《第二章细菌》ppt课件.ppt

* Figure : Bacterial capsules outlined by India ink viewed by light microscopy. This is a true capsule, a discrete layer of polysaccharide surrounding the cells. Sometimes bacterial cells are embedded more randomly in a polysaccharide matrix called a slime layer or biofilm. Polysaccharide films that may inevitably be present on the surfaces of bacterial cells, but which cannot be detected visually, are called glycocalyx. ★ 幻灯片041.042） 有些细菌细胞外存在着一层厚度不定的胶状物质，称为荚膜。根据其厚度和边界情况的不同，常有不同的名称，例如微荚膜（microcapsule）、荚膜或粘液层（slime layer），有的细菌例如Zoogloea（动胶菌属）的菌种回产生有一定形状的大型粘胶物，称为菌胶团（zoogloea），它实质上是细菌群体的一个共同荚膜。荚膜可通过离心沉降（分层），而粘液层则是扩散到培养基中的胞外多糖，通过离心无法使它沉降，有时甚至将培养容器倒置时，培养液还是结成凝胶状。 观察方法：负染色 成分：纯多糖 葡聚糖：肠膜状明串珠菌等 多糖 纤维素：木醋杆菌 杂多糖：几种链球菌（如肺炎链球菌） 多肽： 炭疽芽孢杆菌荚膜成分 蛋白质：鼠疫巴氏杆菌 多肽和多糖：巨大芽孢杆菌 DNA：几种盐杆菌（生长在亚适量盐浓度下时形成） 主要功能：a)???? 抗干燥、抗吞噬；b)???? 贮存养料；c)???? 堆积代谢废物；d)???? 附着作用，如Streptococcus salivarius（唾液链球菌）和S.mutans（变异链球菌）。 应用：代血浆（Leucomostoc mesenteroides ）、葡聚糖凝胶制剂（Sephadex）、黄原胶（xanthan，Xanthomonas campestris）、污水处理等。 为害：食品变质发粘；增强致病力；造成严重龋齿。 ★CAPSULES Capsules are structures that lay outside of an organisms cell wall and thus are in direct contact with the environment. Many, perhaps most, bacteria produce capsules under the right conditions. Bacterial capsules are most often composed of long POLYMERS of sugar (or sugar derivatives) known as POLYSACCHARIDES. Some capsules are composed of POLYALCOHOLS or AMINO ACID POLYMERS. Capsules may be absent, thin, thick, well formed or loose. Capsule formation is dependent upon the nutrient conditions. In general, media rich in sugars and low in nitrogen tend to induce capsule formation. Capsules play a number of roles in the life of microbes. Both prokaryotes and eukaryotes form capsules. Capsules can serve the following functions: Protect the cell from desiccation (drying). Protect the cell from phagocytosis (being engulfed by white blood cells). Provide a food reserve when certain organic compounds are in exc