Comparative genomics and the role of lateral gene transfer in the evolution of bovine adapted Streptococcus agalactiae

Abstract

In addition to causing severe invasive infections in humans, Streptococcus agalactiae, or group B Streptococcus (GBS), is also a major cause of bovine mastitis. Here we provide the first genome sequence for S. agalactiae isolated from a cow diagnosed with clinical mastitis (strain FSL S3-026). Comparison to eight S. agalactiae genomes obtained from human disease isolates revealed 183 genes specific to the bovine strain. Subsequent polymerase chain reaction (PCR) screening for the presence/absence of a subset of these loci in additional bovine and human strains revealed strong differentiation between the two groups (Fisher exact test: p < 0.0001). The majority of the bovine strain-specific genes (~85%) clustered tightly into eight genomic islands, suggesting these genes were acquired through lateral gene transfer (LGT). This bovine GBS also contained an unusually high proportion of insertion sequences (4.3% of the total genome), suggesting frequent genomic rearrangement. Comparison to other mastitis-causing species of bacteria provided strong evidence for two cases of interspecies LGT within the shared bovine environment: bovine S. agalactiae with Streptococcus uberis (nisin U operon) and Streptococcus dysgalactiae subsp. dysgalactiae (lactose operon). We also found evidence for LGT, involving the salivaricin operon, between the bovine S. agalactiae strain and either Streptococcus pyogenes or Streptococcus salivarius. Our findings provide insight intomechanismsfacilitatingenvironmentaladaptationandacquisitionofpotential virulence factors, while highlighting both the key role LGT has played in the recent evolution of the bovine S. agalactiae strain, and the importance of LGT among pathogens within a shared environment.

Genomic analysis of diversity, population structure, virulence, and antimicrobial resistance in Klebsiella pneumoniae , an urgent threat to public health

“…This finding suggests that the utilization of lactose from the cow using this second lactose operon may confer an important selective growth advantage to K. pneumoniae isolates associated with mastitis. Similar associations have been made in Streptococcus agalactiae (58).…”

Sequence type 1 group B Streptococcus , an emerging cause of invasive disease in adults, evolves by small genetic changes

“…The lac.2 operon has previously been shown to be preferentially found in GBS isolated from cattle (24). Given that the presence of the lac.2 operon in bovine GBS strains has been suggested to occur via lateral gene transfer (25), our findings indicate that a single genetic event could account for the majority of differences observed between these two strains.…”

Physiological impact of transposable elements encoding DDE transposases in the environmental adaptation of Streptococcus agalactiae

“…The sequence of the genome of the S. agalactiae strain FSL S3-026 isolated from a cow with mastitis revealed the presence of several copies of an IS of the IS256 family (Richards et al, 2011 (Dodd et al, 1994). To our knowledge, an increase in the transcription of downstream genes after ISSag11 insertion has not yet been described in S. agalactiae.…”