BackgroundWithin the genus Streptococcus, only Streptococcus
thermophilus is used as a starter culture in food fermentations.
Streptococcus macedonicus though, which belongs to the
Streptococcus bovis/Streptococcus equinus complex
(SBSEC), is also frequently isolated from fermented foods mainly of dairy
origin. Members of the SBSEC have been implicated in human endocarditis and
colon cancer. Here we compare the genome sequence of the dairy isolate
S. macedonicus ACA-DC 198 to the other SBSEC genomes in order
to assess in silico its potential adaptation to milk and its
pathogenicity status.ResultsDespite the fact that the SBSEC species were found tightly related based on
whole genome phylogeny of streptococci, two distinct patterns of evolution
were identified among them. Streptococcus macedonicus, Streptococcus
infantarius CJ18 and Streptococcus pasteurianus ATCC 43144
seem to have undergone reductive evolution resulting in significantly
diminished genome sizes and increased percentages of potential pseudogenes
when compared to Streptococcus gallolyticus subsp.
gallolyticus. In addition, the three species seem to have lost
genes for catabolizing complex plant carbohydrates and for detoxifying toxic
substances previously linked to the ability of S. gallolyticus to
survive in the rumen. Analysis of the S. macedonicus genome
revealed features that could support adaptation to milk, including an extra
gene cluster for lactose and galactose metabolism, a proteolytic system for
casein hydrolysis, auxotrophy for several vitamins, an increased ability to
resist bacteriophages and horizontal gene transfer events with the dairy
Lactococcus lactis and S. thermophilus as potential
donors. In addition, S. macedonicus lacks several
pathogenicity-related genes found in S. gallolyticus. For example,
S. macedonicus has retained only one (i.e. the pil3)
of the three pilus gene clusters which may mediate the binding of S.
gallolyticus to the extracellular matrix. Unexpectedly, similar
findings were obtained not only for the dairy S. infantarius CJ18,
but also for the blood isolate S. pasteurianus ATCC 43144.ConclusionsOur whole genome analyses suggest traits of adaptation of S.
macedonicus to the nutrient-rich dairy environment. During this
process the bacterium gained genes presumably important for this new
ecological niche. Finally, S. macedonicus carries a reduced number
of putative SBSEC virulence factors, which suggests a diminished pathogenic
potential.
Streptococcus macedonicus ACA-DC 198, a food-grade isolate from naturally fermented Greek Kasseri cheese, produces a lantibiotic named macedocin that has been previously purified and characterized. In the present study, a 15,171 bp region in the S. macedonicus ACA-DC 198 chromosome, containing the biosynthetic gene cluster of macedocin, has been sequenced. This region consists of 10 ORFs, which correspond to the genes (mcd genes) involved in macedocin biosynthesis, regulation and immunity. The mcd genes are organized in two operons and their role is predicted on the basis of similarities to genes of known lantibiotics. Compared with its closest match, the streptococcin A-FF22 gene cluster, the macedocin one contains an additional structural gene and an insertion sequence between the regulatory and the biosynthetic operons.
Lactobacillus delbrueckii subsp. lactisACA-DC 178, which was isolated from Greek Kasseri cheese, produces a cell-wall-bound proteinase. The proteinase was removed from the cell envelope by washing the cells with a Ca2+-free buffer. The crude proteinase extract shows its highest activity at pH 6.0 and 40°C. It is inhibited by phenylmethylsulfonyl fluoride, showing that the enzyme is a serine-type proteinase. Considering the substrate specificity, the enzyme is similar to the lactococcal PI-type proteinases, since it hydrolyzes β-casein mainly and α- and κ-caseins to a much lesser extent. The cell-wall-bound proteinase from L. delbrueckii subsp. lactisACA-DC 178 liberates four main peptides from β-casein, which have been identified.
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