Controlling Multidrug-Resistant <i>Staphylococcus aureus</i> by Combined Use of Antimicrobials and Phage STPX-6 with Broad Spectrum and High Efficiency

<b><i>Objective:</i></b> The emergence of antibiotic-resistant <i>Staphylococcus aureus</i>, especially methicillin-resistant <i>S. aureus</i> (MRSA), poses a great challenge for animal and public health. This study aimed to isolate a broad-spectrum and high-efficiency MRSA phage and explore the phage-antibiotic synergistic effect on MRSA. <b><i>Results:</i></b> Phage STPX-6 belongs to <i>Caudovirales</i>, <i>Podoviridae</i>. It has a hexahedral head and a short tail. Its genome length was 17,007 bp, and it did not contain resistance genes and virulence genes. STPX-6 lysed 79.6% (133/167) of 167 <i>S. aureu</i>s and 87.96% (95/108) of MRSA from different sources. The titer of phage was 1.18 × 10¹⁰ PFU/mL, the optimal multiplicity of infection was 1, the latent period and lysis period were about 10 min and 60 min, respectively, and the burst amount was 68 PFU/cell. At 50°C and 70-90°C, the titer of STPX-6 was maintained at about 10¹⁰ PFU/mL and at least 10³ PFU/mL, respectively. In the range of pH 4-12, the titer of phage remained above 10⁸ PFU/mL, and it remained above 10⁴ PFU/mL at pH 2, 3, 13, and 14. The combined application of phage STPX-6 and enrofloxacin, doxycycline, ampicillin could reduce the minimum inhibitory concentration (MIC) of the three antibiotics to 1/4 MIC, 1/16 MIC, and 1/2 MIC, respectively. <b><i>Conclusion:</i></b> This study found that for the host MRSA, lytic phage STPX-6 had the characteristics of a broad lytic spectrum, a short latent period, strong adaptability and strong tolerance to high temperature, a strong acid and strong alkali environment, and might maintain certain activity under extreme environment. More importantly, the combination of phage STPX-6 with enrofloxacin, doxycycline, and ampicillin could reduce the antibiotic concentration used for MRSA. In other words, phages as new antibacterial agents have received increasing attention. The combined application of phages and antibiotics provides a new method for controlling multidrug resistant bacteria and reduce the use of antibiotics.