BACKGROUND
propionibacterium acnes is regarded as a commensal of human skin, but is also known to be involved in different infections such as acne vulgaris  <cit>  and infections with orthopaedic implants  <cit> .
treatment of acne vulgaris is complicated by bacterial resistance to commonly used antibiotics  <cit> .
p. acnes infections of joint prostheses are probably much more common than previously thought  <cit> , and treatment is complicated by biofilm formation on the foreign material  <cit> .
the complete genome of a p. acnes isolate was recently sequenced by brüggemann et al  <dig>  <cit> , showing that p. acnes possesses several putative virulence genes including hemolysins and co-hemolysins   <cit> .
the sequenced genome only contains one cryptic prophage, and in general there is limited knowledge about phages from p. acnes.
bacteriophages can enter two principally different life cycles, lytic or lysogenic.
in the lytic life cycle a bacteriophage attaches to the bacterial cell and injects its genetic material.
this genetic material is directly replicated, early and late phage genes are transcribed, proteins are translated, new phage particles are formed, and the bacterium is ultimately lysed to release the progeny.
in the lysogenic life cycle, most known phages integrate their genome into the host genome by specific attachment and recombination events.
there have been reports about phages existing as extrachromosomal circular or linear plasmid prophages, as a part of their lysogenic cycle  <cit> .
these so called prophages become integrated parts of the genome and are replicated together with the bacterial genome during cell division.
prophages account for much of the genetic diversity seen in bacteria and often carry genes that are beneficial for the bacteria, including toxins and other virulence factors  <cit> .
almost  <dig> years ago, it was reported that 18% of p. acnes isolates are carriers of bacteriophages  <cit> , but little is known about these phages and their potential impact on virulence.
studies of p. acnes phages have been undertaken to establish a phage typing system to distinguish between different types of p. acnes  <cit>  as an alternative to use fermentative and serological methods  <cit> .
many studies on phages have been done on other propionibacteria as propionibacterium freudenreichii mainly due to the research impact for dairy industry  <cit> .
recently farrar et al sequenced the first genome of a p. acnes lytic phage  <cit> .
the phage was classified as a siphovirus with a genome of  <dig>  bp encoding  <dig> putative genes.
characterization of phages from p. acnes gives a deeper understanding of the relationship between phages and bacteria, and may eventually lead to a new therapy to treat p. acnes infections.
in this study, we have induced, isolated, and characterized  <dig> temperate bacteriophages from different p. acnes isolates.
the phages are all classified as siphoviruses and can be divided into different groups based on dissimilarities in two genes encoding a putative major head protein and an amidase.
RESULTS
carriage of phages
since not much is known about the presence and carriage rate of bacteriophages in p. acnes, we investigated this in relation to both the site of isolation  and to biotype.
to investigate if p. acnes had prophages that could be induced to enter the lytic life cycle, we stimulated  <dig> different p. acnes isolates  with  <dig> μg/ml mitomycin c to induce prophages, followed by analysis of the plaque forming capacity of lysates on the noninduced parental isolates.
plaques were clear with well-defined edges and had a diameter of 6– <dig> mm.
bacteriophages could be induced in more than 70% of the isolates examined.
in this study, we have used different isolates of p. acnes  from deep tissue , and from the skin .
ad-isolates and as-isolates had a carriage-rate of  <dig> % respectively  <dig> % .
* phages from which high quality tem micrographs were obtained.
bacterial isolates that did not have any inducible phages using mitomycin c were screened for prophages using a pcr-based approach by amplification of the gene encoding a putative major head protein and reca as a positive control.
only one of the isolates, as <dig>  was positive in the major head pcr, indicating that only 1/ <dig> of these isolates has phages with similarities to the known major head gene .
the carriage rate of inducible temperate phages in the different biotypes was also examined.
biotypes had previously been determined by sequencing of reca .
biotype ia had a higher carriage rate than ib , as did biotype ii .
since biotype ib had a lower carriage rate compared to the other two biotypes, we compared the carriage rate of phages in the isolates determined as biotype ib between deep and superficial isolates of p. acnes.
