Germination of unactivated spores of Bacillus cereus T. Effect of preincubation with L-alanine or inosine on the subsequent germination.

Heat-activated spores of Bacillus cereus T germinate rapidly in the presence of L-alanine alone or inosine alone. In contrast, unactivated spores can not germinate in the presence of either germinant alone but rapidly in the presence of both germinants. The highest level of cooperative action of L-alanine and inosine on the germination was observed when they were present in a ratio 1:1. Preincubations of unactivated spores with L-alanine or inosine had opposite effects on the subsequent germination in the presence of both germinants: preincubation with L-alanine stimulated the initiation of subsequent germination, while preincubation with inosine inhibited it. These results suggest that germination of unactivated spores initiated by L-alanine and inosine includes two steps, the first initiated by L-alanine and the second prompted by inosine. The effect of preincubation of unactivated spores with L-alanine was not diminished by washings. The pH dependence of the preincubation of unactivated spores was not so marked as that of the subsequent germination in the presence of inosine.     

Involvement of calcium in germination of coat-modified spores of Bacillus cereus T.

The effect of calcium on germination of coat-modified Bacillus cereus T spores was investigated. Coat-modified spores produced either by chemical extraction (SDS-DTT-treated spores) or by mutagenesis (10LD mutant spores) were unable to germinate in response to inosine. While SDS-DTT-treated spores could germinate slowly in the presence of L-alanine, 10LD mutant spores could not germinate at all. The lost or reduced germinability of coat-modified spores was restored when exogenous Ca2+ was supplemented to the germination media. The calcium requirement of coat-modified spores for germination was fairly specific. The simultaneous presence of germinant with Ca2+ was also required for germination of coat-modified spores. The optimal recovery of germinability was observed in the presence of 1.0 mM of calcium acetate. The calcium requirement itself was remarkably diminished under the condition in which L-alanine and a certain purine nucleoside analog, adenosine or inosine, coexisted. The lost or diminished germinability observed in SDS-DTT-treated spores or 10LD mutant spores may be attributed to the loss of calcium associated with the spore integuments.     

Stimulation of germination of unactivated Bacillus cereus spores by ammonia

Inclusion of ammonia in germinant mixtures containing L-alanine and inosine stimulated germination of unactivated Bacillus cereus spores at rates equal to those obtained using heat-activated spores without ammonia. D-Alanine had little effect on germination of heat-activated spores, but severely inhibited germination of unactivated spores in the presence of ammonia. Ammonia did not replace the requirement for either L-alanine or inosine: all three compounds were required for rapid germination. Kinetic analysis suggested that the functions of ammonia and L-alanine were more closely related than the functions of ammonia and inosine. With rate-saturating concentrations of L-alanine and inosine, germination rates showed saturation kinetics for ammonia with a Km for NH4Cl of 5 mM. Comparisons of the effects of salts, amines and pH on germination rates suggested that NH4OH rather than NH+4 was the rate-limiting form of ammonia. In comparisons of various strains of B. cereus, stimulation of germination by ammonia occurred in all cases, although spores of most soil isolates germinated more rapidly than B. cereus T spores in the absence of ammonia.     

Oxidative Activation of Bacillus cereus Spores

A study was made of the activation of Bacillus cereus strain T spores by using the oxidizing agent sodium perborate. The degree of activation was measured with constant germination conditions by using L-alanine, inosine, adenosine, and L-alanine plus adenosine as germination stimulants. The germinal response following the various treatments was compared with the responses obtained with heat activation. It was concluded that the optimal time for activation with 30 mM sodium perborate at room temperature was about 4 hr. If the exposure time was greatly extended, the spores would germinate spontaneously. When the perborate treatment followed heat activation, the germinal response to L-alanine was stimulated, to inosine retarded and without apparent effect for adenosine or L-alanine plus adenosine. Results of experiments designed to demonstrate deactivation by slow oxidation showed that spores activated with sodium perborate were not deactivated by slow oxidation, whereas those activated by heat were. A deactivation study using mercaptoethanol as the deactivation agent showed that both methods of activation could be deactivated after a 24-hr exposure, but this deactivation was reversible by extending the exposure to mercaptoethanol. The results of heat-sensitivity studies revealed that about 70% of the sodium perborate-activated spores were heat sensitive after 60 min in a germination menstruum of L-alanine plus adenosine, whereas similarly treated heat-activated and nonactivated spores were about 99.99% heat sensitive, respectively.     

