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Triclosan and Antibiotics resistance

4. When are bacteria said to be “resistant”?

    The SCCS opinion states:

    The terms employed in the context of this mandate are defined below in order to avoid confusion in the definitions used to describe the level and type of resistance reported.

    There are several definitions of resistance to antimicrobials biocides or/and antibiotics and several terms used to describe similar phenomena in the literature. A literal/biological definition of resistance is the capacity of bacteria to withstand the effects of a harmful chemical agent.

    The following definitions are based partly on those put forward by Chapman and colleagues (Chapman 1998, Chapman et al. 1998), Russell and colleagues (Hammond et al. 1987, Russell 2003) and Cloete (2003), and the recent SCENIHR opinion (2009).

    The practical meaning of antibiotic resistance is to describe situations where (i) a strain is not killed or inhibited by a concentration attained in vivo, (ii) a strain is not killed or inhibited by a concentration to which the majority of strains of that organism are susceptible or (iii) bacterial cells that are not killed or inhibited by a concentration acting upon the majority of cells in that culture.

    When non-antibiotic agents (i.e. triclosan or other biocides) are considered, the word “resistance” is used in a similar way where a strain is not killed or inhibited by a concentration attained in practice (the in-use concentration) and in situations (ii) and (iii) described above.

    These definitions reflect those given by EFSA whereby “antimicrobial susceptibility or resistance is generally defined on the basis of in vitro parameters. The terms reflect the capacity of bacteria to survive exposure to a defined concentration of an antimicrobial agent, but different definitions are used depending on whether the objective of the investigation is clinical diagnostics or epidemiological surveillance” (EFSA 2008)

    The term 'Multi-Drug Resistant’ (MDR) applies to a bacterium that is simultaneously resistant to a number of antibiotics belonging to different chemical classes by using various mechanisms (Depardieu et al. 2007).

    The term “co-resistant” is used to denote a strain possessing a biochemical mechanism that inhibits the activity of several antibiotics belonging to the same structural family (e.g. ß- lactamase and ß-lactams). When the transfer of resistance determinants occurs, co- resistance specifically refers to genetic determinants (such as integrons, transposons or plasmids) encoding for unrelated resistance mechanisms, that are transferred in a single event and expressed jointly in a new bacterial host.

    The term “cross-resistant” is used to denote a strain possessing a resistance mechanism that enables it to survive the effects of several antimicrobial molecules with mechanism(s) of action that are related or overlap.

    Other terms such as “insusceptibility” and “tolerance” have been used in the published literature. Insusceptibility refers to an intrinsic (innate) property of a micro-organism, such as cell layer impermeability in mycobacteria and Gram-negative bacteria. Tolerance denotes a reduced susceptibility to an antimicrobial molecule characterised by a raised minimum inhibitory concentration (MIC), or a situation in which a preservative system no longer prevents microbial growth.

    Source & ©: SCCS,  Opinion on triclosan (antimicrobial resistance) (2010) , p.14

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