The Mechanism of Bacterial and Fungus Growth Inhibition by 2-Methyl-1,4-Naphthoquinone

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Abstract
The inhibitory effect of certain synthetic naphthoquinones on the growth of Staphylococcus aureus in broth, Escherichia coli in a chemically denned medium and certain molds in malt extract agar was studied. Excesses on a molar basis of [long dash]SH compounds and of certain reducing agents (NaHSO3 and Na2S2O4 suppressed anti-growth properties of 2-methyl-l,4-naphthoquinone and 2-methyl-3-chloro-l,4-naphthoquinone, but not those of 2-methyl-3-methoxy-l,4-naphthoquinone. The weaker reducing agents, Na2S2O3 and KCOOH, had no effect. Excesses of 2-methyl-l,4-naphthoquinone and 2-methyl-3-chloro-1,4-naphthoquinone with cysteine or thioglycolate produced colored precipitates and negative nitroprusside tests for [long dash]SH groups; excesses of the methoxy quinone with cysteine or thioglycolate resulted in no precipitate and positive nitroprusside tests. The reaction between [long dash]SH compounds and 2-methyl-l,4-naphthoquinone, known to occur at the 3-position on the quinone ring, apparently blocks a site important in antibacterial and antifungal potency, suggesting that this quinone exerts its antigrowth effects by combining with essential [long dash]SH enzymes or metabolites at this site. The methoxy group in. this position apparently hinders the addition of[long dash]SH compounds. The methoxy quinone, however, is inhibitory to growth even in the presence of [long dash]SH compounds, indicating that, in spite of its structural similarity, it either functions by a mechanism different from that proposed for 2-methyl-l,4-naphthoquinone, or that the mode of action proposed for the latter is incomplete.