Susceptibility of Streptococcus suis to liamocins from Aureobasidium pullulans

Streptococcus suis is a major pathogen of swine that is also considered an emerging zoonotic pathogen of humans. Because of increasing antibiotic resistance in S. suis, new antimicrobial agents are being sought. Liamocins are unique polyol lipids produced by certain strains of the fungus Aureobasidium pullulans. Liamocins recently have been found to have antibacterial activity with specificity for Streptococcus species. However, liamocins have not previously been tested for inhibition of S. suis. Ten strains of S. suis isolated from domestic pigs were tested for susceptibility to liamocins by the broth microdilution method. All 10 were susceptible, with MICs ranging from 39 to 78 μg/mL. Viability assays using the LIVE/DEAD BacLight Bacterial Viability Kit indicated that liamocins are bactericidal to S. suis. Cell-leakage assays showed that cells treated with liamocins leaked small molecular mass nucleic acids into the extracellular matrix, suggesting that liamocins act via disruption of the cell membrane. Liamocins thus show potential for development as a specific antibacterial to reduce colonization of S. suis in swine.