HiMedia – M1340_HiCrome™ MacConkey Sorbitol Agar Base

Principle and Interpretation
Sorbitol MacConkey Agar is based on the formulation described by Rappaport and Henigh (4). The medium contains sorbitol instead of lactose and it is recommended for the detection of enteropathogenic strains of Escherichia coli O157:H7 that ferments lactose but does not ferment sorbitol (2) and hence produce colourless colonies. Sorbitol fermenting strains of Escherichia coli produce pink-red colonies. The red colour is due to production of acid from sorbitol, absorption of neutral red and a subsequent colour change of the dye when pH of the medium falls below 6.8. Escherichia coli O157:H7 has been recognised as a cause of hemorrhagic colitis (2). March and Ratnam (3) reported that the detection of Escherichia coli O157:H7 had a sensitivity of 100% and specificity of 85% on Sorbitol MacConkey Agar and they recommended this medium as reliable means of screening Escherichia coli O157:H7. B.C. indicator is added to detect the presence of the enzyme b-D-glucuronidase which is specific for Escherichia coli. (1). Strains of Escherichia coli fermenting sorbitol and possessing b-Dglucuronidase appear as blue - purple coloured colonies on the medium. Enteropathogenic strains of Escherichia coli O157:H7 do not possess b-D-glucuronidase activity (5) and thus produce colourless colonies. Tryptone and proteose peptone provide carbonaceous, nitrogenous and other essential growth nutrients. Most of the gram-positive organisms are inhibited by crystal violet and bile salts. Sodium chloride maintains the osmotic equilibrium.

原理與解釋
山梨醇MacConkey瓊脂基於Rappaport和Henigh（4）描述的配方。該培養基含有山梨糖醇而不是乳糖，推薦用於檢測大腸桿菌O157：H7的腸致病性菌株，該菌株發酵乳糖但不發酵山梨糖醇（2），因此產生無色菌落。山梨醇發酵菌株的大腸桿菌產生粉紅色菌落。紅色是由於山梨糖醇產生的酸，中性紅的吸收以及當介質的pH低於6.8時染料隨後的顏色變化。大腸埃希氏菌O157：H7已被認為是出血性結腸炎的原因（2）。 March和Ratnam（3）報導，大腸桿菌O157：H7的檢測靈敏度為100％，對山梨醇MacConkey瓊脂的特異性為85％。 推薦這種培養基作為篩選大腸桿菌O157：H7的可靠方法。公元前加入指示劑以檢測對大腸桿菌特異的酶b-D-葡醣醛酸糖苷酶的存在。 （1）。 發酵山梨糖醇和具有b-Dglucuronidase的大腸桿菌菌株在培養基上顯示為藍紫色菌落。 腸致病性大腸桿菌O157：H7菌株不具有b-D-葡醣醛酸糖苷酶活性（5），因此產生無色菌落。 胰蛋白腖和蛋白腖提供碳質，含氮和其他必需的生長營養素。大多數革蘭氏陽性生物被結晶紫和膽鹽抑制。氯化鈉保持滲透平衡。