334 Modern Food Microbiology
Microbial Interference
The food products covered in this section are favorably affected by selective members of their
microbiota, which do not alter the identity of the food products. In contrast, the fermented products
covered in Chapters 7 and 8 are essentially new and finished products with an identity of their own,
e.g., yogurt from milk; pickles from cucumbers; wine from fruit juice, etc. Although fermented
foods are not included under microbial interference, it should be noted that many of the effective
organisms are also involved in fermentations. The interference biota consists essentially of biocontrol
agents.
Microbial interference refers to the general nonspecific inhibition or destruction of one microorganism by other members of the same habitat or environment. Whereas lactic antagonism is a specific
example of microbial interference, there are other less well-defined ways in which inhibition occurs and some of these are outlined below. The expression “bacterial interference” was suggested
by R. Dubos to describe the early work in this area, which dealt primarily with the antagonism of
certain human pathogens by the normal background biota of the skin. More specifically, a number
of clinical researchers showed in the 1960s and 1970s that the normal harmless staphylococcal biota
of the nares prevented colonization by more virulent staphylococcal strains. This was demonstrated
by spraying or inoculating the nares of newborn infants with live avirulent strains, which prevented
subsequent colonization by virulent strains. Examples of bacterial interference dating back to around
1877 have been noted and reviewed by Florey.
57 Among the earliest published studies of general microbial interference in foods were those of Dack and Lippitz,
37 Peterson et al.,
149 and Goepfert and
Kim.
69 Dack and Lippitz observed that the natural biota of frozen pot pies inhibited inoculated cells
of Staphylococcus aureus, E. coli, and S. Typhimurium. The repression of S. aureus in pot pies by
around 10
5 /g of the normal biota was shown by Peterson et al. The inability of foodborne pathogens
to grow in fresh ground beef with a background biota of ca. 10
5 /g was demonstrated by Goepfert and
Kim. Since these early studies, the antagonism of the normal food biota against L. monocytogenes
and against pathogenic strains of E. coli has been demonstrated. The suppressive effects of a sufficiently large aerobic bacterial biota against the growth of Clostridium botulinum in fresh meats is
well established, as is the suppression of yeasts and molds by the bacterial biota of comminuted fresh
meats.
93
The mechanisms of general microbial interference are not clear, but some observations are worthy
of note. First, the background biota needs to be larger in a number of viable cells than the organism
to be inhibited. Second, the interfering biota is generally not homogeneous, and the specific roles that
individual species play are unclear. Among the explanations offered over the years are (1) competition
for nutrients, (2) competition for attachment/adhesion sites, (3) unfavorable alteration of the environment, and (4) combinations of these. Since interference typically occurs when the APC is at least
10
6 cells/g, it is not inconceivable that biofilm formation and the occurrence of quorum sensing play
some as-yet unknown role(s) in this phenomenon. Some specific examples of interference and lactic
antagonism are presented below.
A somewhat unusual example of what might be called “biotic interference” has been demonstrated
with a soil nematode. The free-living nematode, Caenorhabditis elegans, was shown to disperse
bacteria with an apparent preference for Gram-negative cells over Gram positives although members
of both groups were ingested in a laboratory study.
5 Salmonella Poona was ingested by this nematode,
which resulted in the protection of this bacterium from the effect of sanitizers.
25 The dispersal capacity
of soil-inhabiting nematodes could prove to be significant to the dispersal and persistence of some
foodborne bacterial pathogens in soils, especially among seed sprouts.
Microbial Interference
The food products covered in this section are favorably affected by selective members of their
microbiota, which do not alter the identity of the food products. In contrast, the fermented products
covered in Chapters 7 and 8 are essentially new and finished products with an identity of their own,
e.g., yogurt from milk; pickles from cucumbers; wine from fruit juice, etc. Although fermented
foods are not included under microbial interference, it should be noted that many of the effective
organisms are also involved in fermentations. The interference biota consists essentially of biocontrol
agents.
Microbial interference refers to the general nonspecific inhibition or destruction of one microorganism by other members of the same habitat or environment. Whereas lactic antagonism is a specific
example of microbial interference, there are other less well-defined ways in which inhibition occurs and some of these are outlined below. The expression “bacterial interference” was suggested
by R. Dubos to describe the early work in this area, which dealt primarily with the antagonism of
certain human pathogens by the normal background biota of the skin. More specifically, a number
of clinical researchers showed in the 1960s and 1970s that the normal harmless staphylococcal biota
of the nares prevented colonization by more virulent staphylococcal strains. This was demonstrated
by spraying or inoculating the nares of newborn infants with live avirulent strains, which prevented
subsequent colonization by virulent strains. Examples of bacterial interference dating back to around
1877 have been noted and reviewed by Florey.
57 Among the earliest published studies of general microbial interference in foods were those of Dack and Lippitz,
37 Peterson et al.,
149 and Goepfert and
Kim.
69 Dack and Lippitz observed that the natural biota of frozen pot pies inhibited inoculated cells
of Staphylococcus aureus, E. coli, and S. Typhimurium. The repression of S. aureus in pot pies by
around 10
5 /g of the normal biota was shown by Peterson et al. The inability of foodborne pathogens
to grow in fresh ground beef with a background biota of ca. 10
5 /g was demonstrated by Goepfert and
Kim. Since these early studies, the antagonism of the normal food biota against L. monocytogenes
and against pathogenic strains of E. coli has been demonstrated. The suppressive effects of a sufficiently large aerobic bacterial biota against the growth of Clostridium botulinum in fresh meats is
well established, as is the suppression of yeasts and molds by the bacterial biota of comminuted fresh
meats.
93
The mechanisms of general microbial interference are not clear, but some observations are worthy
of note. First, the background biota needs to be larger in a number of viable cells than the organism
to be inhibited. Second, the interfering biota is generally not homogeneous, and the specific roles that
individual species play are unclear. Among the explanations offered over the years are (1) competition
for nutrients, (2) competition for attachment/adhesion sites, (3) unfavorable alteration of the environment, and (4) combinations of these. Since interference typically occurs when the APC is at least
10
6 cells/g, it is not inconceivable that biofilm formation and the occurrence of quorum sensing play
some as-yet unknown role(s) in this phenomenon. Some specific examples of interference and lactic
antagonism are presented below.
A somewhat unusual example of what might be called “biotic interference” has been demonstrated
with a soil nematode. The free-living nematode, Caenorhabditis elegans, was shown to disperse
bacteria with an apparent preference for Gram-negative cells over Gram positives although members
of both groups were ingested in a laboratory study.
5 Salmonella Poona was ingested by this nematode,
which resulted in the protection of this bacterium from the effect of sanitizers.
25 The dispersal capacity
of soil-inhabiting nematodes could prove to be significant to the dispersal and persistence of some
foodborne bacterial pathogens in soils, especially among seed sprouts.