Biol 20: Chapter 13
Control of Microbial Growth
Importance of Controlling Microbial Growth
Microbial growth can lead to contamination and disease; understanding methods for control is crucial.
Microbes are ubiquitous in various environments, leading to the need for effective control measures.
Study of Microbial Contamination in Cars
Study Overview:
Analyzed 11 locations in 13 different cars for microbial colony forming units (CFUs).
Found that areas like the central console harbor a high variety and number of microorganisms due to infrequent cleaning and the presence of food.
Key Findings:
Central console had the highest microbial counts, followed by door latches and steering wheels.
Ignorance of common areas where microbes thrive emphasizes the need for regular cleaning.
Fomites and Disease Transmission
Definition of Fomites:
Inanimate objects or surfaces that can harbor infectious agents, e.g., doorknobs, toys, towels.
Factors Affecting Cleanliness Levels:
Type of Item: Items frequently touched require higher levels of cleanliness.
Invasiveness: Items that require insertion into the human body need strict hygiene to prevent infections.
Resistance to Antimicrobial Treatment: Some microbes, like endospores, are resistant to cleaning agents, affecting the level of disinfectant needed.
Biological Safety Levels (BSL)
Developed by CDC and NIH for classifying the safety measures necessary for handling various pathogens.
Factors Determining Safety Levels:
Infectivity of the agent.
Ease of transmission.
Potential severity of disease.
Type of work being conducted with the agent.
BSL-1 (Basic Microbiological Practices)
Description: Nonpathogenic microbes (e.g., E. Coli, Staphylococcus epidermidis).
Safety Features:
Standard aseptic techniques.
Regular protective equipment (lab coats, gloves, goggles).
Autoclave for sterilization before and after use.
BSL-2 (Moderate Risk Agents)
Description: Agents pose moderate risk to lab personnel and the community (e.g., Staphylococcus aureus, Salmonella).
Safety Features:
Access restricted to authorized personnel.
Personal protective equipment (PPE) required, including face shields.
Use of biological safety cabinets for procedures that may aerosolize pathogens.
Self-closing doors and eyewash stations for emergencies.
BSL-3 (Potential Lethal Agents)
Description: Agents that can cause severe or fatal diseases by inhalation (e.g., Mycobacterium tuberculosis, HIV).
Safety Features:
BSL-2 features plus:
Medical surveillance and vaccination requirements for workers.
Respirators mandatory.
Hand-free sink and eyewash station near exits.
Two self-closing doors for added security and directional airflow controls to avoid contamination.
BSL-4 (Highly Dangerous Agents)
Description: Work with high-risk agents that can cause fatal diseases (e.g., Ebola virus, Marburg virus).
Safety Features:
Must wear full body suits with positive air pressure.
Must shower upon exiting the lab and disinfect all materials.
High-efficiency particulate air (HEPA) filtration used for air supply.
Separate buildings or isolated areas with distinct air supply and exhaust.
Summary of Biological Safety Levels Chart
Understanding the specific features and requirements of each BSL is critical for laboratory safety and effective microbial growth control.
Each level builds on the previous with enhanced safety precautions suited to the risk level of the agents handled.
The conclusion of this section highlights the importance of controlling microbial environments and understanding the serious implications of microbial safety in various settings, especially in laboratory environments as we progress to explore important terms and concepts in the next part.
video 2
Key Terms in Microbial Control
Sterilization
Complete removal or killing of all microbial cells, including vegetative cells, endospores, and viruses.
Achieves total sterility in items or environments, crucial in clinical settings.
Aseptic Techniques
A set of procedures to maintain sterility and prevent patient contamination.
Example: Cleaning the skin before invasive procedures with alcohol or antiseptic wipes to eliminate microorganisms.
Commercial Sterilization
Uses heat to destroy common pathogens while preserving food quality (taste and texture).
Specifically aimed at killing spores of Clostridium botulinum, which is known for producing heat-resistant endospores.
Disinfection
Inactivation of most microbes on inanimate objects (fomites) using antimicrobial chemicals or heat.
Important for surfaces and medical instruments, but does not achieve complete sterility.
Antiseptics
Chemicals safe for use on living tissues (e.g., skin).
Prevent infections by reducing microbial load on skin and tissues, but they do not always lead to sterility.
De germing
A form of disinfection focusing on reducing microbial numbers on living tissue through gentle scrubbing, usually with mild chemicals.
Example: Handwashing with soap and water or using alcohol swabs; it significantly lowers but does not eliminate all microbes.
Sanitization
Cleaning processes aimed at reducing microbial load on utensils and food service items to safe levels for public health.
Example: Washing dishes or using commercial dishwashers, which use high temperatures to sanitize but not sterilize.
Essential Distinctions
Disinfectants vs. Antiseptics
Disinfectants: Used on non-living objects.
Antiseptics: Used on living tissues.
De germing vs. Sterilization
De germing reduces microbial numbers but doesn’t kill all microbes; it’s about general cleaning (e.g., handwashing).
Sterilization is the complete eradication of all microbes.
Sanitization vs. Sterilization
Sanitization lowers the microbial count to safe levels for public health; it doesn’t achieve absolute sterility.
Sterilization is the ultimate goal of eliminating all microbes.
Conclusion
Understanding these key terms is crucial, as they involve subtle yet significant differences in meaning and application in microbial control.
It's important for exam preparation to remember these definitions accurately and their applications in clinical and public health settings.
video 3
Physical Methods of Microbial Control
Common methods include:
High temperature
Low temperature
Radiation (ionizing and nonionizing)
Filtration
Desiccation
Mechanism:
Disrupts plasma/ cell membranes
Alters permeability
Damages proteins and nucleic acids via denaturation, degradation, and chemical modifications.
