Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL STRESS RESPONSE IN THE FOOD ENVIRONMENT 109
apparent that other microorganisms that have not been studied will most likely also
be injured by sublethal stresses. In general, Gram-negative bacteria are more susceptible
to injury than Gram-positive bacteria, and bacterial spores are much more
resistant than vegetative cells to a particular stress. 1–3
Microbial injury and growth of injured cells have been studied with both bacterial
cells and spores. The material discussed here mainly covers bacterial cell and spore
injury.
B. Manifestation of Bacterial Sublethal Injury
A bacterial population, following exposure to a sublethal stress, contains three physiologically
different subpopulations: the uninjured (normal) cells, reversibly injured
(injured) cells, and irreversibly injured (dead) cells. Their relative percentages vary
greatly and are dependent on the species and strains, nature of suspending media,
nature and duration of stress, and methods of detection. Injured cells differ in the
level of injury and in several characteristics from their normal counterparts. One of
these is increased sensitivity to many compounds to which normal cells are resistant,
such as surface-active compounds (bile salts, deoxycholate, or SDS), NaCl, some
chemicals (LiCl, selenite, bismuthsulfite, or tetrathionate), hydrolytic enzymes
(lysozyme or RNase), antibiotics, dyes (Crystal Violet or Brilliant Green), and low
pH and undissociated acids. They also lose cellular materials, such as K + , peptides,
amino acids, and RNA. When exposed to or maintained in an unfavorable environment,
the injured cells die progressively. Also, the injured cells do not multiply unless
the injury has been repaired. In a nonselective and preferably nutritionally balanced
medium, the cells are able to repair their injury, which can extend 1 to 6 h, depending
on the nature of stress and degree of injury. After an extended repair phase, the cells
regain their normal characteristics and initiate multiplication. The injured cells in a
population can be detected by a suitable nonselective enumeration technique. The
results in Table 9.2 show that normal Esc. coli cells grow almost equally in both the
nonselective and selective media, indicating that the cells are not sensitive to the
surface-active agent, deoxycholate, in the medium. Following freezing and thawing,
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Table 9.1 Cells and Spores of Some Microorganisms Important in Food in Which
Sublethal Injury is Detected
Gram-positive pathogenic bacteria: Sta. aureus, Clo. botulinum, Lis. monocytogenes, Clo.
perfringens, Bac. cereus
Gram-negative pathogenic bacteria: Salmonella, Shigella spp., enteropathogenic Esc.
coli, Esc. coli O157:H7, Vib. parahaemolyticus, Cam. jejuni, Yer. enterocolitica, Aer.
hydrophila
Gram-positive spoilage bacteria: Clo. sporogenes, Clo. bifermentum, Bac. subtilis, Bac.
stearothermophilus, Bac. megaterium, Bac. coagulans
Gram-negative spoilage bacteria: Pseudomonas spp., Serratia spp.
Gram-positive bacteria used in food bioprocessing: Lac. lactis spp., Lab. delbrueckii
subsp. bulgaricus, Lab. acidophilus
Gram-positive indicator bacteria: Ent. faecalis
Gram-negative indicator bacteria: Esc. coli, Ent. aerogenes, Klebsiella spp.
Bacterial spores: Clo. botulinum, Clo. perfringens, Clo. bifermentum, Clo. sporogenes, Bac.
cereus, Bac. subtilis, Bac. stearothermophilus, Des. nigrificans
Yeasts and molds: Sac. cerevisiae, Candida spp., Asp. flavus