Sterilization (microbiology) Study Guide

📖 Core Concepts Sterilization: Complete elimination (or irreversible inactivation) of all microorganisms, spores, prions, and viruses. Objects become sterile or aseptic. Sterility Assurance Level (SAL): Probability that a unit remains non‑sterile after processing (e.g., SAL = 10⁻⁶ means 1 failure in 1,000,000 units). D‑value: Time (or dose) needed to achieve a 1‑log (90 %) reduction in viable count; \(D = 1/k\). First‑order kill kinetics: Microbial death rate proportional to the surviving population, \( \frac{dN}{dt} = -kN \). Overkill method: Sterilize for a margin beyond the measured bioburden to guarantee the target SAL. Aseptic technique: Practices that keep sterile items free from contamination (sterile garments, gloves, cleanroom environment). 📌 Must Remember SAL requirement for high‑risk medical products: ≤ 10⁻⁶ (FDA). Steam autoclave parameters: 121 °C – 134 °C for 3–30 min (gravity‑displacement) or 18 min at 134 °C for prions. Dry heat: 160 °C ≥ 2 h (standard) or 190 °C for 6 min (unwrapped). Ethylene oxide (EO) gas: 30 °C – 60 °C, RH > 30 %, 200–800 mg/L, several hours. Membrane pore sizes: 0.22 µm → most bacteria; 20–50 nm → viruses. Biological indicator for steam: Geobacillus stearothermophilus spores. Radiation dose for sterilization: ≥ 10 MeV to induce radioactivity (not used); typical sterilization energies are below this, so items stay non‑radioactive. 🔄 Key Processes Heat‑based microbial kill (first‑order) \(N = N0 e^{-kt}\) → \(N = N0 10^{-t/D}\). Determine D‑value at a reference temperature, then adjust for other temps using Arrhenius: \(k = A e^{-Ea/(RT)}\). Steam sterilization cycle Load → pre‑vacuum (or gravity‑displacement) → heat to 121–134 °C → hold for prescribed time → depressurize → dry. EO gas sterilization Load → set temperature, RH, and concentration → expose for required duration → aeration to remove residual EO. Sterile filtration Pre‑filter → select appropriate membrane (0.22 µm or 20–50 nm) → pass fluid tangentially → monitor pressure/flow → perform integrity test. 🔍 Key Comparisons Steam vs. Dry Heat Moist heat denatures proteins at lower temps (121 °C) → faster; dry heat requires higher temps (≥ 160 °C) and longer exposure. EO Gas vs. Hydrogen Peroxide EO penetrates porous plastics, low temp, longer cycle, toxic residues. H₂O₂ (vapour) faster, no toxic residues, but limited penetration and material incompatibility (cellulose, nylon). UV vs. Ionizing Radiation UV: surface‑only, non‑penetrating, low dose, ineffective in shadows. Ionizing (γ, e‑beam, X‑ray): deep penetration, higher dose, can sterilize bulk loads. ⚠️ Common Misunderstandings “Sterilization = Disinfection” – Sterilization eliminates all life; disinfection only reduces microbial load. “All prions are killed by standard autoclave cycles” – Prions need longer times (≥ 60 min at 121 °C) or higher temps (134 °C ≥ 18 min). “Filtration removes all viruses” – Only nanofiltration (≤ 50 nm) can reliably remove viruses; 0.22 µm filters do not. 🧠 Mental Models / Intuition Log‑kill “stairs”: Each D‑value is one step down a 10‑fold ladder; 3 D‑values = 99.9 % kill, 6 D‑values = 99.9999 % kill (≈ SAL 10⁻⁶). Temperature‑time trade‑off (FT‑T): Higher temperature → smaller D‑value → fewer minutes needed (inverse exponential via Arrhenius). Barrier vs. Inactivation: Heat/chemicals inactivate microbes; filtration physically blocks them. 🚩 Exceptions & Edge Cases Prion resistance: Require extended high‑temperature steam cycles; many chemical agents (including aldehydes) are ineffective. Material incompatibility: Dry heat can cause oxidation; EO may degrade certain polymers; H₂O₂ can embrittle nylon. Air‑dry cycles: Gravity‑displacement autoclaves may leave air pockets in complex loads → risk of incomplete sterilization. 📍 When to Use Which Heat‑stable metal tools → Steam sterilization (fast, reliable). Heat‑sensitive plastics, electronics → EO gas or VHP (choose EO for deep penetration, VHP for quicker cycle & no toxic residues). Liquid pharmaceuticals that cannot be heated → Sterile filtration (0.22 µm for bacteria, nanofiltration for viruses). Large bulk items or pallets → Gamma or high‑energy X‑ray (deep penetration). Surface decontamination of workspaces → UV germicidal lamps (short‑range, no residues). 👀 Patterns to Recognize “D‑value decreases as temperature rises” → look for Arrhenius‑type statements. “Biological indicator spore = most resistant organism” → expect G. stearothermophilus for steam, similar for other methods. “Cycle time + temperature = target SAL” → if the question gives time and temp, compute log‑kill steps to verify SAL. “Pore size vs. target organism” → bacteria → 0.22 µm; viruses → ≤ 50 nm. 🗂️ Exam Traps Distractor: “Steam sterilization kills all prions at 121 °C for 15 min.” – Wrong; prions need longer or higher temperature. Distractor: “EO sterilization works at any humidity.” – Incorrect; RH must be > 30 % for effective EO diffusion. Distractor: “UV can sterilize opaque liquids.” – UV cannot penetrate opaque or turbid media. Distractor: “A 0.22 µm filter guarantees viral removal.” – Only nanofiltration (20–50 nm) removes viruses. Distractor: “Higher SAL (e.g., 10⁻⁴) is safer than 10⁻⁶.” – Lower numeric SAL indicates higher assurance (10⁻⁶ is safer). --- Keep this guide handy; each bullet is an exam‑ready fact or decision rule.