Food Biopreservatives of Microbial Origin, Bacteriocin, and Nanotechnology ◾ 221
NisB and NisC are involved in the dehydration of serine and threonine to dehydroalanine and
dehydrobutyrine and thioether ring formation; NisT is the ABC transporter involved in the transportation of nisin; NisI is responsible for producer cell immunity to nisin; NisP removes the leader
peptide from prenisin following its transport by NisT; NisR and NisK are regulatory proteins and
modulate transcription of nis-operon; and NisE, NisF, and NisG provide accessory protection to
the producer cells against nisin. Similar genes for regulatory proteins are found in other bacteriocin operons, such as sakacin P.
Biosynthesis of a bacteriocin, involving transcription, translation, structural modification,
and secretion, occur in a concerted way. In pediocin, AcH or PA-1 and similar bacteriocins,
probably a single mRNA, is produced for all four genes, which are then translated into four
separate proteins (Figure 17.2). As soon as the 62-amino-acid prepedicin is translated in the
cytoplasm, the leader peptide directs transport of the molecule outside the cell by the help of Pap
or PedD (ABC transporter) and C (helper). During transportation, the catalytic site of protein D
recognizes –G–G– at the –1 and –2 positions of prepediocin and cleaves the leader peptide (18
amino acids); the transporter then secretes the 44-amino-acid pediocin into the environment. In
an oxidized environment, the four cysteine molecules form two disulfide bonds, one at positions
+9 and +14 and another at +24 and +44. The location and mechanism of the immunity protein
Pap or PedB in the producing cells are not known. In nisin A production, initially, proteins
NisR and NisK induce transcription of the cluster, probably to produce a single mRNA, which
is then translated into 11 separate proteins. NisB dehydrates serine and threonine to their respective dehydroamino acids, and NisC with NisB then enables dehydroamino acids to form thioether rings with cysteine residues in the molecule. The modified molecule is then translocated
through the membrane by NisT, and NisP removes the 14-amino-acid leader peptide, releasing
the 34-amino-acid nisin outside the cells. NisE, NisF, and NisG provide extra protection to
producer cells against nisin.
Mode of Action 10–17
Bacteriocins of lactic acid bacteria kill sensitive bacterial cells very rapidly and the highly potent
one at a very low concentration [minimum inhibitory concentration (MIC) ~0.01 μg/mL for
pediocin AcH to Listeria monocytogenes]. Their antibacterial actions can be summarized as
follows:
1. They differ greatly in the spectra of antibacterial activity against sensitive Gram-positive
bacteria (e.g., nisin and pediocin have wider spectra than leucocin or sakacin).
2. Their relative potency (MIC) against a sensitive strain differs greatly (e.g., pediocin is more
potent than leucocin against Lis. monocytogenes).
3. Bactericidal efficiency of a bacteriocin increases at acidic pH, at higher temperatures, in the
presence of a detergent, and against exponentially growing cells.
4. Even in the population of a most sensitive strain, there are variant cells that are resistant to
a bacteriocin, but this property is not stable because in the absence of the bacteriocin they
become sensitive again.
5. A sensitive strain resistant to one bacteriocin can be sensitive to a second bacteriocin.
6. Gram-negative and resistant Gram-positive bacteria injured by a physical or chemical stress
become sensitive to a bacteriocin.
7. Bacterial spores of a sensitive bacterium are resistant to a bacteriocin but become sensitive
following germination and outgrowth.
NisB and NisC are involved in the dehydration of serine and threonine to dehydroalanine and
dehydrobutyrine and thioether ring formation; NisT is the ABC transporter involved in the transportation of nisin; NisI is responsible for producer cell immunity to nisin; NisP removes the leader
peptide from prenisin following its transport by NisT; NisR and NisK are regulatory proteins and
modulate transcription of nis-operon; and NisE, NisF, and NisG provide accessory protection to
the producer cells against nisin. Similar genes for regulatory proteins are found in other bacteriocin operons, such as sakacin P.
Biosynthesis of a bacteriocin, involving transcription, translation, structural modification,
and secretion, occur in a concerted way. In pediocin, AcH or PA-1 and similar bacteriocins,
probably a single mRNA, is produced for all four genes, which are then translated into four
separate proteins (Figure 17.2). As soon as the 62-amino-acid prepedicin is translated in the
cytoplasm, the leader peptide directs transport of the molecule outside the cell by the help of Pap
or PedD (ABC transporter) and C (helper). During transportation, the catalytic site of protein D
recognizes –G–G– at the –1 and –2 positions of prepediocin and cleaves the leader peptide (18
amino acids); the transporter then secretes the 44-amino-acid pediocin into the environment. In
an oxidized environment, the four cysteine molecules form two disulfide bonds, one at positions
+9 and +14 and another at +24 and +44. The location and mechanism of the immunity protein
Pap or PedB in the producing cells are not known. In nisin A production, initially, proteins
NisR and NisK induce transcription of the cluster, probably to produce a single mRNA, which
is then translated into 11 separate proteins. NisB dehydrates serine and threonine to their respective dehydroamino acids, and NisC with NisB then enables dehydroamino acids to form thioether rings with cysteine residues in the molecule. The modified molecule is then translocated
through the membrane by NisT, and NisP removes the 14-amino-acid leader peptide, releasing
the 34-amino-acid nisin outside the cells. NisE, NisF, and NisG provide extra protection to
producer cells against nisin.
Mode of Action 10–17
Bacteriocins of lactic acid bacteria kill sensitive bacterial cells very rapidly and the highly potent
one at a very low concentration [minimum inhibitory concentration (MIC) ~0.01 μg/mL for
pediocin AcH to Listeria monocytogenes]. Their antibacterial actions can be summarized as
follows:
1. They differ greatly in the spectra of antibacterial activity against sensitive Gram-positive
bacteria (e.g., nisin and pediocin have wider spectra than leucocin or sakacin).
2. Their relative potency (MIC) against a sensitive strain differs greatly (e.g., pediocin is more
potent than leucocin against Lis. monocytogenes).
3. Bactericidal efficiency of a bacteriocin increases at acidic pH, at higher temperatures, in the
presence of a detergent, and against exponentially growing cells.
4. Even in the population of a most sensitive strain, there are variant cells that are resistant to
a bacteriocin, but this property is not stable because in the absence of the bacteriocin they
become sensitive again.
5. A sensitive strain resistant to one bacteriocin can be sensitive to a second bacteriocin.
6. Gram-negative and resistant Gram-positive bacteria injured by a physical or chemical stress
become sensitive to a bacteriocin.
7. Bacterial spores of a sensitive bacterium are resistant to a bacteriocin but become sensitive
following germination and outgrowth.