Control of Microbial Growth

• Disinfection


  • Elimination of most or all pathogens
  • Some viable microbes may remain
• Oral antiseptic Listerine was named for Joseph Lister in 1879 when it was introduced as a surgical antiseptic
• Antiseptics

  • Chemicals used on living tissues
• Disinfectants

  • Chemicals used on inanimate objects
  • Often called germicides
  • Bactericides kill bacteria
  • Fungicides kill fungi
  • Virucides inactivate viruses
• Sterilization


  • Removal or destruction of all microorganisms and viruses
  • Sterile item is free of microbes including endospores
• Other control methods used in daily life include cooking foods, cleaning surfaces, and refrigeration
• Soap aids in the mechanical removal of organisms
• Microbiology Laboratories

  1. Must eliminate microbial contamination to both experimental samples and the environment

  2. Careful treatment both before work (use sterile materials) and after work (sterilize cultures, waste)

  3. Aseptic technique used to prevent contamination of samples, workers, and the environment
• Sanitization


  • Substantially reduces microbial population to meet accepted health standards that minimize the spread of disease
  • Not a specific level of control
• If a patient might be infected with a highly transmissible or epidemiologically important pathogen, Transmission-Based Precautions are also used
• Preservation

  • Process of delaying spoilage of perishable products
  • Choose conditions of storage to slow growth
  • Add bacteriostatic (growth-inhibiting) preservatives
• British physician Joseph Lister (1827 to 1912) revolutionized surgery by introducing methods to prevent infection of wounds
• Controlling microbes in healthcare settings is crucial due to the danger of healthcare-associated infections (HAIs)
• Special care must be taken to control microorganisms in operating rooms to avoid introducing infections during surgery
• Standard Precautions are used in patient care to prevent infection of both patients and personnel
• Pasteurization

  Brief heating to reduce the number of spoilage organisms, destroy pathogens without changing characteristics of the product
• Joseph Lister was impressed with Pasteur’s work and wondered if ‘minute organisms’ might be responsible for infections
• CDC Biosafety Levels


  • The U.S. Centers for Disease Control and Prevention (CDC) has established guidelines known as biosafety levels (BSLs) for laboratories working with microorganisms
  • BSL-1: Procedures for work with microbes not known to cause disease in healthy people
  • BSL-2: Procedures for work with microbes that pose moderate risk to individuals and the environment
• Beneficial skin microbiota reside deeper on underlying layers of skin, hair follicles, and are not adversely affected by regular washing
• Joseph Lister's method


  1. Applied phenol directly onto damaged tissues to prevent infections

  2. Improved methods further by sterilizing instruments and maintaining a clean operating environment
• Washing and scrubbing with soaps and detergents in daily life achieves routine control
• Microbiology Laboratories routinely work with microbial cultures and use rigorous methods of control
• Weakened patients are more susceptible to infection in healthcare settings
• Decontamination


  • Reduces the number of pathogens to a safe level
• Healthcare facilities must protect personnel, as demonstrated during the COVID-19 pandemic
• Number of Microbes


  1. Removing some organisms by washing reduces the time needed to sterilize or disinfect a product

  2. Scrubbing helps remove biofilms, which are important because organisms in a biofilm are resistant to disinfectants

  3. Decimal reduction time (D value) is the time required to kill 90% of the population under specific conditions
• Risk for Infection

  1. Medical instruments are categorized according to the risk for transmitting infectious agents

  2. Critical items, like needles and scalpels, must be sterile as they come into contact with body tissues

  3. Semi-critical instruments, like endoscopes, must be free of microorganisms and viruses as they contact mucous membranes but do not penetrate body tissues

  4. Non-critical instruments and surfaces, like countertops and stethoscopes, have a low risk of transmission as they only contact unbroken skin
• Types of Microbes

  • Bacterial endospores: most resistant, only extreme heat or chemical treatment destroys them
  • Protozoan cysts and oocysts: resistant to disinfectants, excreted in feces, causes diarrheal disease if ingested, destroyed by boiling
  • Mycobacterium species: waxy cell walls make them resistant to many chemical treatments
  • Pseudomonas species: resistant to and can actually grow in some disinfectants
  • Non-enveloped viruses: lack lipid envelope, more resistant to disinfectants
• Some sterilization and disinfection methods are inappropriate for certain items
• Approaches to Control in Food and Food Production Facilities

