The effect of
dicyclohexylamine on seven freshly isolated bacterial strains of
mastitis pathogens was studied. Streptococcus uberis was the most sensitive strain investigated, since 5 mM-
dicyclohexylamine totally arrested its growth and 1.25 mM of the
drug caused 60% growth inhibition. The Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa strains were also sensitive to the
drug, but less so than Strep. uberis, since 5 mM
drug caused only partial inhibition of growth. Micrococcus sp. and Klebsiella sp. grew in the presence of 10.0 mM-
dicyclohexylamine, and, finally the growth of Streptococcus agalactiae was not at all affected by
dicyclohexylamine. These different sensitivities towards
dicyclohexylamine in vivo were paralleled by different sensitivities of the bacteria's
spermidine synthase to the
drug in vitro, and also by the ability of the
drug to lower
spermidine concentration in bacterial cells.
Spermidine synthase from sensitive bacteria was inhibited by more than 90% by 50 microM-
dicyclohexylamine in vitro, and the concentration of
spermidine was decreased in E. coli and Ps. aeruginosa by 70% and in Strep. uberis by 95%, whereas in Strep. agalactiae 5 mM-
dicyclohexylamine did not affect the concentration of
spermidine at all.
Dicyclohexylamine treatment led to the accumulation of
putrescine in Strep. uberis.
Spermidine synthesis catalysed by the extracts of Micrococcus sp. required 500 microM-
dicyclohexylamine for 90% inhibition, and Strep. agalactiae contained a
spermidine synthase that was still active at 1000 microM-
dicyclohexylamine, The observed inhibition of growth was totally reversed by adding 50 microM-
spermidine (final concentration) to the medium.
Putrescine reversed the inhibition only when bacteria had a
spermidine synthase activity insensitive to
dicyclohexylamine.
Spermine did not overcome the inhibition of growth caused by
dicyclohexylamine, probably because it was not taken up by the bacterial cells used in this study. The inhibition of the growth by
dicyclohexylamine (even in the case of Strep. uberis) was reversible in the sense that addition of 50 microM-
spermidine 18 h after
dicyclohexylamine still restored the growth rate of untreated controls.