Abstract
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.
Highlights
Within the genus Streptococcus, only Streptococcus thermophilus is used as a starter culture in food fermentations
Based on the close phylogenetic relationship among the four species, these observations suggest that the genome of S. macedonicus, as well as those of S. pasteurianus and S. infantarius may be evolving under selective pressures that allow gene loss events and genome decay processes when compared to the S. gallolyticus genomes
In this study we presented the analysis of the first complete genome sequence of a dairy isolate of S. macedonicus
Summary
Within the genus Streptococcus, only Streptococcus thermophilus is used as a starter culture in food fermentations. Lactic acid bacteria (LAB) constitute a very important group of microorganisms for the food industry, as well as the health of humans and animals [1,2] Several species in this group have a long history of safe use in fermented foods and belong to the very few bacteria that may qualify for the “generally regarded as safe” (GRAS) or the “qualified presumption of safety” (QPS) status according to FDA and EFSA, respectively [3]. The presence of specific LAB strains, called “probiotics”, in certain niches of the body is considered to promote the health of the host [2] This benign nature of LAB, as well as their economic value, often obscure the existence of notorious LAB pathogens that are among the leading causes of human morbidity and mortality worldwide [4]. It is believed that this is the only streptococcal species that, during its adaptation to the nutrient-rich milk environment, underwent extensive genome decay, resulting in the loss of pathogenicity-related genes present in members of the genus [7,8]
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