Department of Food Science and Technology, Laboratory of Microbiology and Biotechnology of Foods, Agricultural University of Athens, Iera Odos 75, Athens 11855, Greece.
Int J Food Microbiol. 2011 Feb 28;145(2-3):400-6. doi: 10.1016/j.ijfoodmicro.2011.01.001. Epub 2011 Jan 11.
The aim of this study was to investigate the potential effect of adaptive stationary phase acid tolerance response (ATR) of Listeria monocytogenes Scott A cells on their attachment to stainless steel (SS) under low pH or/and high salt conditions and on the subsequent resistance of sessile cells to strong acid challenge. Nonadapted or acid-adapted stationary-phase L. monocytogenes cells were used to inoculate (ca. 10⁸ CFU/ml) Brain Heart (BH) broth (pH 7.4, 0.5% w/v NaCl) in test tubes containing vertically placed SS coupons (used as abiotic substrates for bacterial attachment). Incubation was carried out at 16 °C for up to 15 days, without any nutrient refreshment. L. monocytogenes cells, prepared as described above, were also exposed to low pH (4.5; adjusted with HCl) or/and high salt (5.5% w/v NaCl) stresses, during attachment. On the 5th, 10th and 15th day of incubation, cells attached to SS coupons were detached (through bead vortexing) and enumerated (by agar plating). Results revealed that ATR significantly (p<0.05) affected bacterial attachment, when the latter took place under moderate acidic conditions (pH 4.5, 0.5 or 5.5% w/v NaCl), with the acid-adapted cells adhering slightly more than the nonadapted ones. Regardless of acidity/salinity conditions during attachment, ATR also enhanced the resistance of sessile cells to subsequent lethal acid challenge (exposure to pH 2 for 6 min; pH adjusted with either hydrochloric or lactic acid). The trend observed with viable count data agreed well with conductance measurements, used to indirectly quantify remaining attached bacteria (following the strong acid challenge) via their metabolic activity. To sum, this study demonstrates that acid adaptation of L. monocytogenes cells during their planktonic growth enhances their subsequent attachment to SS under extended exposure (at 16 °C for up to 15 days) to mild acidic conditions (pH 4.5), while it also improves the resistance of sessile cells to extreme acid treatment (pH 2). Therefore, the ATR of bacterial cells should be carefully considered when applying acidic decontamination strategies to eradicate L. monocytogenes attached to food processing equipment.
本研究旨在探究李斯特菌 Scott A 细胞的适应性稳定相酸耐受反应 (ATR) 对其在低 pH 值或/和高盐条件下附着于不锈钢 (SS) 的潜在影响，以及随后对固定细胞耐受强酸冲击的抗性。将未经适应或酸适应的稳定相李斯特菌细胞用于接种 (约 10⁸ CFU/ml) 脑心 (BH) 肉汤 (pH 7.4，0.5% w/v NaCl)，置于含有垂直放置 SS 试片的试管中（用作细菌附着的非生物基质）。在 16°C 下孵育，最长可达 15 天，不进行任何营养更新。如上所述制备的李斯特菌细胞也在附着过程中暴露于低 pH 值 (4.5；用 HCl 调节) 或/和高盐 (5.5% w/v NaCl) 应激下。在孵育的第 5、10 和 15 天，通过珠状涡旋将附着在 SS 试片上的细胞分离 (detached) 并进行计数 (agar plating)。结果表明，ATR 显著 (p<0.05) 影响了细菌附着，当附着发生在中度酸性条件下 (pH 4.5，0.5 或 5.5% w/v NaCl) 时，适应酸的细胞附着略多于未适应酸的细胞。无论附着过程中的酸度/盐度条件如何，ATR 还增强了固定细胞对随后致命酸冲击的抗性 (暴露于 pH 2 6 分钟；用盐酸或乳酸调节 pH)。与存活计数数据一致的趋势与电导测量结果吻合良好，该方法通过代谢活性间接量化强酸冲击后剩余附着的细菌 (quantify)。总之，本研究表明，李斯特菌细胞在浮游生长过程中的酸适应增强了其在温和酸性条件下 (pH 4.5) 对 SS 的后续附着，而在极端酸性处理 (pH 2) 下也提高了固定细胞的抗性。因此，在应用酸性消毒策略根除附着在食品加工设备上的李斯特菌时，应仔细考虑细菌细胞的 ATR。
