Lysogenicity in streptomycetes

Characterization and genome sequencing of two Propionibacterium acnes phages displaying pseudolysogeny

Background

Propionibacterium acnes is a Gram positive rod inhabiting the human skin that also infects orthopaedic implants and is associated with acne vulgaris. Previously, one lytic bacteriophage, PA6, from P. acnes has been sequenced and partially characterized. We recently isolated several inducible phages from P. acnes classified as Siphoviruses based on morphology and partial genome sequencing.

Results

In this study we sequenced the inducible P. acnes phages PAD20 and PAS50, isolated from deep infection and from skin, respectively. The genomes of PAD20 and PAS50 are 29,074 and 29,017 bp, respectively, compared with the 29,739 bp of PA6. The phage genomes have 87.3-88.7% nucleotide sequence identity. The genes are divided into clusters with different levels of similarity between the phages. PAD20 and PAS50 share four genes encoding identical amino acid sequences. Some deletions and insertions in the genomes have occurred, resulting in lack of genes, frame shifts, and possible regulatory differences. No obvious virulence factor gene candidates were found. The phages are inducible, but bacteria can be cured of phages by serial colony isolations and lose their phages during stationary phase, but are still sensitive to new phage infections. Construction of a phylogenetic tree based on more than 459 phage genomes, suggested that P. acnes phages represent a new lineage of Siphoviruses.

Conclusions

The investigated P. acnes Siphovirus genomes share a high degree of homology to other P. acnes phages sequenced, but not to genomes of other phages isolated from Propionibacteria. The phage genomes are not integrated in the bacterial genome, but instead, most likely have a pseudolysogenic life cycle.

Isolation and characterization of two types of actinophages infecting Streptomyces scabies MR13

Two types of actinophages, phi S and phi L, were isolated from soil samples by using Streptomyces scabies MR13, a potato scab pathogen, as an indicator strain. The phages were partially characterized according to their physicochemical properties, plaques and particles morphology and their host-range. The host-range of these phages was narrow for phi S and wide for phi L. The adsorption rate constants of the phi S and phi L were 3.44 x 10(-9) and 3.18 x 10(-9) ml/min, and their burst sizes were 1.61 and 3.75 virions, respectively. One-step growth indicated that phi S and phi L have a latent period of 30 min followed by a rise period of 30 min. The temperate character of these phages was tested in other isolates of Streptomyces. Four of the phages (phi SS3, phi SS12, phi SS13 and phi SS17) were identified as temperate phages, since they were able to lysogenize SS3, SS12, SS13 and SS17. phi SS3, phi SS12 and phi SS13 were homoimmune, and they were heteroimmune with respect to phi SS17. The restriction barriers of lysogenic isolates (SS12, SS13 and SS17) interfered with the blockage of plaques formation by phages (phi SS12, phi SS13 or phi SS17) propagated on them, about 75% of lysogenic isolates had restriction systems. The exposure of the lysogenic isolates (SS12, SS13 and SS17) to UV-irradiation prevented the possible restriction barriers of these isolates, and these barriers could be overcome.

Isolation and characterization of two types of actinophage infecting Streptomyces scabies

Two types of actinophages, phi S and phi L, were isolated from soil samples by using Streptomyces scabies, a potato scab pathogen, as indicator strain. The phages were partially characterized according to their physicochemical properties, plaques and particles morphology, and their host range; this varied from narrow (for phi S) to wide (for phi L). The adsorption rate constants of the phi S and phi L were 3.44 and 3.18 pL/min, and their burst sizes were 1.61 and 3.75 virions per mL, respectively. One-step growth indicated that phi S and phi L have a latent period of 1/2 h followed by a rise period of 1/2 h. The temperate character of these phages was tested in other isolates of Streptomyces. Four of the phages (phi SS3, phi SS12, phi SS13 and phi SS17) were identified as temperate phages, since they were able to lysogenize SS3, SS12, SS13 and SS17. phi SS3, phi SS12 and phi SS13 were homoimmune, and they were heteroimmune with respect to phi SS17. The restriction barriers of lysogenic isolates (SS12, SS13 and SS17) interfered with the blockage of plaque formation by phages (phi SS12, phi SS13 or phi SS17) propagated on them, about 75% of lysogenic isolates had restriction systems. The exposure of the lysogenic isolates (SS12, SS13 and SS17) to UV-irradiation prevented the possible restriction barriers of these isolates so that these barriers could be overcome.

Advances in Streptomyces coelicolor genetics

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Isolation and characterization of a lysogenic strain of Nocardia erythropolis

A stable phage-carrying strain of Nocardia erythropolis was isolated from an infection with the nocardiophage phiEC. Growth of the strain in phage-specific antiserum for 48 hr produced cured organisms at a frequency of about 0.5%. Spontaneous curing, determined by serial single-colony isolations, was less than 0.4%. The strain could not be infected by phage phiEC nor by a closely related phage, phiC, although the cells were able to adsorb these phages. In cell populations, a frequency of 2.5 x 10(-4) cells spontaneously induced. The growth rate of the strain was comparable to that of the uninfected wild-type N. erythropolis. Ultraviolet irradiation or treatment with mitomycin C induced the strain to produce larger numbers of phage. It was concluded that the isolated strain was lysogenic.