Covers bacterial endospores: their dormant, highly resistant nature and why they form during sporulation, followed by their layered structure, staining appearance and the stages of germination back to a vegetative cell. It then distinguishes spore-forming Bacillus and Clostridium species, with their medically important members, from non-spore-formers, and notes how spore position and cell swelling aid identification.
Topic 7 of 113 in the MU Sofia syllabus for Microbiology (Medicine, Year 3). The free AceMed note covers it in 1 section, about 596 words (4 min of reading), with 14 flashcards.
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Bacterial Ultrastructure – Spores
Bacterial Ultrastructure – Spores
Bacterial Spores (Endospores)
Definition and Function: Bacterial spores, specifically endospores, are dormant, tough, non-reproductive structures produced by certain bacteria. They are formed from a bacterial vegetative cell. Their primary function is to ensure survival of the bacterium through periods of environmental stress. This allows the bacteria to exist in unfavourable conditions. Endospores are purely dormant life forms and their development from a vegetative state does not involve assimilation of additional external nutrients.
Resistance: Endospores are the most resistant cells known. They are highly resistant to chemical and physical noxae. This resistance includes intense heat, radiation, attack by most enzymes, most disinfectants, long periods of drought, and lack of nutrients. They can survive in soil and other inanimate objects for years. Bacterial spores are listed as the second most resistant microbial entity to sterilization, after prions. Sterilization typically requires intense heat, such as 160°C for 2 hours of dry heat.
Metabolic Activity: Spores do not exhibit measurable metabolic activity. Consequently, antibiotics don't work against them.
Formation: Spores are typically formed when nutrients are insufficient, or when the bacteria senses a change in the external environment. This process, called sporulation, involves differentiation of the mother cell (vegetative cell). There are eight stages of spore formation.
Structure: An endospore is a dehydrated, multishelled structure. It contains peptidoglycan, cytoplasm, water (very little) and bacterial DNA. The structure protects the genomic DNA. The spore layers, from outside to inside, are: Exosporium (made of lipoproteins), Protein coat (made of keratin-like proteins), Outer membrane, Cortex (the thickest layer, made of tightly cross-linked peptidoglycans), Spore wall (made of peptidoglycan), Inner membrane, and the Core. Resistance is attributed to factors like Calcium dipicolinate in the core, the Keratin spore coat, and heat-resistant enzymes.
Appearance: Spores look refractile (bright) under the microscope. Once stained, dormant spores are quite resistant to decolourization. Using specific spore stains, the endospore appears blue, while the cytoplasm of the vegetative cell appears red. Spores are typically spherical to oval in shape.
Germination: Under more favourable conditions, the spore can be 'Germinated'. Germination is the transformation of spores into the vegetative state. This process involves activation, initiation, and outgrowth. Germination breaks hibernation, usually stimulated by disruption of the outer coat by mechanical stress, pH, heat, or other factors. In outgrowth, the core of the spore makes new chemical components and exits the old spore coat to develop into a fully functional vegetative bacterial cell.
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