Prophages and Streptococcal Genomes

The single most striking feature of all of the sequenced GAS genomes is the number of endogenous phage genomes found in each strain, ranging from two in M49 strain NZ131 to seven or eight in M6 strain MGAS10394, depending upon whether element MGAS10394.4 is considered a real prophage genome or a trans-poson. A comprehensive review of the bacteriophages of group A streptococci can be found in the book Gram Positive Pathogens [77]. While temperate phages exist that target S. pneumoniae and S. agalactiae [78-80], it is remarkable that none were found in the genome strains, providing a dramatic contrast to S. pyogenes. For example, the genome of GBS NEM316 harbors 14 pathogenicity islands containing elements (especially integrases or transposases) related to bacteriophages or IS elements, but no complete or partial phage genomes [37]. Likewise, while a number of IS elements were found in the S. pneumoniae genomes, no prophage genome or significant remnant was present in either. Thus, it appears that the high frequency of phage carriage in the GAS genomes is not stochastic, but reflects a strong selective pressure for possessing the virulence genes carried by these GAS phages, and demonstrates how intimately these phages are linked to the biology of GAS.

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