Members of several bacterial genera can form endospores (Figure 1). The two most common are Gram-positive rods: the obligately aerobic genus Bacillus and the obligately anaerobic genus Clostridium. The other bacteria known to form endospores are Thermoactinomyces, Sporolactobacillus, Sporosarcina, Sporotomaculum, Sporomusa, and Sporohalobacter spp. These organisms undergo a cycle of differentiation in response to environmental conditions: The process, sporulation, is triggered by near depletion of any of several nutrients (carbon, nitrogen, or phosphorous). Each cell forms a single internal spore that is liberated when the mother cell undergoes autolysis. The spore is a resting cell, highly resistant to desiccation, heat, and chemical agents; when returned to favorable nutritional conditions and activated, the spore germinates to produce a single vegetative cell. The location of an endospore within a cell is species-specific and can be used to determine the identity of a bacterium.

Fig1. Sporulating cells of bacillus species. A: Unidentified bacillus from soil. B: B. cereus. C: B. megaterium. (Reproduced with permission from Robinow CF: Structure. In Gunsalus IC, Stanier RY [editors]. The Bacteria: A Treatise on Structure and Function, Vol 1. Academic Press, 1960.)

A. Sporulation

The sporulation process begins when nutritional conditions become unfavorable, near depletion of the nitrogen or carbon source (or both) being the most significant factor. Sporulation occurs massively in cultures that have terminated exponential growth because of this near depletion.

Sporulation involves the production of many new structures, enzymes, and metabolites along with the disappearance of many vegetative cell components. These changes represent a true process of differentiation: A series of genes whose products determine the formation and final composition of the spore are activated. These changes involve alterations in the transcriptional specificity of RNA polymerase, which is determined by the association of the polymerase core protein with one or another promoter-specific proteins called sigma factors. During vegetative growth, a sigma factor designated σA predominates. Then, during sporulation, five other sigma factors are formed that cause various spore genes to be expressed at various times in specific locations.

The sequence of events in sporulation is highly complex: Differentiation of a vegetative cell of B. subtilis into an endospore takes about 7 hours under laboratory conditions.

Different morphologic and chemical events occur at sequential stages of the process. Seven distinct stages have been identified.

Morphologically, sporulation begins with the formation of an axial filament (Figure2). The process continues with an infolding of the membrane to produce a double-membrane structure whose facing surfaces correspond to the cell wall synthesizing surface of the cell envelope. The growing points move progressively toward the pole of the cell to engulf the developing spore.

Fig2. The stages of endospore formation. (Reproduced with permission from Merrick MJ: Streptomyces. In: Parish JH [editor]. Developmental Biology of Procaryotes. Univ California Press, 1979.)

The two spore membranes now engage in the active syn thesis of special layers that will form the cell envelope: the spore wall and the cortex, lying outside the facing mem branes. In the newly isolated cytoplasm, or core, many vegetative cell enzymes are degraded and are replaced by a set of unique spore constituents.

B. Properties of Endospores

 1. Core—The core is the spore protoplast. It contains a complete chromosome, all the components of the protein synthesizing apparatus, and an energy-generating system based on glycolysis. Cytochromes are lacking even in aerobic species, the spores of which rely on a shortened electron transport pathway involving flavoproteins. A number of vegetative cell enzymes are increased in amount (eg, alanine racemase), and a number of unique enzymes are formed (eg, dipicolinic acid synthetase). Spores contain no reduced pyridine nucleotides or ATP. The energy for germination is stored as 3-phosphoglycerate rather than as ATP.

The heat resistance of spores is partly attributable to their dehydrated state and in part to the presence in the core of substantial amounts (5–15% of the spore dry weight) of calcium dipicolinate, which is formed from an intermediate of the lysine biosynthetic pathway. In some way not yet understood, these properties result in the stabilization of the spore enzymes, most of which exhibit normal heat lability when isolated in soluble form.

