Isolation and characterization of phage

In previous study, Jun and colleagues reported two virulent Shigella phages, a Siphoviridae phage pSf-1 infecting S. flexneri11 and a Podoviridae phage pSb-1 infecting S. boydii12. pSf-1 was able to infect all of the S. flexneri and most of S. sonnei strains, forming clear plaques11; pSb-1 was able to infect all of the S. boydii strains and formed clear plaques12. Although the virulent Shigella phages, pSf-1 and pSb-1 showed the potential usefulness against shigellosis as reported previously, the two phages were considered to present restricted effectiveness because S. sonnei ATCC® 11060 was not infected by any of two phages11,12. This study aimed to isolate a virulent phage infecting S. sonnei strains in order to make the best combination of various phages for use in a phage cocktail as previously noted that the isolation of S. sonnei phage was needed12.

Previous research in our laboratory led to the isolation of S. sonnei phage as a first priority rather than the isolation of different phages infecting other Shigella species. A virulent Shigella phage, pSs-1 infecting S. sonnei and S. flexneri was isolated from the Hongjecheon stream in Seoul in April 2012. From the isolated phages, pSs-1 was selected for further studies depending on the clarity of plaque and the phage titer after a single propagation.

The morphology of pSs-1 places it in the family Myoviridae according to the classification system of Ackermann13 (Fig. 1C). The tail length and width were 120 ± 7 nm (mean ± SD) (n = 10) and 18 ± 2 nm (n = 10), respectively, and the head diameter was 98 ± 4 nm (n = 10). pSs-1 inhibited all of the S. sonnei and S. flexneri strains, producing clear plaques on all of the strains except one S. flexneri strain, ATCC® 12022 (turbid plaque). However, no S. boydii strain was infected by pSs-1. The high EOP value was obtained with S. flexneri ATCC® 29903 (Fig. 1B) although no strain had a higher value than the indicator host strain, S. sonnei ATCC® 25931 (Table 1). Furthermore, pSs-1 was not able to infect Escherichia coli strains used in this study. The result of one-step growth analysis revealed that pSs-1 had a short latent period (25 min) and a large burst size (97 PFU/cell) (Fig. 2).

Figure 1: Phage plaques formed in double-layer agar plates and electron micrograph of negatively stained phage pSs-1. (A) phage plaques formed in agar plates with S. sonnei ATCC® 25931, (B) phage plaques formed in agar plates with S. flexneri ATCC® 29903, and (C) electron micrograph of pSs-1. The bar corresponds to 50 nm. Full size image

Table 1 Host range and EOPs of phage pSs-1 against all the bacterial strains used in this study. Full size table

Figure 2: One-step growth curve of pSs-1. The error bars indicate standard deviations. Full size image

The bacteriolytic effect of pSs-1 was tested on early phase cultures of S. sonnei ATCC® 25931 (Fig. 3A) and S. flexneri ATCC® 29903 (Fig. 3B). The OD 600 values of the uninfected control culture (MOI: 0) continued to increase during the incubation period. In contrast, the growth of bacteria infected by pSs-1 was retarded at MOIs of 0.1, 1, and 10; the bacterial growth was inhibited most effectively at an MOI of 10. pSs-1 lysed S. sonnei (3 h after incubation) more readily than S. flexneri (12 h after incubation).

Figure 3: Time course of host cell lysis effect of pSs-1 against S. sonnei ATCC® 25931 (A) and S. flexneri ATCC® 29903 (B). Early exponential phase cultures of S. sonnei ATCC® 25931 and S. flexneri ATCC® 29903 were co-cultured with pSs-1 at MOIs of 0, 0.1, 1, and 10. The results are shown as mean + standard deviations from triplicate experiments. Full size image

The effective bacteriolytic activity and high EOP value of pSs-1 against S. sonnei and S. flexneri strains indicated that pSs-1 could be used for control of both S. sonnei and S. flexneri although pSs-1 was proved to use only S. sonnei strain as a host bacterium; the propagation trials of pSs-1 using S. flexneri strains have not resulted in high titer for further study. These results conclude that the combination of pSf-1, pSb-1, and pSs-1 can inhibit all of the Shigella strains used in this study, increasing the possibility for Shigella control. The bacteriolytic activity of pSs-1 was found to be active for 1 h over the temperature range of 4–50 °C or the pH range of 5.0–9.0 toward S. sonnei and S. flexneri strains although significant reduction of its activity was observed at 50 °C (data not shown). This result emphasized that pSs-1 could be used in various natural environments, especially poor sanitation surroundings.

Comparative genomic analysis of phage

According to the genome sequencing results, the genome size of pSs-1 is 164,999 bp, with a 35.54% total G + C content. The genomic sequence of the Shigella phage pSs-1 which was described in this study has been deposited in the GenBank database under the accession number KM501444. The predominant start codon was ATG; 10 ORFs such as ORF28, ORF51, ORF66, ORF67, ORF84, ORF112, ORF150, ORF178, ORF189, and ORF238 started with an uncommon start codon (GTG). ORFs with a length of at least 34 amino acids were selected. A total of 26 promoters, 14 transcriptional terminator regions, and 266 ORFs were predicted in the genome. However, only 121 ORFs (45.49%) were determined to be functional based on gene predictions and annotation of the genome. Concrete gene information such as positions, directions, sizes, molecular weights, and putative functions of each pSs-1 ORFs are shown in Supplementary Table S1. A total of 121 ORFs were determined to be functional. The functional analysis indicates that pSs-1 has similar functional system to those of T4-like phages. pSs-1 was proved to contain similar host lysis system to Shfl2 (virulent phage against S. flexneri) although orf217 showed higher similarity to SP18 (virulent phage against S. sonnei). Also, the predicted ORFs of phage structural genes were widely scattered across the entire genome although they were mostly located between orf155 and orf204 (74.46%, 35 ORFs in total).

