BACKGROUND
acinetobacter baumanni is a non-fermentative, aerobic, gram-negative bacillus, and is an opportunistic pathogen with global distribution.
it is frequently found in elderly patients and cancer patients with compromised immune function, especially in intensive care units.
with the use of broad-spectrum antibiotics, immunosuppressive drugs, and glucocorticoids, a. baumannii  has become a major nosocomial pathogen species
<cit> .
multidrug-resistant , extensively drug-resistant , and pan drug-resistant  a. baumannii strains are increasingly prevalent
<cit> .
mdr-ab refers to a. baumannii strains that are resistant to at least three of the following five types of antimicrobial agents: cephalosporins, carbapenems, β-lactamase inhibitors , fluoroquinolones, and aminoglycosides
<cit> .
bacteriophage therapy is a potential alternative treatment for multidrug-resistant bacterial infections
<cit> .
a bacteriophage is a bacterial virus that can lyse and kill the host cell.
phage-related studies have gone through three stages.
félix d’herelle discovered bacteriophage for the treatment of bacterial infections in 1917
<cit> .
after the emergence of antibiotics in the 1940s, phages were seldom used for therapeutic purposes, and mainly functioned as molecular and genetic research tools.
with the recent emergence of multidrug-resistant bacteria, however, there has been renewed interest in methods of phage therapy
<cit> .
in this study we isolated a lytic bacteriophage ime-ab <dig>  and compared biological characteristics and genomic sequence with other acinetobacter baumannii phages.
the genomes of a. baumannii phages ime-ab <dig>  a. baumannii ab <dig>  a. baumannii ap <dig>  and a. baumannii phiac- <dig> were compared thoroughly in this study.
to our knowledge this is the first report of comparison of the characteristics and complete genome sequence of acinetobacter baumannii bacteriophages.
a clear understanding of its genes is conducive to the treatment of multidrug-resistant a. baumannii in the future.
RESULTS
isolation of a lytic bacteriophage against multidrug-resistant a. baumannii
a. baumannii strain mdr-ab <dig>  isolated from a sputum sample of a patient with pneumonia at pla hospital  <dig>  was resistant to multiple antibiotics .
the bacteria was used to screen bacteriophages in sewage samples from pla hospital  <dig>  the isolated phage was designated as vb_abam-ime-ab <dig> following the recommendation by international committee on taxonomy of viruses in phage nomenclature
<cit> .
the pahge ime-ab <dig> could form clear plaques in the double-layer assay and clear its host’s suspension in just 4 hours , indicating that it is a lytic phage.
in order to check the development of resistance, we had extended the period of the experiment to 24 h. the result indicated that the bacterial suspension became turbid finally.
the final suspension was plated on solid lb culture and then some single bacterial clones were picked to be used for 16 s rdna sequencing.
the sequences of 16 s rdna proved that the final suspension was a. baumannii that developed resistance to ime-ab <dig>  the phage particles were concentrated with peg <dig> and then purified with a cesium chloride gradients density to a titer of 1 ×  <dig> pfu/ml.
observation under an electron microscope showed that the phage ime-ab <dig> consisted of an icosahedral head and a contractile tail.
the total length of the phage from the top of the head to the bottom of the tail was about 160 nm, with the head measuring approximately  <dig>  nm, and the tail about 90 nm.
this morphology suggested that phage ime-ab <dig> should be classified as a member of the myoviridae family .
among the  <dig> clinical strains of a. baumannii, only three strains of a. baumannii  could be lysed by the phage ime-ab <dig>
a. baumannii
strain mdr-ab2
growth and lytic characteristics of ime-ab2
to determine the optimal multiplicity of infection  of ime-ab <dig>  the phage and its host cells were mixed at various ratios, and incubated for  <dig>  h at 37°c.
the results indicated that a moi of  <dig> gave the highest production of phage progeny .
to examine the host adsorption ability of phage ime-ab <dig>  host bacteria were infected with ime-ab <dig> at a moi of  <dig>  and incubated at 37°c.