the carriage rate in biotype ib was significantly higher in superficial isolates as compared with isolates from deep infections, since none of the deep tissue isolates biotyped as ib had inducible phages .
bacteriophage morphology and classification
the classification of bacteriophages is mainly based on phage morphology and the nature of the nucleic acid  <cit> .
though other classification systems such as sequence similarities within genes encoding structural proteins have been proposed, classification based on morphology and the nature of the nucleic acid is still the most accepted system  <cit> .
forty-nine of the bacteriophages were examined using negative staining and transmission electron microscopy.
all examined phages have an icosahedral head of approximately  <dig> nm in diameter, and a tail composed of  <dig> segments with a total length of 145– <dig> nm and a width of 9– <dig> nm.
the tail is non-contractile and appears flexible.
most phages have a visible base plate on the tail with attached spikes .
these morphological attributes warrant classification of the phages as siphoviridae.
thus, this morphology is identical to pa <dig>  <cit>  and very similar to the p. acnes phages studied by zierdt  <cit> .
also, the phages are very similar morphologically to siphoviruses isolated from other propionibacteria  <cit> .
this classification was further strengthen by amino acid sequence comparison of a part of a putative major head protein with other known phage proteins, using a blastp search against genbank.
the best hit was on gp <dig> from pa <dig>  and the second and third best hit on gp <dig> from mycobacterium phage che9d and gp <dig> from mycobacterium phage halo.
all these phages are classified as siphoviridae.
phage specificity
bacteriophages are generally quite restricted in their host range.
there are phages that can infect over bacterial species boundaries, but some phages are species specific and in many cases also specific for certain subgroups  within a species.
to determine how specific the isolated phages are, different p. acnes isolates were infected with  <dig> different phages .
bacterial isolates with inducible phages were generally easy to lyse, with plaque production in nearly 99%  of cases.
p. acnes isolate as <dig> was significantly more difficult to lyse than other isolates with inducible phages, and only 73%  of the examined phages caused plaque formation.
similarly, phage pad <dig> failed to lyse 19%  of the examined isolates.
in p. acnes isolates without any inducible phages, we could only observe plaque formation in 30%  of the cases.
we also examined the ability of phages to cause lysis in the recently sequenced p. acnes strain kpa <dig>  of biotype ib  <cit> .
this strain was lysed only by 40%  of the phage lysates.
the different phages are not specific to certain biotypes, but seem to be less lytic against biotype ii in isolates with inducible phages, since 23/ <dig> failures to infect and lyse the bacterial isolates were in isolates of biotype ii.
since all of the examined phages have identical morphology, protein pattern and in most cases very high similarity in genes encoding the putative major head protein and an amidase , we examined if phage host-range could be used as a possible tool to differentiate the phages.
based on the host-range analysis we choose p. acnes isolates kpa <dig>  ad <dig>  as <dig> and as <dig> to differentiate the phages and to divide them into different groups.
by using these four isolates, we could divide the phages into  <dig> separate groups .
sixteen of the examined phages could infect and lyse all four bacterial isolates .
there is a tendency that phages isolated from biotype ii and from skin can infect and lyse all four bacterial isolates, and are classified as host-range group pa i. also, the only phages that can lyse the prophage-free isolates ad <dig> and ad <dig>  are classified as belonging to the host-range group pa i. furthermore, none of nine selected phages  were able to infect and lyse p. avidum, p. granulosum or p. freudenreichii .
our results show that isolates lacking inducible temperate phages are more difficult to lyse using phages, than isolates carrying inducible temperate phages and that the phages are specific to p. acnes.
phylogenetic analysis of phage genes and proteins
a part of a putative major head gene was sequenced in all isolated phages.
the nucleotide sequence was aligned and a phylogenetic tree was reconstructed.
the phages could be divided in two distinctly divided groups with the already sequenced phage pa <dig> forming a third group .
the whole gene encoding a putative major head protein was sequenced in nine of these phages, selected based on their partial sequencing and site of isolation , and the amino acid sequences were aligned and a phylogenetic tree was reconstructed .
the p. acnes phage putative major head protein show high similarity to phages isolated from mycobacterium, lactococcus and streptococcus.
