Coccoid morphology as a possible manifestation of Helicobacter pylori adaptation to adverse environments

After characterization of preferred conditions for Helicobacter pylori survival in the sessile state, it was observed that the bacterium transforms from spiral to coccoid under mild circumstances, whereas under extreme ones it is unable to undergo shape modification. This strongly supports the view that transformation into the coccoid form is an active, biologically led process, switched on by the bacterium as a protection mechanism.All living organisms are equipped with mechanisms that allow extended survival under adverse environments. For a number of them, this response involves, besides metabolic adaptations, changes in cell morphology (24). Similarly, the gastrointestinal pathogen Helicobacter pylori is known to mainly present a spiral shape in the natural habitat within the human host, but it converts into a coccoid shape when exposed to detrimental environmental circumstances (2). In this case, however, the pleiomorphic nature of the bacterium has been the subject of intensive debate over the last 10 years, with part of the scientific community still maintaining that the coccoid shape represents a degraded, nonviable form of the cell (8, 13, 19, 25). There are several factors contributing to this situation. (i) When H. pylori transformation into the coccoid form occurs, the cells enter a nonculturable state and are unable to be revived when placed under optimum growth conditions. (ii) Reversion trials (i.e., transformation from the coccoid form to the spiral form) have not been successful so far. (iii) There appears to be little metabolic activity and modification of physiology of the bacterium during conversion. (iv) Transformation to the coccoid form always appears to occur in what are thought to be the most adverse environments when the cells have no chance of survival. On the other hand, several reports have argued that coccoid cells might constitute a survival strategy in adverse environmental conditions (4, 10, 11, 22). The main argument for this is the existence of a state named viable but nonculturable (18, 27, 29). Viable but nonculturable bacteria also tend to possess little activity, which provides an alternative explanation for some of the phenomena observed for H. pylori