Fundamental Food Microbiology, Third Edition - Fuad Fathir

156 FUNDAMENTAL FOOD MICROBIOLOGY
Bac + , production of several bacteriocins (also their respective immunity, processing,
and translocation traits; bacteriocin, like nisin, is encoded in a transposon)
Phage r , resistance to specific bacteriophages
R/M system, restriction/modification
Resistance to several antibiotics (such as Km r , resistance to kanamycin)
Metabolism of several carbohydrates (such as Gal + , galactose utilization)
Muc + , mucin production
• Str. thermophilus. Plasmid linkage of a phenotype not conclusively known
• Leuconostoc Species. Bac + , production of different bacteriocins (also immunity
to them)
• Pediococcus Species.
Suc + , sucrose hydrolysis
Bac + , production of different bacteriocins (also immunity to them and translocation)
• Lactobacillus Species.
Lac + , lactose hydrolysis
Mal + , maltose hydrolysis
Bac + , production of some bacteriocins (also immunity to them)
Muc + , ability to produce mucin
Resistance to some antibiotics (such as erythromycin, Em r )
R/M system, restriction/modification
The same phenotype in a species can be encoded in different-size plasmids.
F. Cryptic Plasmids
Many plasmids of lactic acid bacteria carry genes that encode for specific phenotypes.
An example is the plasmid pSMB74 in Pediococcus acidilactici H, which
encodes genes associated with the production of a bacteriocin, pediocin AcH. However,
there are other plasmids in lactic acid bacteria for which no specific phenotypes
can be assigned, and they are designated as cryptic plasmids. Some of them are able
to integrate in the host chromosome and others are able to replicate in different
homologous and heterologous hosts. For this reason, some of them have been used
to construct vectors for cloning and expression of genes in lactic acid bacteria and
some Gram-positive and Gram-negative bacteria. Plasmids pWVO1 and pSH71 have
been extensively used to develop a series of cloning and expression vectors. 3
G. Plasmid Replication
Plasmids of lactic acid bacteria have a high degree of sequence homology in the
origin of replication (rep A) and the gene encoding the replication protein (rep B).
Depending on the size, a plasmid replicates either by a sigma-type or by a thetatype
mechanism. In the sigma (or rolling circle) replication mode, replication is
initiated by the binding of the plasmid encoding the Rep protein to the origin (plusorigin
site). This is followed by a nick in one and fixation of Rep at the 5¢ end of
the nicked strand. The other strand is replicated from the nick site, causing displacement
of the nicked strand, thus forming the shape of a sigma. The single strand is