Convalescent sera To prevent any contact with infectious agents, SPF Bama minipigs and healthy piglets were housed in independent units with absolute filters. Prior to challenge, all the pigs were negative for SS2-specific antibodies, as determined by an ELISA test. SPF minipigs (n = 8, Guizhou line, 7 weeks old) were randomly grouped into 2 units (4/unit, named as group 1 and 2) and piglets (n = 12, 8 weeks old) into 2 units (6/unit, named as group 3 and 4). Bacterial suspensions in THB with 10% inactivated bovine serum were prepared and adjusted to a concentration of 1 × 108 colony forming units (CFU)/mL of S. suis. These pigs

were challenged with 2 mL of strain ZY05719 HM781-36B mw (1 × 108CFU/mL), intramuscularly (i.v.) for group 1 and 3, and intravenously (i.m.) for group 2 and 4, respectively. The pigs were monitored daily post-inoculation (pi) for clinical signs, notably fever and central nervous system dysfunctions such as opisthotonos, tremors, and nystagmus. The rectal temperature was recorded daily. No inflammation was observed at the injection sites. Intramuscularly challenged pigs died naturally between 4 and 8 days after experimental infection, while intravenously challenged pigs died between 2 and 7 days. The pigs, 3 minipigs (1 for

i.v. group and 2 for i.m.) and 5 piglets (2 for i.v. group and 3 for i.m.), that recovered after being challenged were used in the subsequent experiments performed in this study. The antibody titer against a homologous strain was determined by indirect ELISA every week Cisplatin chemical structure after challenge. At week 4, the animals were sacrificed and bled. The sera were collected and kept frozen at -40°C. The flowchart

of piglet infections was as shown in Additional File 1: Figure S1. Convalescent sera collected from the recovered pigs were used for IVIAT selection. Positive control sera SS2-positive sera were prepared much from 3 SPF minipigs immunized with inactivated ZY05719 whole cell bacteria (2 mL of 1 × 108 CFU each) 4 times at 2-week intervals. Ten days after the last injection, the antisera were pooled and used as the positive control in ELISA tests. Negative control sera To reduce variability animal to animal, serum samples were obtained from healthy SPF minipigs prior to SS2 infection, negative in ELISA test, used as the negative control for IVIAT or ELISA. Adsorption of swine convalescent-phase and control sera To compensate for variations in the immune responses of individual pigs, equal volumes of convalescent sera from 3 minipigs and 5 piglets were pooled and extensively adsorbed with in vitro-derived SS2 antigens to completely remove all antibodies that recognize the antigens that are expressed under the in vitro condition. The adsorption protocol has been described previously [20].