the already sequenced phage pa <dig> forms its own group, while pad <dig>  pad <dig>  pas <dig>  pas <dig>  pas <dig> and pad <dig> form a large group with similar sequences, even though pad <dig> is slightly different.
phages pas <dig> and pas <dig> form a third group and pad <dig> forms a fourth separated group.
this pattern is similar to the pattern seen when only using partial sequencing, except that pad <dig> now seems to form its own group.
we further sequenced the gene encoding a putative amidase in these nine phages .
the phage pattern is similar to what could be seen when aligning the major head protein.
however, pa <dig> is more closely related to pas <dig> and pas <dig> in this protein, and pad <dig> do not form its own group, but have high similarity to the large group of phages.
the closest phage similarity is to mycobacterium phage pg <dig> gp <dig>  but more closely related is a chromosomally encoded p. acnes amidase.
discussion
we show that more than 70% of the investigated p. acnes isolates are carriers of inducible phages, which is a significantly higher carriage rate of inducible phages than that reported by webster et al   <cit>  despite that similar methods were used and isolates were from both skin and deeper infections.
this is most likely due to differences in the geographic origin of isolates, and may reflect a difference in p. acnes susceptibility to mitomycin c or strain acquired resistance to phages.
there was no overall difference in carriage rate of inducible phages between p. acnes isolated from skin or deep infections, indicating that carriage of phage not necessary leads to an increased virulence of the host strain.
thus it appears that the phages studied here do not carry virulence factors, which is in concordance with the lack of putative virulence factors on the recently published genome of a p. acnes phage  <cit> .
it should be noted that at present, we only have information on the presence or absence of inducible phages.
however, future studies may very well reveal differences in the gene composition of phages isolated from skin and deep isolates.
we found that isolates of biotype ib have significantly lower rate of inducible phages as compared to isolates of ia and ii.
interestingly, none of the isolates of biotype ib in isolates from deep infections are carriers of inducible phages , while isolates from skin of biotype ib have a carriage rate of almost 73%.
the low carriage rate of inducible phages in ad-isolates typed as ib is further strengthen by the fact that none of these isolates have similarities to the phage major head gene as judged by failure of amplifying this gene in these isolates , thus indicating that these isolates do not have any inducible phages and not any prophages with similarities to known p. acnes phages.
thereby it seems like either only biotype ib without phages can cause infections, or that biotype ib loses their phages during infection.
if this is the case, it is uncertain why this only happens in biotype ib and not in biotype ia and ii.
the lower carriage rate in biotype ib is neither due to an increased resistance to phage entrance into the bacterial cell, since biotype ib is equally sensitive to phages as the other biotypes, as judged by additional file  <dig>  more research in this specific area is needed to understand this phenomenon.
when we examined the host isolate specificity of the phages, we found that isolate as <dig> was significantly more difficult to lyse than the other isolates with inducible phages.
however, as <dig> is still highly susceptible to pas <dig>  indicating that as <dig> might differ in a protein or receptor essential for most phages to be able to infect and lyse the bacterial cell.
opposite to this, we found that phage pad <dig> did not lyse other isolates as efficiently as other phages.
this difference could be caused by the phage favouring a lysogenic state, or that the phage receptor binding to the bacterial cell is less efficient compared to other p. acnes phages.
when we added phages to p. acnes isolates without inducible phages, we found that most of these isolates were resistant to phage-mediated lysis.
this is not surprising, since these isolates do not have inducible phages and thereby could have a mechanism that makes them resistant against phage infection.
the different phages were not specific to certain biotypes.
however, most of the failures to infect and cause lysis in the bacterial isolates were in biotype ii, possibly indicating that these isolates generally are more resistant to phages, or adapt more easily to them.
grouping of the phages based on host-range did not correlate with biotype, even though there is a tendency that phages from isolates of biotype ii are classified as host-range group pa i, thus indicating that phages from isolates of biotype ii may have broader infective capacity as compared to biotype ia and ib.
the phages also seem to be specific to p. acnes isolates, since none of the examined phages were able to infect and lyse p. avidum, p. granulosum or p. freudenreichii.