aliquots were taken at  <dig>   <dig>   <dig>   <dig>   <dig>   <dig>  and 18 min post-infection and assayed for the absorbed phage by titration using the double-layer method.
the percentages of phage absorption at different time points were plotted .
the results showed that phage ime-ab <dig> had an adsorption rate of 50% within 3 min, 80% within 6 min and 99% within 9 min.
for one-step growth curve analysis, mdr-ab <dig> cells  were infected with phage ime-ab <dig> at a moi of  <dig> .
the bacteriophage was allowed to adsorb for 15 min at 37°c
<cit> .
the mixture was then centrifuged at  <dig>  × g for 30 s to remove unadsorbed phage particles, and the resultant pellet was re-suspended in 5 ml of lb medium.
samples were incubated at 37°c and collected every 10 min during 0–60 min, as well as at  <dig> and 120 min
<cit> .
as shown in figure
3b, the latent period of phage ime-ab <dig> lasted for 20 min, the burst period reached a peak at 30 min, and the phage multiplication reached the final plateau phase at 50 min.
the burst size of phage ime-ab <dig> was determined to be  <dig> pfu/cell
<cit> .
analysis of the phage proteins and genome
purified phage particles were denatured in loading buffer  and heated in a boiling water bath for 5 min, followed by separation of the proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis .
the results indicated that the structural proteins of phage ime-ab <dig> showed a pattern of nine protein bands in 10% sds-page gel, with molecular masses ranging from 15–100 kda .
the most abundant protein band in the gel above 35 kda was analyzed with liquid sampling mass spectrometry  and proved to be the phage putative capsid protein.
the genome analysis indicated that phage ime-ab <dig> has a double-stranded dna genome, approximately 40 kb in size.
the genome of phage ime-ab <dig> could be digested with endonuclease ndei, hincii and hindiii .
it was found that endonuclease enzymes, hindiii and hincii, have the  <dig> and  <dig> cutting sites on the genome of phage ime-ab <dig> respectively by vector nti
<cit> .
compared to other a. baumannii complete genome , the two endonucleases also have most restriction enzyme cutting sites on them.
high-throughput sequencing of the phage genomic dna generated  <dig>  valid reads with which the complete sequence of the genome was assembled using both velvet and clc genomic workbench, with an average coverage of 785 ×
<cit> .
the complete genome of phage ime-ab <dig> consists of  <dig>  bp, with an average gc content of  <dig> % .
annotation results showed that the genome encodes  <dig> coding sequences  .
the classification of the  <dig> cdss is shown in table
<dig> and figure
<dig>  the complete genome of ime-ab <dig> is organized into three functional units which encoding structural proteins, metabolic proteins and packaging-associated proteins respectively.
no trna was found in the genome of ime-ab <dig>  and no significant proteins considered to be markers of temperate bacteriophages were identified.
running blastn showed that the isolated ime-ab <dig> has a high similarity to acinetobacter phage ab <dig> , acinetobacter phage ap <dig>  and acinetobacter phage phiac- <dig> , which were isolated in china, russia and korea respectively.
the phage ab <dig>  ap <dig> or phiac- <dig> has a genome of about 45 kb and owns a coverage of 65%, 50%, 8% respectively when compared with the isolated phage ime-ab <dig>  genomic annotation found that ime-ab <dig> encodes  <dig> cdss, ab <dig>  <dig> cdss, ap <dig>  <dig> cdss, phiac- <dig>  <dig> cdss.
the  <dig> cdss from ime-ab <dig> shared  <dig> homologues with ab <dig>   <dig> homologues with ap <dig> and  <dig> homologues with phiac- <dig> respectively .
totally,  <dig> of the  <dig> cdss encoded by ime-ab <dig> were identified to be putatively functional.
genomic analysis revealed that the bacteriophage ime-ab <dig> was most closely related to ab <dig>  a nucleotide alignment of the four acinetobacter baumannii phages showed that some functional regions are highly homologous, with no significant rearrangements observed .
