Bacterial Cytoplasmic Membrane

Cytoplasmic Membrane

• A semi-permeable, double-layered thin structure lies inside the cell wall, enclosing the cytoplasm.
• Primarily consists of phospholipids (most abundant chemicals) and proteins, including integral and peripheral types.
• Phospholipid molecules form two parallel surfaces known as a lipid bilayer.
• Polar phosphate groups face the outside of the bilayer
• Nonpolar lipid chains are on the inside.
• The membrane acts as a permeability barrier, controlling the entry and exit of molecules.
• Differs from eukaryotic cells by the absence of sterols, except in Mycoplasma, which lacks a cell wall and uses cholesterol for rigidity.

Functions of Cell Membrane

• It is the most dynamic structure of the prokaryotic cell.
• Acts as a selective barrier, controlling material entry and exit from the cell with selective permeability.
• Serves as the location for the transport system for specific solutes like nutrients and ions.
• Involved in energy generation through respiratory and photosynthetic electron transport systems and ATP synthesis.
• Unlike eukaryotes, bacteria lack intracellular organelles, like mitochondria so the cytoplasmic membrane carries out functions such as respiration or photosynthesis.
• Plays a biosynthetic role, synthesizing phospholipids, membrane lipids, lipopolysaccharides in Gram-negative cells, and cell wall peptidoglycan (murin).
• Secretes hydrolytic exoenzymes, enabling the degradation of nutrients into smaller subunits for cytoplasmic membrane penetration.
• Involves the coordination of DNA replication and segregation with septum formation during cell division (binary fission).
• Functions in chemotactic systems, attracting bacteria to nutrients.
• Formation of endospores.
• The cytoplasmic membrane is an important target for antibiotics like polymyxins and disinfectants/antiseptics.

Movement of Materials Across Cell Membranes

• Materials cross plasma membranes through passive and active transport.

Passive Transport

• Substances cross the membrane from high to low concentration areas without cellular energy expenditure.
• Passive processes include simple diffusion, facilitated diffusion, and osmosis.
• Simple diffusion is molecule/ion movement from high to low concentration areas; no carrier or transporter is needed for substances like oxygen, carbon dioxide, nitrogen, ethanol, water, and urea.
• Facilitated diffusion relies on integral membrane proteins as channels or carriers for ion/large molecule movement.
• Facilitated diffusion does not expend energy as substances move from high to low concentration, but requires transporters.

Active Transport

• The cell expends energy to move substances from areas of low concentration to areas of high concentration (against the concentration gradient).
• Energy (ATP) is required enabling cells to accumulate needed materials.
• Active transport systems require carrier proteins or "permeases."
• Active transport includes ion-coupled transport, ABC transport, group translocation, and specific transport.
• During group translocation a substance is chemically altered; an example is the phosphorylation of glucose.
• The phosphorylated form of glucose cannot be transported out, and can then be used in the cell's metabolic pathways.

Mesosomes

• Specialized structures are formed by convoluted invaginations of the cytoplasmic membrane.
• Mesosomes are divided into septal and lateral types.
• Functions to increase the membrane surface area, which is beneficial for enzyme activity in respiration and transportation.
• Plays a role in cell division (septal mesosomes) via the origin of the transverse septum that divides the cell in half, in which the septal mesomesome act as the binding site of the bacterial chromosome that duplicates to provide the genetic material of each daughter cell.

Capsules, Slime Layers, and Glycocalyx

• It is a sticky, viscous material that forms an extracellular layer outside the cell wall.
• Composed of polysaccharides (in most species), polypeptides, or a mixture of both.
• A capsule is tightly bound to the cell with an organized structure, while a slime layer or glycocalyx is loosely bound and amorphous.
• Capsules or glycocalyx aid cells in adhering to surfaces and protecting bacteria from antibodies and phagocytosis.
• They act as diffusion barriers against some antibiotics, contributing to the organism's pathogenicity (virulence factor).
• Bacillus anthracis causes anthrax, a disease that is only encapsulated by (polepeptide; D-glutamic acid).
• Streptococcus pneumoniae causes pneumonia and is only encapsulated by (polysaccharide).

Cytoplasm

• It is a thick, aqueous, semi-transparent, and elastic substance contained inside the plasma membrane.
• Approximately 80% of the cytoplasm is water, containing proteins (enzymes), carbohydrates, lipids, inorganic ions, and low-molecular mass compounds.
• The major structures include the nucleoid (bacterial chromosome), plasmids, ribosomes, and inclusion bodies.

Nucleoid

• It is the area within the cytoplasm where DNA is located. Consisting of a single long, continuous, frequently circular thread of double-stranded DNA called the bacterial chromosome.
• Contains the cell's genetic information required for its structures and functions
• Carries genetic information to daughter cells
• Duplicates before cell division.
• There is no nuclear membrane or nucleus.
• Vibrio cholerae & Brucella melitensis have 2 dissimilar chromosomes.

