CONCLUSIONES FINALES

Ans. Asexual reproduction is very common in microorganisms. The following methods are used by bacteria to reproduce asexually

i. Binary Fission ii. Budding

iii. Spore formation

Bacteria reproduce by binary fission. During binary fission, the cell divides into two daughter cells that are similar in size and shape.

Binary fission begins with the single DNA molecule replicating and both copies attaching to the cell membrane. Next, the cell membrane begins to grow between the two DNA molecules. Once the bacterium just about doubles its original size, the cell membrane begins to pinch inward. A cell wall then forms between the two DNA molecules dividing the original cell into two identical daughter cells.

Bacterial Reproduction: This salmonella bacterium is undergoing the process of binary fission. The cell divides resulting in the formation of two identical cells.

Spore Formation

Sporulation is the formation of nearly dormant forms of bacteria. In a limited number of bacteria, spores can preserve the genetic material of the bacteria when conditions are inhospitable and lethal for the normal (vegetative) form of the bacteria. The commitment of a bacterium to the sporulation process sets in motion a series of events that transform the cell.

Structure of Endospores

Bacteria produce a single endospore internally. The spore is sometimes surrounded by a thin covering known as the exosporium, which overlies the spore coat. The spore coat, which acts like a sieve that excludes large toxic molecules like lysozyme, is resistant to many toxic molecules and may also contain enzymes that are involved ingermination.

The cortex lies beneath the spore coat and consists of peptidoglycan. The core wall lies beneath the cortex and surrounds the protoplast or core of the endospore. The core contains the spore chromosomal DNA is encased in chromatin-like proteins known as SASPs, that protect the spore DNA from UV radiation and heat. The core also contains normal cell structures, such as ribosomes and otherenzymes, but is not metabolically active.

Up to 20% of the dry weight of the endospore consists of calcium dipicolinate within the core, which is thought to stabilize the DNA.Dipicolinic acid could be responsible for the heat resistance of the spore, and calcium may aid in resistance to heat and oxidizing agents. However, mutants resistant to heat but lacking dipicolinic acid have been isolated, suggesting other mechanisms contributing to heat resistance are at work.

Formation of Endospore

Sporulation begins with the duplication of the bacterial genome. The second copy and some of the cytoplasm is then enveloped in an in-growth of the membrane that surrounds the bacterium. The result is essentially a little spherical cell inside the larger bacterium.

The little cell is referred to as the "daughter cell" and the original bacterium is now called the "mother cell." Another membrane layer is laid down around the daughter cell.

Between these two membranes lies a layer of peptidoglycan material, the same rigid material that forms the stress-bearing network in the bacterial cell wall. Finally, a coat of proteins is layered around the outside of the daughter cell. The result is a nearly impregnable sphere.

The above spore is technically termed an endospore, because the formation of the membrane-enclosed daughter cell occurs inside the mother cell.

In a so-called exospore, the duplicated DNA migrates next to a region on the inner surface of the cell membrane and then a bud forms. As the bud protrudes further outward, the DNA is drawn inside the bud.

Examples of endospore forming bacteria

These include those in the genera Bacillus and Clostridium. Endospore forming bacteria include Methylosinus, Cyanobacteria, and Microsporidia.

When still in the mother cell, the location of the spore (e.g., in the center, near one end or at one pole) is often a distinctive feature for a particular species of bacteria, and can be used as a feature to identify the bacteria.

Genetics of Sporulation

The genetic grounding for the commitment to form a spore is a protein called SpoA. This protein functions to promote the transcription of genes that are required for the conversion of the actively growing bacterium to a spore. The formation of an active SpoA protein is controlled by a series of reactions that are themselves responsive to the environmental conditions. Thus, the activation of SpoA comes only after a number of checkpoints have been passed. In this way a bacterium has a number of opportunities to opt out of the sporulation process. Once committed to sporulation, the process is irreversible.

Significance of Sporulation

Sporulation ultimately provides for a multilayered structure can be maintained for a very long time. Relative to the norm life span of the microorganism, spores are designed to protect a bacterium from heat, dryness, and excess radiation for a long time. Endospores of Bacillus subtilis have been recovered from objects that are thousands of years old.

Furthermore, these are capable of resuscitation into an actively growing and dividing cell.

Spores have been recovered from amber that is more than 250 million years old.The sporulation process has been well studied in Bacillus subtilis. The process is stimulated by starvation. Typically, sporulation is a "last resort," when other s fail (e.g., movement to seek new food, production of enzymes to degrade surrounding material, production of antimicrobial agents to wipe out other microbes competing for the food source, etc.).

Q.17 What are main components of bacterial nutrition? Explain the role of carbon source