GerN, an antiporter homologue important in germination of Bacillus cereus endospores

A homologue of the grmA spore germination gene of Bacillus megaterium and of a NaH-antiporter gene (napA) of Enterococcus hirae has been identified in Bacillus cereus 569 (ATCC 10876). The putative protein product has 58 and 43% amino acid identity with GrmA and NapA, respectively. Insertional inactivation of this B. cereus gene, named gerN, did not affect vegetative growth or sporulation. The null mutant spores were 30-fold slower to germinate in inosine (5 mM) but germinated almost normally in response to L-alanine (10 mM). The null mutant spores germinated after several hours with inosine as the sole germinant, but germination was asynchronous and the normal order of germination events was perturbed. At a suboptimal germinant concentration (50 microM), inosine germination was completely blocked in the mutant, while the rate of germination in 50 microM L-alanine was reduced to one-third of that of the wild type. The requirement for GerN function in the response to a particular germinant suggests that a germination receptor may have a specifically associated antiporter, which is required at the initiation of germination and which, in the case of the inosine receptor, is GerN. Since germination in suboptimal concentrations of L-alanine shows a delay, additional germination transporters may be required for optimal response at low germinant concentrations.     

Germination of Unactivated Spores of Bacillus cereus T

Heat-activated spores of Bacillus cereus T germinate rapidly in the presence of l-alanine alone or inosine alone. In contrast, unactivated spores can not germinate in the presence of either germinant alone but rapidly in the presence of both germinants. The highest level of cooperative action of l-alanine and inosine on the germination was observed when they were present in a ratio 1 :1. Preincubations of unactivated spores with l-alanine or inosine had opposite effects on the subsequent germination in the presence of both germinants: preincubation with l-alanine stimulated the initiation of subsequent germination, while preincubation with inosine inhibited it. These results suggest that germination of unactivated spores initiated by l-alanine and inosine includes two steps, the first initiated by l-alanine and the second prompted by inosine. The effect of preincubation of unactivated spores with l-alanine was not diminished by washings. The pH dependence of the preincubation of unactivated spores was not so marked as that of the subsequent germination in the presence of inosine.     

The preparation,germination properties and stability of superdormant spores of Bacillus cereus

Aims: To determine yields, germination and stability of superdormant Bacillus cereus spores. Methods and Results: Superdormant B. cereus spores were isolated by germination with high concentrations of inosine or l ‐alanine in 2–5% yield and did not germinate with high concentrations of either of these germinants, but germinated like starting spores with Ca‐DPA, dodecylamine, l ‐alanine plus inosine or concentrated complete medium. Yields of superdormant spores from germinations with low inosine concentrations were higher, and these spores germinated poorly with low inosine, but relatively normally with high inosine. Yields of superdormant spores were also higher when nonheat‐activated spores were germinated. Superdormant spores stored at 4°C slowly recovered some germination capacity, but recovery was slowed significantly at ?20°C and ?80°C. Conclusions: Factors that influence levels of superdormant B. cereus spores and the properties of such spores are similar to those in B. megaterium and B. subtilis, suggesting there are common mechanisms involved in superdormancy of Bacillus spores. Significance: Superdormant spores are a major concern in the food industry, because the presence of such spores precludes decontamination strategies based on triggering spore germination followed by mild killing treatments. Studies of the properties of superdormant spores may suggest ways to eliminate them.     