all phages examined were able to infect and lyse their non-induced parental host, thus indicating that the prophages do not confer superinfection immunity as many other known prophages do  <cit> .
this is in concordance with the fully sequenced phage pa <dig> that does not seem to have a repressor-like protein  <cit> .
this may benefit p. acnes phage evolution by more efficient gene transfer between prophages and free phages.
the phages examined could be divided into three-four groups using phylogenetic analysis of the gene encoding a putative major head protein and an amidase.
this differences in sequence stress the fact that the phages in the different groups very well may have other genetic dissimilarities, providing advantages for the phages.
it is also obvious that several phages have identical nucleotide sequences and perhaps should be considered as subspecies to certain groups of p. acnes phages.
the changes in the gene encoding the major head do not lead to a changed morphology, as seen when comparing pas <dig> with the other phage micrographs .
when examining the phage proteins on an sds-page, all phages have identical protein patterns with four protein bands .
this indicates that the phages have very similar structural proteins, even though some substitution in amino acids occur, and may reflect an evolutionary pressure to retain the structural proteins.
CONCLUSIONS
we have induced, identified, and characterized  <dig> temperate p. acnes phages, classified as siphoviruses on a morphological basis.
these phages are species specific and do not confer superinfection immunity.
these results give new insight into the relation between p. acnes and its phages, and contribute to a better understanding of the phage-host interaction.

 propionibacterium acnes is regarded as a commensal of human skin, but is also known to be involved in different infections such as acne vulgaris and infections with orthopaedic implants. treatment of acne vulgaris is complicated by bacterial resistance to commonly used antibiotics. p. acnes infections of joint prostheses are probably much more common than previously thought, and treatment is complicated by biofilm formation on the foreign material. the complete genome of a p. acnes isolate was recently sequenced by brüggemann et al, showing that p. acnes possesses several putative virulence genes including hemolysins and co-hemolysins. the sequenced genome only contains one cryptic prophage, and in general there is limited knowledge about phages from p. acnes. bacteriophages can enter two principally different life cycles, lytic or lysogenic. in the lytic life cycle a bacteriophage attaches to the bacterial cell and injects its genetic material. this genetic material is directly replicated, early and late phage genes are transcribed, proteins are translated, new phage particles are formed, and the bacterium is ultimately lysed to release the progeny. in the lysogenic life cycle, most known phages integrate their genome into the host genome by specific attachment and recombination events. there have been reports about phages existing as extrachromosomal circular or linear plasmid prophages, as a part of their lysogenic cycle. these so called prophages become integrated parts of the genome and are replicated together with the bacterial genome during cell division. prophages account for much of the genetic diversity seen in bacteria and often carry genes that are beneficial for the bacteria, including toxins and other virulence factors. almost years ago, it was reported that 18% of p. acnes isolates are carriers of bacteriophages, but little is known about these phages and their potential impact on virulence. studies of p. acnes phages have been undertaken to establish a phage typing system to distinguish between different types of p. acnes as an alternative to use fermentative and serological methods. many studies on phages have been done on other propionibacteria as propionibacterium freudenreichii mainly due to the research impact for dairy industry. recently farrar et al sequenced the first genome of a p. acnes lytic phage. the phage was classified as a siphovirus with a genome of bp encoding putative genes. characterization of phages from p. acnes gives a deeper understanding of the relationship between phages and bacteria, and may eventually lead to a new therapy to treat p. acnes infections. in this study, we have induced, isolated, and characterized temperate bacteriophages from different p. acnes isolates. the phages are all classified as siphoviruses and can be divided into different groups based on dissimilarities in two genes encoding a putative major head protein and an amidase.  carriage of phages since not much is known about the presence and carriage rate of bacteriophages in p. acnes, we investigated this in relation to both the site of isolation and to biotype. to investigate if p. acnes had prophages that could be induced to enter the lytic life cycle, we stimulated different p. acnes isolates with μg/ml mitomycin c to induce prophages, followed by analysis of the plaque forming capacity of lysates on the noninduced parental isolates. plaques were clear with well-defined edges and had a diameter of 6– mm. bacteriophages could be induced in more than 70% of the isolates examined. in this study, we have used different isolates of p. acnes from deep tissue, and from the skin. ad-isolates and as-isolates had a carriage-rate of % respectively %. * phages from which high quality tem micrographs were obtained. bacterial isolates that did not have any inducible phages using mitomycin c were screened for prophages using a pcr-based approach by amplification of the gene encoding a putative major head protein and reca as a positive control. only one of the isolates, as was positive in the major head pcr, indicating that only 1/ of these isolates has phages with similarities to the known major