Uljas H E, Ingham S C
Department of Food Science, University of Wisconsin-Madison 53706, USA.
J Food Prot. 1998 Aug;61(8):939-47. doi: 10.4315/0362-028x-61.8.939.
Extreme acid tolerance of Escherichia coli O157:H7 has raised doubts about the safety of acidic foods. This study examined whether prior storage in acidic and/or cold conditions enhanced survival of E. coli O157:H7 in synthetic gastric fluid (SGF). Three E. coli O157:H7 strains were stored in trypticase soy broth (TSB; acidified with HCl, malic acid, citric acid, or lactic acid) or pH 3.5 and 6.5 (nonacidic control) apple juice at 4 and 21 degrees C for < or = 7 days and then were incubated in pH 2.5 SGF at 37 degrees C for 4 h. Cells survived better in apple juice than in TSB containing organic acids, suggesting that juice constituents other than organic acids protect E. coli O157:H7. Refrigeration combined with low pH best protected cells in apple juice and acidified TSB, but, compared to the nonacidic control, only acidified TSB enhanced subsequent survival in pH 2.5 SGF. Equal survival in SGF occurred after storage in pH 3.5 or 6.5 apple juice at 4 degrees C, suggesting that low temperature alone in apple juice enhanced acid tolerance. Two strains stored at 4 degrees C in TSB containing malic or citric acid subsequently survived better in SGF than cells stored in nonacidified TSB but poorer than cells stored in the presence of HCl. These differences reflect the higher pKa of these organic acids. However, subsequent survival of these strains in SGF was poorer after refrigerated storage in apple juice than in TSB containing citric or malic acids. Cells stored in lactic acid were most likely to be completely eliminated upon transfer to SGF. Differences in survival in storage media or SGF related to strain, storage conditions, or acidifier were consistent and often statistically significant (P < 0.05). Although the survival of E. coli O157:H7 in refrigerated acidic beverages may not be affected by the type of acidifier used, the subsequent survival in SGF of this pathogen may be critically dependent on this factor.
大肠杆菌O157:H7对极端酸性环境的耐受性引发了人们对酸性食品安全性的质疑。本研究考察了在酸性和/或低温条件下预先储存是否能提高大肠杆菌O157:H7在模拟胃液（SGF）中的存活率。将三株大肠杆菌O157:H7菌株分别储存在胰蛋白酶大豆肉汤（TSB；用盐酸、苹果酸、柠檬酸或乳酸酸化）或pH值为3.5和6.5（非酸性对照）的苹果汁中，于4℃和21℃下储存≤7天，然后在37℃的pH值为2.5的SGF中孵育4小时。细胞在苹果汁中的存活情况优于含有有机酸的TSB，这表明除了有机酸之外，果汁中的其他成分也能保护大肠杆菌O157:H7。冷藏结合低pH值能最好地保护苹果汁和酸化TSB中的细胞，但与非酸性对照相比，只有酸化TSB能提高后续在pH值为2.5的SGF中的存活率。在4℃下储存在pH值为3.5或6.5的苹果汁后，在SGF中的存活率相同，这表明仅苹果汁中的低温就能提高耐酸性。两株在含有苹果酸或柠檬酸的TSB中于4℃储存的菌株，随后在SGF中的存活情况优于储存在未酸化TSB中的细胞，但比在盐酸存在下储存的细胞差。这些差异反映了这些有机酸较高的pKa值。然而，这些菌株在苹果汁中冷藏储存后在SGF中的后续存活率比在含有柠檬酸或苹果酸的TSB中更低。储存在乳酸中的细胞转移到SGF后最有可能被完全清除。储存培养基或SGF中存活率的差异与菌株、储存条件或酸化剂有关，是一致的，且通常具有统计学意义（P<0.05）。虽然大肠杆菌O157:H7在冷藏酸性饮料中的存活率可能不受所用酸化剂类型的影响，但该病原体随后在SGF中的存活率可能严重依赖于这一因素。
