58 ◾ Fundamental Food Microbiology
that grow at 4°C. Another situation can arise if a food contains, among the mixed population, two
species initially present in equal numbers with both growing optimally under the specific intrinsic
and extrinsic environments of the food but one having a shorter generation time than the other. After
a storage period, the one with the shorter generation time becomes predominant. Many foodborne
pathogens are poor competitors, and some of them have a longer generation time than the resident
flora; thus their growth is very poor in a mixed culture environment (e.g., Listeria monocytogenes).
Many foods, most often, are spoiled by bacteria rather than by yeasts and molds because, in general,
bacteria have shorter generation times. Thus, in a mixed population, the intrinsic and extrinsic environments dictate which one, two, or few in the initial mixed population will become predominant
and produce specific changes in a food. These aspects are very important in the control of microbial
spoilage of foods and in the production of bioprocessed (or fermented) foods. 3,4
Sequence of Growth
Among the different microbial types normally present in a food, different species (strains) can
become predominant in sequence during storage. 3,4 Initially, depending on the environment, one
or two types may grow optimally and then create an environment in which they can no longer
grow rapidly. However, another type in the mixed population can find this changed environment
favorable for growth and grow rather rapidly. This shift in predominance can occur several times
during the storage of a food. The sequential microbial growth can be seen particularly in foods
stored for a long time. If a food is packaged in a bag with a little air (e.g., ground meat stored in
a vacuum package), the aerobes grow first and utilize the oxygen. The environment then becomes
anaerobic, in which the anaerobes (or facultative anaerobes) grow favorably. In the natural fermentation of some foods, such as sauerkraut, four different bacterial species grow in succession,
one creating the favorable growth condition for the next. The desirable characteristics of the final
product are dependent on the growth of all four species in specific sequence. To identify the existence of such a situation, it is necessary to enumerate and determine the microbial types at different stages of storage or fermentation. 3,4
Growth in Succession or Diauxic Growth
Microorganisms that can metabolize two or more nutrients in a food, one preferred over the other
and present in limiting concentrations, show growth in stages separated by a short lag phase. 1
Initially, a bacterial strain grows by utilizing the preferred nutrient and, after a short lag of adaptation, grows by utilizing the other nutrient. During each stage, the growth curve has exponential
and stationary phases with a lag phase in between. An example is the growth of certain bacterial
strains (such as some lactic acid bacteria and Gram-negative bacteria) in fresh meat. A strain grows
initially by utilizing the limiting concentrations of carbohydrate present, followed by utilization of
nonprotein nitrogenous (NPN, such as amino acids) substances.
Symbiotic Growth
Symbiosis, or helping one another, during growth often occurs in food containing two or more
types of microorganisms. 3,4 One type may produce metabolic products that the second type needs
for proper growth but cannot produce by itself. In turn, the second species produces a nutrient that
stimulates the first one to grow better. This is found in the production of some fermented foods,
such as yogurt. Initially, Streptococcus thermophilus hydrolyzes milk proteins by its extracellular
that grow at 4°C. Another situation can arise if a food contains, among the mixed population, two
species initially present in equal numbers with both growing optimally under the specific intrinsic
and extrinsic environments of the food but one having a shorter generation time than the other. After
a storage period, the one with the shorter generation time becomes predominant. Many foodborne
pathogens are poor competitors, and some of them have a longer generation time than the resident
flora; thus their growth is very poor in a mixed culture environment (e.g., Listeria monocytogenes).
Many foods, most often, are spoiled by bacteria rather than by yeasts and molds because, in general,
bacteria have shorter generation times. Thus, in a mixed population, the intrinsic and extrinsic environments dictate which one, two, or few in the initial mixed population will become predominant
and produce specific changes in a food. These aspects are very important in the control of microbial
spoilage of foods and in the production of bioprocessed (or fermented) foods. 3,4
Sequence of Growth
Among the different microbial types normally present in a food, different species (strains) can
become predominant in sequence during storage. 3,4 Initially, depending on the environment, one
or two types may grow optimally and then create an environment in which they can no longer
grow rapidly. However, another type in the mixed population can find this changed environment
favorable for growth and grow rather rapidly. This shift in predominance can occur several times
during the storage of a food. The sequential microbial growth can be seen particularly in foods
stored for a long time. If a food is packaged in a bag with a little air (e.g., ground meat stored in
a vacuum package), the aerobes grow first and utilize the oxygen. The environment then becomes
anaerobic, in which the anaerobes (or facultative anaerobes) grow favorably. In the natural fermentation of some foods, such as sauerkraut, four different bacterial species grow in succession,
one creating the favorable growth condition for the next. The desirable characteristics of the final
product are dependent on the growth of all four species in specific sequence. To identify the existence of such a situation, it is necessary to enumerate and determine the microbial types at different stages of storage or fermentation. 3,4
Growth in Succession or Diauxic Growth
Microorganisms that can metabolize two or more nutrients in a food, one preferred over the other
and present in limiting concentrations, show growth in stages separated by a short lag phase. 1
Initially, a bacterial strain grows by utilizing the preferred nutrient and, after a short lag of adaptation, grows by utilizing the other nutrient. During each stage, the growth curve has exponential
and stationary phases with a lag phase in between. An example is the growth of certain bacterial
strains (such as some lactic acid bacteria and Gram-negative bacteria) in fresh meat. A strain grows
initially by utilizing the limiting concentrations of carbohydrate present, followed by utilization of
nonprotein nitrogenous (NPN, such as amino acids) substances.
Symbiotic Growth
Symbiosis, or helping one another, during growth often occurs in food containing two or more
types of microorganisms. 3,4 One type may produce metabolic products that the second type needs
for proper growth but cannot produce by itself. In turn, the second species produces a nutrient that
stimulates the first one to grow better. This is found in the production of some fermented foods,
such as yogurt. Initially, Streptococcus thermophilus hydrolyzes milk proteins by its extracellular