HiMedia – M1862 HiCrome™ M-Modified ECO157:H7 Selective Agar Base

Principle and Interpretation
Escherichia coli O157:H7 belongs to the Enterohemorrhagic Escherichia coli (EHEC) group and it predominates as a food borne pathogen. E.coli O157:H7 was first recognized as a human pathogen in 1982 when two outbreaks of hemorrhagic colitis were associated with consumption of undercooked ground beef that has been contaminated with this organism (3) that results from the action of a shiga-like toxin (SLT)(1, 7). This medium is recommended for isolation of enteropathogenic Escherichia coli O157:H7 in meats, poultry, dairy foods, infant formula, liquid eggs, mayonnaise and apple cider (4, 5). The medium is based on three differential biochemical reactions - lysine decarboxylase (positive for typical EHEC O157 strains), sorbitol fermentation and beta-glucuronidase (2). This medium is also used for the enumeration of b- glucuronidase-positive E.coli from foods (6). Peptone and yeast extract provides nitrogenous and carbonaceous compounds, long chain amino acids and other essential growth nutrients. Sodium chloride maintains the osmotic environment of the medium. Bacteria which were able to grow on this medium will ferment dextrose first. Once dextrose has been depleted, sorbitol positive bacteria will begin to ferment sorbitol, producing a drop in pH of medium, which produces yellow colour to the colony due to phenol red which is a pH indicator. Glucuronidase positive E.coli will break down X-Gluc, resulting in the production of an insoluble blue precipitate in the colony. This will combine with the colour of the pH indicator dye to produce a green colony in case of sorbitol positive or lysine negative bacteria. This medium also contains lysine, lysine positive organisms decarboxylates lysine which produces an increase in pH of medium, hence produces pink coloured colonies. Selectivity is achieved through the use of monensin (FD295) which inhibits gram positive bacteria and incubation at 44 - 44.5°C inhibits gram negative bacteria. Most of the other organisms are unable to grow and if any develop yellow colonies.

原理與解釋
大腸桿菌O157：H7屬於腸出血性大腸桿菌（EHEC）組，並且它主要作為食源性病原體。大腸桿菌O157：H7在1982年首次被認為是一種人類病原體，當時兩次爆發的出血性結腸炎與食用被這種生物體污染的未煮熟的碎牛肉有關（3）是由志賀樣毒素的作用引起的（SLT）（1,7）。該培養基推薦用於分離肉類，家禽，乳製品，嬰兒配方奶粉，液體蛋，蛋黃醬和蘋果酒中的腸致病性大腸桿菌O157：H7（4,5）。該培養基基於三種不同的生化反應 - 賴氨酸脫羧酶（典型EHEC O157菌株陽性），山梨糖醇發酵和β-葡醣醛酸酶（2）。該培養基還用於從食物中計數b-葡醣醛酸糖苷酶陽性大腸桿菌（6）。 蛋白腖和酵母提取物提供含氮和含碳化合物，長鏈氨基酸和其他必需的生長營養素。氯化鈉維持培養基的滲透環境。能夠在這種培養基上生長的細菌將首先發酵葡萄糖。一旦葡萄糖耗盡，山梨糖醇陽性細菌將開始發酵山梨糖醇，導致培養基的pH下降，由於酚紅是pH指示劑，其對菌落產生黃色。葡醣醛酸酶陽性大腸桿菌將分解X-Gluc，導致在菌落中產生不溶的藍色沉澱。這將與pH指示劑染料的顏色結合，以在山梨糖醇陽性或賴氨酸陰性的情況下產生綠色菌落 菌。該培養基還含有賴氨酸，賴氨酸陽性生物使賴氨酸脫羧，這會使培養基的pH升高，從而產生粉紅色的菌落。通過使用抑制革蘭氏陽性細菌的莫能菌素（FD295）實現選擇性，並且在44-44.5℃溫育以抑制革蘭氏陰性細菌。大多數其他生物無法生長，如果有任何發育黃色菌落。