Abstract
In Streptococcus pneumoniae the competence-stimulating peptide (CSP), encoded by the comC gene, controls competence development and influences biofilm growth. We explored the influence of pherotype, defined by the two major comC allelic variants (comC1 and comC2), on biofilm development and recombination efficiency. Among isolates recovered from human infections those presenting comC1 show a higher capacity to form in vitro biofilms. The influence of pherotype on biofilm growth was confirmed by experiments with isogenic strains differing in their comC alleles. Biofilm architecture evaluated by confocal laser scanning microscopy showed that strains carrying comC1 form biofilms that are denser and thicker than those carrying the comC2 allele. Isogenic strains carrying the comC1 allele yielded more transformants than those carrying the comC2 allele in both planktonic and biofilm growth. Transformation assays with comC knockout strains show that ComD1 needs lower doses of the signaling peptide to reach the same biological outcomes. In contrast to mixed planktonic growth, within mixed biofilms inter-pherotype genetic exchange is less frequent than that occurring between bacteria of the same pherotype. Since biofilms are a major bacterial lifestyle, these observations may explain the genetic differentiation between populations with different pherotypes reported previously. Considering that biofilms have been associated with colonization our results suggest that strains carrying the comC1 allele may be more transmissible and more efficient at persisting in carriage. Both effects may help explain the higher prevalence of the comC1 allele in the pneumococcal population.
Highlights
Planktonic bacteria are the most frequently studied microbial lifestyle in vitro, it is comparatively rare in vivo, where most prokaryotes grow in matrix-enclosed biofilms
In order to better understand the influence of the two major pherotypes on biofilm growth, we constructed two strains where the pherotypes were switched relative to the ones presented by the wild-type (R36A-CSP2 and TIGR4-CSP1) with the objective of comparing their capacity to form biofilm with that of their otherwise isogenic wild-type strains
An important end point of this activation is the induced lysis of a fraction of the pneumococcal population, a phenomenon known as fratricide [10,13]
Summary
Planktonic bacteria are the most frequently studied microbial lifestyle in vitro, it is comparatively rare in vivo, where most prokaryotes grow in matrix-enclosed biofilms. Life within a biofilm matrix, wherein oxygen is limited and the metabolic rate is altered, results in several benefits, including protection from environmental threats such as host immune defenses, antibiotics, and surfactants [1,2,3] These biological structures have been proposed as the way bacteria organize themselves when colonizing asymptomatically the upper airways of their human hosts [4,5,6]. A set of genes induced at competence functions to trigger the lysis of a fraction of the bacteria within a population [10,12,13] This process of ‘‘fratricide’’ has been postulated to be an important source of DNA for competent cells
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