Fluorescence labeling of bacteria for studies of intracellular pathogenesis

Interactions between intracellular bacterial pathogens and their eukaryotic cellular hosts or targets are often studied with fluorescence-based techniques such as fluorescence microscopy and flow cytometry. We tested whether the intracellular bacterial pathogens L. monocytogenes, M. avium, M. tuberculosis, and S. typhimurium could be labeled by growth in broth containing the fluorochromes carboxy-X-rhodamine (CR), a hydrazine derivative of fluorescein (FH), and Lucifer Yellow CH (LY). Only Listeria were labeled by all three fluorochromes, Salmonella took up only FH, whereas M. avium and M. tuberculosis were labeled by FH and LY. In general, the fluorochromes did not affect bacterial growth or viability, although FH inhibited the growth of M. tuberculosis. Fluorescent Listeria and M. tuberculosis were used to demonstrate that FH- and LY-labeled bacteria were bright enough after phagocytosis by macrophages to distinguish phagocytic from nonphagocytic cells by flow cytometry. To test whether the fluorochromes might alter bacterial interactions with host cells, we measured both phagocytosis of fluorescent Listeria by macrophages and subsequent bacterial replication in these cells. In these experiments, labeled and unlabeled Listeria were phagocytosed similarly by macrophages and were not impaired in their ability to replicate within them. Thus, this method for fluorescence labeling of bacteria is useful for studying physiologic macrophage:bacteria interactions.