Probing the phagolysosomal environment of human macrophages with a Ca2+-responsive operon fusion in Yersinia pestis.

Several microorganisms, including Yersinia sp., Salmonella sp., Brucella sp., Mycobacterium sp. and Leishmania sp., have successfully adapted to grow within macrophage phagolysosomes. Infections caused by these intracellular pathogens are among the most difficult to treat. As part of an antimicrobial strategy directed at modifying the phagolysosomal environment to the disadvantage of these important pathogens, we are defining the ambient conditions within the organism-containing phagolysosome. To probe this environment, we have used Yersinia pestis, whose expression of several virulence attributes is highly dependent on the Ca2+ concentration in its growth environment. We first genetically engineered a strain of Y. pestis which responds to a low-calcium environment by transcription of inserted structural genes of the Escherichia coli lac operon. Using this mutant organism as a relevant biological probe, we demonstrate here that the calcium concentration in Y. pestis-containing phagolysosomes is sufficiently low to permit virulence gene expression; this resolves the question of where Y. pestis might express its Ca2+-regulated genes in vivo.