70 ◾ Fundamental Food Microbiology
temperatures), with an optimum at 15°C and a range of –5°C to 20°C. However, these divisions
are not clear cut and overlap each other.
Two other terms used in food microbiology are very important with respect to microbial growth
at refrigerated temperatures and survival of microorganisms to low heat treatment or pasteurization because both methods are widely used in the storage and processing of foods. Psychrotrophs
are microorganisms that grow at refrigerated temperature (0°C–5°C) irrespective of their optimum range of growth temperature. They usually grow rapidly between 10°C and 30°C. Molds;
yeasts; many Gram-negative bacteria from genera Pseudomonas, Achromobacter, Yersinia, Serratia,
and Aeromonas; and Gram-positive bacteria from genera Leuconostoc, Lactobacillus, Bacillus,
Clostridium, and Listeria are included in this group (see Chapter 21). Microorganisms that survive
pasteurization temperatures are designated as thermodurics. They include species from genera
Micrococcus, Bacillus, Clostridium, Lactobacillus, Pediococcus, and Enterococcus. Bacterial spores
are also included in this group. They have different growth temperatures, and many can grow at
refrigerated temperatures as well as thermophilic temperatures.
When foods are exposed to temperatures beyond the maximum and minimum temperatures
of growth, microbial cells die rapidly at higher temperatures and relatively slowly at lower temperatures. Influence of temperature on microbial growth and viability is important in reducing
food spoilage, enhancing safety against pathogens, and in food bioprocessing. Temperature of
growth is also effectively used in the laboratory to enumerate and isolate microorganisms from
foods.
Quorum Sensing and Bacterial Growth
The intrinsic and extrinsic factors control bacterial growth in food, but little was known about
the molecular mechanism of bacterial behavior in these conditions until recently. Bacterial
quorum sensing (QS) is thought to play a major role in the bacterial response to a changing
environment in food. QS is described as a bacterial cell-to-cell communication system to
develop a strategy for the bacterial population to adapt to the harsh environment in food or
the host. 12 QS is cell-density dependent, that is, when bacteria reach a certain cell number,
they produce small diffusible molecules, termed autoinducers (AIs) as signaling molecules
that are used for cell-to-cell communication during growth. For example, under stressful conditions, such as nutrient starvation, low water activity, high temperatures, antimicrobial compounds, acid, or other microbes, bacterial AIs bind to receptors and activate gene expression
to alter behavior (i.e., adaptation to harsh environment) and morphological characteristics
(i.e., biofilm formation, sporulation). There are four major categories of AIs: (i) autoinducer-1
(AI-1), for example, N-acyl homoserine lactone (Figure 6.2) is produced by Gram-negative
bacteria and is used for intraspecies communication; (ii) autoinducer-2 (AI-2) is produced by
both Gram-positive and Gram-negative bacteria and is used as a universal signaling molecule;
(iii) autoinducer-3 (AI-3) is a signaling molecule used by enterohaemorrhagic Escherichia
coli during infection; and (iv) autoinducing peptides (AIPs) that are produced and used by
Gram-positive bacteria. AI-1 and AI-2 have been found in many different food systems,
including milk, meat, and vegetables, when these foods are associated with pseudomonads,
Enterobactericeae, and lactic acid bacteria.
QS has been recognized as a key regulatory system in both food safety and food spoilage. In
pathogenic bacteria, such as Salmonella, Escherichia coli O157:H7, and Staphylococcus aureus, QS
is involved in virulence gene expression, promoting increased pathogen survival and colonization
in the host. 13 QS is also a regulatory spoilage mechanism. 12 Food spoilage is characterized by
temperatures), with an optimum at 15°C and a range of –5°C to 20°C. However, these divisions
are not clear cut and overlap each other.
Two other terms used in food microbiology are very important with respect to microbial growth
at refrigerated temperatures and survival of microorganisms to low heat treatment or pasteurization because both methods are widely used in the storage and processing of foods. Psychrotrophs
are microorganisms that grow at refrigerated temperature (0°C–5°C) irrespective of their optimum range of growth temperature. They usually grow rapidly between 10°C and 30°C. Molds;
yeasts; many Gram-negative bacteria from genera Pseudomonas, Achromobacter, Yersinia, Serratia,
and Aeromonas; and Gram-positive bacteria from genera Leuconostoc, Lactobacillus, Bacillus,
Clostridium, and Listeria are included in this group (see Chapter 21). Microorganisms that survive
pasteurization temperatures are designated as thermodurics. They include species from genera
Micrococcus, Bacillus, Clostridium, Lactobacillus, Pediococcus, and Enterococcus. Bacterial spores
are also included in this group. They have different growth temperatures, and many can grow at
refrigerated temperatures as well as thermophilic temperatures.
When foods are exposed to temperatures beyond the maximum and minimum temperatures
of growth, microbial cells die rapidly at higher temperatures and relatively slowly at lower temperatures. Influence of temperature on microbial growth and viability is important in reducing
food spoilage, enhancing safety against pathogens, and in food bioprocessing. Temperature of
growth is also effectively used in the laboratory to enumerate and isolate microorganisms from
foods.
Quorum Sensing and Bacterial Growth
The intrinsic and extrinsic factors control bacterial growth in food, but little was known about
the molecular mechanism of bacterial behavior in these conditions until recently. Bacterial
quorum sensing (QS) is thought to play a major role in the bacterial response to a changing
environment in food. QS is described as a bacterial cell-to-cell communication system to
develop a strategy for the bacterial population to adapt to the harsh environment in food or
the host. 12 QS is cell-density dependent, that is, when bacteria reach a certain cell number,
they produce small diffusible molecules, termed autoinducers (AIs) as signaling molecules
that are used for cell-to-cell communication during growth. For example, under stressful conditions, such as nutrient starvation, low water activity, high temperatures, antimicrobial compounds, acid, or other microbes, bacterial AIs bind to receptors and activate gene expression
to alter behavior (i.e., adaptation to harsh environment) and morphological characteristics
(i.e., biofilm formation, sporulation). There are four major categories of AIs: (i) autoinducer-1
(AI-1), for example, N-acyl homoserine lactone (Figure 6.2) is produced by Gram-negative
bacteria and is used for intraspecies communication; (ii) autoinducer-2 (AI-2) is produced by
both Gram-positive and Gram-negative bacteria and is used as a universal signaling molecule;
(iii) autoinducer-3 (AI-3) is a signaling molecule used by enterohaemorrhagic Escherichia
coli during infection; and (iv) autoinducing peptides (AIPs) that are produced and used by
Gram-positive bacteria. AI-1 and AI-2 have been found in many different food systems,
including milk, meat, and vegetables, when these foods are associated with pseudomonads,
Enterobactericeae, and lactic acid bacteria.
QS has been recognized as a key regulatory system in both food safety and food spoilage. In
pathogenic bacteria, such as Salmonella, Escherichia coli O157:H7, and Staphylococcus aureus, QS
is involved in virulence gene expression, promoting increased pathogen survival and colonization
in the host. 13 QS is also a regulatory spoilage mechanism. 12 Food spoilage is characterized by