Summary: Single components of virginiamycin M and S inhibit the growth of sensitive strains of The mixture of M and S halts completely the multiplication of double-sensitive organisms and of S-sensitive M-resistant mutants, but only partly inhibits growth of M-sensitive S-resistant cells, and is without effect on double-resistant mutants. Single virginiamycin components have essentially a bacteriostatic activity, whereas the association of M and S is lethal for double-sensitive bacteria as well as for S-sensitive M-resistant mutants. This lethal effect is a two-step process, which occurs without appreciable lysis. No bactericidal effect can be observed when S-resistant cells are incubated with a mixture of M and S. The patterns of inhibition of protein synthesis in different mutants are similar to those of growth inhibition and viability loss: the latter effects are therefore consequences of the former. Virginiamycin S can have different effects on protein synthesis: it enhances the action of M in S-sensitive cells but prevents the inhibitory action of M in S-resistant mutants. It can be concluded that both virginiamycin components interfere with growth, protein formation and cell viability. However, only the gene for S-sensitivity is essential (i) for the lethal action of the association of virginiamycin M and S, (ii) for the inactivation of cells submitted to an alternate treatment with the two virginiamycin components and (iii) for rendering permanent the inhibition of protein synthesis.


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