Microbes and Viruses: Understanding Their Structure and Life Cycles
Microbes: The Microscopic World
Microbes are a diverse group of living organisms. They are so small that they can only be seen with a microscope. Microbes exist in two main forms:
- Acellular forms: Viruses, viroids, and prions.
- Cellular forms: Bacteria, protozoa, algae, and fungi.
Microbes share several key features:
- Small volume of cytoplasm.
- Accelerated metabolic reactions that occur very quickly.
Viruses: Obligate Intracellular Parasites
Viruses are microscopic acellular entities composed of a nucleic acid core surrounded by a protein structure. They are obligate intracellular parasites, meaning they can only replicate inside a host cell.
Structure of a Virus
- Nucleic acid: Viruses can contain either DNA or RNA as their genetic material. This can be circular or linear, double-stranded or single-stranded, and sometimes contains unusual nucleobases.
- Capsid: The capsid is the protein coat that surrounds the nucleic acid. It is composed of subunits called capsomeres that assemble into geometric shapes.
- Envelope (Protein Complex): In some viruses, the capsid is surrounded by a lipid or protein envelope.
- Enzymes: Viruses may contain enzymes such as polymerases for replicating and transcribing their nucleic acid, and lysozymes for penetrating the host cell membrane.
Viral Life Cycles
Viruses have two main types of life cycles:
A) Lytic Cycle
In the lytic cycle, viruses infect a cell and produce many copies of themselves using the host cell’s machinery.
- Entry phase: The virus attaches to the cell surface and enters the cell. There is specificity in the attachment, involving interactions between proteins on the viral capsid and glycoproteins on the host cell’s plasma membrane.
- Eclipse phase: After entry, the virus seemingly disappears as no viral activity is detected.
- Assembly phase: Viral components are assembled, producing new viruses.
- Release phase: New viruses are released from the cell, often through cell lysis, and go on to infect new cells.
B) Lysogenic Cycle
In the lysogenic cycle, the viral DNA integrates into the host cell’s DNA and remains in a latent state. The integrated viral DNA is called a provirus.
Viroids and Prions
- Viroids: Viroids are small, circular, single-stranded RNA molecules that lack a protective coat. They are primarily found in plants.
- Prions: Prions are infectious proteins that induce changes in the conformation of normal proteins, transforming them into abnormal forms. They are associated with proteins in the neuronal membrane of mammals.
Bacteria: Prokaryotic Microorganisms
Bacteria are prokaryotic organisms that possess all the necessary components for nutrition and metabolic reproduction.
Types of Bacteria
Bacteria can be classified based on their shape:
- Bacilli (rod-shaped)
- Cocci (spherical)
- Spirilla (spiral-shaped)
- Vibrios (comma-shaped)
Some bacteria remain joined after division, forming:
- Streptococci (chains of cocci)
- Diplococci (pairs of cocci)
- Staphylococci (clusters of cocci)
Structure of Bacteria
- Capsule: An outer layer with a non-defined structure, rich in carbohydrates. It acts as a barrier, providing resistance to drying and containing a large amount of water. It also hinders bacterial destruction by antibodies and phagocytes, and allows for the formation of bacterial colonies.
- Bacterial Wall: A rigid and strong structure that protects bacteria from hostile osmotic environments. It is composed of peptidoglycan, a glycoprotein that forms a framework of interwoven polysaccharide chains linked by peptides. Destruction of the bacterial wall leaves the bacteria defenseless. The Gram stain differentiates two types of bacteria based on the structure of their cell wall.
- Plasma Membrane: Surrounds the cytoplasm and is similar in nature to that of eukaryotic cells. It contains invaginations called mesosomes, which are rich in enzymes involved in bacterial DNA replication, cellular respiration, photosynthesis (in photosynthetic bacteria), and nitrogen assimilation.