fairly uniform size (diameter approximately 100 nm) are present in the cytoplasm at the restrictive temperature. They are especially concentrated at only one pole of each filament. Filamentous growth of the mutant is less sensitive to penicillin than growth in the rod form. Growth in either form is equally sensitive to d-cycloserine at the concentrations used for selection of the mutant. Temperature shift-up experiments suggest that one to two rounds of deoxyribonucleic acid (DNA) replication occur before the phenotypic expression of the mutation occurs. The septations after these replication events can be either two-division septations or a single-division septation plus a subsequent sporulation septation. This conclusion, coupled with previously reported work, supports the hypothesis that the early stages of sporulation represent a modified cell division.

Phospholipid metabolism and membrane synthesis during sporulation in Bacillus megaterium

In view of previously published reports of localized membrane growth in exponentially growing Bacillus megaterium and in sporulating Bacillus cereus, an attempt was made to describe phospholipid metabolism and the topology of membrane synthesis during sporulation in B. megaterium. The cells were pulsed with radioactive glycerol or acetate at the time of septum formation, and the specific activity of the lipid fraction was measured at various times through the free spore stage. The bulk of the material labeled during septation could not be recovered in the spore. Rather, it was found that the labeled lipid fraction underwent considerable turnover during spore development. Additionally, other experiments revealed that the lipid made before the initiation of sporulation was also subject to extensive turnover. In order to minimize both the confounding effects of lipid turnover and the possible presence of lateral diffusion of labeled lipid in the membrane, autoradiography of cells pulse labeled with radioactive glycerol at the time of septation was performed; a symmetrical grain distribution resulted. Thus, despite previously published suggestions to the contrary, the current experimental techniques could not demonstrate the existence of localized membrane synthesis in B. megaterium during sporulation.

Binding of radioactive benzylpenicillin to asporogenous mutants of Bacillus subtilis during postexponential growth

The specific penicillin binding capacity of a postexponential culture of Staphylococcus aureus remains constant, but that of a sporulating Bacillus subtilis culture fluctuates dramatically. An initial decrease in binding capacity during presporulation events is followed by two distinct intervals of enhanced specific binding capacity during the postlogarithmic growth of a sporulating B. subtilis culture. The first peak of enhanced binding occurs during septation, when enzymes for germ cell wall formation are present; and the second peak coincides with cortical biosynthesis. The specific postlogarithmic binding capacities of a number of Spo(-) mutants of B. subtilis were examined to ascertain if specific asporogenous mutations altered the binding pattern observed with the wild-type organism. Four distinct postexponential binding patterns were recognized: (i) a low, constant binding capacity resembling the binding pattern of S. aureus, (ii) a decrease in binding capacity with no subsequent significant peaks, (iii) a decrease in binding capacity followed by a single peak corresponding to the first peak seen with the wild type, (iv) a pattern similar to the wild type. The fourth pattern was observed in a mutant blocked during stage III of sporogenesis which produced forespores that never became refractile. Mutations blocking either one or both periods of enhanced postlogarithmic binding were interspersed throughout a linkage group of spore genes next to lys-2 on the B. subtilis chomosome.

Ultrastructural analysis of sporulation in a conditional serine protease mutant of Bacillus subtilis

The morphological characteristics of wild-type Bacillus subtilis and a temperature-sensitive serine protease derivative have been observed during vegetative and sporulation time periods. At 30 C wild-type and mutant cells grow and sporulate identically. At 47.5 C wild-type and mutant cells grow identically, but the mutant cells are blocked at stage 0 or I in the sporulation sequence. Wild-type cells sporulate normally at 47.5 C.

Acid-soluble nucleotides in an asporogenous mutant of Bacillus subtilis

An asporogenous mutant of Bacillus subtilis Sp(-)H12-3, which is considered to have a block at stage 0, showed general growth characteristics similar to those of sporulating cultures. However, a sudden increase in the total amount of acid-soluble nucleotides observed at t(2) in sporulating bacteria was completely absent in this mutant. In sporulating cells, a marked increase in two nucleotides which were identified to be uridine diphosphate (UDP)-galactose and UDP-N-acetylglucosamine was noted, whereas UDP-glucose appeared to be accumulated in the mutant cells at t(2). No unusual nucleotides were found in the strains of B. subtilis examined. The possible role of these UDP derivatives in early stages of sporulation in B. subtilis is discussed.

Sporulation of tricarboxylic acid cycle mutants of Bacillus subtilis

A mutant of Bacillus subtilis 168 lacking aconitase (EC 4.2.1.3) was found to be blocked at stage 0 or I of sporulation. Although adenosine triphosphate levels, which normally decrease in tricarboxylic acid cycle mutants at the completion of exponential growth, could be maintained at higher levels by feeding metabolizable carbon sources, this did not permit the cells to progress further into the sporulation sequence. When post-exponential-phase cells of mutants blocked in the first half of the tricarboxylic acid cycle were resuspended with an energy source in culture fluid from post-exponential-phase wild-type B. subtilis or Escherichia coli, good sporulation occurred. The spores produced retained the mutant genotype and were heat stable but lost refractility and heat stability several hours after their production.

Binding of radioactive benzylpenicillin to sporulating Bacillus cultures: chemistry and fluctuations in specific binding capacity

The chemistry of the binding of (14)C-benzylpenicillin to sporulating cultures of Bacillus megaterium and B. subtilis is similar to that in a 4-hr vegetative culture of Staphylococcus aureus. Unlabeled penicillins prevent the binding of (14)C-benzylpenicillin, but benzylpenicilloic acid and benzylpenilloic acid do not. Bound antibiotic can be removed from cells with neutral hydroxylamine at 25 C. Sporulating cultures display two intervals of enhanced binding, whereas binding to stationaryphase S. aureus cells remains constant. The first period of increased binding activity occurs during formation of the spore septum or cell wall primordium development, and the second coincides with cortex biosynthesis.