lecture 6

Control of Microbial Growth

• Microbial growth control methods have a history, starting with Joseph Lister's use of carbolic acid to prevent surgical wound infections.
• Effective microbial control is crucial for public health and preventing food spoilage.
• A variety of methods exist to manage microbial growth on inanimate objects and surfaces, which are typically non-selective, impacting various life forms.

Approaches to Control

• Physical methods include heat, irradiation, filtration, and mechanical removal (washing).
• Chemical methods utilize antimicrobial chemicals.
• Some processes combine physical and chemical methods.
• The effectiveness of these methods varies against different organisms.
• The chosen method depends on circumstances and desired level of control.

Principles of Control

• Sterilization: Removes or destroys all microorganisms and viruses; an absolute term. Sterile items are completely free of microbes, endospores, and viruses. Achieved through filtration, heat, chemicals, or irradiation. Sterilization does not eliminate prions. Combustion is the most reliable method for prion destruction.
• Disinfection: Uses antimicrobial chemicals to eliminate most pathogens. Disinfectants are biocides used on inanimate objects and surfaces. Disinfectants are commonly germicides, which target microorganisms, including bacteria, endospores, fungi, and viruses. Bactericidal means killing bacteria, while antiseptics are non-toxic disinfectants suitable for skin application.
• Pasteurization: Brief heat treatment that reduces spoilage bacteria and pathogens in foods and inanimate objects, without complete sterilization. Methods include high temperature-short time (HTST), and ultra-high temperature (UHT).
• Decontamination: Reduces pathogens to a safe handling level, often using heat or disinfectants.
• Sanitization: Substantially reduces microbial populations to meet health standards. Doesn't specify a precise control level.
• Preservation: Delays spoilage of perishable items through the addition of microbial growth inhibitors/chemicals (bacteriostatic agents).

Situational Considerations

• Daily life control measures (e.g., washing, cooking) may not suffice in specific situations like hospitals or labs.
• Simple hand washing prevents many infectious diseases.
• Hospitals require rigorous microbial control measures due to healthcare-associated infections (HAIs). Sterilization is crucial in operating rooms. Prions represent a major concern in hospitals.
• Microbiology labs need strict, rigorous microbial control methods and aseptic techniques. Materials must be sterile and correctly discarded.
• Food and food processing facilities rely on heat treatment, irradiation, and chemicals to control microbes and prevent contamination.
• Water treatment facilities should be free of pathogenic microbes to avoid contamination. Chlorine is a common disinfectant, but can generate disinfection by-products (DBPs). Certain pathogens, like Cryptosporidium parvum oocysts, are resistant to traditional disinfection processes.

Selecting an Antimicrobial Procedure

• No single method satisfies all requirements, so a tailored approach is needed.
• The selection depends on the microbe type, number of microbes, environmental conditions (e.g., temperature, pH, presence of organic material), risk of infection, and composition of the target item.
• Different microbes have varying resistances. Endospores (e.g., Bacillus and Clostridium) require extreme heat or chemicals to eliminate. Protozoan cysts and oocysts are eliminated rapidly by boiling. Mycobacterium species need stronger disinfectants due to their waxy cell walls. Non-enveloped viruses, like polio, are more resistant to disinfectants than enveloped viruses, such as HIV.
• The time needed to eliminate a population of microbes depends on its initial count. Washing/scrubbing can lessen the time required for sterilization/disinfection.
• Decimal reduction time (D-value) is the time needed to reduce a microbial population by 90%.
• Environmental factors (e.g., temperature, organic materials) influence microbial death rates.
• Items posing a significant threat of disease transmission (e.g., critical items in a hospital) require more rigorous sterilization methods.

Physical Methods to Destroy/Remove Microbes

• Heat: Provides a reliable, safe, fast, and economical means of removing microbes.

  • Moist heat: Irreversibly denatures proteins of microorganisms via boiling, pasteurization, and pressurized steam (autoclaving).
    • Boiling (100°C at sea level) for 10 minutes kills most microbes and viruses (except endospores).
    • Pasteurization is a brief heat treatment to reduce spoilage bacteria in food and drinks.
    • Autoclaving (pressurized steam) sterilizes items that can withstand heat and moisture, such as medical instruments and supplies.
  • Dry heat: Burns cell constituents or denatures proteins via dry heat ovens or incineration. Requires higher temperatures and longer times compared to moist heat.

• Filtration: Used for heat-sensitive materials.

  • Membrane/microfilters are used to remove bacteria and sometimes viruses. The pore size determines the microbes the filter can catch.
  • Depth filters trap microbes in a thick porous material (like cellulose).
  • HEPA filters remove particles larger than 0.3μm from air (like hospital rooms, labs).

• Irradiation: Damages microbial DNA, or produces reactive molecules.

  • Ionizing radiation (e.g., gamma rays, X-rays) is effective for sterilizing heat-sensitive materials like medical supplies, foods. It can be done after packaging in certain instances. It disrupts microbial DNA or produces reactive molecules that harm the microbial cells.
  • Ultraviolet (UV) radiation (non-ionizing) destroys microbes by directly damaging their DNA. It is mainly used for disinfection of surfaces, air, and water. Use in close range is important but can harm skin and eyes.
  • Microwaves heat items, killing microbes by the heat the item produces rather than directely affecting the microbes. Food is heated unevenly with microwaves, so microbes can survive.

• High-pressure processing: Used for commercial food products. Pressures up to 130,000 psi denature proteins and alters microbial cell permeability. Products typically retain their fresh color and flavor.

Next steps

• Refer to chapter 5 for further study of this topic

Explore the various methods used to control microbial growth, from physical techniques like heat and filtration to chemical agents. Understanding these methods is crucial for public health and preventing food spoilage. Dive into the principles of sterilization and the effectiveness of different approaches.