head gene. the carriage rate of inducible temperate phages in the different biotypes was also examined. biotypes had previously been determined by sequencing of reca. biotype ia had a higher carriage rate than ib, as did biotype ii. since biotype ib had a lower carriage rate compared to the other two biotypes, we compared the carriage rate of phages in the isolates determined as biotype ib between deep and superficial isolates of p. acnes. the carriage rate in biotype ib was significantly higher in superficial isolates as compared with isolates from deep infections, since none of the deep tissue isolates biotyped as ib had inducible phages. bacteriophage morphology and classification the classification of bacteriophages is mainly based on phage morphology and the nature of the nucleic acid. though other classification systems such as sequence similarities within genes encoding structural proteins have been proposed, classification based on morphology and the nature of the nucleic acid is still the most accepted system. forty-nine of the bacteriophages were examined using negative staining and transmission electron microscopy. all examined phages have an icosahedral head of approximately nm in diameter, and a tail composed of segments with a total length of 145– nm and a width of 9– nm. the tail is non-contractile and appears flexible. most phages have a visible base plate on the tail with attached spikes. these morphological attributes warrant classification of the phages as siphoviridae. thus, this morphology is identical to pa and very similar to the p. acnes phages studied by zierdt. also, the phages are very similar morphologically to siphoviruses isolated from other propionibacteria. this classification was further strengthen by amino acid sequence comparison of a part of a putative major head protein with other known phage proteins, using a blastp search against genbank. the best hit was on gp from pa and the second and third best hit on gp from mycobacterium phage che9d and gp from mycobacterium phage halo. all these phages are classified as siphoviridae. phage specificity bacteriophages are generally quite restricted in their host range. there are phages that can infect over bacterial species boundaries, but some phages are species specific and in many cases also specific for certain subgroups within a species. to determine how specific the isolated phages are, different p. acnes isolates were infected with different phages. bacterial isolates with inducible phages were generally easy to lyse, with plaque production in nearly 99% of cases. p. acnes isolate as was significantly more difficult to lyse than other isolates with inducible phages, and only 73% of the examined phages caused plaque formation. similarly, phage pad failed to lyse 19% of the examined isolates. in p. acnes isolates without any inducible phages, we could only observe plaque formation in 30% of the cases. we also examined the ability of phages to cause lysis in the recently sequenced p. acnes strain kpa of biotype ib. this strain was lysed only by 40% of the phage lysates. the different phages are not specific to certain biotypes, but seem to be less lytic against biotype ii in isolates with inducible phages, since 23/ failures to infect and lyse the bacterial isolates were in isolates of biotype ii. since all of the examined phages have identical morphology, protein pattern and in most cases very high similarity in genes encoding the putative major head protein and an amidase, we examined if phage host-range could be used as a possible tool to differentiate the phages. based on the host-range analysis we choose p. acnes isolates kpa ad as and as to differentiate the phages and to divide them into different groups. by using these four isolates, we could divide the phages into separate groups. sixteen of the examined phages could infect and lyse all four bacterial isolates. there is a tendency that phages isolated from biotype ii and from skin can infect and lyse all four bacterial isolates, and are classified as host-range group pa i. also, the only phages that can lyse the prophage-free isolates ad and ad are classified as belonging to the host-range group pa i. furthermore, none of nine selected phages were able to infect and lyse p. avidum, p. granulosum or p. freudenreichii. our results show that isolates lacking inducible temperate phages are more difficult to lyse using phages, than isolates carrying inducible temperate phages and that the phages are specific to p. acnes. phylogenetic analysis of phage genes and proteins a part of a putative major head gene was sequenced in all isolated phages. the nucleotide sequence was aligned and a phylogenetic tree was reconstructed. the phages could be divided in two distinctly divided groups with the already sequenced phage pa forming a third group. the whole gene encoding a putative major head protein was sequenced in nine of these phages, selected based on their partial sequencing and site of isolation, and the amino acid sequences were aligned and a phylogenetic tree was reconstructed. the p. acnes phage putative major head protein show high similarity to phages isolated from mycobacterium, lactococcus and streptococcus. the already sequenced phage pa forms its own group, while pad pad pas pas pas and pad form a large group with similar sequences, even though pad is slightly different. phages pas and pas form a third group and pad forms a fourth separated group. this pattern is similar to the pattern seen when only using partial sequencing, except that pad now seems to form its own group. we further sequenced the gene encoding a putative amidase in these nine phages. the phage pattern is similar to what could be seen when aligning the major head protein. however, pa is more closely related to pas and pas in this protein, and pad do not form its own group, but have high similarity to the large group of