Complete Genome Sequence of Bacteriophage Deep-Blue Infecting Emetic Bacillus cereus

Getting Outside the Cell: Versatile Holin Strategies Used by Distinct Phages to Leave Their Bacillus thuringiensis Host

Holins are small transmembrane proteins involved in the final stage of the lytic cycle of double-stranded DNA (dsDNA) phages. They cooperate with endolysins to achieve bacterial lysis, thereby releasing the phage progeny into the extracellular environment. Besides their role as membrane permeabilizers, allowing endolysin transfer and/or activation, holins also regulate the lysis timing. In this work, we provide functional characterization of the holins encoded by three phages targeting the Bacillus cereus group. The siphovirus Deep-Purple has a lysis cassette in which holP30 and holP33 encode two proteins displaying holin properties, including a transmembrane domain. The holin genes were expressed in Escherichia coli and induced bacterial lysis, with HolP30 being more toxic than HolP33. In Bacillus thuringiensis, the simultaneous expression of both holins was necessary to observe lysis, suggesting that they may interact to form functional pores. The myoviruses Deep-Blue and Vp4 both encode a single candidate holin (HolB and HolV, respectively) with two transmembrane domains, whose genes are not located near the endolysin genes. Their function as holin proteins was confirmed as their expression in E. coli impaired cell growth and viability. The HolV expression in B. thuringiensis also led to bacterial lysis, which was enhanced by coexpressing the holin with its cognate endolysin. Despite similar organizations and predicted topologies, truncated mutants of the HolB and HolV proteins showed different toxicity levels, suggesting that differences in amino acid composition influence their lysis properties. IMPORTANCE The phage life cycle ends with the host cell lysis, thereby releasing new virions into the environment for the next round of bacterial infection. Nowadays, there is renewed interest in phages as biocontrol agents, primarily due to their ability to cause bacterial death through lysis. While endolysins, which mediate peptidoglycan degradation, have been fairly well described, the pore-forming proteins, referred to as holins, have been extensively characterized in only a few model phages, mainly infecting Gram-negative bacteria. In this work, we characterized the holins encoded by a siphovirus and two myoviruses targeting members of the Gram-positive Bacillus cereus group, which comprises closely related species, including the well-known Bacillus anthracis, B. cereus sensu stricto, and Bacillus thuringiensis. Overall, this paper provides the first experimental characterization of holins encoded by B. cereus phages and reveals versatile lysis mechanisms used by these phages.

Characterization of PlyB221 and PlyP32, Two Novel Endolysins Encoded by Phages Preying on the Bacillus cereus Group

Endolysins are phage-encoded enzymes implicated in the breaching of the bacterial cell wall at the end of the viral cycle. This study focuses on the endolysins of Deep-Blue (PlyB221) and Deep-Purple (PlyP32), two phages preying on the Bacillus cereus group. Both enzymes exhibit a typical modular organization with an enzymatically active domain (EAD) located in the N-terminal and a cell wall binding domain (CBD) in the C-terminal part of the protein. In silico analysis indicated that the EAD domains of PlyB221 and PlyP32 are endowed with peptidase and muramidase activities, respectively, whereas in both proteins SH3 domains are involved in the CBD. To evaluate their antimicrobial properties and binding specificity, both endolysins were expressed and purified. PlyB221 and PlyP32 efficiently recognized and lysed all the tested strains from the B. cereus group. Biochemical characterization showed that PlyB221 activity was stable under a wide range of pHs (5-9), NaCl concentrations (up to 200 mM), and temperature treatments (up to 50 °C). Although PlyP32 activity was less stable than that of PlyB221, the endolysin displayed high activity at pH 6-7, NaCl concentration up to 100 mM and the temperature treatment up to 45 °C. Overall, PlyB221 and PlyP32 display suitable characteristics for the development of biocontrol and detection tools.

Novel motB as a potential predictive tool for identification of B. cereus, B. thuringiensis and differentiation from other Bacillus species by triplex real-time PCR

Quantitative triplex real-time PCR (qPCR) offers an alternative method for detection of bacterial contamination. It provides quantitation of the number of gene copies. In our study, we established a qPCR assay to detect and quantify the specificity towards Bacillus cereus and B. thuringiensis. The assay was designed to detect a 280 bp fragment of motB gene encoding the flagellar motor protein, specific for detection of B. cereus and B. thuringiensis, excluding other group species B. pseudomycoides, B. mycoides and B. weihenstephanensis. Specificity of the assay was confirmed with 111 strains belonging to Bacillus cereus group and performed against 58 B. cereus, 50 B. thuringiensis, 3 other Bacillus bacteria and 9 non-Bacillus bacteria. Detection limit was determined for each assay. Direct analysis of samples revealed the specificity towards identification and characterization of B. cereus group cultured in nutrient media. Based on results, it was observed that motB showed 97% specificity towards B. cereus strains, 98% for B. thuringiensis but other B. cereus group showed less sensitivity (0%), thus, provides an efficient tool to identify B. cereus and B. thuringiensis. Further, environmental and food samples do not require band isolation, re-amplification or sequence identification. Thus, reducing the time and cost of analysis.