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
There are several definitions of
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
The practical meaning of
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.
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
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
surveillance” (EFSA 2008)
The term 'Multi-Drug Resistant’ (MDR) applies to a
bacterium that is
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
to the same structural family (e.g. ß- lactamase and ß-lactams).
When the transfer of
occurs, co- resistance specifically refers to
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
“cross-resistant” is used
to denote a strain possessing a
that enables it to survive the effects of several
molecules with mechanism(s) of action that are related or
Other terms such as “insusceptibility” and
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
denotes a reduced
susceptibility to an
molecule characterised by a raised minimum inhibitory
concentration (MIC), or a situation in which a
no longer prevents microbial growth.