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Impact of Physiological Neutral pH on Morphology of Francisella tularensis – An Electron Microscopic Study

Francisella tularensis is an infectious, gram-negative, coccobacilli bacterium that is responsible for causing the disease Tularemia, via varying routes in both animal and human. F. tularensis is a facultative intracellular bacterium which grows in the cytosol of host cells and in media of pH ~5.5 to 7.5. The overall physiological neutral pH of most niches of the human body is about 7.4, therefore, any effective pathogen must be able to not only survive but also proliferate at this slightly alkaline pH, especially in many intracellular conditions. Our study demonstrates that F. tularensis subsp holarctica grows at a reduced rate at pH7.4 compared to pH6.4, suggesting that pH7.4 presents a challenge to which the bacterium must adapt. Although ultrastructure of F. tularensis in acidic endosomes and lysosomes as well as in neutral pH of cytosol, particularly in macrophages, has been reported, the impact of neutral pH environment on morphology of the bacteria has not been reported. In this study, we used different EM techniques to reveal morphological differences between F. tularensis grown in media at pH6.4 and 7.4.