Essay: Bacterial endospores

Bacterial endospores are highly resistant structures that can withstand many forms of treatments, including heat and UV (Atrih & Foster, 2002), and this characteristic is facilitated by their unique spore structure. Bacterial capsules play an important role in the virulence of bacteria for their host, and enable bacterial cells to evade host defense mechanisms and survive hostile environments. The structure and function of endospores and capsules work specifically to benefit the microbial cell; furthermore, various physiological changes occur in these structures as a result of environmental stress (Sahin, Yong, Driks, & Mahadevan, 2012). The specific mechanism of these physiological changes, the types of environmental stresses that cause the changes, and how these correlate with endospore and capsular structure and function are essential to the understanding of this topic.
Bacterial endospores are dormant, non-reproductive, and sometimes disease-causing cell structures that are typically formed in Gram-positive bacteria under a process called sporulation. Endospores exhibit high resistance to environmental stresses, and these structures are able to resist conditions that are unfavourable to most organisms, enabling bacteria to lie dormant for extended periods of time (hundreds to thousands of years). In addition, endospores are highly durable, as dormant spores return to an actively growing state a process called germination) when nutrients return to their environment. Sporulation is initiated in spore-forming bacteria under conditions of nutrient depletion in the growth medium(mostly when Carbon and Nitrogen sources are lacking), accumulation of the products of growth, or when an unfavourable reaction renders the medium unsuitable for further growth of the organism(Atrih & Foster, 2002). Endospores are commonly found in soil and water, and they are able to resist many extreme conditions including ultraviolet radiation, high temperatures, desiccation, chemical disinfectants, and freezing. Furthermore, endospores are resistant to antibiotics, most disinfectants, and physical agents such as boiling, drying, and radiation; the impermeability of the spore coat is responsible for the endospore’s resistance to chemicals, and its ability to withstand extreme heat is due to a variety of structural factors(Cook 2008).
Bacterial capsules are polysaccharide layers that lie outside of the cell envelope. Capsule formation occurs in both Gram-positive and Gram-negative bacteria, and confer a variety of functions including: attachment to surfaces, interaction with other organisms, protection against phagocytic engulfment (enhancement of cell virulence), and protection against desiccation. However, capsules are most importantly recognized as virulence factors, as they enable cells to evade or counteract various host defense mechanisms, including the inhibition of phagocytosis. In addition, the role of capsules as virulence factors enables them to cause infection within the host organism. The immunological significance of capsules is seen in many pathogenic bacteria, and encapsulated bacterial species lead to severe mortality and morbidity (Schembri, Dalsgaard, & Klemm, 2004). Capsules are important determinants of the behavior of bacteria within the animal host; to survive within the host, the bacteria must be able to evade a diverse array of defense mechanisms and survive in hostile environments, which, as will be further discussed, is facilitated by encapsulation (Cook 2008).
Structural variations in the composition of endospore and capsular structure contribute to their virulence properties. The spore is essentially composed of a series of shells; the innermost compartment, the core, holds DNA and chromosomal material, and the desiccation of the core is essential for heat resistance. The inner membrane surrounds the core, after which the next layer, the cortex, can be found. The cortex is a layer of peptidoglycan, and along with the inner membrane, these layers keep the spore core dry. A multilayered protein shell called the coat surrounds the cortex, which prevents the entry of large degradative molecules as well as the toxic activity of small reactive molecules (such as glutaraldehyde) and predation by other microbes. Together, the protective functions of these structural layers allow bacterial cells to remain dormant for extended periods of time, or years (Driks 2002). The order of assembly of the endospore coat relies mainly on protein-protein interactions; proteolytic events, protein-protein crosslinking, and protein glycosylation contribute to the assembly process. Carbohydrate and protein content play an essential role in the composition of endospore coat structure and contribute to their structural integrity as well as enhance their resistance properties, including lysozyme and chemical resistance (Piggot & Hilbert, 2004). Small acid-soluble proteins (SASPs) are found only in endospore cores. These proteins tightly bind and condense the DNA material and are responsible for resistance to UV light and DNA-damaging chemicals. The chemical composition of the spore has not been defined; however, it is thought that the spore core contains dipicolinic acid. Dipicolinic acid is a spore-specific chemical that allows the spore core to maintain dormancy. Analysis of the membranes or ‘walls’ of the endospore reveal that they consist predominantly of protein material. The composition of the insoluble integuments, or walls, also contain diaminopimelic acid (DAP) and hexosamine, which are referred to as ‘spore peptide’ material, and this material confers structural integrity as well as heat resistance (Henriques & Moran, 2000). Bacterial capsules, or polysaccharide capsules, are characteristically composed of long gelatinous polysaccharide chains known as capsular polysaccharides, which consist of homopolymers and heteropolymers of polysaccharides and are negatively charged, therefore generating a highly hydrated capsule layer. Capsules may also contain glycoproteins or polypeptides. The capsule is firmly attached to the cell wall(Driks 2002). The chemical nature of capsules is diverse, but the vast majority of them are polysaccharides; these polymers are composed of repeating oligosaccharide units. Cytochemical methods have revealed that capsules contain a striated structure made of various layers that differ in chemical composition. The membrane-like outer layer of the capsule consists of peptides and neutral mucopolysaccharides. The middle layer of the capsule contains complex substances of polysaccharides and proteins, and the innermost layer consists of acid mucopolysaccharides. Capsular antigens are localized on different parts of the capsule, and a given bacterial species can produce a range of capsular antigens with different chemical compositions, which can be distinguished by serotyping. The chemical nature of the capsule is important in the functions that the capsule plays in the infection process; different capsular antigens confer various infectious properties, and these antigens interact with the host immune system to produce an immunological response (Hyams et. Al, 2010).
