PCV3 capsid protein (Cap) is an important antigen for diagnosis and vaccine development. To achieve high-level expression of recombinant PCV3 Cap in Escherichia coli (E. coli), the gene of wild-type entire Cap (wt-eCap) was amplified from clinical samples, and three optimized entire Cap (opti-eCap) and one optimized Cap deleted nuclear location signal (NLS) (opti-dCap) gene fragments encoding the same amino acid sequence with wt-eCap were synthesized based on the codon bias of E. coli. Those gene fragments were inserted into the pET30a expression vector. One recombinant strain with the highest expressed soluble eCap from four entire Cap (one wt-eCap and three opti-eCap) and one recombinant strain expressed opti-dCap were selected for further purification. The purified eCap and dCap were identified by transmission electron microscopy (TEM), a large number of round hollow particles with a diameter of 10 nm virus-like particles (VLPs) were observed in eCap, whereas irregular aggregation of proteins observed in dCap. After formation the VLPs were applied as a coating antigen to establish an indirect ELISA (I-ELISA) for detection of PCV3-specific antibody in swine serum. 373 clinical swine serum samples from China collected in 2019 were tested utilizing the VLP-based I-ELISA method under optimized conditions. To the best of our knowledge, this is the first report of self-assembly into VLPs of PCV3 recombinant Cap. Our results demonstrated that the VLP-based I-ELISA will be a valuable tool for detecting the presence of PCV3 antibodies in serum samples and will facilitate screening of large numbers of swine serum for clinical purposes.

Streptococcus suis causes disease in pigs and is implicated increasingly in human disease worldwide. Although most clinical cases are associated with serotype 2, infections by other serotypes have sometimes been reported. Here, we sequenced the genome of a multidrug-resistant S. suis serotype 28 (strain 11313) and a multidrug-resistant S. suis serotype 31 (strain 11LB5). Strain 11313 was apathogenic in mouse infection models, whereas strain 11LB5 displayed ganglion demyelination, meningeal thickening, congestion, mononuclear cell infiltration, massive proliferation of cortical glial cells, and bacteria (>104 CFU/g) in the spinal cord and ganglia in mice. Furthermore, immunohistochemistry found that the heavily infiltrated glial cells were astrocytes. Strain 11313 harbored the resistance genes ant(6)-Ia, erm(B), optrA, tet(l), tet(o), and strain 11LB5 harbored the resistance genes ant(6)-Ia, erm(B), tet(40), tet(o/w/32/o), aac(6′)-aph(2″). Mouse studies showed that strain 11LB5 exhibited a similar virulence to serotype 2 strain 700794, highlighting the need for surveillance of the other serotype S. suis isolates, in addition to serotype 2, in farms. This is the first report of the aminoglycoside resistance gene ant(6)-Ia in S. suis from animals. This suggests that S. suis might serve as an antibiotic resistance reservoir, which spreads the resistance gene ant(6)-Ia or optrA to other streptococcal pathogens on farms.