Potential use of an unencapsulated and aromatic amino acid-auxotrophic Streptococcus suis mutant as a live attenuated vaccine in swine

Streptococcus suis is responsible for severe economic losses to the swine industry. Prevention of the diseases caused by this pathogen is hampered by the inability of available vaccines to generate a protective response in pigs. A non-virulent, aromatic amino acid-auxotrophic and unencapsulated mutant strain of S. suis was generated in this study and a preliminary evaluation of its protective capacities was conducted in swine. Deletion of the cognate promoter of the aro operon in S. suis virulent strain S735 resulted in the abolishment of the expression of the four members of the operon, aroA, aroK, pheA and orf10. The resulting mutant strain BD101 was auxotrophic for aromatic amino acids as demonstrated by its failure to grow in a chemically defined medium unless it was supplemented with these compounds. In addition, as a result of the deletion of the cognate promoter of the aro operon, mutant BD101 lost its encapsulated phenotype. A protection assay was performed by immunisation of pigs with live strain BD101. Vaccination resulted in minor clinical signs but did not substantially impair the growth of vaccinated animals. Immunisation of animals with live mutant BD101 induced a considerable antibody response against S. suis. Vaccinated pigs presented only minor clinical signs and a survival rate of 100%, while 57% of non-vaccinated animals died, after a challenge with the virulent parent strain S735. In order to prevent S. suis infections in swine, it may be useful to further evaluate strain BD101 as a vaccine candidate.