The Effect of Plasmids and Other Biomolecules on the Effectiveness of Antibiofilm Agents - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 163

had the most notorious effect in reducing the surfactant activity. Ishiguro
et al. ( 2007 ) reported that cationic surfactants bind intensely to humic substances
and Koopal et al. ( 2004 ) also verified the formation of complexes of humic acid-
cationic surfactant.
Respiratory activity potentiation happened when the QACs were used on
P. fluorescens and the bacterial consortium in the presence of humic acids and
yeast extract (Ara ´ jo et al. 2013 ). Humic acids were proposed to replace synthetic
surfactants in industrial applications (Visser 1985 ), so it is possible that their
inclusion in a solution of QACs may interfere with the chemical characteristics of
the solution, leading to an apparent reduced antimicrobial efficacy. Since QACs are
membrane active agents, their use at sub-lethal concentrations could improve
membrane permeability and consequently the nutrient influx. Humic acids might
be broken down to smaller molecules that could be utilised by cells as a carbon
(Camper 2004 ) or nutrient source (Salati et al. 2011 ). In fact, it was found that the
growth rates of many anaerobic and aerobic microorganisms are increased by
humic substances that stimulate enzyme activity (Hartung 1992 ; Pouneva 2005 ).
In a similar way, yeast extract is a nitrogen source widely used as component of
growth media (Hakobyan et al. 2012 ).
3 Conclusions
The main aim of this work was to analyse the impacts of cell transformation with a
recombinant plasmid and also the presence of organic compounds on the antimi-
crobial activity of different biocides. In the case of plasmids, biocide treatment
results indicate that transformed cells are more resistant to short exposure periods
and that BDMDAC is ineffective for E. coli biofilm removal but effective for
biofilm inactivation. This result can be explained by a high protein content of the
biofilm formed by the plasmid-free strain. From the tested interfering substances,
humic acids have the most severe impact on the biocide activity, even causing
metabolic activation in some circumstances. These results highlight the importance
of specific biomolecules on the efficacy of biocides that are currently used in
medical applications.
Acknowledgements The authors acknowledge the financial support provided by Operational
Programme for Competitiveness Factors—COMPETE, European Fund for Regional
Development—FEDER and by the Portuguese Foundation for Science and Technology—FCT,
through Projects PTDC/EBB-BIO/102863/2008 and PTDC/EBB-EBI/105085/2008. Luciana
Gomes acknowledges the receipt of a Ph.D. grant from FCT (SFRH/BD/80400/2011).