Strep B Genome Yields Surprising Diversity

Researchers have published the complete genome sequence for the pathogenic bacterium Streptococcus agalactiae (also known as group B streptococcus or strep B), which is a major cause of pneumonia and meningitis in newborns and the source of life-threatening illnesses in a growing number of adults with deficient immune systems. The research was reported in the August 28, 2002, online edition of the Proceedings of the National Academy of Sciences.

As part of the study, investigators from The Institute for Genomic Research (Rockville, MD, USA; www.tigr.org) compared the genome of the ‘serotype V' isolate of S agalactiae--the most common capsule type that is associated with invasive infection among adults other than pregnant women--to the genetic makeup of other S agalactiae strains and also to two different species of streptococci (S pneumoniae and S pyogenes) that cause diseases in humans.

S agalactiae has a circular genome of about 2.16 million base pairs. Researchers predicted that there would be 2,176 genes in that genome, and about 65% of the proteins expressed by those genes were of known function. The authors of the current study found that the three streptococcal species shared 1,060 genes, about half of their genes, but that 683 genes were unique to S agalactiae.

"We were surprised to find so many differences among the isolates of this important pathogen, said the study's first author, Dr. Herve Tettelin, an associate investigator at The Institute for Genomic Research. "Those differences could help explain why some strains of S agalactiae are much more virulent than others.”

Dr. Tettelin said the microarray experiments that compared those related genomes found numerous differences, even among strains with the same serotype. For example variations exist in the type of polysaccharides that make up the capsule that surrounds each bacterium. Furthermore, the S agalactiae genome differed from that of other streptococci in several of the microbe's metabolic pathways and in related transport systems through the bacterium's cell membrane. The researchers also found unique genes in S agalactiae related to surface proteins, capsule synthesis, and the hemolysin enzyme.



Related Links:
The Institute for Genomic Research