Bacterial metabolism

Energy & Carbon Sources - Bacterial Diets

Classification based on how bacteria acquire energy and carbon.

• Energy Source
  • Phototrophs: Use light.
  • Chemotrophs: Use chemical compounds.
    • Organotrophs: Organic chemicals.
    • Lithotrophs: Inorganic chemicals.
• Carbon Source
  • Autotrophs: Use inorganic $CO_2$.
  • Heterotrophs: Use organic compounds.

⭐ Nearly all medically important bacteria are chemoheterotrophs. They derive both energy and carbon from organic compounds, typically sourced from their host.

Oxygen Requirements - To Breathe or Not

• Obligate Aerobes: Require O₂ for aerobic respiration; produce ATP via cellular respiration.
  • Possess catalase and superoxide dismutase (SOD) to neutralize reactive oxygen species (ROS).
  • 📌 Nagging Pests Must Breathe: Nocardia, Pseudomonas, Mycobacterium, Bacillus.
• Facultative Anaerobes: Use O₂ when present, but can switch to fermentation or anaerobic respiration.
  • Possess catalase and SOD.
  • e.g., Staphylococci, Enterobacteriaceae (E. coli).
• Obligate Anaerobes: O₂ is toxic; they lack catalase and/or SOD.
  • 📌 Anaerobes Can't Breathe Fresh Air: Actinomyces, Clostridium, Bacteroides, Fusobacterium.
• Microaerophiles: Require O₂ at low concentrations (2-10%).
  • e.g., Campylobacter jejuni, Helicobacter pylori.

⭐ Aminoglycosides are ineffective against anaerobes because their uptake across the bacterial cell membrane is an oxygen-dependent process.

Metabolic Pathways - ATP Factories

• Core Goal: Generate ATP & precursor molecules.
• Three Central Pathways:
  • Glycolysis: Cytoplasmic. Glucose → 2 Pyruvate. Net: 2 ATP, 2 NADH.
  • Krebs Cycle (TCA): Links to glycolysis via PDC (Pyruvate → Acetyl-CoA). Generates GTP, NADH, FADH₂.
  • Oxidative Phosphorylation: Main ATP generator. ETC on the inner cell membrane creates a proton motive force ($H^+$ gradient) to power ATP synthase.

• Final Electron Acceptor Dictates Lifestyle:
  • Aerobes: Use O₂. High ATP yield (~32 ATP/glucose).
  • Anaerobes: Use inorganic molecules (e.g., $NO_3^-$, $SO_4^{2-}$). Lower ATP yield.
  • Fermentation: Regenerates $NAD^+$ for glycolysis; no net ATP gain.

⭐ Obligate anaerobes lack catalase and superoxide dismutase (SOD), making O₂ toxic. They cannot handle reactive oxygen species.

Key Enzymes & Tests - Metabolic Fingerprints

• Catalase: Degrades $H_2O_2$ into $H_2O$ and $O_2$. Differentiates Staphylococci (Cat+) from Streptococci (Cat-).
  • Test: Bubbles upon adding $H_2O_2$.
• Coagulase: Converts fibrinogen to fibrin. Identifies S. aureus (Coag+) from other Staph.
  • Test: Clot formation in plasma.
• Urease: Hydrolyzes urea to ammonia ($NH_3$) and $CO_2$.
  • 📌 Pee CHUNKSS: Proteus, Cryptococcus, H. pylori, Ureaplasma, Nocardia, Klebsiella, S. epidermidis, S. saprophyticus.
• Oxidase: Detects cytochrome c oxidase. Differentiates Neisseria & Pseudomonas (Oxidase+) from Enterobacteriaceae (Oxidase-).

⭐ Catalase-positive organisms can survive the oxidative burst within phagocytes, a key virulence factor. The test is a critical initial step for identifying Gram-positive cocci.

High‑Yield Points - ⚡ Biggest Takeaways

• Obligate aerobes like Pseudomonas & Mycobacterium require O₂ for maximal ATP generation.
• Obligate anaerobes like Clostridium & Bacteroides lack catalase and superoxide dismutase, making O₂ toxic.
• Most pathogenic bacteria are facultative anaerobes, capable of growing with or without O₂.
• Lactose fermentation is a key differential on MacConkey agar, turning colonies pink.
• Key identifying enzymes include catalase, urease, coagulase, and oxidase.