ANTIMICROBIAL ACTIVITY OF MICROORGANISMS ISOLATED FROM KEFIR GRAINS

Abstract Kefir is the product of milk fermentation by kefir grains. These grains contain a mixture of symbiotic bacteria and yeasts embedded in a matrix composed of polysaccharides and proteins. Many human health benefits have been attributed to kefir, including antimicrobial activity against Gram positive and Gram negative bacteria. We studied the antimicrobial activity of sixty bacterial strains isolated from kefir grains compared to Escherichia coli O157:H7, Salmonella enterica subsp. enterica serotypes Typhimurium and Enteritidis, Staphylococcus aureus, and Listeria monocytogenes through antagonism test. We tested the antimicrobial activity of lactic acid bacteria supernatants that were active in the antagonism test. The experiment was repeated using supernatants with neutralized pH. Salmonella Typhimurium and Enteritidis survived for 24 hours in kefir fermentation. E. coli O157:H7, S. aureus and L. monocytogenes were recovered up to 72 hours after the start of fermentation. All isolates showed antimicrobial activity against at least one of the pathogens used in the test of antagonism. Supernatants of 25 isolates showed inhibitory activity, and three maintained this activity with neutralized pH. Pathogenic bacteria studied survived for longer than the time normally used for fermentation of artisanal kefir, characterizing a potential danger to the consumer when the raw material does not provide health security. Lactobacillus isolated from kefir grains exhibit antimicrobial activity against strains of E. coli O157:H7, Salmonella serotypes Typhimurium and Enteritidis S. aureus, and L. monocytogenes in addition to that exerted by the lower pH values.

摘要：开菲尔（Kefir）是开菲尔粒（kefir grains）发酵牛乳的产物，该颗粒由包埋于多糖与蛋白质构成的基质中的共生细菌与酵母菌混合菌群组成。开菲尔已被证实具有多项对人体有益的生理功效，其中包括对革兰氏阳性菌（Gram positive）与革兰氏阴性菌（Gram negative）的抗菌活性。本研究通过拮抗实验，探究了60株从开菲尔粒中分离得到的细菌菌株的抗菌活性，实验对照菌株包括大肠埃希氏菌O157:H7（Escherichia coli O157:H7）、肠炎沙门氏菌肠炎亚种鼠伤寒血清型与肠炎血清型、金黄色葡萄球菌（Staphylococcus aureus）以及单核细胞增生李斯特菌（Listeria monocytogenes）。本研究进一步对拮抗实验中显现活性的乳酸菌上清液的抗菌活性进行了检测，并使用pH中和后的上清液重复了上述实验。鼠伤寒沙门氏菌与肠炎沙门氏菌在开菲尔发酵体系中可存活24小时；大肠埃希氏菌O157:H7、金黄色葡萄球菌及单核细胞增生李斯特菌则可在发酵启动后长达72小时内被分离检出。所有分离菌株均对本拮抗实验中使用的至少一种致病菌具有抗菌活性。其中25株分离菌株的上清液具有抑菌活性，且有3株菌株的上清液在pH中和后仍保留抑菌活性。本研究涉及的致病菌存活时长均长于手工开菲尔的常规发酵时长，这意味着当原料未达到健康安全标准时，会对消费者构成潜在健康风险。从开菲尔粒中分离得到的乳杆菌属菌株，除了通过降低发酵体系pH发挥抑菌作用外，还对大肠埃希氏菌O157:H7、鼠伤寒与肠炎血清型沙门氏菌、金黄色葡萄球菌以及单核细胞增生李斯特菌具有抗菌活性。