Clinical microbiologists use susceptibility tests to predict the probability of an antibiotic's therapeutic success. To say
that bacteria are susceptible to an antibiotic means that, under laboratory conditions, the minimum inhibitory concentration
(MIC) of that antibiotic complied with the recommended standard for the target microorganism. But therapeutic success is not
assured because the interactions between the host and the microorganism also influence therapeutic success.
Resistance occurs when the required amount of antibiotic exceeds the minimum MIC and the likelihood of therapeutic success
is low. Reduced susceptibility to an antibiotic occurs when the recommended concentration to achieve therapeutic success is
not effective and a higher concentration is required to achieve therapeutic success. When an antibiotic complies with the
minimum MIC but is ineffective against an infection by a microorganism, the microorganism is said to be resistant, regardless
of whether the failure was caused by the host, antibiotic, or clinician.
Classes of disinfectants and examples of resistance
Studies of disinfectants' mechanisms of action generally indicate that, unlike antibiotics, whose high levels of target-specificity
facilitate selective action against specific cell targets, disinfectants work at many sites within the cell. The damage caused
by disinfectants is seldom accomplished through a single injury and is a direct consequence of exposure to or chemical attack
by the disinfectant's active ingredients. That is, the actions of disinfectants are, in most cases, not pharmacologically
precise (1). In the case of bacteriostatic antibiotics, the host's immune response plays an important role in eradicating
the invading microorganisms. Disinfectant formulations contain biocidal chemicals at concentrations high enough to affect
multiple rather than unique targets. Supplementary ingredients are also included in disinfectant formulations to enhance the
The activity of disinfectants is measured by MIC or MBC. In this sense, the antibacterial ingredient could be an antibiotic,
an antiseptic, or a disinfectant. The MIC is determined by visual inspection of the media used in the test, and the MBC requires
plate counts to quantify the amount of inactivated or killed microorganisms.
Disinfectants are classified in two broad categories: oxidizing disinfectants and nonoxidizing disinfectants. These two categories
are further subdivided. Tables II and III list common disinfectants (and some antiseptics) and their modes of action. These
lists are not exhaustive; only the most relevant examples are presented.
Table II: Oxidizing disinfectants.
As Tables II and III show, most disinfectants' mechanism of action is nonspecific and damages cells at different targets.
The most effective disinfectants are oxidizing agents, followed by reducing agents.
Table III: Nonoxidizing disinfectants.
Helicobacter pylori has shown resistance to hydrogen peroxide at a use concentration of ~3% v/v with an exposure time of 60 min (7). This tolerance
to use concentrations of hydrogen peroxide has been attributed to the presence of several enzymes such as catalase (7). H. pylori is an uncommon bacterium, however, because it is only found in the human stomach.
On the other hand, Bacillus subtilis mutants (MBC = 0.5%) are more resistant to hydrogen peroxide than wild strains are (MBC = 0.34%) (8). This example shows
that "resistant" means having reduced susceptibility. Mutant B. subtilis is still killed by hydrogen peroxide at use concentrations.