Bacterial Cell Structure and Shapes

• Bacteria was first observed by Antony von Leeuwenhoek.
• Bacterium has a prokaryotic cell structure with no nucleus or organelles.
• Bacteria are typically 0.4 to 1.5µ in diameter.
• Bacteria are unicellular organisms which can be found in specific arrangements.
• Bacteria have characteristic shapes.

Bacterial Shapes:

• Coccus (round)
• Bacillus (rod)
• Vibrio (comma)
• Spirillum (spiral)
• Bacteria cell structure consists of a chromosome, pilus aka fimbria, ribosomes, inclusions, flagellum, plasmid, cytoplasm, a cell wall, a cellular plasma membrane, and a capsule or slime layer.

Cell Structural Features

• Cell structural features consist of: Internal organization, Cell envelope, and External structures.

Internal Organization

• Includes cytoplasm, nucleoid, plasmid, ribosome, and inclusion bodies.
• Cytoplasm lacks unit membrane-bound organelles.
• The cytoplasmic matrix is the substance lying between the plasma membrane and the nucleoid.
• The Cytoplasmic matrix mostly consists of 70% water, enzymes, and ions.
• Bacteria lack a true cytoskeleton, but have a system of proteins to support the cell.

The Nucleoid

• Genetic material is located in this irregularly shaped region
• The Nucleoid is not membrane-bound, or as well-defined as a nucleus
• Many bacteria possess plasmids in addition to a nucleoid and they are circular, double stranded DNA molecules
• Plasmids are extra-chromosomal DNA, which confers drug resistance to bacteria.
• Plasmids are not required for host growth and reproduction.
• Plasmids replicate independently of the chromosomal DNA.
• There can be 1-700 copies of plasmid molecules present in a bacterial cell.
• Ribosomes are made up of RNA and protein, and they are present freely in the matrix.
• 70S ribosomes consist of 50S and 30S subunits.
• Ribosomes are the sites of protein synthesis.
• Inclusion bodies are for storage in form of glycogen, phosphates, lipids, etc.

Cell Envelope

• Includes plasma membrane and cell wall
• The cell membrane is a phospholipid bilayer membrane surrounding the cytoplasm.
• Cell membranes are selectively permeable and control the movement of substances in and out.
• The Periplasmic space exists between the cytoplasmic membrane and the cell wall.
• Cell walls are strong, rigid with some elasticity, porous and freely permeable.
• Cell walls provide support and shape to the cell.
• Cell walls consist of peptidoglycan [NAM and NAG]: (polysaccharide + protein).
• Gram positive and Gram negative bacteria are categorised on the basis of cell wall composition.
• Gram staining categorizes bacteria.

External Structures of Bacteria

• Includes Flagella, Fimbriae, and Pili
• Fimbriae are short, fine, hair-like appendages that help in attachment to solid surfaces in many gram negative bacteria
• Sex Pili range from 1 to 10 per cell, are larger than fimbriae, and are involved in conjugation.
• Flagella are slender, rigid, provide motility, and range from 1 to many per cell.

Viruses

• Viruses are obligate, intracellular parasites.
• Viruses are very small in size, 20 - 300nm, and can pass through bacteria proof filters.
• Viruses can't be cultured in nutrient media, and require a living system for cultivation.
• Viruses are acellular and do not have cellular structure.
• The virus is made up of nucleic acid, enclosed within a protein coat called the capsid.
• The core structure contains, only one type of nucleic acid which can be either DNA or RNA.
• Nucleic acid is enclosed within a protein coat called capsid.
• Each capsid is composed of protein subunits - capsomers.
• Capsids may or may not have an outer envelope.
• Naked viruses are non-enveloped viruses.

Viral Types

• DNA Viruses - Enveloped and Non-enveloped
• RNA Viruses - Single stranded and Double stranded
• Many viruses possess a viral envelope that may have additional spikes.
• Spikes are carbohydrate-protein complexes and help the virus to attach to the host cell.
• An example is the Sars-CoV-2 virus that causes Covid-19, and also the HIV virus has spikes.