phages. the closest phage similarity is to mycobacterium phage pg gp but more closely related is a chromosomally encoded p. acnes amidase. discussion we show that more than 70% of the investigated p. acnes isolates are carriers of inducible phages, which is a significantly higher carriage rate of inducible phages than that reported by webster et al despite that similar methods were used and isolates were from both skin and deeper infections. this is most likely due to differences in the geographic origin of isolates, and may reflect a difference in p. acnes susceptibility to mitomycin c or strain acquired resistance to phages. there was no overall difference in carriage rate of inducible phages between p. acnes isolated from skin or deep infections, indicating that carriage of phage not necessary leads to an increased virulence of the host strain. thus it appears that the phages studied here do not carry virulence factors, which is in concordance with the lack of putative virulence factors on the recently published genome of a p. acnes phage. it should be noted that at present, we only have information on the presence or absence of inducible phages. however, future studies may very well reveal differences in the gene composition of phages isolated from skin and deep isolates. we found that isolates of biotype ib have significantly lower rate of inducible phages as compared to isolates of ia and ii. interestingly, none of the isolates of biotype ib in isolates from deep infections are carriers of inducible phages, while isolates from skin of biotype ib have a carriage rate of almost 73%. the low carriage rate of inducible phages in ad-isolates typed as ib is further strengthen by the fact that none of these isolates have similarities to the phage major head gene as judged by failure of amplifying this gene in these isolates, thus indicating that these isolates do not have any inducible phages and not any prophages with similarities to known p. acnes phages. thereby it seems like either only biotype ib without phages can cause infections, or that biotype ib loses their phages during infection. if this is the case, it is uncertain why this only happens in biotype ib and not in biotype ia and ii. the lower carriage rate in biotype ib is neither due to an increased resistance to phage entrance into the bacterial cell, since biotype ib is equally sensitive to phages as the other biotypes, as judged by additional file more research in this specific area is needed to understand this phenomenon. when we examined the host isolate specificity of the phages, we found that isolate as was significantly more difficult to lyse than the other isolates with inducible phages. however, as is still highly susceptible to pas indicating that as might differ in a protein or receptor essential for most phages to be able to infect and lyse the bacterial cell. opposite to this, we found that phage pad did not lyse other isolates as efficiently as other phages. this difference could be caused by the phage favouring a lysogenic state, or that the phage receptor binding to the bacterial cell is less efficient compared to other p. acnes phages. when we added phages to p. acnes isolates without inducible phages, we found that most of these isolates were resistant to phage-mediated lysis. this is not surprising, since these isolates do not have inducible phages and thereby could have a mechanism that makes them resistant against phage infection. the different phages were not specific to certain biotypes. however, most of the failures to infect and cause lysis in the bacterial isolates were in biotype ii, possibly indicating that these isolates generally are more resistant to phages, or adapt more easily to them. grouping of the phages based on host-range did not correlate with biotype, even though there is a tendency that phages from isolates of biotype ii are classified as host-range group pa i, thus indicating that phages from isolates of biotype ii may have broader infective capacity as compared to biotype ia and ib. the phages also seem to be specific to p. acnes isolates, since none of the examined phages were able to infect and lyse p. avidum, p. granulosum or p. freudenreichii. all phages examined were able to infect and lyse their non-induced parental host, thus indicating that the prophages do not confer superinfection immunity as many other known prophages do. this is in concordance with the fully sequenced phage pa that does not seem to have a repressor-like protein. this may benefit p. acnes phage evolution by more efficient gene transfer between prophages and free phages. the phages examined could be divided into three-four groups using phylogenetic analysis of the gene encoding a putative major head protein and an amidase. this differences in sequence stress the fact that the phages in the different groups very well may have other genetic dissimilarities, providing advantages for the phages. it is also obvious that several phages have identical nucleotide sequences and perhaps should be considered as subspecies to certain groups of p. acnes phages. the changes in the gene encoding the major head do not lead to a changed morphology, as seen when comparing pas with the other phage micrographs. when examining the phage proteins on an sds-page, all phages have identical protein patterns with four protein bands. this indicates that the phages have very similar structural proteins, even though some substitution in amino acids occur, and may reflect an evolutionary pressure to retain the structural proteins.  we have induced, identified, and characterized temperate p. acnes phages, classified as siphoviruses on a morphological basis. these phages are species specific and do not confer superinfection immunity. these results give new insight into the relation between p. acnes and its phages, and contribute to a better understanding of the phage-host interaction. 
