A biocide may be defined as a chemical or physical agent which kills viable organisms, both pathogenic and non-pathogenic. This broad definition clearly includes microoganisms, but is not restricted to them.
LABORATORY
EVALUATION OF ANTIMICROBIAL AGENTS
DEFINITIONS
Key terms such as disinfection,
preservation, antisepsis and sterilization can be seen in upcoming articles. A
number of other important terms used to describe the antimicrobial activity of
agents are also commonly used. A biocide may be
defined as a chemical or physical agent which kills viable organisms, both
pathogenic and non-pathogenic. This broad definition clearly includes microoganisms, but
is not restricted to them. The term microbicide is therefore also used to refer
specifically to an agent which kills microorganisms (germicide may also be used
in this context, but generally refers to pathogenic microorganisms). The
terms biocidal, bactericidal, fungicidal and viricidal therefore describe
an agent with killing activity against a specific class or classes of organism
indicated by the prefix, whereas the terms bacteriostatic and fungistatic refer to agents which inhibit the
growth of bacteria or fungi (Figure 18.1),
but do not necessarily kill them. It should be noted, however, that some
microorganisms that appear non-viable and non-cultivable fol-lowing
antimicrobial challenge may be revived by appropriate methods, and that
organisms incapable of multiplication may retain some enzymatic activity.
In the laboratory evaluation of
antibacterial agents, the terms minimum inhibitory concentration (MIC)
and minimum bactericidal concentration (MBC) are most
commonly used. Recently published British Society for Chemotherapy (BSAC)
guidelines for the determination of minimum inhibitory
concentrations (see Further Reading) define the MIC as the
lowest concentration of antimicrobial which will inhibit the visible growth of
a microorganism after overnight cultivation and the MBC as the lowest
concentration of antimicrobial that will prevent the growth of a microorganism
after subculture onto antibiotic-free media. Generally, MIC and MBC values are
recorded in milligrams per litre or per millilitre (mg/L or mg/ml). With most
cidal antimicrobials, the MIC and MBC are frequently near or equal in value,
although with essentially static agents (e.g. tetracycline), the lowest
concentration required to kill the microorganism (i.e. the MBC) is invariably
many times the MIC and often clinically unachievable without damage to the
human host. As with microbicides, cidal terms can be applied to studies
involving not just bacteria but other microbes, e.g. when referring to cidal
antifungal agents the term minimum fungicidal
concentration (MFC) is used. Recently, thanks to developments
in the design of high-throughput laboratory screens for biofilm susceptibility,
the minimum biofilm eradication concentration (MBEC)
can be accurately determined for organisms grown as single or mixed species
biofilms. The MBEC is the minimum concentration of an antimicrobial agent
required to kill a microbial biofilm. For conventional antibiotics and biocides
the MBEC value may be 1000-fold higher than the MBC value for the same
planktonic microorganisms. Further studies have shown that often no correlation
exists between the MIC and the MBEC, indicating the potential limitations of
therapeutic antibiotic selection based on determined MIC values.
The term tolerance implies the ability of some bacterial
strains to survive (without using or expressing resistance mechanisms), but not
grow, at levels of antimicrobial agent that should normally be cidal. This
applies particularly to systems employing the cell-wall-active β-lactams and
glycopeptides, and to Gram-positive bacteria such as streptococci. Normally,
MIC and MBC levels in such tests should be similar (i.e. within one or two
doubling dilutions); if the MIC/MBC ratio is 32 or greater, the term tolerance
is used. Tolerance may in some way be related to the Eagle phenomenon
(paradoxical effect), where increasing concentrations of antimicrobial result
in reduced killing rather than the increase in cidal activity expected
(see Figure 18.2).
Tolerance to elevated antimicrobial challenge concentrations is also a
characteristic of microbial biofilm populations. Finally, the term resistance
has several definitions within the literature, however, it generally refers to
the ability of a microorganism to withstand the effects of a harmful chemical
agent, with the organism neither killed nor inhibited at concentrations to
which the majority of strains of that organism are susceptible. Resistance mechanisms
generally involve modification of the normal target of the antimicrobial agent
either by mutation, enzymatic changes, target substitution, antibiotic
destruction or alteration, antibiotic efflux mechanisms and restricted
permeability to antibiotics.
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