Measuring the pH of Pathogen-Containing Phagosomes

The environmental pH exerts a strong or even controlling effect on the survival and growth of microorganisms, both free living and intracellular. This reflects the importance of pH, both intracellular and environmental, to all organisms. Thus the acid pH of the phagosorne is a major contributor in the hostile environment presented to potential pathogens. This chapter uses pathogens that survive intracellularly to study the environment of the phagosome that occurs during the normal infection. This requires that both the pathogen and the host cell survive for the course of the experiment. This is a minimum requirement for the experiment to achieve its desired result and allow the study of the host-parasite relationship. The chapter uses the parasite labeled with reporting molecules to study the phagosomal pH during a cellular infection. Using the parasite to introduce the pH probe provides biologic specificity for its introduction into the parasite-phagosome space and thereby eliminates spurious signals from uninfected cells and from nearby but uninvolved vesicles in the infected cell. This is a critical advantage of the method and is not easily duplicated with other techniques.