These mutually exclusive R/M systems were shown to protect against viral infection by viruses produced in cells of the opposite genotype, reducing infection frequency to < 10-5 [35]. The R/M cassette has a size compatible with horizontal transfer by transformation, so we wondered if

the distribution of the R/M cassettes could be correlated to the pherotype and thereby contribute to promote asymmetries of horizontal gene transfer within the pneumococcal population. To pursue this hypothesis, the R/M cassette carried by pneumococcal see more isolates previously characterized by MLST was determined. The proportion of CSP-2 isolates with the dpnII cassette (3/23) is lower than the proportion of CSP-1 isolates with that same cassette (25/67) and the association between

pherotype and the R/M system is significant (p = 0.037, selleckchem Fisher exact test), suggesting that phage transduction may be indirectly arbitrated PI3K inhibitor by the pherotype via the R/M systems, such that the spread of large genetic elements that rely on this mechanism of horizontal gene transfer could also be limited by pherotype. Pherotype is a marker of population segregation MLST data has been used to characterize the clonality of bacterial populations and to explore the impact of recombination and mutation in bacterial evolution [4]. For S. pneumoniae

the recombination rate has been estimated to be 3-10 times the mutation rate per locus [28, 36]. To test if the pherotype could be limiting the genetic exchanges within pneumococci, we took the simple approach of testing among all pairs of sequence types that diverge at the allele of a single locus (single-locus variants – SLV) and that should represent the initial stages of diversification dominated by recombination, if the allele that differed was more frequent among sequence types sharing the same pherotype or among isolates of a different pherotype. Considering the observed SLV pairs in our study, the probability that the changing allele Tryptophan synthase came from a different pherotype is 0.11. In a panmictic population, the expected probability would be 0.38 (p < 10-4, permutation test), again suggesting that recombination between pherotypes is reduced. To test if the populations defined by each pherotype showed genetic differentiation we analyzed the concatenated sequences of six of the genes used in MLST, excluding ddl since it was previously shown that this gene showed a hitchhiking effect with pbp2b involved in penicillin resistance[37] and could thus bias the results.