Bacterium Study Guide

📖 Core Concepts Bacteria – Single‑celled prokaryotes, 0.5–5 µm long, lack a membrane‑bound nucleus. Cell wall – Made of peptidoglycan; thickness determines Gram‑positive (thick) vs. Gram‑negative (thin + outer membrane). Metabolism classification – By energy source (phototrophic vs. chemotrophic), electron donor (lithotroph vs. organotroph), carbon source (autotroph vs. heterotroph), and terminal electron acceptor (aerobic vs. anaerobic). Horizontal gene transfer (HGT) – Acquisition of DNA via transformation, transduction, or conjugation; drives antibiotic‑resistance spread. Biofilm – Surface‑attached bacterial community encased in glycocalyx; confers up to 500‑fold antibiotic resistance. Endospore – Dormant, highly resistant structure formed by certain Gram‑positive genera (e.g., Bacillus, Clostridium). Gram stain – Differentiates bacteria by peptidoglycan thickness; purple = Gram‑positive, pink = Gram‑negative. --- 📌 Must Remember Generation time (g) – Time for a population to double; fastest bacteria ≈ 17 min. Gram‑positive wall – Thick peptidoglycan + teichoic acids; target of penicillin. Gram‑negative wall – Thin peptidoglycan + outer LPS membrane; resistant to many antibiotics. Mycobacteria – Thick peptidoglycan + outer lipid layer → acid‑fast staining. Key HGT mechanisms – Transformation (DNA uptake), Transduction (phage‑mediated), Conjugation (pilus‑mediated). Endospore resistance – Survive UV, γ‑radiation, heat, desiccation, disinfectants, even vacuum. Quorum sensing – Population‑density‑dependent gene regulation via autoinducers; controls biofilm formation, toxin production. Antibiotic categories – Bactericidal (kill) vs. bacteriostatic (stop growth). --- 🔄 Key Processes Binary fission Cell grows → DNA replicated → septum forms → two identical daughter cells. Gram staining Crystal violet → iodine fixative → alcohol decolorization (thick wall retains) → safranin counterstain. Endospore formation (sporulation) Vegetative cell → asymmetric division → forespore engulfed → cortex & coat deposition → dormancy. Horizontal gene transfer Transformation: competence → DNA uptake → recombination. Transduction: bacteriophage infection → packaging of host DNA → transfer to new cell. Conjugation: pilus formation → plasmid (or chromosomal) DNA transfer. Biofilm development Initial attachment → microcolony formation → EPS (glycocalyx) production → mature 3‑D structure with channels. --- 🔍 Key Comparisons Gram‑positive vs. Gram‑negative Thick peptidoglycan vs. thin peptidoglycan + outer membrane. Teichoic acids vs. lipopolysaccharide (LPS). Retains crystal violet vs. loses it during decolorization. Endospore‑forming vs. Non‑forming bacteria Bacillus/Clostridium produce dormant, highly resistant spores vs. other genera lack this survival stage. Transformation vs. Transduction vs. Conjugation DNA taken up from environment vs. DNA packaged by phage vs. DNA transferred via pilus. Phototrophic vs. Chemotrophic Energy from light vs. energy from oxidation of chemical compounds. Autotroph vs. Heterotroph CO₂ fixation for carbon vs. use of organic carbon sources. --- ⚠️ Common Misunderstandings “All Gram‑negative bacteria are resistant to penicillin.” – True because penicillin targets peptidoglycan synthesis, but some β‑lactams (e.g., ampicillin) can penetrate the outer membrane. “Endospores are just thick‑walled cells.” – Endospores are metabolically dormant, contain a dehydrated core, and have multiple protective layers, not merely a thick wall. “All bacteria are either Gram‑positive or Gram‑negative.” – Mycobacteria and some others have atypical cell walls and stain acid‑fast, not fitting classic categories. “Horizontal gene transfer only occurs in the lab.” – Transformation, transduction, and conjugation happen naturally in many environments, driving rapid evolution. --- 🧠 Mental Models / Intuition “Wall thickness = stain color” – Visualize a sponge (thick wall) that holds onto dye vs. a thin‑walled balloon that lets dye wash out. “Biofilm = city” – Bacteria are citizens living in a protective “city wall” (EPS), sharing resources through “streets” (nutrient channels). “Endospore = survival capsule” – Think of a spacecraft sealed for harsh space travel; the core is protected until conditions improve. “Metabolism = fuel choice” – Like cars: some run on sunlight (phototrophs), others on gasoline (chemotrophs), and some can switch (facultative anaerobes). --- 🚩 Exceptions & Edge Cases Mycobacteria – Thick peptidoglycan plus a high‑lipid outer layer; acid‑fast staining, intermediate between Gram‑positive and Gram‑negative. Linear chromosomes – Present in Streptomyces and Borrelia; most bacteria have circular chromosomes. Multiple chromosomes – Certain Vibrio species contain more than one chromosome. Gram‑variable organisms – Some species (e.g., Gardnerella) may appear Gram‑positive or Gram‑negative depending on age of culture. --- 📍 When to Use Which Gram stain – First‑line classification in clinical labs; decide on empiric therapy (e.g., β‑lactams for Gram‑positive cocci). Acid‑fast stain – Suspect Mycobacterium or Nocardia (e.g., pulmonary infections, TB). Selective/differential media – Identify pathogens based on growth/appearance (e.g., MacConkey for Gram‑negative enterics). PCR / DNA sequencing – Rapid species identification, especially for non‑culturable organisms or outbreak tracing. MALDI‑TOF MS – Quick protein‑fingerprint identification in well‑equipped labs. --- 👀 Patterns to Recognize Shape + Gram reaction → likely genus Gram‑positive cocci in clusters → Staphylococcus. Gram‑positive cocci in chains → Streptococcus. Gram‑negative rods with oxidase‑positive → Pseudomonas. Biofilm‑related infections – Chronic device‑associated infections, persistent UTIs, or lung infections in cystic fibrosis often involve biofilms. Spore‑forming Gram‑positive rods – Presence of anthrax or tetanus symptoms points to Bacillus/Clostridium. Anaerobic growth with gas production – Suggests fermentative pathways (e.g., Clostridioides difficile). --- 🗂️ Exam Traps “All Gram‑negative bacteria are aerobic.” – False; many are facultative or obligate anaerobes (e.g., Bacteroides). “Penicillin works on all bacteria with peptidoglycan.” – False; Gram‑negative outer membrane can block access, and some Gram‑positives produce β‑lactamases. “Transformation requires a plasmid.” – Incorrect; bacteria can take up naked chromosomal DNA. “All biofilm cells are metabolically inactive.” – Wrong; cells exhibit heterogeneous activity; only the deep layers are often dormant. “Acid‑fastness means the organism is a fungus.” – Misleading; acid‑fastness is a bacterial trait (e.g., Mycobacterium), not fungal. ---