Biomedical Engineering Reference
In-Depth Information
other hand, it is known that the antimicrobial mode of action of certain cationic
surfactants as QACs has been attributed to their positive charge, which promotes an
electrostatic interaction with negatively charged sites on cell membrane (Cloete
et al.
1998
). So, it is believable that electrostatic interactions could increase cell-to-
cell cross-linking by preventing cell removal, resulting in more compact biofilms.
Possibly, the QAC cations may also cross-link the anionic groups of biofilm poly-
mers (such as polysaccharides), providing greater binding force in a developed
biofilm. The content of the main EPS components in bacterial biofilms (proteins
and polysaccharides) was analysed, and a higher protein content and similar
polysaccharide percentage was determined for the non-transformed strain
(Teod´sio et al.
2012
). The results showed that protein content is probably a better
indicator for biofilm resistance than the polysaccharide composition, as suggested
by Pereira and Vieira (
2001
).
Despite being unsuccessful at removing the biofilms, BDMDAC was very
effective on killing the cells since complete inactivation was attained for all the
strains. However, cells transformed with pUC8 were the most resistant to inactiva-
tion with this biocide, indicating that antimicrobial susceptibility of biofilms can be
plasmid dependent (Teod´sio et al.
2012
).
2.2 Case Study 2: The Effects of the Presence of BSA,
Alginate, Yeast Extract and Humic Acids on Biofilm
Resistance
The antibacterial activity of BAC, CTAB and their combination was determined by
respirometry (Sim
˜
es et al.
2005b
) in the absence and presence of four potentially
interfering substances (Ara
´
jo et al.
2013
). Higher inactivation rates were observed
for
B. cereus
when compared to
P. fluorescens
at the same QAC concentration.
B. cereus
is more susceptible due to the fact that it is a Gram-positive bacterium that
lacks an outer membrane, which typically provides increased protection to Gram-
negative bacteria (Ara´ jo et al.
2013
). This result was also reported by Lawrence
(
1950
), who has shown that Gram-positive bacteria are more affected by cationic
surfactants because of their higher ratio between acidic and basic groups,
e.g. nucleus and enzyme systems, than Gram-negative bacteria.
The selected interfering substances influenced the antimicrobial activity of the
QACs to some extent. The inactivation of
B. cereus
was not significantly affected
by the presence of any interfering substance, except that the presence of humic
acids increased concentrations necessary for total inactivation (Ara´ jo et al.
2013
).
The antimicrobial action of the QACs against
P. fluorescens
was not significantly
influenced by the presence of most potential interfering substances, except for
humic acids, and the antimicrobial activity of the QACs against the bacterial
consortium was significantly affected by the presence of the tested interfering
substances (Ara´ jo et al.
2013
). Humic acids were the interfering substances that
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