Introduction to Foodborne Pathogens
529
Apparent Role of Quorum Sensing
The first published demonstration of the possible role of quorum sensing in biofilms was that of
McLean et al.
73 who recovered AHLs from aquatic biofilms growing on submerged stones in the
San Marcos River in Texas. The direct involvement of AHLs was demonstrated with Pseudomonas
aeruginosa where a mutant that could not synthesize AHL produced atypical biofilms (contained no
water channels) that were sensitive to sodium dodecyl sulfate in contrast to wild-type strains.
30
Biofilm formation on indwelling medical devices and by organisms such as P. aeruginosa and
Burkholderia cepacia that wreak havoc on cystic fibrosis patients is well documented (as is biofilm
formation by L. monocytogenes in food processing environments). The relationship between biofilm
formation, quorum sensing, virulence or pathogenicity of foodborne pathogens is unclear, but it is not
inconceivable that relationships exist.
SIGMA (δ) FACTORS
Sigma is one of the four subunits of RNA polymerase, and its role is in the recognition of the
promoter (where RNA polymerase binds to DNA and transcription begins). Sigma is involved only in
the initial RNA polymerase-DNA complex. After a small portion of mRNA has formed, δ dissociates.
δ
A (or δ
70 , the number refers to the molecular size in kilodaltons) recognizes the majority of genes
that encode essential cell functions, and its closest homologue is δ
S . Among the known sigma factors
are the following: δ
28 is involved in flagellar synthesis in Salmonella, and also in the type III secretion
system. δ
32 (RpoH) is involved in heat shock proteins (HSPs), some of which are molecular chaperones
or proteases that eliminate those that cannot be repaired. δ
54 (RpoN) regulates harp (hypersensitive
response and pathogenicity) genes in at least some Pseudomonas syringae pathovars. δ
B endows
L. monocytogenes with resistance to lethal acidic conditions. It is more abundant at 25
◦ C than at 42
◦ C
in E. coli. It is involved in stress responses in Bacillus subtilis. δ
S is present in the γ -subclass of the
Proteobacteria including the vibrios. It aids V. vulnificus in its resistance to adverse environmental
conditions. It is discussed further below under Alternative Sigma Factors. A change in the cell’s
environment that is stressful (starvation, low pH, increased osmotic pressure, etc.) leads to the induction
of alternative sigma factors that aid the cell in coping with its unfavorable state (see reference 5).
The general responses that bacteria make upon exposure to acidic conditions consist of the
following:
27 (1) proton pumps come into play where a proton motive force (PMF) can facilitate
the extrusion of protons from the cytoplasm, which results in a drop in intracellular pH; (2) repair
of macromolecules such as DNA, and proteins such as RecA; (3) changes in cell membrane components (e.g., fatty acids); (4) regulation of gene expression by alternative sigma factors; (5) cell density and biofilm formation (which protects cells from certain adverse environmental influences); and
(6) alteration of metabolic pathways. Biofilms are covered in the previous section of this chapter, and
alternative sigma factors are discussed further below.
Alternative Sigma Factors
The alternative sigma factor, δ
38 (sigma-38, δ
S ) is encoded by the rpoS gene, and it regulates at
least 30 proteins, and it along with δ
B is discussed in this section. Exposure to acid stress leads to
the synthesis of proteins that protect the bacterium. With log-phase cells at or below pH 4.5, at least
529
Apparent Role of Quorum Sensing
The first published demonstration of the possible role of quorum sensing in biofilms was that of
McLean et al.
73 who recovered AHLs from aquatic biofilms growing on submerged stones in the
San Marcos River in Texas. The direct involvement of AHLs was demonstrated with Pseudomonas
aeruginosa where a mutant that could not synthesize AHL produced atypical biofilms (contained no
water channels) that were sensitive to sodium dodecyl sulfate in contrast to wild-type strains.
30
Biofilm formation on indwelling medical devices and by organisms such as P. aeruginosa and
Burkholderia cepacia that wreak havoc on cystic fibrosis patients is well documented (as is biofilm
formation by L. monocytogenes in food processing environments). The relationship between biofilm
formation, quorum sensing, virulence or pathogenicity of foodborne pathogens is unclear, but it is not
inconceivable that relationships exist.
SIGMA (δ) FACTORS
Sigma is one of the four subunits of RNA polymerase, and its role is in the recognition of the
promoter (where RNA polymerase binds to DNA and transcription begins). Sigma is involved only in
the initial RNA polymerase-DNA complex. After a small portion of mRNA has formed, δ dissociates.
δ
A (or δ
70 , the number refers to the molecular size in kilodaltons) recognizes the majority of genes
that encode essential cell functions, and its closest homologue is δ
S . Among the known sigma factors
are the following: δ
28 is involved in flagellar synthesis in Salmonella, and also in the type III secretion
system. δ
32 (RpoH) is involved in heat shock proteins (HSPs), some of which are molecular chaperones
or proteases that eliminate those that cannot be repaired. δ
54 (RpoN) regulates harp (hypersensitive
response and pathogenicity) genes in at least some Pseudomonas syringae pathovars. δ
B endows
L. monocytogenes with resistance to lethal acidic conditions. It is more abundant at 25
◦ C than at 42
◦ C
in E. coli. It is involved in stress responses in Bacillus subtilis. δ
S is present in the γ -subclass of the
Proteobacteria including the vibrios. It aids V. vulnificus in its resistance to adverse environmental
conditions. It is discussed further below under Alternative Sigma Factors. A change in the cell’s
environment that is stressful (starvation, low pH, increased osmotic pressure, etc.) leads to the induction
of alternative sigma factors that aid the cell in coping with its unfavorable state (see reference 5).
The general responses that bacteria make upon exposure to acidic conditions consist of the
following:
27 (1) proton pumps come into play where a proton motive force (PMF) can facilitate
the extrusion of protons from the cytoplasm, which results in a drop in intracellular pH; (2) repair
of macromolecules such as DNA, and proteins such as RecA; (3) changes in cell membrane components (e.g., fatty acids); (4) regulation of gene expression by alternative sigma factors; (5) cell density and biofilm formation (which protects cells from certain adverse environmental influences); and
(6) alteration of metabolic pathways. Biofilms are covered in the previous section of this chapter, and
alternative sigma factors are discussed further below.
Alternative Sigma Factors
The alternative sigma factor, δ
38 (sigma-38, δ
S ) is encoded by the rpoS gene, and it regulates at
least 30 proteins, and it along with δ
B is discussed in this section. Exposure to acid stress leads to
the synthesis of proteins that protect the bacterium. With log-phase cells at or below pH 4.5, at least