aerobe

**Definition:** Aerobes are organisms that use molecular oxygen as the terminal electron acceptor in the electron transport chain (oxidative phosphorylation), yielding far more ATP per glucose (~30-32) than fermentation (2).

**Classification by O2 requirement:** - **Obligate aerobes:** Require O2 for growth. They have superoxide dismutase (SOD) and catalase to detoxify reactive oxygen species. Mnemonic: "Nagging Pests Must Breathe" – Nocardia, Pseudomonas, Mycobacterium tuberculosis, Bacillus (B. cereus/anthracis are often described as obligate or facultative), plus Burkholderia, Brucella, Bordetella, Legionella, Francisella (Neisseria is also an obligate aerobe, although it can use alternatives in some contexts). - **Facultative anaerobes:** Grow with or without O2 (aerobic respiration when O2 is present, fermentation or anaerobic respiration when it is absent). Examples: Staphylococcus, Streptococcus, E. coli, Enterobacteriaceae, Listeria. - **Microaerophiles:** Need low O2 (about 2-10%) and are damaged by full atmospheric O2. Examples: Campylobacter, Helicobacter pylori. - **Aerotolerant anaerobes:** Do not use O2 but tolerate it (e.g., Clostridium perfringens, Lactobacillus, Cutibacterium acnes, Streptococcus mutans). - **Obligate anaerobes:** Lack SOD and catalase, and O2 is toxic to them. Examples: Clostridium, Bacteroides, Actinomyces, Fusobacterium, Peptostreptococcus.

**Pathophysiology and clinical significance:** - **Oxygen-rich tissues:** Obligate aerobes favor high-O2 sites. M. tuberculosis causes reactivation disease in the upper lobes of the lung (high V/Q, high PaO2). Pseudomonas colonizes the lungs in cystic fibrosis and causes infections in burn wounds. - **Reactive oxygen species:** Aerobic metabolism produces superoxide (O2⁻), H2O2, and hydroxyl radicals. SOD converts O2⁻ to H2O2, and catalase converts H2O2 to H2O and O2. Catalase-positive organisms (Staph, Pseudomonas, Nocardia, Listeria, Serratia, E. coli, Burkholderia, Candida, Aspergillus) cause infections in chronic granulomatous disease (CGD), which results from NADPH oxidase deficiency. - **Aminoglycosides:** These require an oxygen-dependent transport system to enter bacteria. They are therefore ineffective against anaerobes. This is a high-yield pharmacology point. - **Anaerobic infections:** Abscesses, foul-smelling discharge, and infections below the diaphragm or in poorly perfused, necrotic tissue point to anaerobes. Metronidazole and clindamycin treat them. - **Culture:** Obligate aerobes grow at the top of a thioglycolate broth tube (where O2 is highest). Facultative anaerobes grow throughout, with the most growth at the top. Obligate anaerobes grow at the bottom. - **Eukaryotic aerobes:** Human cells are aerobes, with mitochondrial oxidative phosphorylation. Cyanide and CO inhibit cytochrome c oxidase (complex IV).

**Key associations:** Aerobic respiration → TCA cycle + ETC → high ATP yield; obligate aerobes have SOD + catalase; aminoglycosides need O2; M. tuberculosis (upper lobe), Pseudomonas (CF, burns), Nocardia (weakly acid-fast, branching filamentous rods).