Production and properties of bacteriocin-like inhibitory substances from the swine pathogen Streptococcus suis serotype 2

Abstract

Streptococcus suis serotype 2 is a major pathogen found in the upper respiratory tract of swine. In this study, isolates of this bacterial species were tested for the production of bacteriocin-like inhibitory substances (BLIS). Of the 38 strains tested, four inhibited the growth of other S. suis isolates according to a deferred-antagonism plate assay. Interestingly, three of the strains were originally isolated from healthy carrier pigs and were considered nonvirulent. Three isolates (94-623, 90-1330, and AAH4) that produced BLIS in liquid broth were selected for further characterization. None of the inhibitory activities was related to the production of either organic acids or hydrogen peroxide. The BLIS produced by these strains were heat stable and proteinase K, pronase, and elastase sensitive but were trypsin and chymotrypsin resistant. They were stable at pH 2 and 12 and had molecular masses in the range of 14 to 30 kDa. Maximum production was observed during the mid-log phase. Following a curing procedure with novobiocin, only 90-1330 lost the ability to produce BLIS, suggesting that the BLIS might be plasmid encoded. Analysis of the inhibitory spectra revealed that the BLIS-producing strains also inhibited the growth of Actinobacillus minor, Actinobacillus porcinus, Enterococcus durans, Micrococcus luteus, Streptococcus agalactiae, Streptococcus dysgalactiae subsp. dysgalactiae, Streptococcus equi subsp. zooepidemicus, and S. dysgalactiae subsp. equisimilis. This study reports for the first time the ability of the swine pathogen S. suis serotype 2 to produce BLIS with the characteristics of classic bacteriocins. Further studies are required to investigate the possibility of using bacteriocin-producing strains to prevent swine infections caused by virulent strains of S. suis serotype 2.

FIG. 1.

BLIS production by S. suis 94-623 (1), S. suis 90-1330 (2), S. suis AAH4 (3), S. suis 65 (4), and S. suis 31-533 (5) (negative control). Fresh overnight cultures were spotted on THA plates and were incubated overnight. Plates were then overlaid with THA soft agar that had been inoculated with the indicator strain used (S. suis 24), and their contents were further incubated overnight.

FIG. 2.

Kinetics of BLIS production during growth of S. suis 94-623. The growth of S. suis 94-623 was evaluated by measuring the OD₆₆₀, and the BLIS titer against S. suis 24 was expressed as AU per milliliter as described in Materials and Methods.

FIG. 3.

Mode of action of the BLIS produced by S. suis 90-1330 (A), 94-623 (B), and AAH4 (C) on the growth of S. suis S735. The BLIS-containing supernatant (test culture) or the supernatant from a nonproducing strain (negative control) was added in the early exponential growth phase.