Controlling microbial growth is crucial in healthcare, food safety, and research. From household cleaners to high-tech sterilization methods, various approaches target different microbes. Understanding these techniques helps prevent infections and maintain sterile environments.

Biosafety levels guide lab practices for handling microbes. From BSL-1 for harmless bacteria to BSL-4 for deadly viruses, each level has specific protocols. These measures protect researchers and the public from potential biohazards.

Microbial Growth Control Methods

Disinfectants vs antiseptics vs sterilants

• Disinfectants reduce microbial populations to safe levels on inanimate objects and surfaces (household cleaners, bleach, hydrogen peroxide)
• Antiseptics prevent infection by reducing microbial populations on living tissue (hand sanitizers, iodine, hydrogen peroxide)
• Sterilants eliminate all forms of microbial life, including spores, on inanimate objects (ethylene oxide gas, glutaraldehyde, peracetic acid)

Principles of sterilization and disinfection

• Physical methods
  • Heat sterilization
    1. Moist heat: autoclave uses steam under pressure (121°C, 15 psi, 15-20 minutes)
    2. Dry heat: hot air oven employs higher temperatures and longer exposure times (160-180°C, 2-4 hours)
  • Filtration removes microorganisms from liquids and gases using membrane filters with pore sizes 0.2-0.45 μm
  • Radiation
    • Ionizing radiation (gamma rays, X-rays) damages DNA
    • Non-ionizing radiation (UV light) causes thymine dimers in DNA
• Chemical methods
  • Alcohols (ethanol, isopropanol) denature proteins and disrupt cell membranes, effective against vegetative bacteria, fungi, and enveloped viruses
  • Halogens (chlorine, iodine) oxidize cellular components and denature proteins, effective against a wide range of microorganisms
  • Phenolics denature proteins and disrupt cell membranes, effective against vegetative bacteria and enveloped viruses
  • Hydrogen peroxide and peracetic acid are strong oxidizing agents that damage cellular components, effective against a wide range of microorganisms, including spores
  • Aldehydes (formaldehyde, glutaraldehyde) cross-link proteins and inactivate enzymes, effective against vegetative bacteria, fungi, and viruses

Antimicrobial Agents and Microbial Resistance

• Antimicrobial agents are substances that kill or inhibit the growth of microorganisms
• Selective toxicity is the property of antimicrobial agents that allows them to harm microorganisms without significantly damaging host cells
• Minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial agent that inhibits visible growth of a microorganism
• Microbial resistance occurs when microorganisms develop mechanisms to survive exposure to antimicrobial agents, making treatments less effective

Biological Safety Levels and Handling Techniques

Biological safety levels and handling

• Biosafety Level 1 (BSL-1)
  • Low-risk microorganisms not known to consistently cause disease in healthy adults (non-pathogenic E. coli, Bacillus subtilis)
  • Standard microbiological practices
• Biosafety Level 2 (BSL-2)
  • Moderate-risk microorganisms that can cause human disease (Staphylococcus aureus, Salmonella, Hepatitis B virus)
  • BSL-1 practices plus limited access to the lab, biohazard warning signs, sharps precautions, and a biosafety manual defining any needed waste decontamination or medical surveillance policies
• Biosafety Level 3 (BSL-3)
  • Indigenous or exotic microorganisms with potential for aerosol transmission and lethal disease (Mycobacterium tuberculosis, SARS-CoV, Francisella tularensis)
  • BSL-2 practices plus controlled access, decontamination of all waste and lab clothing before laundering, and negative airflow into the laboratory
• Biosafety Level 4 (BSL-4)
  • Dangerous and exotic microorganisms with a high risk of aerosol-transmitted infections, frequently fatal, or with no available vaccines or treatments (Ebola virus, Marburg virus, Lassa virus)
  • BSL-3 practices plus clothing change before entering, shower on exit, and all material decontaminated on exit from the facility
  • Aseptic technique is crucial to prevent contamination and maintain sterile conditions