Agriculture Reference
In-Depth Information
Bacterial Protective Cultures and Related Antimicrobial Metabolites
The use of protective (or antagonistic) cultures is a relatively well-established and
consumer-accepted biocontrol approach, particularly for fermented meats and dairy
products (Breidt and Fleming 1997). For various reasons including a previous lack of
association of produce with foodborne illness and the nonfermented nature of these
foods, specifi c applications related to fresh fruits and vegetables have, until recently,
been rarely documented. Applied research activity is on the increase, and reviews
addressing the use of protective cultures (Kostrzynska and Bachand 2006; Leverentz
and others 2002) and bacteriocins (Settanni and Corsetti 2008) in fresh produce have
arisen as a consequence.
The concept of protective cultures is derived from the acknowledged contribution
the native microfl ora makes to intrinsic factors governing the growth of microorgan-
isms, including pathogens, in that food. The ability of certain microbial species,
particularly lactic acid bacteria (LAB), to behave antagonistically toward closely
related organisms, including pathogens such as L. monocytogenes , is well established
and extensively reviewed, as are the mechanisms whereby antagonism occurs, e.g.,
competition for space and nutrients, acidifi cation of the environment, and production
of antimicrobial metabolites (see Leverentz and others 2002).
Of the antimicrobial metabolites produced by bacteria, bacteriocins have been
particularly well studied. Nisin is the best known example of a commercially applied
biocontrol system in low pH foods susceptible to contamination by L. monocytogenes
and other Gram-positive bacteria such as Bacillus and Clostridium spp. Nisin has an
unusually broad range of activity, hence its preferred status for food applications, but
it is ineffective against Gram-negative bacteria.
Antagonism can be achieved either by addition of GRAS protective cultures or
directly via commercial bacteriocin preparations such as Nisaplin ® . The former option
is preferable because it offers a number of advantages including potential in situ
production of antimicrobial metabolites (assuming growth conditions are met) and
lower associated costs, and it may also alleviate labeling requirements. See Gálvez
and others (2007) for a more complete review. However, optimizing either approach
requires understanding of a number of complex aspects, including 1) the food matrix
and related processing and storage conditions; 2) the composition, diversity, and
quantity of the competing microfl ora; and 3) the susceptibility of the target species.
Ideally, protective cultures should survive under typical storage conditions, grow
at abuse temperatures without negatively impacting organoleptic characteristics and
shelf life (Kostrzynska and Bachand 2006), and actively reduce populations of a broad
range of pathogens (although the limitation of growth may be suffi cient in some cases).
Effi cacious strains indigenous to the food in question are therefore of most interest.
To this end, a number of groups have isolated and characterized antagonistic bacteria
(predominantly lactococci, lactobacilli, and pediococci) from the microfl ora of various
produce items and reported on their ability to inhibit produce-related pathogens
(Bennik and others 1997; Cai and others 1997; Franz and others 1998; Kelly and
others 1998; Liao and Fett 2001; Schuenzel and Harrison 2002; Wilderdyke and others
2004 ; Yildirim and Johnson 1998 ).
It is evident from these reports that there is signifi cant disparity in the spectrum of
activity of antagonists against pathogens tested in vitro , particularly Gram - negative
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