it revealed a stable area.
stability is suggested from the high level of nucleotide identity, lack of inversions and other major rearrangements, and the stabilizing selection inferred for virtually all genes harboring synonymous and non-synonymous mutations
<cit> .
functional related genes are sequential, yet there are a lot of breakpoint modules obviously and some sections of these strains of phages are nonhomologous .
it illustrated that these structural genes had occurred in the extensive structural rearrangements during evolution.
bacteriophages are the most diverse and abundant biological entities in nature environment.
most of them can hardly to be found homologous to another bacteriophage, which means evolutionary success obtained by bacteriophages.
furthermore, the diversity is such that even genes with required functions cannot always be recognized.
during bacteriophage evolution,the elimination or recombination of genes result in the diversity and meanwhile confer a selective advantage to survive and infect so that phages can better adapt to host bacteria.
a. baumannii
phage ime-ab <dig>
a. baumannii
phage ab <dig>
a. baumanni i
phage ap <dig>  and
a <dig> baumannii
phage phiac- <dig>
ime-ab <dig> is the reference for alignments and comparisons to the three other strains .
structural proteins
seven cdss encoding structural proteins were identified in the phage ime-ab <dig>  the putative capsid protein  is similar to that of phage ab <dig> , phage ap <dig> , phage phiac- <dig> .
phage ab <dig> , phage ap <dig> , phage phiac- <dig>  also share homology to the putative phage head protein encoded by ime-ab <dig> cds.
<dig>  cds.
<dig> and cds.
<dig> of phage ime-ab <dig> are identified to be associated with tail fiber protein.
cds.
<dig> and cds.
<dig> of phage ime-ab <dig> are predicted to encode proteins responsible for baseplate.
these two related proteins can be found similar area in the other three phages.
the results also demonstrate the phage tail related proteins generally cluster together .
metabolic proteins
a unique feature of the ime-ab <dig> genome is that it encodes cobalt transport protein .
notably, cobalt is a cofactor and is required by enzymes from bacteria
<cit> .
it is possible that these metabolic enzymes benefit phage by enhancing the metabolism of the infected bacterial cell, which could in turn increase phage proliferation.
no similar cobalt proteins were found in the other three phages sharing homology with ime-ab <dig>  cds.
<dig> encodes a putative rna polymerase protein.
it is necessary for constructing rna chains using dna genes as templates, a process called transcription.
transcription of most double-stranded dna bacteriophages rely on their host bacteria
<cit> .
the putative cds.
<dig> of ime-ab <dig> is predicted to encode hth domains which have been recruited to a wide range of functions beyond transcription regulation, such as dna repairing and replication, rna metabolism and protein-protein interactions in diverse signaling contexts.
beyond their basic role in mediating macromolecular interactions, the hth domains have also been incorporated into the catalytic domains of diverse enzymes
<cit> .
in functional terms, the hnh endonuclease domain  is found in crispr-related proteins.
crispr functions as a prokaryotic immune system, in that it confers resistance to exogenous genetic elements
<cit> .
cds.
<dig> is predicted to be a putative bacteriophage-associated immunity protein, which was considered to be responsible for phage superinfection immunity
<cit> .
cds.
<dig> and cds.
<dig> are putative lysozyme family protein.
unlike, cds.
<dig>   <dig>   <dig> encoded by phiac- <dig>  which are putative lysozyme-like domain protein and adjacent to each other, the two lysozyme-like proteins from ime-ab <dig> are not clustered.
packaging-associated proteins
in the four similar acinetobacter baumannii phages, the genetic elements encoding the products involved in the packaging system are commonly found adjacent to one another in the phage genomes.
packaging system is generally composed of big subunit  and small subunit .
usually, the two subunits of the terminase and head portal protein  are closely connected in the packaging system, while the portal protein is a bacteriophage component that forms a hole, or portal, enabling dna passage during packaging and ejection
<cit> .
it also forms the junction between the phage head  and the tail proteins.
