Peptidoglycan Structural Dynamics during Germination of Bacillus subtilis 168 Endospores

Peptidoglycan structural dynamics during endospore germination of Bacillus subtilis 168 have been examined by muropeptide analysis. The first germination-associated peptidoglycan structural changes are detected within 3 min after the addition of the specific germinant l-alanine. We detected in the spore-associated material new muropeptides which, although they have slightly longer retention times by reversed-phase (RP)-high-pressure liquid chromatography (HPLC) than related ones in dormant spores, show the same amino acid composition and molecular mass. Two-dimensional nuclear magnetic resonance (NMR) analysis shows that the chemical changes to the muropeptides on germination are minor and are probably limited to stereochemical inversion. These new muropeptides account for almost 26% of the total muropeptides in spore-associated material after 2 h of germination. The exudate of germinated spores of B. subtilis 168 contains novel muropeptides in addition to those present in spore-associated material. Exudate-specific muropeptides have longer retention times, have no reducing termini, and exhibit a molecular mass 20 Da lower than those of related reduced muropeptides. These new products are anhydro-muropeptides which are generated by a lytic transglycosylase, the first to be identified in a gram-positive bacterium. There is also evidence for the activity of a glucosaminidase during the germination process. Quantification of muropeptides in spore-associated material indicates that there is a heterogeneous distribution of muropeptides in spore peptidoglycan. The spore-specific residue, muramic δ-lactam, is proposed to be a major substrate specificity determinant of germination-specific lytic enzymes, allowing cortex hydrolysis without any effect on the primordial cell wall.The extreme heat resistance of dormant bacterial endospores has made them an important problem in the production of safe foodstuffs (3). The spore cell wall peptidoglycan is considered to play a major role in the maintenance of heat resistance and dormancy (6). Bacillus subtilis spore peptidoglycan is composed of two layers. A thin, inner layer called the primordial cell wall retains the basic vegetative cell peptidoglycan structure. The primordial cell wall represents 2 to 4% of the total endospore peptidoglycan, is not digested during germination, and serves as the initial cell wall during outgrowth (2, 5, 25, 29). The outer thick layer of peptidoglycan, known as the cortex, is characterized by several unique spore-specific features. Approximately 50% of the muramic acid residues in the glycan strands are present in the δ-lactam form (2, 24). Muramic acid side chains are composed of 26 and 23% of tetrapeptide and single l-alanine, respectively (2).Despite their extreme dormancy and thermostability, bacterial endospores retain an alert sensory mechanism enabling them to respond within minutes to the presence of specific germinants. Spores of B. subtilis respond to at least two different types of germinative stimuli: (i) l-alanine and (ii) a combination of l-asparagine, glucose, fructose, and KCl (AGFK) (34). The germination response is initiated by the interaction of a receptor protein with specific germinants which triggers the loss of spore-specific properties and the transformation of a dormant resistant bacterial spore into a metabolically active vegetative cell. The germination process is characterized by sequential, interrelated biochemical events. The specific hydrolysis of peptidoglycan in the spore cortex layer is an essential event in germination (2, 25). Its degradation removes the physical constraints of the cortex and allows core expansion and outgrowth (9, 25). As a consequence of cortex hydrolysis, peptidoglycan fragments can be detected in the germination exudate (13, 33).A number of bacterial spore germination-specific cortex-lytic enzymes (GSLEs) have been reported to be involved in cortex hydrolysis (9, 18–20). A gene homologous to that encoding the GSLE from Bacillus cereus has been identified and inactivated in B. subtilis, and the resulting mutant germinates more slowly than the wild type (22). Recently a germination-specific muramidase isolated from a germination extract of Clostridium perfringens S40 has been purified and characterized (4).GSLEs have a high substrate specificity, requiring intact spore cortex for activity (9, 23). The muramidase from C. perfringens S40, however, hydrolyzes cortical fragments but has a strict requirement for the presence of the muramic δ-lactam residues (4). Thus, the GSLEs are highly specialized and may exist as proforms which are specifically activated during germination (9).Very little is known about the mechanism by which the cortex is hydrolyzed during germination and the autolytic enzymes involved. Muropeptide analysis provides a method for fine chemical structural determination of spore cortex (2, 24, 25). In this paper, we report the use of muropeptide analysis to determine the peptidoglycan structural dynamics which occur during spore germination of B. subtilis 168 and the evidence for a number of different enzyme activities.