Disinfection, Antisepsis, and Sterilization

• Disinfection and Antisepsis are important for controlling microorganisms
• Sterilization, pasteurization, disinfection, decontamination, and sanitization all serve different purposes
• Understanding the differences is vital

Resistant Microbial Structures

• Three bacterial structures and one bacterial/eukaryotic structure are hard to kill
• Naked viruses are more difficult to kill than enveloped viruses
• Water activity affects microbial survival
• Some organisms thrive best at low water activity
• Decimal reduction time varies based on the organism, surface, and decontaminant

Medical Instruments

• Medical instruments can be classified as critical, semicritical, or non-critical
• Moist heat is a more effective decontaminant than dry heat

Autoclaves and Pasteurization

• Understanding how an autoclave functions is important
• Liquids can be pasteurized using three different methods
• Key differences exist between membrane filters and depth (HEPA) filters

Radiation and Chemical Disinfectants

• Some types of radiation are effective for killing bacteria and other types are less efficient
• Understanding which types kill bacteria is important
• Targets for chemical disinfectants include a variety of bacterial structures

Disinfectant Levels and Action

• High-level, intermediate-level, and low-level disinfectants differ in their effectiveness
• Different levels of disinfectants will kill different microbes
• Understanding the modes of action of various chemicals is usefull (alcohols, aldehydes, chlorhexidine, etc.)

Historical Outbreaks

• The Cryptosporidium outbreak in Milwaukee in 1993 was a significant water-associated disease event
• Overuse of triclosan has led to increased bacterial resistance

Key Vocabulary

• Sterilization: Eliminating all microorganisms.
• Pasteurization: Reducing the number of microbes.
• Disinfection: Killing MOST pathogens.
• Decontamination: Reducing pathogens to a "safe" level based on standards.
• Sanitization: Reducing all organisms to a "safe" level; not specific to pathogens.
• Preservation: Lowering the growth rate of organisms for long-term food stability.

Decontamination Targets

• Endospores, protist cysts, naked viruses, and Hepatitis E have high resistance.
• Sexual fungal spores have medium resistance.
• Asexual fungal spores, lipid enveloped viruses, and vegetative bacteria have medium to low resistance.

Enveloped vs. Naked Viruses

• Enveloped viruses have a phospholipid bilayer derived from the host cell membrane.
• Naked viruses lack this envelope and have spikes extending from their capsid.

Resistant Microbe Forms

• Bacterial endospores are highly resistant and can be killed by autoclaving or glutaraldehyde.
• Protozoal cysts (Giardia, Cryptosporidium) are resistant to chemicals but killed by boiling.
• Mycobacterium are resistant to low-level disinfectants but killed by high-level ones.
• Pseudomonas is a common nosocomial organism resistant to some disinfectants and antibiotics.
• Naked viruses are resistant to drying and disinfectants due to the lack of a lipid envelope.

Water Activity (aw)

• Water is essential for membrane integrity
• Water activity is the vapor pressure of a solution divided by the vapor pressure of pure water (Pv/Pvo)
• Moisture content is related to a
• Most bacteria need high a to grow
• Fungi can survive at the lowest a
• Pure water has an a of ~1.0. Xerophilic fungi like Xeromyces can survive in water activity as low as 0.68

Decimal Reduction Time (D)

• Decimal reduction time represents a log decrease of 1 on a survivor curve.

Disinfection Process Factors

• The type of surface, the composition of the surface, and the intended use dictate the process to use
• Oils and greases interfere with disinfectant action
• Biofilms require more disinfection time
• Heat or moisture sensitive electronics, or instruments, must be carefully processes
• Critical instruments involve direct contact with tissues, semicritical with mucous membranes, and non-critical with external surfaces.

Disinfection Methods

• Disinfection involves destroying or removing microorganisms from surfaces
• Physical methods involve high temperature, radiation, filtration, and sound waves
• Chemical methods involve reactants that alter important cell components

Physical Heat Methods

• Physical methods involving heat include dry and moist heat
• Dry heat sterilization occurs at 170°C for 3 hours or 1700°C for 5 seconds
• Moist heat is more penetrating
• Boiling disinfects, but doesn't sterilize
• Autoclaving at 121°C (15 psig steam) for 15 to 45 minutes

Pasteurization Methods

• Pasteurization leads to deacreased bacterial counts without sterilization
• Flash Pasteurization (HTST) uses 72°C for 15 seconds to achieve a 5-log reduction.
• Batch Pasteurization uses 63°C for 30 minutes to better retain flavor.
• Ultrapasteurization (UHT) uses 150°C for 1 second and results in sterilization

Physical Methods at Room Temperature

• Filtration uses 0.2 micrometer pore filters to remove bacteria
• Viruses cannot be filtered out
• HEPA filters are depth filters without pores
• HEPA filters trap organisms in the filter matrix.

Ionizing Radiation

• Ionizing radiation is strong enough to remove electrons and create unstable ions.
• Gamma irradiation can cause DNA strand breakage.
• UV radiation has shorter wavelengths/ is more penetrating, especially at sterilization levels of < 300 nm.
• UV radiation can cause cyclobutane thymine dimers which leads to termination of the DNA chain synthesis

Radiation Methods

• Ionizing radiation, i.e. X-rays/y‐rays can achieve excellent penetration, and can be used for foods and heat-sensitive materials.
• UV rays are good for surfaces but do not penetrate deeply; they are used in wastewater facilities.
• Microwaves doesn't have enough energy to kill bacteria
• Heat may kill bacteria with adequate time

Chemical Methods targets

• Chemical methods target proteins, DNA, and the cytoplasmic membrane
• Proteins are targeted by alcohols, aldehydes, halogens, metals, ozone, peroxygens, and phenolics
• DNA is targeted by ethylene oxide and aldehydes
• The cytoplasmic membrane is targeted by biguanides, phenolics, and Quats

Chemical Disinfectant levels

• Sterilants kill everything, including spores, with enough exposure
• High-level disinfectants kill all but spores and are suitable for semicritical instruments
• Intermediate-level disinfectants kill all but spores and some viruses, suitable for non-critical instruments
• Low-level disinfectants kill mycobacteria, naked viruses, and spores and can be for routine household use

Disinfectant Actions

• Alcohols damage lipid membranes and denature proteins.
• Aldehydes crosslink and rigidify proteins and are high-level sterilants.
• Biguanides (chlorhexidine) make membranes leaky and are low-level.
• Ethylene oxide is a gaseous sterilant that oxidizes DNA. A toxic residue must be evaporated after use.

Halogens and Heavy Metals

• Halogens oxidize proteins and are used as sterilants.
• Iodine is an intermediate-level disinfectant used in tinctures
• Heavy metals denature cysteine, preventing protein disulfide bonds. Silver nitrate and sulfadiazine are still in use.

Disinfectant Characteristics

• Oxidizing agents oxidize proteins
• Ozone is generated at point of use
• Peroxides are high-level disinfectants on inanimate surfaces but not viable to be used on the skin
• Phenolics dissolve lipid membranes and are commonly used in cosmetics.
• Quaternary Ammonium salts (QUATs) dissolve membranes but are low-level.

Triclosan

• Triclosan targets a specific gene product and causes resistance.
• Triclosan is not antibacterial but rather a narrow spectrum antibiotic
• Spontaneous mutations cause Triclosan resistance, mutations horizontally transferred via F-pili.
• Resistance occurs due to drug efflux pumps.