Plasmid

• Bacteria often contain these small, usually circular, double-stranded DNA molecules in addition to the bacterial chromosome.
• Extrachromosomal genetic elements, which replicate independently of chromosomal DNA.
• Not essential for the bacterium's survival under normal environmental conditions.
• May carry genes for antibiotic resistance, tolerance to toxic metals, toxin production, and enzyme synthesis.
• It can be transferred from one bacterium cell to another and gained or lost without harming the cell.

Ribosomes

• The site where they perform taking place protein.
• A prokaryotic cell's cytoplasm has tens of thousands of ribosomes, giving the cytoplasm a grainy look.
• Consist of two subunits, each containing protein and ribosomal RNA (rRNA).
• Prokaryotic ribosomes are known as 70S ribosomes.
• The subunits of the 70S ribosome are a small 30S subunit and a larger 50S subunit.
• Certain antibiotics block protein synthesis at prokaryotic ribosomes, including streptomycin, gentamicin, erythromycin, and chloramphenicol.

Inclusion bodies

• They are the prokaryotic cells cytoplasm's reserves of nutrients.
• Cells store nutrients when plentiful and use them when deficient.
• Cells avoid the increase in osmotic pressure that would result of the molecules were dispersed in the cytoplasm by using the nutrients.
• Volutin granules serve as a reserve of polyphosphate that is used in ATP synthesis.
• Polysaccharide granules are a reserve for glycogen and starch.
• Lipid inclusions appear in various species, including Mycobacterium and Bacillus.

Bacterial Appendages: Flagella

• The flagella (singular: flagellum) are long, hollow tubular structures attached to the bacterial cell wall and cytoplasmic membrane by a basal body.
• Organs of motility in most motile bacteria.
• Composed of the protein flagellin, which is antigenic and differs among bacterial species.
• A flagellum consists of the basal body, which is a system of rings in the cell envelope, a hook-like structure near the cell surface that attaches the filament and the flagellar filament as the long outermost region.
• The number and distribution of flagella are constant for each species.
• Constant distributions include:
  • Peritrichous: Distributed over the entire cell.
  • Monotrichous: A single flagellum at one pole.
  • Lophotrichous: A tuft or bunch of flagella from one pole.
  • Amphitrichous: Flagella at both poles of the cell.
• The functions of flagella include:
  • To act as an organ of locomotion to enable movement of a bacterium toward or away from a stimulus is called taxis
  • Chemotaxis: Movement towards nutrients.
  • Aerotaxis: Movement towards optimal O2 concentration.
  • Phototaxis: Movement of photosynthetic bacteria towards light.
• As antigenic components due to the prescence of The H antigen, which is used for serotyping and diagnosis.
• To assist in penetrating viscid mucus secretions and epithelial barriers, spreading throughout body fluids and tissues.

Pili and Fimbriae

• Hair-like appendages that are shorter, straighter, and thinner than flagella.
• They are present in Gram-negative bacilli and responsible for adhesion & attachment to mucosa, working as a virulence factor.
• They are structures consist of a protein called pilin.
• They are two types
  • Ordinary pili or fimbriae: Also known as colonization antigens. They mediate adherence of bacteria to specific receptors on the human cell surface for infection (e.g., mutants of Neisseria gonorrhoeae that do not have pili are non-pathogenic).
  • Sex pili or F-pili: They are highly specialized hair-like structures that transfer DNA from one cell (F+ donor) to another (F- recipient) by direct physical contact via six pilus, a process called conjugation.

Bacterial Endospores

• Resting or dormant-phase cells formed inside the original cell by gram-positive bacteria like Clostridium and Bacillus when essential nutrients are depleted
• They are released from the original cell as free spores.
• Some members of the genus Clostridium cause diseases such as gangrene, tetanus, botulism, and food poisoning.
• Some members of the genus Bacillus cause anthrax and food poisoning.
• They resist extreme heat, a lack of water, toxic chemicals, and radiation if released.
• Spores are formed only in vitro in unfavourable conditions, not in animal tissues where nutrition is available.
• They are have no metabolic activity and can remain dormant for years.
• Sporulation can be thought of as repackaging a copy of bacterial DNA into a new form that contains very little water, has no metabolic activity/division, and restructured with a highly impermeable, multilayered envelope.
• The core has high levels of the compound calcium dipicolinate, which is thought to be helpful in protecting the spore DNA from environmental damage.
• A thick spore coat of protein surrounds the outside membrane, responsible for resisting endospores from harsh chemicals.
• To return to the vegetative state, spores need stimulation that will allow germination.
• If the activated spore is in a nutritious environment, which it senses by monitoring various key metabolites, it begins to germinate.
• The process involves destruction of the cortex by lytic enzymes, uptake of water, release of calcium dipicolinate from the cell, and resumed metabolic activity.

Explore the structure and function of the bacterial cytoplasmic membrane. Learn about its chemical composition, differences from eukaryotic membranes, and its role in energy production and cell division. Understand its contribution to bacterial pathogenicity and transport mechanisms.