Dairy Research Institute, General Directorate of Agricultural Research, Hellenic Agricultural Organization DEMETER, Katsikas, 45221 Ioannina, Greece.
J Food Prot. 2020 Mar 1;83(3):542-551. doi: 10.4315/0362-028X.JFP-19-430.
Mixed thermophilic and mesophilic commercial starter cultures (CSCs), particularly those including Streptococcus thermophilus as a primary milk acidifier, have been found to reduce growth and counteract in situ nisin A (NisA+) antilisterial effects by the novel, indigenous Lactococcus lactis subsp. cremoris M78 costarter in traditional Graviera thermized milk cheese curds. Therefore, this model challenge study evaluated growth and in situ NisA+ activity of strain M78 in coculture with S. thermophilus ST1 singly in sterilized raw milk (SRM). Strain ST1, derived from a CSC for cheese, was challenged at two inoculation levels (5 and 7 log CFU/mL) in SRM against 6 and 3 log CFU/mL of strain M78 and Listeria monocytogenes, respectively. Pure cultures of each strain and cocultures of strain ST1 with the CSC L. lactis LL2, in replacement of strain M78, served as controls. At the high (7-log) inoculation level, the rapid, competitive growth (>9.3 log CFU/mL) of S. thermophilus ST1 reduced growth of both L. lactis by at least 10-fold; the industrial strain LL2 retained slightly higher relative population densities (7.4 to 9.1%) than the wild NisA+ strain M78 (3.8 to 5.6%) after 6 h at 37°C, followed by an additional 66 h of incubation at 22°C. In full contrast, at the low (5-log) inoculation level, S. thermophilus ST1 failed to predominate in SRM at 6 h; thus, the starter lactic acid bacteria populations were reversed in favor of L. lactis. Notably, strain M78 retained higher relative population densities (83.0 to 90.1%) than the CSC strain LL2 (80.3 to 85.2%) at 22°C. Moreover, at the 5-log ST1 level, the direct and deferred in situ NisA+ activities of strain M78 were at similar levels with its pure culture with L. monocytogenes in SRM, whereas at the 7-log ST1 level, the respective NisA+ effects were counteracted. Hence, 10- to 100-fold lowered inoculation levels of CSC S. thermophilus are required to enhance the performance of the M78 costarter in traditional Greek cheese technologies.
混合嗜热和中温商业起始培养物（CSC），特别是那些包含嗜热链球菌作为主要牛奶酸化剂的起始培养物，已被发现通过新型本土乳球菌乳脂亚种。在传统的 Graviera 热牛奶凝乳奶酪中，减少生长并抵消原位乳链菌肽 A（NisA+）抗李斯特菌的作用。因此，这项模型挑战研究评估了在无菌生牛乳（SRM）中单培养和与嗜热链球菌 ST1 共培养时，M78 株的生长和原位 NisA+活性。ST1 株来源于奶酪 CSC，在 SRM 中以 5 和 7 log CFU/mL 的两个接种水平分别对 6 和 3 log CFU/mL 的 M78 株和单核细胞增生李斯特菌进行挑战。每种菌株的纯培养物和以 L. lactis LL2 代替 M78 株的 CSC 乳球菌 LL2 的共培养物作为对照。在高（7-log）接种水平下，嗜热链球菌 ST1 的快速、竞争性生长（>9.3 log CFU/mL）使乳球菌至少减少 10 倍；工业菌株 LL2 在 37°C 下孵育 6 小时后，相对种群密度（7.4%至 9.1%）略高于野生 NisA+菌株 M78（3.8%至 5.6%），然后在 22°C 下再孵育 66 小时。与此形成鲜明对比的是，在低（5-log）接种水平下，嗜热链球菌 ST1 在 6 小时内未能在 SRM 中占优势；因此，起始乳酸菌的种群有利于乳球菌。值得注意的是，M78 株在 22°C 时的相对种群密度（83.0%至 90.1%）高于 CSC 菌株 LL2（80.3%至 85.2%）。此外，在 ST1 5-log 水平下，M78 株的直接和延迟原位 NisA+活性与其在 SRM 中与单核细胞增生李斯特菌的纯培养物相似，而在 ST1 7-log 水平下，各自的 NisA+效应则被抵消。因此，在传统的希腊奶酪技术中，需要降低 CSC 嗜热链球菌的接种水平 10-100 倍，以增强 M78 共培养物的性能。
