Zilelidou Evangelia, Manthou Evanthia, Skandamis Panagiotis
Agricultural University of Athens, Department of Food Science and Human Nutrition, Laboratory of Food Quality Control and Hygiene, Iera odos 75, 11855 Athens, Greece.
Agricultural University of Athens, Department of Food Science and Human Nutrition, Laboratory of Food Quality Control and Hygiene, Iera odos 75, 11855 Athens, Greece.
Int J Food Microbiol. 2016 Oct 17;235:60-70. doi: 10.1016/j.ijfoodmicro.2016.07.016. Epub 2016 Jul 12.
Listeria monocytogenes strains are widespread in the environment where they live well mixed, often resulting in multiple strains contaminating a single food sample. The occurrence of different strains in the same food might trigger strain competition, contributing to uneven growth of strains in food and to bias during selective procedures. We tested the growth of seven L. monocytogenes strains (C5, 6179, ScottA, PL24, PL25, PL26, PL27) on ham slices and on nutrient-rich agar at 10°C, singly and in combinations. Strains were made resistant to different antibiotics for their selective enumeration. In addition, growth of single strains (axenic culture) and competition between strains in xenic cultures of two strains was evaluated in enrichment broth and on selective agar. According to ISO 11290-1:1996/Amd 1:2004 standard protocol for detection of L. monocytogenes, two enrichment steps both followed by streaking on ALOA were performed. Strain cultures were directly added in the enrichment broth or used to inoculate minced beef and sliced hams which were then mixed with enrichment broth. 180-360 colonies were used to determine the relative percentage of each strain recovered on plates per enrichment step. The data showed a significant impact of co-cultivation on the growth of six out of seven strains on ham and a bias towards certain strains during selective enrichment. Competition was manifested by: (i) cessation of growth for the outcompeted strain when the dominant strain reached stationary phase, (ii) reduction of growth rates or (iii) total suppression of growth (both on ham and in enrichment broth or ALOA). Outgrowth of strains by their competitors on ALOA resulted in limited to no recovery, with the outcompeting strain accounting for up to 100% of the total recovered colonies. The observed bias was associated with the enrichment conditions (i.e. food type added to the enrichment broth) and the strain-combination. The outcome of growth competition on food or nonselective agar surface did not necessarily coincide with the results of competition during enrichment. The results show that certain strains present in foods may be missed during classical detection due to strain competition and such likelihood should be taken into consideration when resolving a listeriosis outbreak.
单核细胞增生李斯特菌菌株广泛存在于环境中，它们在环境中混合良好地生存，常常导致多种菌株污染单个食品样本。同一食品中不同菌株的出现可能引发菌株竞争，导致食品中菌株生长不均以及选择性检测过程中出现偏差。我们测试了七株单核细胞增生李斯特菌（C5、6179、ScottA、PL24、PL25、PL26、PL27）在10°C下单独及混合培养时在火腿片和营养丰富的琼脂上的生长情况。通过使菌株对不同抗生素产生抗性来进行选择性计数。此外，还在增菌肉汤和选择性琼脂上评估了单菌株（纯培养物）的生长以及两菌株混合培养物中菌株之间的竞争。根据ISO 11290-1:1996/Amd 1:2004检测单核细胞增生李斯特菌的标准方案，进行了两个增菌步骤，随后都在ALOA上划线。将菌株培养物直接加入增菌肉汤中，或用于接种碎牛肉和火腿片，然后将它们与增菌肉汤混合。每个增菌步骤用180 - 360个菌落来确定平板上回收的各菌株的相对百分比。数据显示，共培养对七株菌株中六株在火腿上的生长有显著影响，并且在选择性增菌过程中对某些菌株存在偏差。竞争表现为：（i）当优势菌株达到稳定期时，被竞争淘汰的菌株停止生长；（ii）生长速率降低；或（iii）生长完全受到抑制（在火腿上以及在增菌肉汤或ALOA上）。在ALOA上，竞争菌株的生长导致被竞争淘汰的菌株恢复有限或无法恢复，竞争菌株占回收菌落总数的比例高达100%。观察到的偏差与增菌条件（即添加到增菌肉汤中的食品类型）和菌株组合有关。在食品或非选择性琼脂表面上的生长竞争结果不一定与增菌过程中的竞争结果一致。结果表明，由于菌株竞争，经典检测过程中可能会遗漏食品中存在的某些菌株，在解决李斯特菌病暴发时应考虑到这种可能性。
