Fundamental Food Microbiology, Third Edition - Fuad Fathir

104 FUNDAMENTAL FOOD MICROBIOLOGY
I. INTRODUCTION
Foodborne bacterial cells (and other microbes) are usually exposed to different
physical and chemical environments during production, processing, preservation,
storage, transportation, and consumption of foods as well as during microbiological
quality evaluation of foods and food ingredients by the recommended procedures.
As a consequence, bacterial cells may become stressed and manifest several types
of altered characteristics. A stressed environment can be both in the suboptimal
growth range and beyond the growth range (Figure 9.1). Depending on the nature
and level of a stress, cells in a population can develop a higher level of resistance
to the same as well as several other types of stresses, or suffer reversible sublethal
injury or even apparently lose culturability (ability to multiply) in some recommended
bacteriological media and methods or lose viability (or ability to multiply)
permanently. Researchers have used many different terminologies for these altered
states of bacterial cells, some of which are not well defined or are for the same
condition or even scientifically contradictory. This has generated confusion among
interested individuals and controversy among researchers. Currently, there is a move
among some scientists researching in the area of microbial stress to conduct direct
basic studies to understand the response of bacterial cells under different levels and
the nature of stresses and to develop consensus scientific opinion for suitable definitions
and specific terminologies.
In the last 50 years, three different broad terminologies have appeared in
microbiology literature to describe altered characteristics of bacterial cells following
exposure to some physical and chemical stresses: sublethal injury (in the
1960s to 1980s), 1–3 viable-but-nonculturable state (in the 1980s and 1990s), 4 and
stress adaptation (in the 1990s) 5 . In most publications, each aspect has been
treated as a separate and unique phenomenon. In this chapter, the three have been
described separately, and then it is suggested that all three are probably related,
differing only in the degree of bacterial response following exposure to different
levels of a stress. Several edited books have been published in each of the three
areas. 1–5
Stress Stress
Lethal Sublethal Suboptimal Optimal Suboptimal Sublethal Lethal
Growth Range
Figure 9.1 Different levels or degrees of environmental stresses to which bacterial cells can
be exposed during processing and preservation of food. Bacterial cells exposed
to a suboptimal growth condition show stress adaptation. Beyond the growth range,
the cells are usually either sublethally or lethally stressed. See text for further
explanations.