Characterization of MRSA strain MR10
Bacterial strain (MR10: accession no. MT272781) isolated from pus discharge was presumptively identified as S aureus based on microscopic examination, biochemical tests, and 16SrRNA sequence analysis. According to CLSI criteria, the MR10 showed resistance to large number of drugs, however, it was sensitive to vancomycin, linezolid and tigecycline (Supplementary material Table 1). BLAST analysis of its 16s rRNA gene sequence showed 98-99% similarity to S aureus strains. Furthermore, mecA gene (310bp) was successfully amplified from its genome (Publication in process). Phenotypically and genotypically confirmed MRSA strain (MR10) was used for isolation of TSP phage.
Morphological characterization of TSP bacteriophage
A novel lytic phage TSP was isolated from hospital wastewater against MR10. TSP phage formed tiny clear, round plaques (1mm) in diameter (Fig. 1.)
TSP phage showed broad host range against MRSA strains
TSP phage showed broad lytic activity against MRSA (24 of 32 strains, 78%) and MSSA (3 of 8 strains, 37%). However, it was unable to lyse the tested S. epidermidis strains and isolates of gram negative organisms (E. coli, Klebsiella pneumoniae, Serratia marsecence, Pseudomonas aeruginosa, Acinetobacter baumannii and Enterobacter cloacae). The plaque formation ability of TSP phage was observed against 18 isolates of MRSA and 1 isolate of MSSA. The efficiency of plating (EOP) of TSP was grouped into four categories; EOP>0.5 for high production, 0.1<EOP<0.5 for medium production, 0.001<EOP<0.1 for low production, and EOP<0.001 for very low production. The higher EOP values of TSP phage against MR5, MR19 and MR26 suggest that these are more susceptible to phage compared with MR10, while remaining 15/19 isolates have low efficiency of plating as compared to the host strain (Supplementary material Table 2).
TSP bacteriophage showed strong bacteriolytic activity till 12 hours post-inoculation
TSP phage inhibit the bacterial growth for initial 12 hours at MOI-1 and 10 leading to increased bacterial growth after this time in the phage treated mixture but it was still less than growth in the untreated control (Fig 2).
TSP bacteriophage highest stability observed at 37ºC and varying pH (5-9) while maximum storage stability at 4ºC
To assess the stability of bacteriophage TSP for therapeutic use in the future, its thermal, pH and storage stabilities were analyzed. TSP bacteriophage showed highest stability at temperature 25ºC and 37ºC, however at high temperature (45ºC, 50ºC and 60ºC) a progressive decrease in phage titer was observed which destroyed phage activity at temperature above 60ºC (Fig 3A). The TSP stayed highly active at wide pH range (5 to 9), but under extreme pH (below 5 and above 10) conditions, a marked decrease in phage titer was observed (Fig. 3B). Long term storage stabilities showed that TSP phage was more viable at refrigerator temperature (4ºC) as compared to frozen temperatures (-20ºC and -80ºC). However, TSP phage showed better survival at -80ºC (1.95 × 1010) while a significant reduction in phage titer was observed at -20ºC and 25ºC (Fig 3C).
TSP bacteriophage revealed short latent period and higher burst size
According to phage adsorption assay, almost 99% of phage TSP could adsorb to the host cell surface within 9 min at 25ºC (Figure 5A). Adsorption rate constant of phage calculated within the interval of 3 to 9 minute is 4.3 x 10-12 pfu/ml/min. One step growth curve analysis showed short latent period of 20 minutes and average burst size of 103 virions per infected cells (Fig 5B). These results indicated that this phage can rapidly infect the host and replicate.
The TSP have a linear genome of 18Kb long
To further determine whether the genome of TSB is linear or circular, we determined the 1 site cutter in the phage genome through Neb cutter and found that restriction through NcoI & EcoRI produce four fragments of 9.1, 5.7, 3 and 0.1 kb sizes, if the genome is linear, as shown in figure S1. Digestion of TSP phage DNA through NcoI & EcoRI produced restriction pattern like the proposed pattern by Neb cutter, which confirm that TSP phage DNA is linear (Fig. 5). Also, the restriction pattern confirms that the isolated phage DNA is pure with no other DNA contamination.
Genome sequence analysis demonstrates lytic nature of TSP phage
Whole genome sequencing and annotation showed that TSP phage consists of a double stranded, linear DNA with a genomic length of 17,987 bp and an average GC content of 29.7%. It contains 20 predicted open reading frames (ORFs) and no tRNA gene. According to BLASTn analysis, the complete TSP phage genome sequence showed 98% identity to S aureus lytic phages SCH1 (Accession No. KY000084.1), SCH11 (Accession No. KY000085.1) and vB SauP-436A1 (Accession No. MN150710.1) with 94% query coverage. The detailed genomic characterization of TSP phage is given in Supplementary material Table S3. All ORFs presented an ATG start codon. Among all 20 ORFs, 12 had assigned functions while the remaining 8 ORFs were annotated as hypothetical proteins. Annotation and functional analysis of predicted ORFs revealed four functional groups: structural (major capsid and scaffold protein, major and minor tail protein, tail fibers protein, collar proteins, structural protein) host lysis (endolysin, holin and CHAP domain-containing protein), DNA manipulation (single stranded DNA-binding protein, DNA polymerase) and DNA packaging protein. Structural proteins and lysis protein are present on the plus strand while DNA manipulation, DNA packaging and maximum hypothetical proteins are on negative strand. The TSP phage genome consists of 5 potential rho-independent transcription terminators. There were no virulence gene detected in phage TSP genome. The open reading frame ORF7 (Endolysin) was considered to be involved in lytic activity against peptidoglycan of host bacterium. According to Pfam and InterPro Scan analysis, endolysin has two polypeptide domains, one is catalytic domain at N terminus called cysteine, histidine-dependent amidohydrolases/peptidase (CHAP) (pfam05257) and (IPR007921), and other is cell wall binding domain at C terminus named as SH3_5 (pfam08460) and (IPR003646). TSP phage endolysin is located between the structural proteins similar to phage CSA13 and this is the unique characteristic of P68 like viruses.
TSP phage showed genetic similarity with genus P68virus of family Podoviridae
The complete genome sequence of TSP phage was assessed for homology for other S aureus phages (Figure 7A). According to BLASTn analysis, TSP phage showed highest similarity (98%) to phage genomes SCH1, SCH11 and vB SauP-436A1 with 94% query coverage. Comparative genomic analysis indicated that TSP genome showed highest homology with the phages of Podoviridae family, while it showed a distant relationship to the members of other families. According to BLASTp search, major capsid protein of TSP phage showed 99.9% identity to vB SauP-436A1, SCH1 and S13 with 100% query coverage, while DNA polymerase of TSP phage showed 98.95% identity to SCH1 sequence with 100% query coverage (Fig 7B and 7C).