Cooperativity and interference of germination pathways in Bacillus anthracis spores

Spore germination is the first step to Bacillus anthracis pathogenicity. Previous work has shown that B. anthracis spores use germination (Ger) receptors to recognize amino acids and nucleosides as germinants. Genetic analysis has putatively paired each individual Ger receptor with a specific germinant. However, Ger receptors seem to be able to partially compensate for each other and recognize alternative germinants. Using kinetic analysis of B. anthracis spores germinated with inosine and L-alanine, we previously determined kinetic parameters for this germination process and showed binding synergy between the cogerminants. In this work, we expanded our kinetic analysis to determine kinetic parameters and binding order for every B. anthracis spore germinant pair. Our results show that germinant binding can exhibit positive, neutral, or negative cooperativity. Furthermore, different germinants can bind spores by either a random or an ordered mechanism. Finally, simultaneous triggering of multiple germination pathways shows that germinants can either cooperate or interfere with each other during the spore germination process. We postulate that the complexity of germination responses may allow B. anthracis spores to respond to different environments by activating different germination pathways.     

Inhibition by potassium ion of the pregerminative response to L-alanine of unactivated spores of Bacillus cereus T.

The effect of potassium ion on L-alanine-inosine-induced germination of unactivated spores of Bacillus cereus T was studied. Unactivated spores germinated in 0.1 M sodium phosphate buffer (NaPB), but not 0.1 M potassium phosphate buffer (KPB), at pH 8.0 and at 30 C. Inhibition of germination was also observed on incubation of unactivated spores in NaPB containing potassium chloride. Previously it was demonstrated that germination of unactivated spores involves at least two steps, one induced by L-alanine, and the other by inosine. Potassium ion seems to inhibit the response of the spores to inosine, because: (1) Spores that had been preincubated with L-alanine in NaPB or KPB, germinated in NaPB but not KPB in the presence of inosine. (2) During germination in NaPB, incorporation of L-[14C]alanine showed bimodal kinetics with a rapid first phase and a second continuous phase, but in KPB the second phase of incorporation did not occur. The events occurring before germination of unactivated spores are discussed with reference to the initiation of germination.     

Germination properties as marker events characterizing later stages of Bacillus subtilis spore formation

At various stages during spore formation sporangia were shocked by cold treatment or with toluene, and the germination requirements of the prespores were examined. Up to 5 h after induction of sporulation (t5) germination was spontaneous; i.e., it occurred without any added germinants. After t5, during stages V and VI, the capacity for spontaneous germination diminished progressively, and the spores acquired a need for externally added germinants. At t6 this need was satisfied by either L-alanine or a mixture of KCl, glucose, and fructose. By t8, the latter response had disappeared. The spores germinated only with L-alanine, and the response was much slower. Experiments with chloramphenicol showed that the germination properties of the spores appearing between t6 and t8 were the expression of events in protein synthesis that had occurred before t5. Although the germination requirements developed at about the same time as heat resistance, they could be dissociated from heat resistance in wild-type and mutant cells. The germination properties of the developing spores are additional marker events characterizing the later stages of sporulation, as follows: (i) spontaneous germination (up to the end of stage IV); (ii) germination requirements that are satisfied by KCl-glucose-fructose or L-alanine (stage V); and (iii) slow germination response with L-alanine only (stage VI).     

Bacillus cereus Spores Release Alanine that Synergizes with Inosine to Promote Germination

Background

The first step of the bacterial lifecycle is the germination of bacterial spores into their vegetative form, which requires the presence of specific nutrients. In contrast to closely related Bacillus anthracis spores, Bacillus cereus spores germinate in the presence of a single germinant, inosine, yet with a significant lag period.

