Biology Chapter 25: Bacteria and Archaea

Chapter 25: Bacteria and Archaea

• Prokaryotes are divided into two domains: bacteria and archaea
• Most prokaryotes are unicellular, though some form colonies
• Prokaryotic cells are typically 0.5-5 μm, significantly smaller than eukaryotic cells (10-100 μm)
• Some bacteria are pathogens, causing disease; however, most bacteria are benign or beneficial to our bodies
• Prokaryotic cells commonly exhibit three shapes: spheres (cocci), rods (bacilli), and spirals
• Cocci are spherical prokaryotes; some cocci form chains called streptococci
• Bacilli are rod-shaped; some can be threadlike or filamentous (e.g., E. coli)
• Vibrio are curved rods (e.g., cholera)
• Spirilla are short, rigid spirals; spirochetes are longer, flexible spirals (e.g., treponema pallidum, causing syphilis)
• Several hundred bacterial species live on and in our bodies, aiding in functions like decomposing dead skin, producing vitamins, and preventing pathogenic organisms
• Prokaryotes in soil decompose dead organisms sustaining chemical cycles
• A key feature of prokaryotic cells is their cell wall, which maintains shape, provides protection, and prevents bursting in hypotonic environments
• Eukaryotic cell walls are composed of cellulose or chitin
• Bacterial cell walls contain peptidoglycan, a network of sugar polymers cross-linked by polypeptides
• Gram stain categorizes bacteria into Gram-positive and Gram-negative groups based on cell wall composition
• Gram-negative bacteria have less peptidoglycan, an outer membrane that may be toxic, and tend to be more antibiotic-resistant
• Many antibiotics target peptidoglycan damaging bacterial cell walls
• The prokaryotic genome is significantly smaller (one-thousandth of the size) compared to eukaryotic genomes
• The prokaryotic genome is typically a single circular chromosome located in a nucleoid region
• Some bacteria have smaller rings of DNA called plasmids
• Prokaryotes reproduce asexually rapidly by binary fission, often every 1-3 hours
• Many prokaryotes form endospores, metabolically inactive structures that remain dormant and viable in harsh conditions for centuries
• Genetic diversity in prokaryotes is driven by rapid reproduction, mutation, and genetic recombination
• Genetic recombination occurs via transformation (uptake of foreign DNA), transduction (gene transfer by bacteriophages, or viruses), and conjugation (DNA transfer between bacterial cells via sex pili)
• F-factor, a piece of DNA, is needed to produce sex pili used in conjugation and may exist as a plasmid or within the chromosome; conjugation transfers this factor to other bacterial cells
• R plasmids carry genes that provide antibiotic resistance
• Nutritional diversity observed in prokaryotes manifests in varied energy and carbon sources (examples include phototrophs, chemoautotrophs, heterotrophs, photoheterotrophs and chemoheterotrophs)
• Prokaryotes vary in their needs for Oxygen. A distinction is made between obligate aerobes, obligate anaerobes and facultative anaerobes
• Prokaryotes such as nitrogen fixers play important roles in nitrogen fixation and recycling chemical elements in the biosphere
• Symbiotic relationships among prokaryotes and other species are essential interactions within the biosphere; mutualism, commensalism, and parasitism are prevalent forms of symbiosis in which prokaryotes play an important role.
• The presence of diverse toxins such as exotoxins and endotoxins influences the effect of pathogenic bacteria