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Abstract
Streptococcus iniae is a Gram-positive bacterium and a severe pathogen to a wide range of economically important fish species. In addition, S. iniae is also a zoonotic pathogen and can cause serious infections in humans. In this study, we identified from a pathogenic S. iniae strain a putative bacteriocin, Sil, and examined its biological activity. Sil is composed of 101 amino acid residues and shares 35.6% overall sequence identity with the lactococcin 972 of Lactococcus lactis. Immunoblot analysis showed that Sil was secreted by S. iniae into the extracellular milieu. Purified recombinant Sil (rSil) exhibited a dose-dependent inhibitory effect on the growth of Bacillus subtilis but had no impact on the growths of other 16 Gram-positive bacteria and 10 Gram-negative bacteria representing 23 different bacterial species. Treatment of rSil by heating at 50°C abolished the activity of rSil. rSil bound to the surface of B. subtilis but induced no killing of the target cells. Cellular study revealed that rSil interacted with turbot (Scophthalmus maximus) head kidney monocytes and inhibited the innate immune response of the cells, which led to enhanced cellular infection of S. iniae. Antibody blocking of the extracellular Sil produced by S. iniae significantly attenuated the infectivity of S. iniae. Consistent with these in vitro observations, in vivo study showed that administration of turbot with rSil prior to S. iniae infection significantly increased bacterial dissemination and colonization in fish tissues. Taken together, these results indicate that Sil is a novel virulence-associated bacteriostatic and an immunoregulator that promotes S. iniae infection by impairing the immune defense of host fish.
Highlights
Streptococcus iniae is a Gram-positive bacterium and a major pathogen to a large number of farmed fish, notably rainbow trout, tilapia, sea bass, channel catfish, barramundi, Japanese flounder, and turbot [1,2]
In an in vitro cellular infection study, we found that when S. iniae was incubated with turbot head kidney monocytes (HKM) that had been pre-treated with recombinant Sil (rSil), the number of bacterial cells succeeded in infection was significantly increased (Fig. 5)
Sil is a structural homologue of lactococcin 972 but shares low sequence identity with the latter, which suggests a molecular basis for potential functional difference between these two proteins
Summary
Streptococcus iniae is a Gram-positive bacterium and a major pathogen to a large number of farmed fish, notably rainbow trout, tilapia, sea bass, channel catfish, barramundi, Japanese flounder, and turbot [1,2]. Heavy economic losses due to S. iniae infection have been reported in many countries including China [3,4]. A number of virulence-associated factors have been identified in S. iniae, including capsular polysaccharides, extracellular proteases, host factor-interacting proteins, and transcription regulators [5,6,7,8,9,10,11,12,13,14,15]. Bacteriocins are ribosomally synthesized peptides or proteins with antibacterial activity. They are produced by a wide range of bacteria against bacteria of the same or closely related species [16]. Class II bacteriocins are non-lantibiotic peptides that are further subdivided into several subclasses [20]
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