Background: Porcine reproductive and respiratory syndrome virus (PRRSV) and Streptococcus suis are common pathogens in pigs. In samples collected during the porcine high fever syndrome (PHFS) outbreak in many parts of China, PRRSV and S. suis serotype 7 (SS7) have always been isolated together. To determine whether PRRSV-SS7 coinfection was the cause of the PHFS outbreak, we evaluated the pathogenicity of PRRSV and/or SS7 in a pig model of single and mixed infection. Results: Respiratory disease, diarrhea, and anorexia were observed in all infected pigs. Signs of central nervous system (CNS) disease were observed in the highly pathogenic PRRSV (HP-PRRSV)-infected pigs (4/12) and the coinfected pigs (8/10); however, the symptoms of the coinfected pigs were clearly more severe than those of the HP-PRRSV-infected pigs. The mortality rate was significantly higher in the coinfected pigs (8/10) than in the HP-PRRSV- (2/12) and SS7-infected pigs (0/10). The deceased pigs of the coinfected group had symptoms typical of PHFS, such as high fever, anorexia, and red coloration of the ears and the body. The isolation rates of HP-PRRSV and SS7 were higher and the lesion severity was greater in the coinfected pigs than in monoinfected pigs. Conclusion: HP-PRRSV infection increased susceptibility to SS7 infection, and coinfection of HP-PRRSV with SS7 significantly increased the pathogenicity of SS7 to pigs.

Staphylococcus aureus (S. aureus) is a common and dangerous pathogen that causes various infectious diseases. Skin damage, such as burn wounds, are at high risk of Staphylococcus aureus colonization and infection, which increases morbidity and mortality. The phage lysin LysGH15 exhibits highly efficient lytic activity against methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) strains. Apigenin (api) significantly decreases haemolysis of rabbit erythrocytes caused by S. aureus and shows anti-inflammatory function. LysGH15 and api were added to Aquaphor to form an LysGH15-api-Aquaphor (LAA) ointment. The LAA ointment simultaneously exhibited bactericidal activity against S. aureus and inhibited haemolysis. In an LAA-treated mouse model of an MRSA-infected skin wound, the mean bacterial colony count decreased to approximately 10² CFU/mg at 18 h after treatment (and the bacteria became undetectable at 96 h), whereas the mean count in untreated mice was approximately 10⁵ CFU/mg of tissue. The LAA ointment also reduced the levels of pro-inflammatory cytokines (TNF-α, IL-1β, and IFN-γ) and accelerated wound healing in the mouse model. These data demonstrate the potential efficacy of a combination of LysGH15 and api for use as a topical antimicrobial agent against S. aureus.

Bacteriophage endolysin is one of the most promising antibiotic substitutes, but in Gram-negative bacteria, the outer membrane prevents the lysin from hydrolyzing peptidoglycans and blocks the development of lysin applications. The prime strategy for new antibiotic substitutes is allowing lysin to access the peptidoglycan from outside of the bacteria by reformation of the lysin. In this study, the novel Escherichia coli (E. coli) phage lyase lysep3, which lacks outside-in catalytic ability, was fused with the N-terminal region of the Bacillus amyloliquefaciens lysin including its cell wall binding domain D8 through the best manner of protein fusion based on the predicted tertiary structure of lysep3-D8 to obtain an engineered lysin that can lyse bacteria from the outside. Our results showed that lysep3-D8 could lyse both Gramnegative and Gram-positive bacteria, whereas lysep3 and D8 have no impact on bacterial growth. The MIC of lysep3-D8 on E. coli CVCC1418 is 60 μg/ml; lysep3-D8 can inhibit the growth of bacteria up to 12 h at this concentration. The bactericidal spectrum of lysep3-D8 is broad, as it can lyse of all of 14 E. coli strains, 3 P. aeruginosa strains, 1 Acinetobacter baumannii strain, and 1 Streptococcus strain. Lysep3-D8 has sufficient bactericidal effects on the 14 E. coli strains tested at the concentration of 100 μg/ml. The cell wall binding domain of the engineered lysin can destroy the integrity of the outer membrane of bacteria, thus allowing the catalytic domain to reach its target, peptidoglycan, to lyse the bacteria. Lysep3-D8 can be used as a preservative in fodder to benefit the health of animals. The method we used here proved to be a successful exploration of the reformation of phage lysin.