discussion
with the emergence of a growing number of drug-resistant bacterial species, and the difficulties surrounding the development of novel antibiotics
<cit> , exploring novel or alternative therapeutic methods is imperative.
the recent renaissance of bacteriophage therapy may provide new treatment strategies for combatting drug-resistant bacterial infections.
although a large number of work on phage therapy in human disease had been done
<cit> , the host-specific infection and the relatively narrow lytic spectrum of phage is one of the obstacles to their further application.
individualized phage therapy may represent the future of phage therapy, where bacterial infections will be treated with phage combinations that have already been shown as effective for that particular bacterium.
to overcome these limitations, strategies such as screening more lytic phages, combining phages with antibiotics, or administrating phages cocktails should be investigated
<cit> .
therefore, it is very important to isolate novel and sensitive phages to enrich the phage arsenal
<cit> .
all known acinetobacter baumannii bacteriophages were summarized and compared in this research.
there are nearly  <dig> a. baumannii phage strains reported in the literature mainly in  <dig>  most of the phages genome length are about 40 kb.
just only thirteen complete a. baumannii phage genomes were sequenced and deposited in the genbank database currently.
five of those genomes consist of an approximately 160 kb linear dna molecule , and are annotated as t4-like phage
<cit> .
the remaining eight phages contain a genome of 30–50 kb, and may be classified into two different groups according to sequence similarity.
in one group, there are four phages with a linear genome, including phiab <dig>  which was already classified as a ϕkmv-like virus
<cit> , phage ymc/09/02/b1251_aba_bp
<cit> , acinetobacter phage ab <dig> and acinetobacter phage abp <dig>  the other group includes four phages: ab <dig> , ap <dig> , phiac- <dig> , and our phage ime-ab <dig>  we compared the growth characteristics and the genome of a. baumannii phage as shown in table
<dig>  the results indicated that the newly isolated phage ime-ab <dig> has a shorter latent and burst period than ab <dig> and ap <dig>  which implied that the ime-ab <dig> was more lytic and the burst size produced by ime-ab <dig> was smaller than those phages
<cit> .
all the listed a. baumannii bacteriophages or its lysin had been tested in treating bacterial infection such as inhibiting biofilm formation or effecting on host cell survival.
although the bacteriophage ime-ab <dig> could lyse the host bacteria and cleared the bacterial suspension in 4 hours, we observed that the resistant bacteria appeared and made the suspension turbid again in 24 hours.
the easily emerging resistant bacteria after infection with phage might be an obstacle when fighting against bacterial infection with bacteriophage in the future.
acinetobacter baumannii
bacteriophages
ab1
ap22
phiac-1
ab7-ibb2
ab7-ibb1
zz1
abp1
ab3
ymc/09/ 02/b <dig> aba bp
phikm18p
abp53
phiab1
ac <dig> acj <dig> acj <dig>
ab1
phi ab2
CONCLUSIONS
a lytic a. baumannii bacteriophage ime-ab <dig> was isolated and characterized in this research.
the complete genome of ime-ab <dig> was sequenced and compared to those of a. baumannii phage ab <dig>  a. baumannii phage ap <dig>  and a. baumannii phage phiac- <dig> in detail.
the genome of ime-ab <dig> was replete with novel genes without known relatives, which indicated that ime-ab <dig> was a novel and unique a. baumannii bacteriophage.
although the resistant a. baumannii appeared finally after infection with ime-ab <dig>  the comprehensive understanding of the phage’s characteristics is conducive to the treatment of multidrug-resistant a. baumannii in the future.