Endospores have unique functional and morphological characteristics that enable them to resist damage caused by lysozyme and harsh chemicals. The proteinaceous coat that surrounds the endospore acts as a shield to protect the bacterium from these chemicals and from extreme heat (Henriques & Moran, 2000). The endospore coat is a multilayered protein structure that serves two main roles: protection against bactericidal enzymes and chemicals and influencing the spore to monitor its environment and germinate when exposed to appropriate nutrients. A major determinant of endospore heat resistance is the core water content; spores display an inverse relationship between heat resistance and core water content, and thus, spores with increased heat resistance have decreased core water content, and vice versa. Chemical resistance of endospores is due in part to the spore coat. The spore coat plays a significant role in evading damage caused by oxidizing chemicals, and along with SASPs, which bind to DNA and protect it from harmful chemicals that enter the core as well as UV radiation (which can cause mutations), chemical resistance is enabled in endospores (Melnick et. Al., 2011). The various host defense mechanisms that can be evaded by capsular bacteria greatly enhance their virulence properties. Complement-mediated bacteriolysis, uptake and killing by phagocytes (also known as phagocytosis), and cell-mediated immune mechanisms are the most common host defense mechanisms that can be evaded. In humans and animals, the first line of defense against invading pathogens is the innate immune system; however, encapsulation of successful pathogens allows them to interfere with the innate immune system at several levels. Complement and serum killing and interference with humoral immunity are additional benefits that occur with encapsulation (Nizet 2006). Capsules impair bacterial opsonization with C3b/iC3b by both the alternative and classical complement pathways; the also inhibit the conversion of C3b bound to the bacterial surface to iC3b. This opsonization is essential to the host immune response as it is the process by which the pathogen is marked for destruction by the phagocyte. Unencapsulated bacteria are more susceptible to immunoglobin G(IgG) and C-reactive protein(CRP), which indicates that the capsule may inhibit classical pathway complement activity by masking antibody recognition of sub-capsular antigens, as well as by inhibiting CRP binding. In other words, unencapsulated bacteria are more prone to attack by host defense mechanisms and thus cannot easily cause infectious disease in the host. In addition, bacteria that do not produce capsules are more readily engulfed by neutrophil phagocytosis(Campos et. Al., 2004).
Nutrient deficiency and dry conditions trigger morphological and physiological changes in endospores. In spite of dormancy, spores are able to resume metabolic activity and return to vegetative growth, which is a process that requires the coat to be broken apart and shed. In response to humidity, spores dynamically contract and expand; this requires the spore structure to be quite flexible, due in part to a wrinkled configuration of the spore. These wrinkles persist during spore dormancy and allow to spore to accommodate changes in volume without compromising structural and biochemical integrity. The volume and surface area of the spore core decrease during sporulation; the mechanical processes involved in germination govern the unfolding of the spore coat via degradation of the cortex peptidoglycan(Sahin, Yong, Driks, & Mahadevan, 2012). Germination-specific lytic enzymes(GSLEs) are responsible for spore coat hydrolysis during germination. The unfolded and relaxed coat has a larger volume, which allows the core to absorb a larger volume of water(this hydration is a critical step in spore germination). In addition, the spore must synthesize new chemical components during the process of germination. Endospores contain higher levels of sulphur in comparison to vegetative cells, which is concentrated in the form of the amino acid, cystine, and are responsible for maintaining the dormant state. The protein coat is held together by disulfide linkages, and a reduction in these linkages causes the protein coat to unfold, exposing enzymatic sites that are necessary for germination(Liu et. Al., 2004).
Further research on endospores and capsules will have implications on medicine and human health. The variations that occur in endospores and capsules under environmental stresses can be further studied to understand the molecular/genetic basis of these processes. Isolation of endospore and capsule-forming pathogens to study nucleic acid composition, structural components, and physiological mechanisms can be greatly beneficial to the fields of microbiology, medicine, and human health as a whole, as scientists can discover protective mechanisms against these pathogens. The food industry is greatly impacted by endospore-forming bacteria, and adequate elimination processes for these contaminants is yet to be discovered. Pasteurization and autoclaving are two commonly utilized sterilization methods; however, more and more pathogens that can survive these techniques are emerging(Melnick et. Al., 2011). Endospores are usually able to survive a wide array of sterilization methods in the clinical and food industries, thus producing fatal toxins and disease. Encapsulation of bacteria allow them to be protected from moisture, heat, or other extreme conditions that are seen in food industrial sterilization techniques, therefore allowing them to enhance their stability and maintain their viability. The medical industry would also greatly benefit of endospore and capsule research. Numerous fatalities occur due to pathogenic endospore and capsule-forming bacteria, and many antibiotics have little or no effect on these pathogens due to their increasing resistance properties(Campos et. Al., 2004).
Endospores and capsules are unique bacterial structures that enable these microorganisms to enhance their viability in various ways. These highly resistant structures are able to withstand high heat and ultraviolet light, as well as protect themselves from desiccation and harsh chemicals. Capsules play a significant role in enhancing the virulence of bacteria and enable bacterial cells to evade a vast array of host defense mechanisms, including complement-mediated bacteriolysis and phagocytosis. Variations in the composition of endospore and capsular structure, which include carbohydrate and protein content, contribute to their resistance properties. In addition, the morphological and physiological changes that occur in endospores during sporulation and germination are noteworthy; these changes are crucial to understanding the structural and biochemical integrity of these cells. Furthermore, research on these structures is crucial for determining new sterilization methods that can eliminate pathogenic bacteria. The high-resistance of these structures has caused concern mainly in the food and medical industries. A better understanding of the molecular and genetic basis for these resistance properties will greatly benefit microbiology and human health as a whole.