Methods and Findings

We found that the initial lag period of inosine-treated germination of B. cereus spores disappeared in the presence of supernatants derived from already germinated spores. The lag period also dissipated when inosine was supplemented with the co-germinator alanine. In fact, HPLC-based analysis revealed the presence of amino acids in the supernatant of germinated B. cereus spores. The released amino acids included alanine in concentrations sufficient to promote rapid germination of inosine-treated spores. The alanine racemase inhibitor D-cycloserine enhanced germination of B. cereus spores, presumably by increasing the L-alanine concentration in the supernatant. Moreover, we found that B. cereus spores lacking the germination receptors gerI and gerQ did not germinate and release amino acids in the presence of inosine. These mutant spores, however, germinated efficiently when inosine was supplemented with alanine. Finally, removal of released amino acids in a washout experiment abrogated inosine-mediated germination of B. cereus spores.

Conclusions

We found that the single germinant inosine is able to trigger a two-tier mechanism for inosine-mediated germination of B. cereus spores: Inosine mediates the release of alanine, an essential step to complete the germination process. Therefore, B. cereus spores appear to have developed a unique quorum-sensing feedback mechanism to monitor spore density and to coordinate germination.     

Functional relationships between L- and D-alanine, inosine and NH4Cl during germination of spores of Bacillus cereus T

The results of a physiological study of the interaction between NH4Cl, inosine, and the stereoisomers of alanine during germination of spores of Bacillus cereus T are presented. Detailed kinetics for the germination of unheated spores in moderate concentrations of L-alanine (in the absence of auto-inhibition due to alanine racemase) are established, as is the specificity of the stimulatory effect of NH4Cl in relation to other salts, amines, and germinants. The results suggest that NH4Cl and inosine affect an early step in germination closely related to the function of an L-alanine receptor.     

Inhibition by ammonium ion of germination of unactivated spores of Bacillus cereus T induced by l-alanine and inosine.

Studies were carried out on the inhibitory effect of NH4+ on germination of spores of Bacillus cereus T induced by L-alanine and inosine. Kinetic analysis showed that NH4+ inhibited the germination competitively. Its inhibitory effect was greater when the unactivated spores had been preincubated with L-alanine. NH4+ did not inhibit the response of unactivated spores to L-alanine during preincubation. These results suggest that L-alanine sensitizes the spores to the inhibitory effect of NH4+.     

Germination of Bacillus anthracis spores

The influence of amino acids, nucleosides and inorganic components on the kinetics and effectiveness of the germination of B. anthracis spores was studied. The study revealed that the rapid germination of the spores took place after their activation at 65 degrees C in tris buffer with L-alanine in combination with inosine or adenosine added; less pronounced germinative action was caused by the addition of alanine only and the combination of phenylalanine, tyrosine and tryptophan. The rapidity of germination and the sets of effective germinants for spores of different strains were different. All B. anthracis strains under study had nucleotide sequences, of gene gerX in their genome.     

Cooperativity between different nutrient receptors in germination of spores of Bacillus subtilis and reduction of this cooperativity by alterations in the GerB receptor

The GerA nutrient receptor alone triggers germination of Bacillus subtilis spores with L-alanine or L-valine, and these germinations were stimulated by glucose and K+ plus the GerK nutrient receptor. The GerB nutrient receptor alone did not trigger spore germination with any nutrients but required glucose, fructose, and K+ (GFK) (termed cogerminants) plus GerK for triggering of germination with a number of L-amino acids. GerB and GerA also triggered spore germination cooperatively with l-asparagine, fructose, and K+ and either L-alanine or L-valine. Two GerB variants (termed GerB*s) that were previously isolated by their ability to trigger spore germination in response to D-alanine do not respond to D-alanine but respond to the same L-amino acids that stimulate germination via GerB plus GerK and GFK. GerB*s alone triggered spore germination with these L-amino acids, although GerK plus GFK stimulated the rates of these germinations. In contrast to l-alanine germination via GerA, spore germination via L-alanine and GerB or GerB* was not inhibited by D-alanine. These data support the following conclusions. (i) Interaction with GerK, glucose, and K+ somehow stimulates spore germination via GerA. (ii) GerB can bind and respond to L-amino acids, although normally either the binding site is inaccessible or its occupation is not sufficient to trigger spore germination. (iii) Interaction of GerB with GerK and GFK allows GerB to bind or respond to amino acids. (iv) In addition to spore germination due to the interaction between GerA and GerK, and GerB and GerK, GerB can interact with GerA to trigger spore germination in response to appropriate nutrients. (v) The amino acid sequence changes in GerB*s reduce these receptor variants' requirement for GerK and cogerminants in their response to L-amino acids. (vi) GerK binds glucose, GerB interacts with fructose in addition to L-amino acids, and GerA interacts only with L-valine, L-alanine, and its analogs. (vii) The amino acid binding sites in GerA and GerB are different, even though both respond to L-alanine. These new conclusions are integrated into models for the signal transduction pathways that initiate spore germination.     

