Gram-negative bacteria Study Guide

📖 Core Concepts Gram‑negative bacteria: do not retain crystal violet; appear pink/red after Gram stain. Diderm (two‑membrane) architecture: inner cytoplasmic membrane → thin peptidoglycan → outer membrane. Outer membrane composition: outer leaflet = lipopolysaccharide (LPS); inner leaflet = phospholipids. Periplasmic space: gel‑like compartment between the two membranes; houses thin peptidoglycan and enzymes. LPS structure: lipid A (endotoxin) + core polysaccharide + O‑antigen. Porins: protein channels that allow selective diffusion of small hydrophilic molecules. Braun’s lipoprotein: covalently tethers outer membrane to peptidoglycan, stabilizing the envelope. Endotoxin effect: lipid A triggers innate immune response → cytokines, fever, hypotension, septic shock. 📌 Must Remember Outer membrane = barrier to most β‑lactams, detergents, and lysozyme. Effective drug classes: aminopenicillins, ureidopenicillins, cephalosporins, carbapenems, β‑lactam/β‑lactamase inhibitor combos, aminoglycosides, monobactams (aztreonam), quinolones, macrolides, chloramphenicol, folate antagonists. Resistance hot‑spot: periplasmic β‑lactamases degrade antibiotics before they reach the target PBPs. Endotoxin = lipid A; released on lysis → septic shock (↓BP, ↑RR, lactic acidosis). Monoderm vs Diderm: monoderm = Gram‑positive (single membrane); diderm = Gram‑negative (two membranes). Key Gram‑negative pathogens: E. coli, Salmonella, Shigella, P. aeruginosa, M. catarrhalis, H. pylori, L. pneumophila, A. baumannii. 🔄 Key Processes Gram‑staining outcome Crystal violet → iodine complex → wash with alcohol → Gram‑negative cells lose stain → counterstain (safranin) → pink/red. LPS‑mediated endotoxin cascade Bacterial lysis → lipid A released → binds TLR4 on immune cells → NF‑κB activation → cytokine storm → fever, vasodilation, shock. β‑lactam resistance via periplasmic enzymes Antibiotic crosses outer membrane via porin → reaches periplasm → β‑lactamase hydrolyzes β‑lactam ring → inactive drug → survival. 🔍 Key Comparisons Outer membrane vs Inner membrane Outer: LPS outer leaflet, barrier to antibiotics, contains porins. Inner: phospholipid bilayer, houses respiratory chain, ATP synthase. Lipid A vs O‑antigen Lipid A: membrane‑anchored, toxic endotoxin. O‑antigen: polysaccharide side chain, determines serotype, less toxic. β‑lactam vs β‑lactam/β‑lactamase inhibitor combo β‑lactam alone: vulnerable to β‑lactamases. Combo: inhibitor (e.g., tazobactam) blocks enzyme, restores activity. ⚠️ Common Misunderstandings “All Gram‑negative bacteria are resistant to penicillin.” False – many are susceptible to aminopenicillins and β‑lactam/β‑lactamase inhibitor combos; resistance depends on porin loss or β‑lactamase production. “LPS is only a structural component.” Incorrect – lipid A is a potent endotoxin that drives septic shock. “Porins let any molecule through.” They preferentially allow small, hydrophilic molecules; large antibiotics may be excluded. 🧠 Mental Models / Intuition “Two‑wall castle”: Think of Gram‑negative bacteria as a castle with an inner wall (inner membrane) and an outer wall (outer membrane). The moat (periplasm) holds the thin defensive fence (peptidoglycan). The outer wall’s bricks (LPS) are both protective and, when shattered, release a toxic arrow (lipid A). “Filter‑first, enzyme‑second”: An antibiotic must first filter through porins (size‑check) and then survive periplasmic enzymes before reaching its target. 🚩 Exceptions & Edge Cases Gram‑variable bacteria (e.g., M. catarrhalis sometimes stains weakly) – may show mixed Gram‑stain results. LPS‑deficient mutants: Some Gram‑negative species can survive with altered LPS, affecting endotoxin activity and immune detection. β‑lactamase inhibitors not universal: Inhibitors like clavulanic acid work on many, but not all, β‑lactamases (e.g., metallo‑β‑lactamases). 📍 When to Use Which Suspected Gram‑negative infection → start with broad‑spectrum β‑lactam/β‑lactamase inhibitor (piperacillin‑tazobactam) or carbapenem if ESBL producers are likely. Penicillin‑allergic patient → use aztreonam (monobactam) or a fluoroquinolone (ciprofloxacin). Multidrug‑resistant P. aeruginosa → combine antipseudomonal cephalosporin (cefepime) + aminoglycoside + fluoroquinolone for synergy. 👀 Patterns to Recognize Question stem mentions “outer membrane barrier” → answer likely involves porin limitation or β‑lactamase activity. Clinical picture of septic shock + Gram‑negative organism → select lipid A endotoxin as the culprit. Drug list includes “β‑lactamase inhibitor” → indicates β‑lactamase‑producing strain. 🗂️ Exam Traps Choosing “penicillin” alone for Gram‑negative infection – distractor; penicillin is usually degraded by β‑lactamases. Selecting “teichoic acid” as a Gram‑negative feature – teichoic acids are absent in Gram‑negative cells. Assuming all Gram‑negative bacteria are “spore‑forming” – false; they do not form spores. Confusing LPS with peptidoglycan – LPS is outer membrane; peptidoglycan is thin and located in the periplasm. --- Use this guide for rapid recall before your microbiology exam – focus on the bolded keywords and the cause‑effect chains!