 acinetobacter baumanni is a non-fermentative, aerobic, gram-negative bacillus, and is an opportunistic pathogen with global distribution. it is frequently found in elderly patients and cancer patients with compromised immune function, especially in intensive care units. with the use of broad-spectrum antibiotics, immunosuppressive drugs, and glucocorticoids, a. baumannii has become a major nosocomial pathogen species. multidrug-resistant, extensively drug-resistant, and pan drug-resistant a. baumannii strains are increasingly prevalent. mdr-ab refers to a. baumannii strains that are resistant to at least three of the following five types of antimicrobial agents: cephalosporins, carbapenems, β-lactamase inhibitors, fluoroquinolones, and aminoglycosides. bacteriophage therapy is a potential alternative treatment for multidrug-resistant bacterial infections. a bacteriophage is a bacterial virus that can lyse and kill the host cell. phage-related studies have gone through three stages. félix d’herelle discovered bacteriophage for the treatment of bacterial infections in 1917. after the emergence of antibiotics in the 1940s, phages were seldom used for therapeutic purposes, and mainly functioned as molecular and genetic research tools. with the recent emergence of multidrug-resistant bacteria, however, there has been renewed interest in methods of phage therapy. in this study we isolated a lytic bacteriophage ime-ab and compared biological characteristics and genomic sequence with other acinetobacter baumannii phages. the genomes of a. baumannii phages ime-ab a. baumannii ab a. baumannii ap and a. baumannii phiac- were compared thoroughly in this study. to our knowledge this is the first report of comparison of the characteristics and complete genome sequence of acinetobacter baumannii bacteriophages. a clear understanding of its genes is conducive to the treatment of multidrug-resistant a. baumannii in the future.  isolation of a lytic bacteriophage against multidrug-resistant a. baumannii a. baumannii strain mdr-ab isolated from a sputum sample of a patient with pneumonia at pla hospital was resistant to multiple antibiotics. the bacteria was used to screen bacteriophages in sewage samples from pla hospital the isolated phage was designated as vb_abam-ime-ab following the recommendation by international committee on taxonomy of viruses in phage nomenclature. the pahge ime-ab could form clear plaques in the double-layer assay and clear its host’s suspension in just 4 hours, indicating that it is a lytic phage. in order to check the development of resistance, we had extended the period of the experiment to 24 h. the result indicated that the bacterial suspension became turbid finally. the final suspension was plated on solid lb culture and then some single bacterial clones were picked to be used for 16 s rdna sequencing. the sequences of 16 s rdna proved that the final suspension was a. baumannii that developed resistance to ime-ab the phage particles were concentrated with peg and then purified with a cesium chloride gradients density to a titer of 1 ×  pfu/ml. observation under an electron microscope showed that the phage ime-ab consisted of an icosahedral head and a contractile tail. the total length of the phage from the top of the head to the bottom of the tail was about 160 nm, with the head measuring approximately  nm, and the tail about 90 nm. this morphology suggested that phage ime-ab should be classified as a member of the myoviridae family. among the clinical strains of a. baumannii, only three strains of a. baumannii could be lysed by the phage ime-ab a. baumannii strain mdr-ab2 growth and lytic characteristics of ime-ab2 to determine the optimal multiplicity of infection of ime-ab the phage and its host cells were mixed at various ratios, and incubated for  h at 37°c. the results indicated that a moi of gave the highest production of phage progeny. to examine the host adsorption ability of phage ime-ab host bacteria were infected with ime-ab at a moi of and incubated at 37°c. aliquots were taken at and 18 min post-infection and assayed for the absorbed phage by titration using the double-layer method. the percentages of phage absorption at different time points were plotted. the results showed that phage ime-ab had an adsorption rate of 50% within 3 min, 80% within 6 min and 99% within 9 min. for one-step growth curve analysis, mdr-ab cells were infected with phage ime-ab at a moi of. the bacteriophage was allowed to adsorb for 15 min at 37°c. the mixture was then centrifuged at  × g for 30 s to remove unadsorbed phage particles, and the resultant pellet was re-suspended in 5 ml of lb medium. samples were incubated at 37°c and collected every 10 min during 0–60 min, as well as at and 120 min. as shown in figure  3b, the latent period of phage ime-ab lasted for 20 min, the burst period reached a peak at 30 min, and the phage multiplication reached the final plateau phase at 50 min. the burst size of phage ime-ab was determined to be pfu/cell. analysis of the phage proteins and genome purified phage particles were denatured in loading buffer and heated in a