  1. Perishables retain quality longer when contaminating microbes are destroyed, removed, inhibited

  2. Heat treatment is the most common and reliable mechanism, but can alter flavor and appearance of products

  3. Irradiation and high pressure are used to treat certain foods without altering them

  4. Chemical additives can prevent spoilage, but are regulated by FDA due to the risk of toxicity

  5. Food-processing facilities must keep surfaces and machinery clean and relatively free of microorganisms
• Approaches to Control in Water Treatment Facilities

  1. Ensure drinking water is free of pathogens

  2. Chlorine is traditionally used to disinfect water, but can react with naturally occurring chemicals to form disinfection by-products (DBPs) linked to long-term health risks

  3. Some organisms are resistant to chemical disinfectants

  4. Cryptosporidium hominis (intestinal pathogen that causes diarrhea) must be minimized in treated water

  5. Regulations require facilities to minimize DBPs and C. hominis in treated water
• Moist Heat

  1. Moist heat irreversibly denatures proteins

  2. Boiling destroys most microorganisms and viruses, but does not sterilize as endospores can survive

  3. Pasteurization destroys heat-sensitive pathogens and spoilage organisms

  4. High-temperature–short-time (HTST) method: Milk at 72 degrees Celsius for 15 seconds, ice cream at 82 degrees Celsius for 20 seconds

  5. Ultra-high-temperature (UHT) method: shelf-stable boxed juice and milk, known as "ultra-pasteurization"
• Only a fraction of the population dies during a given time interval, so it takes more time to destroy a large population than a small one
• Biosafety levels (BSLs)


  • BSL-1: Procedures for work with microbes not known to cause disease in healthy people
  • BSL-2: Procedures for work with moderate-risk microbes that cause disease but have limited potential for transmission
  • BSL-3: Procedures for work with pathogens that cause serious or potentially fatal disease through inhalation
  • BSL-4: Procedures for work with easily transmitted deadly pathogens
• High-temperature–short-time (HTST) method

  Milk - 72 degrees Celsius for 15 seconds; ice cream - 82 degrees Celsius for 20 seconds
• Ultra-high-temperature (UHT) method


  Shelf-stable boxed juice and milk; known as “ultra-pasteurization”. Milk - 140 degrees Celsius for a few seconds, then rapidly cooled
• Irradiation


  Ultraviolet radiation destroys microbes directly. Damages DNA. Used to destroy microbes in air, water, and on surfaces. Poor penetrating power. Thin films or coverings can limit effect. Cannot kill microbes in solids or turbid liquids. Most glass and plastic block rays. Must be carefully used since damaging to skin, eyes. Microwaves do not affect microorganisms, but the heat they generate can be lethal. Microwave ovens heat food unevenly, so cells can survive
• Dry Heat

  Incineration (destruction by burning) oxidizes cell components to ashes. In microbiology laboratories, the wire loops continually reused to transfer bacterial cultures are sterilized by flaming or heating in a benchtop incinerator. Incineration is also used to destroy medical wastes and contaminated animal carcasses. Hot air ovens kill microbes by destroying cell components and denaturing proteins. Requires higher temperature and longer times than moist heat because dry heat takes longer to penetrate and is less efficient at killing microbes
• Filtration

  Membrane filtration in a liquid medium, retains bacteria while allowing the fluid to pass through. Used extensively to remove organisms from heat-sensitive fluids. Membrane filters or microfilters. Small pore size (0.2 micrometer) to remove bacteria. Vacuum used to move fluid through filter. Depth filters. Thick porous filtration material (for example, cellulose). Electrical charges trap cells. High-efficiency particulate air (HEPA) filters remove nearly all microbes over 0.3 micrometer from air
• Chemicals Methods Used to Destroy Microorganisms and Viruses

  Germicidal chemicals can disinfect and, in some cases, sterilize. React irreversibly with proteins, DNA, cytoplasmic membranes, or viral envelopes. Exact mechanisms of action are often poorly understood. Less reliable than heat but