Apart from MK’s role in shuttling electrons between complexes in the electron transport chain, MK and its biosynthetic intermediates may play greater physiological roles in bacteria than previously appreciated. Shewanella oneidensis utilizes a derivative of DHNA to degrade carbon tetrachloride (53). In S. aureus, MK potentiates spermine and heme toxicity independently of MK’s function in the electron transport chain. Small-colony variants with mutations specifically in the MK biosynthesis pathway lead to increased resistance to spermine and heme (36, 54). Given that MK, DMK, and DHNA share naphthoquinone rings with similar redox characteristics, we surmise that these molecules may act as cofactors in chemical reactions crucial to the survival of L. monocytogenes in macrophages. It is also possible that L. monocytogenes uses DHNA as a substrate for synthesis of noncanonical MK (55) or another yet-to-be-characterized molecule. Alternatively, it has been proposed that two-component systems, such as ArcAB in Escherichia coli and SrrAB in Staphylococcus aureus, may sense changes in the redox state of MK as signals for environmental changes (56, 57). We posit that MK, DMK, and/or DHNA may be used as a sensor(s) of cytosolic stress. In the absence of these molecules, L. monocytogenes may not be able to adapt to this specific host environment.