Abstract
The acquisition of novel genetic traits through natural competence is a strategy used by bacteria in microbe-rich environments where microbial competition, antibiotics, and host immune defenses threaten their survival. Here, we show that virulent strains of Streptococcus suis, an important zoonotic agent and porcine pathogen, become competent for genetic transformation with plasmid or linear DNA when cultured in active porcine and human serum. Competence was not induced in active fetal bovine serum, which contains less complement factors and immunoglobulins than adult serum and was strongly reduced in heat-treated or low-molecular weight fractions of active porcine serum. Late competence genes, encoding the uptake machinery for environmental DNA, were upregulated in the active serum. Competence development was independent of the early competence regulatory switch involving XIP and ComR, as well as sigma factor ComX, suggesting the presence of an alternative stress-induced pathway for regulation of the late competence genes required for DNA uptake.
Highlights
Natural competence, the process by which bacteria acquire environmental DNA with the potential for homologous recombination in the genome [1], may confer selective advantages to bacterial pathogens for the transmission, colonization, and infection of the host, leading to the emergence of more pathogenic clones [2,3].In some streptococcal species, competence for DNA transformation is a quorumsensing trait regulated by a secreted peptide pheromone [4]
Not all SS strains can be transformed by their cognate XIP, including virulent isolates belonging to serotype 7 (SS7) [12]; other methods for inducing competence for DNA transformation would open up possibilities for efficient genetic manipulation
Transcription of comX encoding the sigma factor regulating the expression of the late competence operon was decreased ~0.59-log2 fold in the serum compared with THY (Table 1)
Summary
The process by which bacteria acquire environmental DNA with the potential for homologous recombination in the genome [1], may confer selective advantages to bacterial pathogens for the transmission, colonization, and infection of the host, leading to the emergence of more pathogenic clones [2,3]. The use of serotyping alone as a predictor of virulence has the limitation that strains of the same serotype can vary substantially [8,9] This reflects the high genetic variability of different strains and the risk for the emergence of novel hypervirulent and zoonotic strains, as well the transmission of antibiotic resistance [10,11]. Not all SS strains can be transformed by their cognate XIP, including virulent isolates belonging to serotype 7 (SS7) [12]; other methods for inducing competence for DNA transformation would open up possibilities for efficient genetic manipulation. Invasive isolates of SS can grow in active serum, we hypothesized that the stress induced by exposure to complement, SS-binding antibodies and antimicrobial peptides in serum might induce competence for DNA transformation at sites of infection [13]. We investigated (1) competence induction in active and heat-inactivated porcine and human serum, and (2) the role of early and late regulatory genes comR and comX, on competence development in active serum
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