Angert E R, Clements K D, Pace N R
Department of Biology, Indiana University, Bloomington 47405.
Nature. 1993 Mar 18;362(6417):239-41. doi: 10.1038/362239a0.
The large, morphologically peculiar microorganism Epulopiscium fishelsoni inhabits the intestinal tract of Acanthurus nigrofuscus, a brown surgeonfish (family Acanthuridae) from the Red Sea. Similar microorganisms have been found in surgeonfish species from the Great Barrier Reef. As these microorganisms have only been seen in surgeonfish and no free-living forms have been found, they are considered to be specific symbionts of surgeonfish, although the nature of the symbiosis is unclear. Initial reports considered them to be eukaryotic protists, based primarily on their size, with individuals being larger than 600 microns by 80 microns. But their cellular morphology in the electron microscope is more like that of bacterial than eukaryotic cells. To resolve the nature of these symbionts, we have isolated the genes encoding the small subunit ribosomal RNA from two morphotypes and used them in a phylogenetic analysis. In situ hybridization with oligonucleotide probes based on the cloned rRNA sequences confirmed the source of the rRNA genes. Our result identify the symbionts as members of the low-(G+C) Gram-positive group of bacteria. They are therefore the largest bacteria to be described so far.
大型、形态独特的微生物费氏刺骨鱼菌栖息于黑背鼻鱼的肠道中，黑背鼻鱼是一种来自红海的棕色刺尾鱼（刺尾鱼科）。在大堡礁的刺尾鱼物种中也发现了类似的微生物。由于这些微生物仅在刺尾鱼中被观察到，且未发现自由生活的形态，因此它们被认为是刺尾鱼的特定共生体，尽管共生的性质尚不清楚。最初的报告主要基于其大小，认为它们是真核原生生物，个体大小超过600微米乘80微米。但它们在电子显微镜下的细胞形态更类似于细菌而非真核细胞。为了确定这些共生体的性质，我们从两种形态类型中分离出编码小亚基核糖体RNA的基因，并将其用于系统发育分析。基于克隆的rRNA序列的寡核苷酸探针原位杂交证实了rRNA基因的来源。我们的结果确定这些共生体为低（G+C）革兰氏阳性细菌群的成员。因此，它们是迄今为止所描述的最大的细菌。