however, there was a significant lower carriage rate of phages in p. acnes biotype ib, mostly attributed to the low carriage rate of inducible phages in biotype ib isolated from deep tissue. there was no difference in carriage rate of phages between p. acnes isolates from deep infections and isolates from skin. our findings indicate that temperate phages are common in p. acnes, and that they are a genetically and functionally homogeneous group of siphoviruses. propionibacterium acnes is a commensal of human skin but is also known to be involved in certain diseases, such as acne vulgaris and infections of orthopaedic implants. treatment of these conditions is complicated by increased resistance to antibiotics and/or biofilm formation of p. acnes bacteria. p. acnes can be infected by bacteriophages, but until recently little has been known about these viruses. the aim of this study was to identify and characterize inducible phages from p. acnes on a genetic and morphological basis. more than 70% of p. acnes isolates investigated have inducible phages, classified morphologically as siphoviruses. most phages have a strong lytic activity against all p. acnes isolates with inducible phages, but have less lytic activity against isolates that have no prophages. phages only infected and lysed p. acnes and not other closely related propionibacteria. the phages are specific for p. acnes and do not seem to confer superinfection immunity. finally, sequencing of two phage genes encoding a putative major head protein and an amidase and showed that the phages could be divided into different groups on a genetic basis. all phages could infect and lyse their non-induced parental host, indicating that these prophages do not confer superinfection immunity.
 propionibacterium acnes is a commensal of human skin but is also known to be involved in certain diseases, such as acne vulgaris and infections of orthopaedic implants. treatment of these conditions is complicated by increased resistance to antibiotics and/or biofilm formation of p. acnes bacteria. p. acnes can be infected by bacteriophages, but until recently little has been known about these viruses. the aim of this study was to identify and characterize inducible phages from p. acnes on a genetic and morphological basis.  more than 70% of p. acnes isolates investigated have inducible phages, classified morphologically as siphoviruses. the phages have a head of nm in diameter and a tail of 145– nm in length and 9– nm in width. there was no difference in carriage rate of phages between p. acnes isolates from deep infections and isolates from skin. however, there was a significant lower carriage rate of phages in p. acnes biotype ib, mostly attributed to the low carriage rate of inducible phages in biotype ib isolated from deep tissue. most phages have a strong lytic activity against all p. acnes isolates with inducible phages, but have less lytic activity against isolates that have no prophages. phages only infected and lysed p. acnes and not other closely related propionibacteria. all phages could infect and lyse their non-induced parental host, indicating that these prophages do not confer superinfection immunity. the phages have identical protein pattern as observed on sds-page. finally, sequencing of two phage genes encoding a putative major head protein and an amidase and showed that the phages could be divided into different groups on a genetic basis.  our findings indicate that temperate phages are common in p. acnes, and that they are a genetically and functionally homogeneous group of siphoviruses. the phages are specific for p. acnes and do not seem to confer superinfection immunity. 