The extracellular metalloprotease of Vibrio tubiashii directly inhibits its extracellular haemolysin.

Vibrio tubiashii is a re-emerging pathogen of molluscs that secretes a variety of extracellular products (ECPs), including a metalloprotease and a cytolysin/haemolysin. Previously, we reported that the V. tubiashii haemolysin locus consists of two ORFs (vthB and vthA), similar to that of the homologous haemolysin genes (vvhB and vvhA) found in Vibrio vulnificus. Here, we demonstrate that the concomitant expression of both V. tubiashii genes resulted in significantly higher haemolytic activity than the vthA gene alone. In addition, we created a VthAB- mutant strain of V. tubiashii that was virtually devoid of haemolytic activity in liquid media. Interestingly, significant production of an additional haemolysin(s) was observed on blood plates. Moreover, we have previously reported that in V. tubiashii, proteolytic and haemolytic activities are inversely produced during bacterial growth. Here, we study this correlation in more detail and present evidence that the VtpA metalloprotease inhibits haemolytic activity in culture supernatants, based on the following evidence: (i) loss of metalloprotease activity by either mutation or EDTA inhibition resulted in increased haemolytic activity; (ii) overexpression of the vtpA gene resulted in decreased haemolytic activity; (iii) purified VtpA metalloprotease directly diminished haemolytic activity by purified VthA haemolysin. Importantly, we found not only that vthAB gene expression remained high throughout growth but also that there were no dramatic differences in vthAB gene expression between the parent and VtpA- mutant strains. Thus, our results strongly suggest that the V. tubiashii metalloprotease directly targets its haemolysin.