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Chapter 1 : Growth, Survival, and Death of Microbes in Foods
Category: Applied and Industrial Microbiology; Food Microbiology
Food microbiologists must understand microbiology and food systems and be able to integrate them to solve problems in complex food ecosystems. This chapter addresses this in three parts by (i) examining foods as ecosystems and discussing intrinsic and extrinsic environmental factors that control bacterial growth, (ii) explaining first-order or pseudo-first-order kinetics which govern the log phase of microbial growth and many types of lethality, and (iii) focusing on physiology and metabolism of foodborne microbes. Growth of Clostridium botulinum in foods such as potatoes and sauteed onions exposed to air has caused botulism outbreaks. Bacteria are classified as psychrophiles, psychrotrophs, mesophiles, and thermophiles according to the way in which temperature influences their growth. Additional barriers to microbial growth should be incorporated into refrigerated foods containing no other inhibitors. Food microbiology is concerned with all four phases of microbial growth. Growth curves showing the lag, exponential logarithmic or log, stationary, and death phases of a culture are normally plotted as the number of cells on a logarithmic scale or log10 cell number versus time. These plots represent the states of microbial populations rather than individual microbes. Thus, both the lag phase and stationary phase of growth represent periods when the growth rate equals the death rate to produce no net change in cell numbers. Food microbiologists frequently use doubling times (td) to describe growth rates of foodborne microbes. Developments in molecular biology and microbial ecology will change or deepen the perspective about the growth of microbes in foods.
Relative growth rates of bacteria at different temperatures.
Major catabolic pathways used by foodborne bacteria.
Proton motive force can be generated by respiration, ATP hydrolysis, end-product efflux, or anion exchange mechanisms. Modified from reference 137 .
Transport can be at the direct expense of high-energy phosphate bonds or can be linked to the proton gradient of the proton motive force.
Data indicative of injury and repair. (a) When bacteria are plated on selective (○) or nonselective (●) media during exposure to some stressor (e.g., heat), the decrease in CFU on a nonselective medium represents the true lethality, while the difference between the values obtained on each medium is defined as “injury.” (b) During “repair,” resistance to selective agents is regained, and the value obtained on the selective medium approaches that of the nonselective medium. Unstressed controls are shown at the top of panel b. Modified and redrawn from reference 27 .
Data showing changes of plate count and cell morphology during development of the VNC state induced by temperature downshifts (at time 0 and ↓) and resuscitation by temperature upshifts (↑). Reprinted from reference 105 with permission.
First-order kinetics can be used to describe exponential growth and inactivation
Representative specific growth rates and doubling times of microorganisms
Examples of quorum sensing in food microbiology
Influence of thermal history and enumeration protocols on experimentally determined D values at 55°C for L. monocytogenes ( 83 )