2. Spore wall—The innermost layer surrounding the inner spore membrane is called the spore wall. It contains normal peptidoglycan and becomes the cell wall of the germinating vegetative cell.

3. Cortex—The cortex is the thickest layer of the spore envelope. It contains an unusual type of peptidoglycan, with many fewer cross-links than are found in cell wall peptidoglycan. Cortex peptidoglycan is extremely sensitive to lysozyme, and its autolysis plays a role in spore germination.

4. Coat—The coat is composed of a keratin-like protein containing many intramolecular disulfide bonds. The impermeability of this layer confers on spores their relative resistance to antibacterial chemical agents.

5. Exosporium—The exosporium is composed of proteins, lipids, and carbohydrates. It consists of a paracrystalline basal layer and a hairlike outer region. The function of the exosporium is unclear. Spores of some Bacillus species (eg, B. anthracis and B. cereus) possess an exosporium, but other species (eg, B. atrophaeus) have spores that lack this structure.

C. Germination The germination process occurs in three stages: activation, initiation, and outgrowth.

1. Activation—Most endospores cannot germinate immediately after they have formed. But they can germinate after they have rested for several days or are first activated in a nutritionally rich medium by one or another agent that dam ages the spore coat. Among the agents that can overcome spore dormancy are heat, abrasion, acidity, and compounds containing free sulfhydryl groups.

2. Initiation—After activation, a spore will initiate germination if the environmental conditions are favorable. Different species have evolved receptors that recognize different effectors (ie, germinants) as signaling a rich medium: Thus, initiation is triggered by l-alanine in one species and by adenosine in another. Binding of the effector activates an autolysin that rapidly degrades the cortex peptidoglycan. Water is taken up, calcium dipicolinate is released, and a variety of spore constituents are degraded by hydrolytic enzymes.

3. Outgrowth—Degradation of the cortex and outer layers results in the emergence of a new vegetative cell consisting of the spore protoplast with its surrounding wall. A period of active biosynthesis follows; this period, which terminates in cell division, is called outgrowth. Outgrowth requires a supply of all nutrients essential for cell growth.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم صناعة الشبابيك يواصل أعمال تصنيع بابٍ لحرم مرقد الإمامين الجوادين (عليهما السلام)

قسم العلاقات ينظّم برنامجًا ثقافيًّا لوفد من طلبة مجموعة العميد

قسم الشؤون الفكرية وجامعة العميد يصدران العدد 37 من مجلة تسليم المحكمة

متحف الكفيل يواصل أعمال تركيب عارضات مقتنيات متحف العتبة العسكرية المقدسة

المزيد

الآخبار الصحية

علاج طبيعي لمشكلات البشرة وحب الشباب

اكتشاف ارتباط عنصر شائع في نظامنا الغذائي بحصوات المرارة

كل ليلة.. 11 دقيقة إضافية من النوم لها مفعول السحر

علاج طبيعي يخفف أعراض سكري الحمل

المزيد

الآخبار التكنلوجية

قمر وردي ومذنب يلامس الشمس.. 8 ظواهر فلكية تزين السماء طوال شهر أبريل

تفسير حديث لتحيز التوحد بين الجنسين.. هل تمتلك الإناث "درعا جينيا" ضد المرض؟

الجلوس الذهني النشط.. مفتاح تقليل خطر الخرف لدى كبار السن

خسائر غير مسبوقة للجليد في أيسلندا وسواحل المحيط الهادئ خلال 2025

المزيد

مواضيع ذات صلة

Morphologic Changes During Growth

Staining

The Acid-Fast Cell Wall

Special Components of Gram-Negative Cell Walls

Special Components of Gram-Positive Cell Walls

The Peptidoglycan Layer

Bacterial Cultivation

Principles of Bacterial Cultivation : Environmental Requirements

Structure and Function of the Bacterial Cell

Bacterial Metabolism

Bacterial Genetics: Nucleic Acid Structure and Organization

Detection of Bacteremia: Culture Techniques