who demonstrated
that malic acid caused an agglutination of cytoplasmic content of
L. monocytogenes and E. coli O157:H7 as a consequence of a decrease
in intracellular pH. A study carried out by Lou and Yousef (1999)
indicated that the antimicrobial action of organic acids is attributed
to cytoplasm acidification, as well as to the specific antimicrobial
effect of anionic species. These authors also suggested that
undissociated organic acids can pass through the bacterial cell
membrane, dissociate inside the cytoplasm, and interfere with
metabolic processes of the cell. On the other hand, significant
alteration in cell walls with changes on the cytoplasm membrane
can be observed for both microorganisms in samples subjected to
PL only (Figs. 3c and 4c). It is generally accepted that chemical and
thermal effects caused by UVevisible light are behind the damage
observed at the cell level. The antimicrobial effects of UV light have
been attributed to the absorption of UV wavelengths by conjugated
carbonecarbon double bonds in proteins and nucleic acids, with
the subsequent DNA structural changes (Ramos-Villarroel et al.,
2012). TEM observations support the inactivation results and also
confirm greater cell damage for E. coli (as Gram negative) compared
with L. innocua (Gram positive).