Microbial Ecology of Spoilage 399
chopped garlic in soybean was produced by rehydration of dry, chopped garlic to
which soybean oil was added. The product was filled into 8-ounce glass jars. Each
jar contained 125 g of garlic and 90 mL of water-in-oil emulsion, pH 5.8 to 5.9.
Inoculation studies showed that type A and proteolytic type B strains multiplied and
formed toxin in the product during incubation at 35°C [55]. The product remained
organoleptically acceptable even when highly toxic. Toxin was also formed when
the product was incubated at room temperature. (It is of interest that a type B strain
from a patient involved in the outbreak produced a relatively high level of toxin in
the product.) One or two nonproteolytic type B strains grew and formed toxin in
the chopped garlic in oil incubated at room temperature. Thus, both onions and
garlic in an oil menstruum supported the growth of and toxin formation by
C. botulinum types A and B from a minimal inoculum of one to five spores per gram
while remaining organoleptically acceptable. In 1989 three cases of type A botulism
were attributed to the consumption of a chopped garlic-in-oil product [122]. The
remains of the products contained high concentrations of the organism and the toxin.
After this second outbreak the U.S. Food and Drug Administration ordered compa-
nies to stop making any garlic-in-oil mixes that are protected only by refrigeration.
Such products must now contain specific labels of microbial inhibitors or the acid-
ifying agents, such as phosphoric or citric acid.
12.4 INTERACTIONS BETWEEN SPOILING BACTERIA
Microbial food spoilage is a process involving growth of microorganisms to numbers
(10^7 to 10^9 cfu/g) at which the microorganisms also must be assumed to interact and
influence the growth of one another [123]. The interactions between microorganisms
may be classified on the basis of their effects as being detrimental or beneficial
[124]. Several types of interactions have been studied in food ecosystems, including
both antagonistic and coordinated behavior and interactions where growth or a
particular metabolism of one organism is favored by the growth of another organism.
12.4.1 ANTAGONISM
Changes in environmental conditions (e.g., by lowering of pH) can be a powerful
way for a microorganism to antagonize other bacteria and create a selective advan-
tage. Also, competition for nutrients may select for the organisms best capable of
scavenging the limiting compounds. Several microorganisms important in food spoil-
age have such antagonistic abilities. Thus, the lactic acid bacteria cause a lowering
of pH and may produce antibacterial peptides (bacteriocins) [125]. The spoilage
reactions of certain Gram-negative bacteria may produce NH 3 and trimethyl amine,
which are toxic to a number of other bacteria and sometimes to the producing organism
itself. Pseudomonas spp., in particular the fluorescent group, produce a range of anti-
bacterial and antifungul compounds such as antibiotics and cyanide, and at the same
time they compete very efficiently for iron [126]. Iron is essential for most microor-
ganisms and is used in bacterial respiration (as electron shuttler) and in redox enzymes.
Due to the high oxidative power of Fe+3, iron is mostly bound in insoluble complexes
in the environment and in mammals and plants. Most microorganisms have therefore