boiling water bath for 5 min, followed by separation of the proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. the results indicated that the structural proteins of phage ime-ab showed a pattern of nine protein bands in 10% sds-page gel, with molecular masses ranging from 15–100 kda. the most abundant protein band in the gel above 35 kda was analyzed with liquid sampling mass spectrometry and proved to be the phage putative capsid protein. the genome analysis indicated that phage ime-ab has a double-stranded dna genome, approximately 40 kb in size. the genome of phage ime-ab could be digested with endonuclease ndei, hincii and hindiii. it was found that endonuclease enzymes, hindiii and hincii, have the and cutting sites on the genome of phage ime-ab respectively by vector nti. compared to other a. baumannii complete genome, the two endonucleases also have most restriction enzyme cutting sites on them. high-throughput sequencing of the phage genomic dna generated valid reads with which the complete sequence of the genome was assembled using both velvet and clc genomic workbench, with an average coverage of 785 × . the complete genome of phage ime-ab consists of  bp, with an average gc content of %. annotation results showed that the genome encodes coding sequences. the classification of the cdss is shown in table  and figure  the complete genome of ime-ab is organized into three functional units which encoding structural proteins, metabolic proteins and packaging-associated proteins respectively. no trna was found in the genome of ime-ab and no significant proteins considered to be markers of temperate bacteriophages were identified. running blastn showed that the isolated ime-ab has a high similarity to acinetobacter phage ab, acinetobacter phage ap and acinetobacter phage phiac-, which were isolated in china, russia and korea respectively. the phage ab ap or phiac- has a genome of about 45 kb and owns a coverage of 65%, 50%, 8% respectively when compared with the isolated phage ime-ab genomic annotation found that ime-ab encodes cdss, ab cdss, ap cdss, phiac- cdss. the cdss from ime-ab shared homologues with ab homologues with ap and homologues with phiac- respectively. totally, of the cdss encoded by ime-ab were identified to be putatively functional. genomic analysis revealed that the bacteriophage ime-ab was most closely related to ab a nucleotide alignment of the four acinetobacter baumannii phages showed that some functional regions are highly homologous, with no significant rearrangements observed. it revealed a stable area. stability is suggested from the high level of nucleotide identity, lack of inversions and other major rearrangements, and the stabilizing selection inferred for virtually all genes harboring synonymous and non-synonymous mutations. functional related genes are sequential, yet there are a lot of breakpoint modules obviously and some sections of these strains of phages are nonhomologous. it illustrated that these structural genes had occurred in the extensive structural rearrangements during evolution. bacteriophages are the most diverse and abundant biological entities in nature environment. most of them can hardly to be found homologous to another bacteriophage, which means evolutionary success obtained by bacteriophages. furthermore, the diversity is such that even genes with required functions cannot always be recognized. during bacteriophage evolution,the elimination or recombination of genes result in the diversity and meanwhile confer a selective advantage to survive and infect so that phages can better adapt to host bacteria. a. baumannii phage ime-ab a. baumannii phage ab a. baumanni i phage ap and a baumannii phage phiac- ime-ab is the reference for alignments and comparisons to the three other strains. structural proteins seven cdss encoding structural proteins were identified in the phage ime-ab the putative capsid protein is similar to that of phage ab, phage ap, phage phiac-. phage ab, phage ap, phage phiac- also share homology to the putative phage head protein encoded by ime-ab cds. cds. and cds. of phage ime-ab are identified to be associated with tail fiber protein. cds. and cds. of phage ime-ab are predicted to encode proteins responsible for baseplate. these two related proteins can be found similar area in the other three phages. the results also demonstrate the phage tail related proteins generally cluster together. metabolic proteins a unique feature of the ime-ab genome is that it encodes cobalt transport protein. notably, cobalt is a cofactor and is required by enzymes from bacteria. it is possible that these metabolic enzymes benefit phage by enhancing the metabolism of the infected bacterial cell, which could in turn increase phage proliferation. no similar cobalt proteins were found in the other three phages sharing homology with ime-ab cds. encodes a putative rna polymerase protein. it is necessary for constructing rna chains using dna genes as templates, a process called transcription. transcription of most double-stranded dna bacteriophages rely on their host bacteria. the putative cds. of ime-ab is predicted