Antisepsis, Disinfection, and Sterilization: Types, Action, and Resistance

Disinfection is the antimicrobial reduction of the number of viable microorganisms on or in a product or surface to a level previously specified as appropriate for its intended further handling or use. This chapter considers the most widely used methods of physical disinfection, including heat (moist- and dry-heat methods), cold, radiation, and filtration. It discusses heat convection and conduction, with further consideration of radiation (for disinfection). Heat treatment leads to the release of dipicolinic acid and calcium from spores; dipicolinic acid and calcium are considered to play roles in protecting proteins in the inner core from heat damage and are examples of the multiple resistance mechanisms that protect spores from the effects of heat. Electromagnetic radiation is energy transmitted in the form of waves or rays, including X rays and UV and infrared (IR) radiation, which are considered to be within the electromagnetic spectrum. Nonionizing-radiation methods used for disinfection include UV and IR radiation and microwaves. Filtration is one of the oldest and most widely used physical methods for the removal of contaminants from liquids and gases. Filter types can also be classified as screen or depth filters.

Antiseptics are biocidal products used for antisepsis. They include washes (which contain soaps or other detergents and are used with water) and rubs (which are applied directly to the skin with no washing, e.g., tinctures and alcohols). Antiseptic hand washes or hand rubs for health care workers are fast acting, with minimal irritation, and designed for frequent use on the skin, particularly for the reduction of transient microorganisms. It should be remembered that the purpose of antiseptics is to reduce the level of contamination; although antiseptics can vary considerably in antimicrobial activity on the skin, they do not completely remove all transient and resident microorganisms. There are many reports of cross-transmission by pathogens from contaminated hands in these situations, and studies have shown that the use of antiseptics can reduce the risk of transmission; their effects vary from formulation to formulation, despite the presence of similar concentrations of various biocides. Hand washes include a range of biocides, usually in soap- or detergent-based formulations, such as chlorhexidine, triclosan, chloroxylenol, triclocarban, essential oils, benzalkonium chloride, and some iodophors. This chapter also discusses the biocides used as antiseptics and talks about the major types used in antiseptic skin washes and rinses.

This chapter provides an overview on chemical sterilization. Sterilization is a validated process that ensures that a surface or product is free from viable microorganisms, and evidence should be provided to support such a designation of a process. Despite the wide variety of chemical biocides, only a limited number have actually been developed for use in sterilization processes. They include the epoxides (particularly ethylene oxide (EO)), formaldehyde, hydrogen peroxide-based systems, and other oxidizing-agent-based liquid and gaseous processes. These systems are primarily used as alternatives to physical sterilization methods, particularly due to material compatibility concerns, for example, as alternatives to steam for the sterilization of temperature-sensitive materials. The type and application, spectrum of activity, advantages, disadvantages, and mode of action of each is discussed. EO is one of the most widely used products for industrial sterilization. Low temperature steam-formaldehyde (LTSF) systems are used for medical, dental, and some industrial sterilization processes. High-temperature formaldehyde-alcohol sterilization is a process that combines the biocidal activities of heat with those of formaldehyde and alcohol. Hydrogen peroxide solutions are not generally used in sterilization processes, but gaseous hydrogen peroxide is rapidly sporicidal at much lower concentrations and is considered significantly less damaging to surfaces. A section discusses other oxidizing agent-based sterilization processes that have been described or that are widely used.

This chapter discusses the various mechanisms of biocide resistance described in microorganisms. Resistance can be either a natural property of an organism (intrinsic) or acquired by mutation or by the acquisition of plasmids (self-replicating extrachromosomal DNA) or transposons (chromosomal or plasmid-integrating transmissible DNA cassettes). The biocide concentration is an important variable and must at least be at the MIC or, preferably, at the minimum biocidal concentration to have a significant effect. Mechanisms of intrinsic resistance are described with further consideration of the various types of bacteria. The first acquired resistance mechanisms reported were against mercury compounds and other metallic salts. In recent years, acquired mechanisms of resistance to other types of biocides have been observed, notably in gram-positive staphylococci. The proposed mechanisms of prion resistance are summarized. In comparison with bacteria, very little is known about the ways in which fungi can circumvent the actions of biocides and biocidal processes. As with bacteria, two general mechanisms of resistance can be identified: intrinsic resistance, a natural property or development of the organism during normal growth, and acquired resistance, with examples of both identified or proposed.