The comparative genome analysis of pSs-1 with T4-like phages, such as Shfl2 and SP18 revealed that pSs-1 had approximately 97% nucleotide sequence identity with Shfl2 and 186 homologous ORFs among its 266 ORFs (Fig. 4). In addition, pSs-1 showed approximately 93% nucleotide sequence identity with SP18 and 54 homologous ORFs (Fig. 4). According to the ACT comparison results, the pSs-1 genome revealed a higher degree of similarity to phage Shfl2 than phage SP18; the results showed the forward matches in the order of the genome of Shfl2 and SP18 (Fig. 5).

Figure 4: Functional genome map of phage pSs-1. Hypothetical functions of encoded proteins were determined by comparison of amino acid sequences to the non-redundant databank using BLASTP. The + and − stranded ORFs were colored as grey and white, respectively. The CoreGenes between pSs-1 and Shfl2, between pSs-1 and SP18, and between pSs-1 and T4 were colored as yellow, green, and red, respectively. The predicted tRNA was indicated with thick arrow. Putative promoters and terminators were indicated by a bent arrow and inverted triangle on a vertical line, respectively. Full size image

Figure 5: Genome comparison of pSs-1 to its related phages (Shfl2 and SP18) using Artemis Comparison Tool (ACT). Translated BLAST (tblastx, score cutoff: 40) was used to align translated genome sequences of phages. The blue and red lines represent the reverse and forward matches, respectively, and color intensity is proportional to the sequence homology. Nucleotide base-pairs were indicated between grey lines for each phage genome. Full size image

The high bacteriolytic activity of pSs-1 against both S. sonnei and S. flexneri may be attributed to the high nucleotide identity with Shfl2 and SP1814. All of the ORFs in pSs-1 exhibited homology to sequences of T4-like phages, such as Shfl2, SP18, T4, T6, AR1, RB14, RB32, RB51, and vB_EcoM_ACG-C40, reported in the GenBank database. T4-like phages, one of the best-characterized groups of phages present common characteristics: (i) morphology of the Myoviridae family; (ii) similar host range, the family Enterobacteriacae; (iii) large genome size in the range between 160 kbp and 250 kbp; (iv) similar G + C content, ranging from 35% to 43%15,16,17,18. Based on these results, pSs-1 is considered as a member of the T4-like phage group since it belongs to the Myoviridae family, infects Shigella species, contains relatively large genome (164,999 bp), and possesses G + C content of 35.54%. More than 200 T4-like phages have been examined and about 90% of T4-like phages grow on Escherichia coli or other enterobacteria, especially its close relatives such as Klebsiella and Shigella17,18.

A total of 10 tRNA genes (cove score 41.18–78.18) were identified19, which is more than the average number of tRNAs in T4-like phage group. The exceptionally large number of tRNAs (24 tRNAs) in Aeh1 may be attributed to its significantly large genome (233,234 bp) compared to those of the other T4-like phages17. Likewise, the genome of KVP40 (244,834 bp) encodes a large number of tRNAs20. Although the exact function of tRNA in phage is still not clear, it demonstrates that it may contribute to short latent period and large burst size of pSs-1 since tRNA in phage is known to influence its reproduction in the host, and facilitate the improvement of propagation and the reduction of latent period21. The genome of pSs-1 did not contain lysogeny genes and all of the ORFs had nothing in common with pathogenic factors.

In summary, it is considered as a universal problem as a lot of shigellosis cases are reported in developed countries as well as developing countries, although shigellosis had been regarded as a problem only in developing countries. In developing countries especially where poor hygiene standards occur, a safe year-round supply of drinking water remains a problem because the effective water treatment facility is often beyond their financial capacity22. The successful protection using phage against shigellosis was reported with animal experiments and the safety of phage administration through drinking water was reported with phage safety test in humans8,23,24,25. Shigella phage has potential uses to control or eradicate epidemic shigellosis in frequently affected area as phage is known to be cost-effective26. In developed countries where there are increasing concerns about antibiotic resistance, phages can be alternatives to conventional antibiotics. Phage research has been conducted continuously in Eastern Europe and Former Soviet Union countries, with leadership of the G. Eliava Institute of Bacteriophages, Microbiology and Virology in Tbilisi, Georgia. For many years, the Eliava Institute had developed and produced phages for the treatment and prophylaxis of bacterial infections including intestinal infections27. In a series of clinical trials, the therapeutic effectiveness of phages against infectious diseases was evaluated and no harmful effects were reported27. As the prevalence of a particular species of Shigella is generally considered to differ in various geographical areas5, a large-scale future screenings using Shigella environmental isolates from different geographical areas are required. This study may serve as a momentum to enhance the international collaborative research aiming control of waterborne infections, particularly shigellosis, on different continents.