on 05-Dec-2024 (Thu)

Chemical biocides are used for the prevention and control of infection in health care, targeted home hygiene or controlling microbial contamination for various industrial processes including but not limited to food, water and petroleum. However, their use has substantially increased since the implementation of programmes to control outbreaks of methicillin-resistant Staphylococcus aureus, Clostridioides difficile and severe acute respiratory syndrome coronavirus 2

Inappropriate usage or low concentrations of a biocide may act as a stressor while not killing bacterial pathogens, potentially leading to antimicrobial resistance.

Although some may be used for either application, the terms disinfect- ant and antiseptic, respectively, refer to biocides used on non-living surfaces and living tissues (for example, the skin).

The use of biocides has been documented for centuries 1 , well before the germ theory of diseases by Pasteur 2 and postulates of Koch and co-workers 3 . The work of Ignaz Semmelweis represents an important moment in the modern use of disinfection and antisepsis, as it introduced chlorinated lime water for hand disinfection 4 , leading to a reduction in the incidence of puerperal fever following births

The COVID-19 pandemic contributed to an escalation of surface, air and skin disinfection. The persistence of severe acute respiratory syndrome coronavirus 2 on surfaces, at least for a few hours, not only highlighted the need to improve surface and hand hygiene compli- ance but also provided a reason for disinfectant manufacturers to provide long-lasting antimicrobial protection of surfaces.

Increasing product usage for disinfection and antisepsis means increasing bacterial exposure to biocides.

Unlike chemotherapeutic antibiot- ics, biocides at their in-use concentration exert bactericidal activity by affecting multiple targets on the bacterial cell.

The poor understanding of manufacturers regarding the different chemistries, including factors that affect efficacy, and inappropriate usage and/or misuse of products (such as incorrect dilution or insufficient contact time) can lead to bacterial survival, potential selection or adaptation.

Decreased bacterial susceptibility to biocides, often referred to as resistance, has been reported since the 1950s and has now been reported for all major types of biocides8.

In contrast to chemothera- peutic antibiotics, in which clinical breakpoints can be used to clearly define ‘resistance’, the definition of resistance for biocides is more open to interpretation.

In this Review, the term biocide resistance is used holistically and does not distinguish among decreased susceptibility (a change in susceptibility profile measured by bacteriostasis or growth inhibition), resistance (measured by bac- tericidal protocols) or tolerance (the ability of bacteria to survive a biocide at an in-use concentration)

Some reported outbreaks originated from con- tamination of specific disinfectant or antiseptic products by intrinsi- cally resistant bacteria; for example, contamination of chlorhexidine solution with Burkholderia cepacia9, benzalkonium chloride solutions with Serratia marcescens 10 or alcohol solutions with Bacillus cereus spores11.

Biocides are chemically diverse, with more than 900 chemistries avail- able in the European market.

n the European Union, biocides are regulated by the European Chemicals Agency (ECHA) under the Biocidal Pro- ducts Regulations (BPR) and are differentiated into 22 product types depending on their intended application

Generally, the impact of formulated biocides (biocide chemistries and excipients) on efficacy is not as well reported as the efficacy of unformulated biocides. Yet, when formulated bio- cides are studied, for example, formulated benzalkonium chloride, their bactericidal efficacy is improved and emerging antibiotic resist- ance decreased19

Because of their wide range of applications, some biocides will enter the environment and impact AMR 24 . In this Review, we discuss some examples, but we will not consider their breakdown products or reaction by-products

More reactive biocides, such as oxidizers (for example, chlorine or peroxygen-based disinfectants) and alkylating agents (for example, glutaraldehyde), are more efficacious and are used in applications in which target microorganisms are considered less susceptible to biocides (Fig. 1), as in the case of bacterial endospores that require high-level disinfection25 (Supplementary Box 1). This comes at the cost of increased toxicity, incompatibility with some surface types and reduced residual activity

The number of targets that are affected by the biocide and the severity of the damage imparted to these targets result in bacteriostatic or bactericidal effects 8 (Fig. 2)

Microbial inactivation by biocides is complex and can be understood by using multiple approaches.

Understanding the genotypic and phenotypic determinants that contribute to susceptibil- ity, particularly in the case of sporicides32,33, is crucial.

As a rule, biocides must interact with bacteria and reach their target sites in sufficient quantities to exert biocidal effect. For example, the outer membrane of some Gram-negative species can provide intrinsic resistance to QACs, by acting as a barrier that prevents interaction with the cytoplasmic membrane. This is discussed further in the following sections.

The general mechanisms of action of biocides can be divided into different groups. Alkylating agents (for example, aldehydes and ethylene oxide) act via crosslinking hydroxyl, amino, carboxyl and sulf- hydryl groups, impacting enzyme function and nucleic acid structure, resulting in microbiocidal effects

Another group is constituted by oxidizing agents such as chlorine, iodine and peroxygens that oxidize various chemical groups (amino, sulfhydryl and thiol) associated with lipids, proteins and nucleic acids, thus disrupting major cytoplasmic membrane function, enzyme func - tion and DNA synthesis.

Polymeric biguanides such as polyhexamethylene biguanide are also membrane active and interact with the lipopolysaccharide in the outer membrane of Gram-negative bacteria, promoting self-penetration and inducing phase separation of phospholipids in the cytoplasmic membrane. The fine interaction of QAC with the membrane depends on the QAC chemistry

Hexachlorophene can inhibit meta- bolic activity by interfering with the electron transport chain, whereas organic acids and their esters can impact membrane potential, which affects cells proton motive force and results in the disruption of active transport and oxidative phosphorylation

The bactericidal activity of alcohols is probably linked to denatura- tion of essential membrane proteins, affecting membrane function, as well as cytoplasmic enzymatic functions.

The initial interaction of a biocide with a bacterial cell is reversible, triggering adaptation and repair mechanisms and ultimately bacterial survival (Fig. 2). A prolonged interaction would result in severe damage to the bacterial cytoplasmic membrane, leading to an irreversible effect and eventually bacterial death8. Metabolically inactive bacteria or bacteria with reduced metabolic activity are generally less susceptible to biocides40,41

The efficacy of a biocidal product can be influenced by several factors. Some of these factors are inherent to the product, such as its concentration, pH and formulation excipients. Others are related to the application of the product, such as the duration of contact, soiling and the type of surfaces. There are also factors that are inherent to the microorganisms being targeted (Table 2)

Concentration is arguably the most important, as it determines the extent and severity of damage imparted to the bacterial cell42,43