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For infection prophylaxis and to fight outbreaks of diseases animal husbandry utilizes
enormous amounts of disinfectants each year. Not only humane medicine finds itself confronted
with a growing number of reports about resistances against specific disinfectant
agents, which additionally are partially accompanied by resistances against antibiotics.
This apprehension is shared as well by disinfectant use in animal husbandry. Focus of this
thesis was therefore to research to what extent subinhibitory concentrations of disinfectants,
which are likely to be caused by numerous interfering factors in animal husbandry,
will lead to the induction of resistances against those agents, or co-induction of resistances
To research this question the minimum inhibition concentration (MIC) of three typical
disinfectants used in animal husbandry (peracetic acid, glutaraldehyde and formic acid) of
Escherichia coli as gram-negative and Enterococcus spp. as gram-positive ubiquitous bacteria
found in livestock husbandry was determined. Duplicate measurement following
DVG guidelines for disinfectant review was employed for both the corresponding reference
microbial strains and the nine field isolates from livestock husbandry of each group.
Additionally, for comparison, ESBL-producing and ESBL-negative E. coli were tested to
research the influence of preexisting resistance genes against antibiotics on resistances of
E. coli against disinfectants.
After all tested germs had been exposed ten times to subinhibitory concentrations of
disinfectants a second MIC determination was conducted in order to find out how the MIC
changed. After a following period of unexhibited growth the MIC was again determined in
order to find out the stability of the observed change in sensibility.
Only temporary changes in levels of tolerance were observed, never amounting to
more than one MIC level, which suggests a relatively low risk of inducing resistances
against the researched disinfectants. Contrary to that a convergence, respectively a temporary
transgression of the MIC in comparison to the concentrations ready to use for two of
the three disinfectants was observed. The latter case can be labeled as an induction of an
instable resistance, which was observed using the glutaraldehyde based disinfectant Permanent
neu® for several Enterococcus isolates. By aligning the MIC to the concentrations
ready to use under ideal laboratory conditions it can be deducted that under the harsh conditions
of a barn environment disinfectant effectivity will presumably quickly deteriorate.
Additionally, it is alarming that in the experiments using the glutaraldehyde based disinfectant the Enterococcus field isolates showed a higher tolerance against that agent than
the corresponding Enterococcus reference strain from disinfectant review.
The equivalent behavior of ESBL-negative and ESBL-producing E. coli shows that at
least the existence of multiresistances against β-lactam antibiotics in E. coli will not lead to
reduced sensibility against disinfectants (peracetic acid, glutaraldehyde, formic acid).
To analyze the influence of the changed MIC of the disinfectants on antibiotic sensibility
of the bacteria an additional agar diffusion test with twelve different antibiotic agents
was made before and after each exhibition of selected test germs to subinhibitory concentrations
of disinfectants. In doing so no indication of a correlation between the observed
rise of the MIC of the disinfectants and a loss of sensibility against the tested antibiotics
was found. Nonetheless, it was noticed that the field isolates of the different groups of bacteria
always showed a higher resistance against the tested antibiotic agents than the corresponding
Since the development of resistances of the different species of bacteria is most likely
based on specific rather than generic resistance mechanisms, it can only be deduced from
the conducted research that an induction of stable or highly effective resistances against the
disinfectants peracetic acid, glutaraldehyde and formic acid in the researched group of test
germs is unlikely in a barn environment. Such a development of a resistance can still not
be completely ruled out since it is deemed possible that a different setup of the experiment
might lead to a different conclusion.
In consideration of the partially rather small safety margins of the products used in this
thesis a failure of disinfection under the difficult conditions in animal husbandry is a realistic
danger. Therefore, the importance of a correct and responsible application of disinfectants
should be especially noted.