The effect of progesterone on the growth and virulence of Listeria monocytogenes

Progesterone was examined for action on the virulence of Listeria monocytogenes and the toxicity of its haemolysin. Progesterone at concentrations between 5 and 20 micrograms/ml reduced the numbers of L. monocytogenes over the first two hours of growth. Virulence and haemolysin toxicity were assessed using the allantoic sac route of inoculation into embryonated hens eggs. Increasing the concentrations of progesterone in the culture medium resulted in a decrease in the mortality of chick embryos inoculated with either organisms, or cell-free extracts or purified haemolysin. Progesterone had no effect on the lethality of pre-formed haemolysin.