Legionella pneumophila filamentous morphological form and the generation of novel spherical form upon interaction with protozoan hosts

• Main Author: Chan, Sock Hoai
• Other Authors: Sze Chun Chau
• Format: Theses and Dissertations
• Language: English
• Published: 2012
• Subjects: DRNTU::Science::Biological sciences::Microbiology::Microbial ecology; DRNTU::Science::Biological sciences::Microbiology::Bacteria
• Online Access: https://hdl.handle.net/10356/50797
• Institution: Nanyang Technological University

The filamentous form of Legionella pneumophila is acknowledged to be an alternative morphological form of the bacterium in nature, but its characteristics and ecological relevance in the life cycle of L. pneumophila is poorly understood. In this study, we explored the environmental factors that can trigger formation of the filamentous form as well as the interaction of this morphological form with L. pneumophila’s natural protozoan hosts. Our investigation showed that filamentation cannot be neatly pegged to a single dominant factor but is promoted under a concerted influence of environmental conditions leaning towards (i) higher than ambient temperatures, at 37-42°C, (ii) static fluid movement, and (iii) with at least a minimal organic content of 5% buffered yeast extract, or equivalent. In the course of investigating interaction of the filamentous form with protozoan hosts, a spherical form was discovered. Ultrastructural analysis confirmed the cellular nature of this novel entity and revealed morphological variations that are suggestive of a transitional relationship between the filamentous and the spherical forms. In vitro characterization demonstrated that emergence of the spherical form is (i) preceded by the filamentous form, (ii) rapidly triggered by some soluble factors secreted by protozoa, and (iii) independent of de novo protein synthesis. Exposure of a filament-rich bacterial population to secreted factors of protozoa resulted in the decline in abundance of the filamentous form that is paralleled by a transient surge in the spherical form and a gradual prevalence of the bacillary form; indicating that the spherical form may be an intermediate in the filament-to-rod morphological transition in L. pneumophila. Collectively, these findings suggest that under an intricate balance of environmental factors, the predominantly bacillary L. pneumophila is capable of differentiating into the filamentous form, which upon interaction with protozoan hosts may be triggered to differentiate rapidly back into the bacillary form via a spherical intermediate.