Commitment of bacterial spores to germinate. A measure of the trigger reaction.

The rate of commitment of bacterial spores to germinate after short exposure to L-alanine increases exponentially from the time of addition of L-alanine. This absence of a lag facilitates kinetic analysis and allows the dependence of commitment on temperature and pH to be determined. The pH profile of commitment has been compared with that obtained from measurements of absorbance decreases during germination, and the two profiles exhibit differing pK values. It is suggested that because the decrease in A600 of spore suspensions is a late event in germination, it is an unsuitable parameter for studying germination-triggering reactions. Commitment has been shown to be temperature-dependent, with an optimum at approx. 37 degrees C and an activation energy (mu) of 1.08 X 10(5) J/mol. The data obtained from the present studies have been used to develop a model for the triggering of germination.     

Inhibition of germinant binding by bacterial spores in acidic environments.

Commitment to germinate occurred in both Clostridium botulinum and Bacillus cereus spores during 0.5 min of exposure to 100 mM L-alanine or L-cysteine, measured by the inability of germination inhibitors (D form of amino acid) to inhibit germination. Spore germination at pH 4.5 was inhibited because the germinant did not bind to the trigger sites. C. botulinum spores exposed to 100 mM L-alanine or L-cysteine at pH 4.5 remained sensitive to D-amino acid inhibition at pH 7, indicating that no germinants had bound to the trigger site at pH 4.5. Inhibition of germinant binding at pH 4.5 was reversible but lagged in commitment to germinate upon transfer to pH 7. Spores sequentially exposed to pH 4.5 buffer and pH 7 buffer with the germinant also demonstrated a lag in commitment to germinate. The pH at which binding was inhibited was not significantly affected by composition of the buffer or by reduced germinant concentrations (10 mM). Nonspecific uptake of L-[3H]alanine by C. botulinum spores was not inhibited at pH 4.5. Inhibition of germinant binding in acidic environments appeared to be due to protonation of a functional group in or near the trigger site. This may represent a general mechanism for inhibition of spore germination in acidic environments.     

Inhibition of germinant binding by bacterial spores in acidic environments

Commitment to germinate occurred in both Clostridium botulinum and Bacillus cereus spores during 0.5 min of exposure to 100 mM L-alanine or L-cysteine, measured by the inability of germination inhibitors (D form of amino acid) to inhibit germination. Spore germination at pH 4.5 was inhibited because the germinant did not bind to the trigger sites. C. botulinum spores exposed to 100 mM L-alanine or L-cysteine at pH 4.5 remained sensitive to D-amino acid inhibition at pH 7, indicating that no germinants had bound to the trigger site at pH 4.5. Inhibition of germinant binding at pH 4.5 was reversible but lagged in commitment to germinate upon transfer to pH 7. Spores sequentially exposed to pH 4.5 buffer and pH 7 buffer with the germinant also demonstrated a lag in commitment to germinate. The pH at which binding was inhibited was not significantly affected by composition of the buffer or by reduced germinant concentrations (10 mM). Nonspecific uptake of L-[3H]alanine by C. botulinum spores was not inhibited at pH 4.5. Inhibition of germinant binding in acidic environments appeared to be due to protonation of a functional group in or near the trigger site. This may represent a general mechanism for inhibition of spore germination in acidic environments.     