to encode hth domains which have been recruited to a wide range of functions beyond transcription regulation, such as dna repairing and replication, rna metabolism and protein-protein interactions in diverse signaling contexts. beyond their basic role in mediating macromolecular interactions, the hth domains have also been incorporated into the catalytic domains of diverse enzymes. in functional terms, the hnh endonuclease domain is found in crispr-related proteins. crispr functions as a prokaryotic immune system, in that it confers resistance to exogenous genetic elements. cds. is predicted to be a putative bacteriophage-associated immunity protein, which was considered to be responsible for phage superinfection immunity. cds. and cds. are putative lysozyme family protein. unlike, cds. encoded by phiac- which are putative lysozyme-like domain protein and adjacent to each other, the two lysozyme-like proteins from ime-ab are not clustered. packaging-associated proteins in the four similar acinetobacter baumannii phages, the genetic elements encoding the products involved in the packaging system are commonly found adjacent to one another in the phage genomes. packaging system is generally composed of big subunit and small subunit. usually, the two subunits of the terminase and head portal protein are closely connected in the packaging system, while the portal protein is a bacteriophage component that forms a hole, or portal, enabling dna passage during packaging and ejection. it also forms the junction between the phage head and the tail proteins. discussion with the emergence of a growing number of drug-resistant bacterial species, and the difficulties surrounding the development of novel antibiotics, exploring novel or alternative therapeutic methods is imperative. the recent renaissance of bacteriophage therapy may provide new treatment strategies for combatting drug-resistant bacterial infections. although a large number of work on phage therapy in human disease had been done, the host-specific infection and the relatively narrow lytic spectrum of phage is one of the obstacles to their further application. individualized phage therapy may represent the future of phage therapy, where bacterial infections will be treated with phage combinations that have already been shown as effective for that particular bacterium. to overcome these limitations, strategies such as screening more lytic phages, combining phages with antibiotics, or administrating phages cocktails should be investigated. therefore, it is very important to isolate novel and sensitive phages to enrich the phage arsenal. all known acinetobacter baumannii bacteriophages were summarized and compared in this research. there are nearly a. baumannii phage strains reported in the literature mainly in most of the phages genome length are about 40 kb. just only thirteen complete a. baumannii phage genomes were sequenced and deposited in the genbank database currently. five of those genomes consist of an approximately 160 kb linear dna molecule, and are annotated as t4-like phage. the remaining eight phages contain a genome of 30–50 kb, and may be classified into two different groups according to sequence similarity. in one group, there are four phages with a linear genome, including phiab which was already classified as a ϕkmv-like virus, phage ymc/09/02/b1251_aba_bp, acinetobacter phage ab and acinetobacter phage abp the other group includes four phages: ab, ap, phiac-, and our phage ime-ab we compared the growth characteristics and the genome of a. baumannii phage as shown in table  the results indicated that the newly isolated phage ime-ab has a shorter latent and burst period than ab and ap which implied that the ime-ab was more lytic and the burst size produced by ime-ab was smaller than those phages. all the listed a. baumannii bacteriophages or its lysin had been tested in treating bacterial infection such as inhibiting biofilm formation or effecting on host cell survival. although the bacteriophage ime-ab could lyse the host bacteria and cleared the bacterial suspension in 4 hours, we observed that the resistant bacteria appeared and made the suspension turbid again in 24 hours. the easily emerging resistant bacteria after infection with phage might be an obstacle when fighting against bacterial infection with bacteriophage in the future. acinetobacter baumannii bacteriophages ab1 ap22 phiac-1 ab7-ibb2 ab7-ibb1 zz1 abp1 ab3 ymc/09/ 02/b aba bp phikm18p abp53 phiab1 ac acj acj ab1 phi ab2  a lytic a. baumannii bacteriophage ime-ab was isolated and characterized in this research. the complete genome of ime-ab was sequenced and compared to those of a. baumannii phage ab a. baumannii phage ap and a. baumannii phage phiac- in detail. the genome of ime-ab was replete with novel genes without known relatives, which indicated that ime-ab was a novel and unique a. baumannii bacteriophage. although the resistant a. baumannii appeared finally after infection with ime-ab the comprehensive understanding of the phage’s characteristics is conducive to the treatment of multidrug-resistant a. baumannii in the future. 
