Objectives: Helicobacter pylori (H. pylori), present in a half of the world's population, is a very successful pathogen. The infection by this bacterium causes several gastric diseases in human. H. pylori is morphologically divided into two types; a spiral and a coccoid form. Both types are observed in human stomach. Although the former is converted into the latter in vitro, the process of coccoid formation remains obscure. Furthermore, whether coccoid forms possess viability arouses much controversy among scientists. We investigated both the process of coccoid formation by electron microscopy and the viability of coccoid H. pylori.

Methods: A laboratory strain, ATCC43504, was cultured in liquid medium (Brucella Broth medium with 10% heat inactivated horse serum) for seven days. In each culture day, the organisms were observed by scanning-, conventional transmission- and immuno-electron microscopy and were simultaneously examined the culturability of H. pylori to identify the viability of coccoid form.

Results: As the days went by, spiral forms were replaced by coccoid forms in each medium which possessed culturability to some degree until the 4th day. By the detailed observation of ultrastructural features, coccoid forms were classified into two types on the ground that represents different transformation-processes and distinctive ultrastructures. One was the coccoid form that we named Type A with an irregularly surface- and intracytoplasmic-structure which adhered to one another. The other was Type B having a smooth surface with flagella coiled around its own body and the strictly membranous structure. This type was not adhered to another organism. In later days, each type came to be similar external structure and lost the culturability.

Conclusions: The coccoid forms ofH. pylori can be classified into two types by electron microscopy which represent different transformation-processes and consist of the dying bacteria, the living ones with culturability and the viable but non-culturable ones.