Heads or tails? An insight into the nature of antibacterial proteins of entomopathogenic bacteria.

Microbial pesticides derived from entomopathogenic bacteria occupy the greater share of the global biopesticides market. Since the late 1990s, the exploration of such bacterial species has intensified and expanded beyond the well-described Bacillus thuringiensis to other spore-forming Gram-positive bacteria including Brevibacillus laterosporus and Lysinibacillus sphaericus. Among the non-spore-forming Gram-negative bacteria Chromobacterium spp., Serratia spp., and Pseudomonas spp. are of interest for their insect active properties. Unfortunately, all these bacterial species are susceptible to the effects of some putative antibacterial proteins (ABPs), including bacteria-eating viruses (phages), encapsulins, and phage tail-like bacteriocins (PTLBs). Phage-derived bacteriocins can be either contractile phage tail-like (R-type) or non-contractile tail-like (F-type) structures. Encapsulins, a class of high molecular-weight (HMW) putative ABPs resembling phage capsid or head-like structures have been identified in different bacterial species. These putative ABPs are known to pose a serious threat to the mass production of these useful bacteria by causing a collapse of the culture through lysis of the cells. For instance, B. thuringiensis specific phages can cause production batch failures ranging from 15 to 100%. Recently, the stunted growth of the insect pathogenic B. laterosporus strains 1821L and 1951 from New Zealand has been associated with production of HMW putative ABPs of 31.4 kDa, Linocin M18, and ∼48 kDa, phage-like element PBSX-protein XkdK. This article provides an overview of the biological attributes of the putative ABPs and their implications in harnessing the insecticidal potential of B. thuringienesis and the emerging biocontrol agent B. laterosporus.