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The nucleotide sequence of 16S rRNA gene of strainMYB41-22 has been deposited in the NCBI with accessionnumber KC430113. Based on morphology, physiology and 16S rRNA gene phylogenetic tree analysis (Figure 1), MYB41-22 was identified as B. cereus MYB41-22.It was rod-shaped, spore-forming and belonged to theGram-positive class of bacteria (Figure 2). This psychrophilicbacterium could grow between 4–42 ℃, with optimumgrowth between 15 and 20 ℃.
Figure 1. Phylogenetic analysis of strain MYB41-22 based on 16S rRNA gene sequences available from the NCBI GenBank database.
Figure 2. Scanning electron micrograph of strain MYB41-22. The scale bar marked automatically represents 5 μm.
A phage infecting B. cereus MYB41-22 was isolated and named VMY22. Clear plaques of 2 mm were observedafter incubation of phage with bacteria at 4 ℃ for36 h on a double-layer plate. Cold-active bacteriophagescan infect and reproduce at temperatures ≤ 4 ℃, so it wastherefore recognized as a typical cold-active phage. TEMexamination revealed that VMY22 has an icosahedralhead wh ich is 31.9 nm in diameter and 59.2 nm in length, and a tail that is 43.2 nm in length (Figure 3), a typicalmorphology characteristic of the family Podoviridae.
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B. cereus MYB41-22 strains could produce clearplaques after infection with VMY22 when incubated at 4–37 ℃, and showed maximal phage production at 15–20 ℃. Table 1 shows that the optimal MOI of VMY22was 0.1. Figure 4A shows that VMY22 could lyze almost100% of MYB41-22 cells in 15 min. A one-step growthcurve of VMY22 on MYB41-22 showed that the latentperiod was 70 min and the burst period was 70 min, with a burst size of about 78 bacteriophage particles per infectedcell (Figure 4B).
Figure 4. Characterization of VMY22. (A) The adsorption of phage VMY22 to host cells. (B) One-step growth curve of phage VMY22 on B. cereus MYB41-22. Values are the mean of three determinations. (C) Thermostability of VMY22 at 20 ℃ (open diamond), 40 ℃ (closed diamond), 50 ℃ (open triangle), 60 ℃ (closed triangle) and 70 ℃ (open circle). Values are the mean of three determinations. (D) pH sensitivity of VMY22. Values are the mean of three determinations.
Table 1. The optimal multiplicity of infection of B. cereus MYB41-22 by VMY22
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Thermolability was the most salient physical featureof cold-active phage VMY22 (Figure 4C) (Wells and Deming, 2006). The VMY22 could tolerant more than60 min at 20 ℃ with minimal losses, and decreased rapidlywhen the temperature exceeded 60 ℃. VMY22 wasshown to be stable between pH 5.0 and 9.0, with maximumsurvival at pH 8.0 (Figure 4D).
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Sensitivity to organic solvent and detergents were tested, and VMY22 was found insensitive to chloroform; itretained more than 80% infection activity after exposureto 15% chloroform. However, VMY22 infectivity wascompletely destroyed by treatment with protease K or by incubation with SDS or Triton X-100. The insensitivityto chloroform suggested that the capsid of VMY22 didnot contain lipids (Wells and Deming, 2006).
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VMY22 DNA was extracted and digested with restrictionendonucleases EcoR Ⅰ, BamH Ⅰ, Hind Ⅲ and Pst Ⅰ (Figure 5A). The results showed that VMY22 wasdsDNA with an estimated size of 18–20 kb. Purifiedphage particles were analyzed by SDS-PAGE (Figure 5B). Three main protein bands were observed, with molecularmasses 22 kDa, 52 kDa and 57 kDa. Based on thegenome sequence of VMY22, these could correspond tolower collar protein, Podovirus_Gp16 superfamily protein and tail protein, respectively (data not shown). Themost abundant band was the 52 kDa protein, which maybe the major coat protein of cold-active phage VMY22.
Figure 5. Analysis of VMY22 DNA and protein. (A) Restriction endonuclease digestion patterns of VMY22. M1: DNA marker; 1: VMY22 DNA digested with EcoR Ⅰ; 2: VMY22 DNA digested with BamH Ⅰ; 3: VMY22 DNA digested with Hind Ⅲ; 4: VMY22 DNA digested with Pst Ⅰ; M2: DNA marker; 5: Purified VMY22 DNA. (B) SDSPAGE of purified VMY22. M: protein marker; 1: Purified VMY22.