<b>Identif</b><b>ication and characterization of a novel </b><b>prophage lysin</b><b> against </b><b><i>Streptococcus dysgalactiae</i></b>

<i>Streptococcus dysgalactiae</i> infection could cause bovine mastitis and lead to huge economic losses to dairy industry. The abuse of antibiotics has resulted in growing drug-resistance of <i>S. dysgalactiae</i> that<i> </i>cause hard-to-treat infections. Bacteriophage lysin, as a novel antibacterial agent, has great potential for application against drug-resistant gram-positive bacteria. However, few studies have been conducted on the prophage lysin of <i>S. dysgalactiae</i>. In this study, we mined a novel prophage lysin, named Lys1644, from a clinical <i>S. dysgalactiae </i>isolate, by genome sequencing and bioinformatic analysis. Lys1644 was expressed and purified, and the lytic activity, antibacterial spectrum, optimal pH and temperature, lytic activity in milk, synergistic bacteriostasis with antibiotics was assessed. The Lys1644 prophage lysin showed high bacteriolysis activity specifically on <i>S. dysgalactiae</i>, which results in CFU 100-fold reduction in milk. Moreover, Lys1644 maintained high activity over a wide pH range (pH 5–10) and a wide temperature range (4–42 ℃). Synergistic bacteriostatic experiments showed that the combination of low dose Lys1644 (50 μg/mL) and subinhibitory concentration of aminoglycoside antibiotics (kanamycin or spectinomycin) can completely inhibit bacterial growth, suggesting that the combination of Lys1644 and antibiotics could be an effective therapeutic strategy against <i>S. dysgalactiae </i>infection.停乳链球菌（Streptococcus dysgalactiae）感染可引发奶牛乳腺炎，给乳品行业造成巨额经济损失。抗生素滥用导致停乳链球菌（S. dysgalactiae）的耐药性日益增强，引发难治性感染。噬菌体溶素（bacteriophage lysin）作为新型抗菌剂，在对抗耐药革兰氏阳性菌方面具备极大应用潜力，但目前针对停乳链球菌的前噬菌体溶素（prophage lysin）的研究仍较为匮乏。本研究通过基因组测序与生物信息学分析，从一株临床分离的停乳链球菌菌株中挖掘得到一种新型前噬菌体溶素，将其命名为Lys1644。随后对Lys1644完成了表达与纯化，并对其溶菌活性、抗菌谱、最适pH与温度、牛奶中的溶菌活性以及与抗生素的协同抑菌效果进行了系统性评估。实验结果显示，Lys1644对停乳链球菌具有特异性的高效溶菌活性，可使牛奶中的菌落形成单位（CFU）降低100倍。此外，Lys1644在宽泛的pH范围（pH 5~10）与温度范围（4~42 ℃）内均能维持较高活性。协同抑菌实验表明，低剂量Lys1644（50 μg/mL）与亚抑菌浓度的氨基糖苷类抗生素（aminoglycoside antibiotics，卡那霉素或大观霉素）联合使用时，可完全抑制细菌生长，这提示Lys1644与抗生素联用有望成为治疗停乳链球菌感染的有效策略。