Utilization of External Growth Factors by Intracellular Microbes: Mycobacterium paratuberculosis and Wood Pigeon Mycobacteria

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Abstract
The extent to which the intracellular growth of microbes is dependent upon capacities for growth in vitro has been investigated by use of organisms which require a specific factor, mycobactin, for isolation and growth on conventional mycobacterial media. Similarities between growth on bacteriological media and within tissue cells have been demonstrated by examining the responses of Mycobacterium paratuberculosis and wood pigeon mycobacteria to mycobactin, iron, CO2, and glycerol, and to the low pH which characterizes phagocytic vacuoles. The results indicate that success of intracellular infections depends upon the independent growth of microbes and that the phagocytic vacuoles of sheep and mouse macrophages seem to be freely accessible to bacterial growth factors, minerals and substrates. Because these factors did not modify the appearance, overall metabolism, or surface properties of macrophages, it was concluded that the intracellular growth of microbes is not determined solely by the components, metabolism, or immunological properties of host cells, but is influenced to an important degree by compounds and conditions provided by extracellular environments.