baumannii clinical isolates. Results Isolation of ZZ1 and its morphology Twenty-three A. baumannii clinical isolates were screened for phage present in a sample of fishpond water. Among these, only the strain AB09V could serve as an indicator for ZZ1 in the initial screening. This phage formed clear plaques of approximately 1-2 mm in diameter on AB09V lawns. AB09V was thus used to propagate, purify and characterize the phage. As shown in Figure 1, the phage ZZ1 has a 100-nm icosahedral head and a 120-nm long contractile tail. Morphologically, phage ZZ1 can be tentatively classified as a member of the Myoviridae

family in the order of Caudovirales. Most of the input phages rapidly adsorbed to AB09V cells. Appearance of ghost particles 5 min after mixing phages with bacteria histone deacetylase activity suggested that ejection of DNA from the phage head occurred rapidly. Figure 1 Electron micrographs of ZZ1 and infected  A. baumannii  AB09V. A mixture of ZZ1 phages and A. baumannii AB09V cells was negatively stained. The phage ZZ1 contained a baseplate with fibers (indicated by the white arrow) and an icosahedral head with a contractile tail (indicated by the large black arrow), which allowed for its inclusion in the Myoviridae

family of the order Caudovirales. Intact phages had a head filled with DNA, and ghost particles (indicated by the small black arrows) had an empty head, showing that ejection of DNA from the phage head had taken place within 5 min. Host range of ZZ1 and identification of bacterial beta-catenin mutation strains Two additional natural bacterial hosts, AB0901 and AB0902, were found when the

other 22 of the 23 A. baumannii clinical isolates were used to investigate the host range of ZZ1 by spot test. This test used a higher concentration of phage (108 PFU/ml) than the original screen. Interestingly, some differences were observed in the ability of the phage to lyse the 3 bacterial hosts (AB09V, AB0901, and AB0902). For example, as shown Figure 2, ZZ1 was capable of forming transparent areas on lawns of the strains AB09V, AB0901, and AB0902. However, the minimum phage see more concentrations Interleukin-2 receptor required to form clear spots on each lawn were different: AB09V required 105 PFU/ml, AB0902 required 106 PFU/ml, and AB0901 required 108 PFU/ml. The values suggest that under the same culture conditions, the antibacterial activity of ZZ1 was highest in strain AB09V, followed by AB0902 and then AB0901. There might be natural resistance mechanisms in AB0901 and AB0902; thus, the strain AB09V is likely the most sensitive indicator of the phage titer of the 3 strains and is the best host for phage propagation. Figure 2 Antibacterial activity of phage ZZ1 against three  A.   baumannii strains.  Serial 10-fold dilutions of phage ZZ1 were spotted onto lawns of strains AB09V, AB0901, and AB0902 in 0.7% agar nutrient broth at 37°C. AB09V was used as the indicator for determination of the phage titer.