it could form clear plaques in the double-layer assay and clear its host’s suspension in just 4 hours. we compared the characteristics and complete genome sequence of all known acinetobacter baumannii bacteriophages. whole genome of vb_abam-ime-ab was sequenced and annotated and the results showed that its genome is a double-stranded dna molecule consisting of nucleotides. a clear understanding of its characteristics and genes is conducive to the treatment of multidrug-resistant a. baumannii in the future. the recent renaissance of bacteriophage therapy may provide new treatment strategies for combatting drug-resistant bacterial infections. with the use of broad-spectrum antibiotics, immunosuppressive drugs, and glucocorticoids, multidrug-resistant acinetobacter baumannii has become a major nosocomial pathogen species. there are only three that have been sequenced acinetobacter baumannii phages ab ap and phiac- which have a relatively high similarity and own a coverage of 65%, 50%, 8% respectively when compared with our phage vb_abam-ime-ab a nucleotide alignment of the four acinetobacter baumannii phages showed that some cdss are similar, with no significant rearrangements observed. the isolated bacteriophage vb_abam-ime-ab has an icosahedral head and displays morphology resembling myoviridae family. gel separation assays showed that the phage particle contains at least nine protein bands with molecular weights ranging 15–100 kda. vb_abam-ime-ab could adsorb its host cells in 9 min with an adsorption rate more than 99% and showed a short latent period and a small burst size. vb_abam-ime-ab was a novel and unique a. baumannii bacteriophage.
 with the use of broad-spectrum antibiotics, immunosuppressive drugs, and glucocorticoids, multidrug-resistant acinetobacter baumannii has become a major nosocomial pathogen species. the recent renaissance of bacteriophage therapy may provide new treatment strategies for combatting drug-resistant bacterial infections. in this study, we isolated a lytic bacteriophage vb_abam-ime-ab has a short latent period and a small burst size, which clear its host’s suspension quickly, was selected for characterization and a complete genomic comparative study.  the isolated bacteriophage vb_abam-ime-ab has an icosahedral head and displays morphology resembling myoviridae family. gel separation assays showed that the phage particle contains at least nine protein bands with molecular weights ranging 15–100 kda. vb_abam-ime-ab could adsorb its host cells in 9 min with an adsorption rate more than 99% and showed a short latent period and a small burst size. it could form clear plaques in the double-layer assay and clear its host’s suspension in just 4 hours. whole genome of vb_abam-ime-ab was sequenced and annotated and the results showed that its genome is a double-stranded dna molecule consisting of nucleotides. the genome has a g + c content of % and putative coding sequences. we compared the characteristics and complete genome sequence of all known acinetobacter baumannii bacteriophages. there are only three that have been sequenced acinetobacter baumannii phages ab ap and phiac- which have a relatively high similarity and own a coverage of 65%, 50%, 8% respectively when compared with our phage vb_abam-ime-ab a nucleotide alignment of the four acinetobacter baumannii phages showed that some cdss are similar, with no significant rearrangements observed. yet some sections of these strains of phage are nonhomologous.  vb_abam-ime-ab was a novel and unique a. baumannii bacteriophage. these findings suggest a common ancestry and microbial diversity and evolution. a clear understanding of its characteristics and genes is conducive to the treatment of multidrug-resistant a. baumannii in the future. acinetobacter baumanniibacteriophagecharacteristicsgenome 