Abstract
The recent description and comparative genome analysis of the lactic acid bacterium Streptococcus thermophilus (Bolotin et al., 2004) provided insights into adaptive evolutionary mechanisms that led to the assembly of a generally recognized as safe (GRAS) species, starting from closely related deadly human pathogens such as Streptococcus pneumoniae, Streptococcus pyogenes and Streptococcus agalactiae. Bolotin et al. (2004) argued that the species S. thermophilus evolved from close phylogenetically related pathogenic streptococci through loss-of-function events. The regressive evolution of S. thermophilus was supported by the finding of an inactive vestige of pathogenesis-related genes (10 % of the total sequence), counterbalanced by the acquisition of relevant traits, like lactose utilization, that have allowed the assembly of a new genome organization suitable for the colonization of the dairy niche.
Surprisingly, in the S. thermophilus genome analysis, Bolotin et al. (2004) made no reference to the urease gene cluster that accounts for 0·4 % of the total coding sequence and 0·6 % of the total coding sequence belonging to known functional categories. This urease gene cluster, composed of eight genes organized in an operon (Mora et al., 2004), has been found in all S. thermophilus strains characterized until now, including natural urease-negative mutants that, however, have only been isolated with relatively low frequency (Monnet et al., 2004). Since the close pathogenic streptococci considered by Bolotin et al. (2004) do not carry ureolytic determinants, we could postulate that the urease gene cluster in S. thermophilus could be a function complementation, probably derived from other bacterial pathogens and fixed on a genomic backbone ancestrally regressed from pathogenic streptococci.