Microbial Growth Characteristics ◾ 59
proteinases and generates amino acids that 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
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 byproduct 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 approximately 8–10 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
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 species or strains, between bacteria and yeasts,
between yeasts and molds, and between bacteria and molds. This occasionally occurs as a result of
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 or metabolic
byproducts that can kill or suppress the growth of 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 in using this phenomenon to control growth and enhance the
viability loss of undesirable spoilage and pathogenic microorganisms in food.
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 in 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.
QUESTIONS
1. Describe the process of microbial growth with a diagram.
2. Define generation time as related to bacterial growth. If a pure culture of a bacterial population during incubation at 35°C in a nutritionally rich broth increases to 5 × 10 6 /mL from an
initial population of 2.5 × 10 2 in 300 minutes, what is the generation time of the strain?
proteinases and generates amino acids that 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
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 byproduct 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 approximately 8–10 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
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 species or strains, between bacteria and yeasts,
between yeasts and molds, and between bacteria and molds. This occasionally occurs as a result of
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 or metabolic
byproducts that can kill or suppress the growth of 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 in using this phenomenon to control growth and enhance the
viability loss of undesirable spoilage and pathogenic microorganisms in food.
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 in 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.
QUESTIONS
1. Describe the process of microbial growth with a diagram.
2. Define generation time as related to bacterial growth. If a pure culture of a bacterial population during incubation at 35°C in a nutritionally rich broth increases to 5 × 10 6 /mL from an
initial population of 2.5 × 10 2 in 300 minutes, what is the generation time of the strain?