Prokaryotic Diversity and Antibiotic Resistance

• Prokaryotes, comprised of Bacteria and Archaea, are single-celled, adapt well to diverse and extreme conditions, and are the most abundant organisms on Earth.
• Most prokaryotes are unicellular, but some species form colonies.
• The three most common shapes of prokaryotes are spheres (cocci), rods (bacilli), and spirals (spirogyra).

Cell Surface Structures

• All protist cell walls are important, as they maintain shape, provide protection, and prevent bursting in hypertonic environments.
• Bacterial cell walls contain peptidoglycan, a network of sugar polymers cross-linked by polypeptides.
• Archaeal cell walls contain various polysaccharides and proteins but lack peptidoglycan.
• The Gram Stain is used to classify bacteria into gram-positive and gram-negative groups based on cell wall composition.
• Gram-positive bacteria have simpler walls with a large amount of peptidoglycan
• Gram-negative bacteria are more complex, with less peptidoglycan and an outer membrane containing lipopolysaccharides.
• Gram-negative bacteria are generally more resistant to antibiotics than gram-positive bacteria.
• Many antibiotics target peptidoglycan, damaging gram-positive bacterial cell walls.
• Human cells lack peptidoglycan and are unaffected by these antibiotics.
• Some prokaryotes have a polysaccharide or protein layer outside the cell wall
• A dense and well-defined layer is called a capsule
• A not well-organized layer is called a slime layer.
• Some bacteria form metabolically inactive endospores when water or nutrients are scarce
• The cell copies its chromosome and surrounds it with a multilayered structure.
• Endospores can withstand extreme conditions and remain viable for centuries.
• Fimbriae are hair-like appendages that allow some prokaryotes to stick to surfaces or other individuals in a colony.
• Pili (sex pili) are longer than fimbriae, function to pull cells together, and enable DNA exchange.

Motility

• About half of prokaryotes exhibit taxis, the ability to move toward or away from stimuli; an example being chemotaxis.
• Flagella are common structures used by prokaryotes for movement and can be scattered or concentrated at the cell's ends.
• Prokaryotic and eukaryotic flagella differ in structure, propulsion mechanism, and molecular composition.

Internal Organization and DNA

• Prokaryotic cells typically lack complex compartmentalization, though some have specialized membranes formed by cell membrane infoldings to perform metabolic functions.
• Compared to eukaryotes, prokaryotes have less DNA and produce fewer proteins.
• Prokaryotes have one circular chromosome, whereas eukaryotes have multiple linear chromosomes.
• Plasmids are smaller rings of independently replicating DNA that may be present in prokaryotes.

Reproduction

• Prokaryotes reproduce quickly via binary fission, dividing every 1-3 hours in optimal conditions.
• During binary fission, the chromosome replicates, starting from the original replication point, and the daughter chromosomes actively separate.
• Three key features of prokaryote biology are their small size, reproduction through binary fission, and short generation times.

High Genetic Variation

• Rapid reproduction, mutation, and genetic recombination contribute to the high genetic variation in prokaryotic populations.
• Binary fission produces generally identical cells, but differences can arise through mutation
• Though mutation rates are typically low themselves, the short generation times of large populations allow mutations to accumulate quickly
• Genetic recombination increases prokaryotic diversity by combining DNA from two sources.
• Horizontal gene transfer is the movement of genes among individuals from different species.
• Transformation occurs when prokaryotic cells incorporate foreign DNA from their surroundings
• Transduction involves bacteriophages transferring prokaryotic genes from one host cell to another
• Conjugation is DNA transfer between two prokaryotic cells, with DNA transfer always being one-way
• The F factor (fertility factor), required for pili production, can exist as a plasmid or within the bacterial chromosome.
• Cells with the F plasmid act as DNA donors during conjugation, while those without are recipients
• The F factor is transferable during conjugation.
• A cell with the F factor integrated into its chromosome functions as a donor, known as an Hfr cell, resulting in the recipient becoming a recombinant bacterium.
• R plasmids carry resistance genes, including those for multiple antibiotics, and have genes encoding pili for DNA transfer, facilitating the rapid spread of resistance.

Energy and Carbon

• Phototrophs obtain energy from light
• Chemotrophs obtain energy from chemicals
• Autotrophs require CO2 or related compounds as a carbon source
• Heterotrophs require an organic nutrient to make other organic compounds
• Energy and carbon sources combine to create the four main nutritional modes: photoautotrophy, chemoautotrophy, photoheterotrophy, and chemoheterotrophy.

Oxygen in Metabolism

• Prokaryotic metabolism varies with respect to O2.
• Obligate aerobes require O2 for cellular respiration.
• Obligate anaerobes are poisoned by O2 and live by fermentation or anaerobic respiration.
• Facultative anaerobes can use O2 if present or carry out fermentation/anaerobic respiration.

Nitrogen Metabolism

• Nitrogen is essential for amino acid and nucleic acid production, and prokaryotes metabolize it in various ways.
• During nitrogen fixation, some prokaryotes convert atmospheric nitrogen (N2) to ammonia (NH3).

Metabolic Cooperation

• Cooperation enables prokaryotes to use environmental resources unavailable to individual cells.
• Photosynthetic and nitrogen-fixing cells (heterocysts) exchange metabolic products in filamentous cyanobacteria like Anabaena.

Biofilms

• In some prokaryotic species, metabolic cooperation occurs in surface-coating colonies called biofilms, which are common but can cause issues such as corrosion, device contamination, tooth decay, and chronic infections.

Archaea

• Archaea share traits with bacteria and eukaryotes but also possess unique characteristics.
• Extremophiles, a type of archaea, thrive in extreme environments uninhabitable for most organisms.
• Extreme halophiles tolerate or require highly saline conditions (e.g., Halobacterium)
• Extreme thermophiles have adaptations for DNA and protein stability at high temperatures (e.g., Sulfolobus, Pyrococcus furiosus)
• Methanogens are obligate anaerobes that produce methane and are found in diverse environments
• Prokaryotes play a crucial role in chemical recycling, breaking down dead organisms and releasing carbon/elements back into the environment.

Ecological Interactions

• Symbiosis involves close contact between a larger host and a smaller symbiont.
• Prokaryotes form symbiotic relationships with larger organisms.
  • Mutualism benefits both organisms
  • Commensalism benefits one organism without affecting the other
  • Parasitism harms one organism (the host) through a parasite
• Human intestines host 500–1,000 bacterial species, outnumbering human cells tenfold
• bacteria break down undigested food and are mutualistic
• All known pathogenic prokaryotes are bacteria, and disease-causing
• For example, Mycobacterium cause tuberculosis, a lung disease with over 1.5 million deaths annually.

Explore prokaryotic diversity, including differences between Gram-positive and Gram-negative bacteria. Learn about chemotaxis, pili function, cell wall structures, and the roles of endospores and capsules. Understand how R plasmids contribute to antibiotic resistance.