Fundamental Food Microbiology, Third Edition - Fuad Fathir

64 FUNDAMENTAL FOOD MICROBIOLOGY
This is found in the production of some fermented foods such as yogurt. Initially,
Streptococcus thermophilus hydrolyzes milk proteins by its extracellular proteinases
and generates amino acids, which are necessary for good growth of Lactobacillus
delbrueckii subsp. bulgaricus. Lactobacillus, in turn, produces formate, which stimulates
the growth of Streptococcus species. Both are necessary to produce a desirable
product. 3,4
E. Synergistic Growth
This is observed during symbiotic growth of two or more microbial types in a food. 3,4
In synergistic growth, each type is capable of growing independently and producing
some metabolites at lower rates. However, when the types are allowed to grow in a
mixed population, both the growth rate and the level of by-product formation greatly
increase. The increase is more than the additive of the amounts produced by growing
the two separately. For example, both Str. thermophilus and Lab. delbrueckii subsp.
bulgaricus, when growing in milk independently, produce ca. 8 to 10 ppm acetaldehyde,
the desirable flavor component of yogurt. However, when growing together
in milk, 30 ppm or more of acetaldehyde is produced, which is much higher than
the additive amounts produced independently by the two species and is necessary
for a good-quality yogurt. In the production of a desirable fermented food, two
separate strains and species can be used to induce synergistic growth. 3,4
F. Antagonistic Growth
Two or more types of microorganisms present in a food can adversely affect the
growth of each other, or one can interfere with the growth of one or more types;
sometimes one can kill the other. This has been found among many bacterial strains,
between bacteria and yeasts, between yeasts and molds, and between bacteria and
molds. This occasionally occurs due to the production of one or more antimicrobial
compounds by one or more strains in the mixed population. Some Gram-positive
bacteria produce antibacterial proteins or bacteriocins that can kill many other types
of Gram-positive bacteria. Similarly, some yeasts can produce wall-degrading
enzymes and reduce the growth of molds. Some strains have probably developed
these specific traits for a growth advantage in a mixed-population situation. There
is now an interest to use this phenomenon to control growth and enhance viability
loss of undesirable spoilage and pathogenic microorganisms in food.
IV. CONCLUSION
The study of microbial growth provides basic information that is important to understand
the mechanisms of food spoilage, foodborne diseases, food bioprocessing and
strain improvement, and their detection from food. Microorganisms are present in
mixed cultures in food and can interact with each other during growth. Their growth
is influenced by the environment of a food, which is discussed in Chapter 6.