9.B.139 The pmf-dissipating Cannibalism Toxin SdpC (SdpC) Family
Bacillus subtilis SDP (SdpC) is a peptide toxin that kills cells outside the biofilm to support continued growth. Purified SDP, like endogenously produced SDP, delays sporulation, and the SdpI immunity protein (TC# 9.A.32.1.1) confers SDP resistance. SDP kills a variety of Gram-positive Firmicutes as well as Escherichia coli with a compromised outer membrane, suggesting that it participates in defence of the B. subtilis biofilm against cannibalism and kills other Gram-positive bacteria. Fluorescence microscopy revealed that the effect of SDP on cells differs from that of nisin, nigericin, valinomycin and vancomycin-KCl but resembles that of CCCP, DNP and azide by rapidly collapsing the pmf. This triggers the slower process of autolysis (Lamsa et al. 2012). This secondary consequence of SDP treatment is not required for cell death since an autolysin-defective lytC, lytD, lytE, lytF strain failed to lyse but is nevertheless killed by SDP. The mechanism of pmf collapse is unknown, but collapsing it is an ideal mechanism for a toxin involved in cannibalism and biofilm defence, since this would incapacitate neighbouring cells by inhibiting motility and secretion of proteins and toxins and by inducing autolysis, thereby releasing nutrients that promote biofilm growth. May be distantly related to proponicin F (255 aas) and other members of TC family 9.B.86.