Mechanisms of sorbate inhibition of Bacillus cereus T and Clostridium botulinum 62A spore germination.

The mechanism by which potassium sorbate inhibits Bacillus cereus T and Clostridium botulinum 62A spore germination was investigated. Spores of B. cereus T were germinated at 35 degrees C in 0.08 M sodium-potassium phosphate buffers (pH 5.7 and 6.7) containing various germinants (L-alanine, L-alpha-NH2-n-butyric acid, and inosine) and potassium sorbate. Spores of C. botulinum 62A were germinated in the same buffers but with 10 mM L-lactic acid, 20 mM sodium bicarbonate, L-alanine or L-cysteine, and potassium sorbate. Spore germination was monitored by optical density measurements at 600 nm and phase-contrast microscopy. Inhibition of B. cereus T spore germination was observed when 3,900 micrograms of potassium sorbate per ml was added at various time intervals during the first 2 min of spore exposure to the pH 5.7 germination medium. C. botulinum 62A spore germination was inhibited when 5,200 micrograms of potassium sorbate per ml was added during the first 30 min of spore exposure to the pH 5.7 medium. Potassium sorbate inhibition of germination was reversible for both B. cereus T and C. botulinum 62A spores. Potassium sorbate inhibition of B. cereus T spore germination induced by L-alanine and L-alpha-NH2-n-butyric acid was shown to be competitive in nature. Potassium sorbate was also a competitive inhibitor of L-alanine- and L-cysteine-induced germination of C. botulinum 62A spores.     

Effects of overexpression of nutrient receptors on germination of spores of Bacillus subtilis

The rates of germination of Bacillus subtilis spores with L-alanine were increased markedly, in particular at low L-alanine concentrations, by overexpression of the tricistronic gerA operon that encodes the spore's germinant receptor for L-alanine but not by overexpression of gerA operon homologs encoding receptors for other germinants. However, spores with elevated levels of the GerA proteins did not germinate more rapidly in a mixture of asparagine, glucose, fructose, and K(+) (AGFK), a germinant combination that requires the participation of at least the germinant receptors encoded by the tricistronic gerB and gerK operons. Overexpression of the gerB or gerK operon or both the gerB and gerK operons also did not stimulate spore germination in AGFK. Overexpression of a mutant gerB operon, termed gerB*, that encodes a receptor allowing spore germination in response to either D-alanine or L-asparagine also caused faster spore germination with these germinants, again with the largest enhancement of spore germination rates at lower germinant concentrations. However, the magnitudes of the increases in the germination rates with D-alanine or L-asparagine in spores overexpressing gerB* were well below the increases in the spore's levels of the GerBA protein. Germination of gerB* spores with D-alanine or L-asparagine did not require participation of the products of the gerK operon, but germination with these agents was decreased markedly in spores also overexpressing gerA. These findings suggest that (i) increases in the levels of germinant receptors that respond to single germinants can increase spore germination rates significantly; (ii) there is some maximum rate of spore germination above which stimulation of GerA operon receptors alone will not further increase the rate of spore germination, as action of some protein other than the germinant receptors can become rate limiting; (iii) while previous work has shown that the wild-type GerB and GerK receptors interact in some fashion to cause spore germination in AGFK, there also appears to be an additional component required for AGFK-triggered spore germination; (iv) activation of the GerB receptor with D-alanine or L-asparagine can trigger spore germination independently of the GerK receptor; and (v) it is likely that the different germinant receptors interact directly and/or compete with each other for some additional component needed for initiation of spore germination. We also found that very high levels of overexpression of the gerA or gerK operon (but not the gerB or gerB* operon) in the forespore blocked sporulation shortly after the engulfment stage, although sporulation appeared normal with the lower levels of gerA or gerK overexpression that were used to generate spores for analysis of rates of germination.