High Temperature
Heat: Most common form of microbial control.
Kills microbes by changing membranes and denaturing proteins.
Boiling:
Effective for vegetative cells and some viruses.
Less effective against endospores (can survive up to 20 hours of boiling).
Important to consider endospore survival when using heat.
Heating Protocols:
Dry Heat Sterilization:
Example: Incineration (like flaming inoculating loops).
Effective for complete sterilization at high temperatures over time.
Moist Heat Sterilization:
Example: Autoclave, more effective due to better penetration.
Used to exceed water boiling point to kill vegetative cells and endospores.
Autoclave:
Developed by Charles Chamberlain in 1879, remains an essential sterilization tool.
Uses autoclave tape that changes color to indicate successful sterilization.
Biological indicator (e.g., Geobacillus steatohermophilus) used to test autoclave efficacy.
Pasteurization
Kills pathogens and reduces spoilage organisms without sterilizing the food.
Developed by Louis Pasteur for beer and wine in the 1860s.
Types of Pasteurization:
High Temperature Short Time (HTST): About 72°C for 15 seconds.
Ultra High Temperature (UHT): About 138°C for 2-3 seconds; enhances shelf-life without refrigeration.
Low Temperature
Reduces bacterial growth, not completely kills.
Refrigeration (0-7°C) slows metabolism, preserves food/medicals but must be monitored to avoid spoilage.
Freezing:
Long-term storage at -70°C or lower, using liquid nitrogen or dry ice.
Pressure
High Pressure Processing:
Kills various microbes while maintaining food quality (100-800 MPa).
Triggering protein denaturation, though some endospores may survive.
Hyperbaric Oxygen Therapy:
Used in treatment of infections through increased oxygen saturation at higher pressures (1-3 atm).
Desiccation
Uses salt/sugar to create a hypertonic environment, drawing water from cells.
Examples: salted fish, jellies.
Lyophilization:
Freeze-drying under vacuum methods for preservation.
Radiation
Two categories:
Nonionizing Radiation:
Example: UV light; forms thymine dimers causing lethal mutations.
Used with germicidal lamps for equipment sterilization.
Ionizing Radiation:
Example: Gamma radiation, effective for food sterilization; clear labeling required (Radura symbol).
Filtration
Used for heat-sensitive liquids.
Membrane Filters:
Remove microorganisms while preserving liquid quality.
HEPA Filters:
Clean air in laboratories and healthcare settings by filtering out microbes and spores.
Summary of Physical Control Methods:
Heat: Boiling, dry heat sterilization, moist heat (autoclaving).
Pasteurization: Different methods to control microbial load in food.
Low Temperature: Refrigeration/freezing to inhibit microbial growth.
Pressure: High pressure processing for food preservation and hyperbaric treatments.
Desiccation: Reduces water activity through solute addition.
Radiation: UV and gamma radiation for decontamination.
Filtration: Membrane and air filtration for sterile processing.
Next Topic: Chemical control methods will be discussed in the following section.
video 4
Chemical Control of Bacterial Growth
The role of chemicals in controlling bacterial growth is crucial in various settings, particularly in hospitals.
Phenols and Phenolic Compounds
Joseph Lister: First to use a chemical for sterilization—carbolic acid (phenol).
Carbolic Acid: A phenol used historically in surgical procedures for antisepsis.
Derivatives of Phenol:
Phenylphenol: Common disinfectant.
Hexachlorophene (Bisphenol): Used as an active ingredient in hospital cleansers.
Triclosan:
Common in antibacterial soaps, despite health/environmental risks.
Offers no significant health benefits over conventional soaps.
Heavy Metals as Antimicrobial Agents
Metals like copper, silver, and zinc have antimicrobial properties.
Brass Doorknobs: Oligodynamic effect—ability to kill microorganisms in small quantities.
Impairs cell function of microbes.
Silver:
Silver spoons can reduce microbial load; excessive use can lead to argyria (skin color changes).
Halogens
Chlorine and Iodine: Effective antimicrobial agents.
Betadine:
An iodine solution used as an antiseptic.
Chloramines:
e.g., Monochloramine (derived from ammonia).
Effective disinfectant.
Alcohols
70% Alcohol (Isopropyl or Ethyl): Most effective for disinfection and antiseptic uses.
Rubbing Alcohol: Commonly used in healthcare settings.
Soaps
Definition: Salts of fatty acids, usually from sodium.
Function: Emulsifies lipids and removes oils due to hydrophilic and hydrophobic structure.
Biguanides
Includes Chlorhexidine and Lhexidine: Used as surgical scrubs.
Important in skin antisepsis in medical settings.
Catalase
Hydrogen peroxide as a wound antiseptic.
Catalase breaks down hydrogen peroxide to water and oxygen (e.g., bubbling effect at the wound site).
Antibiotics
Produced by microorganisms to inhibit or kill bacteria.
Penicillin: First antibiotic, derived from the fungus Penicillium.
Handwashing
Emphasized as a critical hygiene practice.
Five essential steps of handwashing:
Wet hands.
Apply soap.
Lather.
Rinse.
Dry hands.
Importance highlighted in preventing infections.
Conclusion:
This chapter reinforces the significance of various chemicals in controlling bacterial growth and infection prevention in healthcare settings.
Visual Aids Used:
Diagrams of chemical structures, handwashing steps, and action mechanisms of disinfectants.