Manganese modulates metabolic activity and redox homeostasis in translationally-blockedLactococcus cremoris, impacting metabolic persistence, cell-culturability, and flavor formation

Abstract

Manganese (Mn) is an essential trace element that is supplemented in microbial media with varying benefits across species and growth conditions. We found that growth of Lactococcus cremoris was unaffected by manganese omission from the growth medium. The main proteome adaptation to manganese omission involved increased manganese transporter production (up to 2000-fold), while the remaining 10 significant proteome changes were between 1.4 and 4 fold. Further investigation in translationally-blocked (TB), non-growing cells showed that Mn supplementation (20 μM) led to approximately 1.5X faster acidification compared to Mn-free conditions. However, this faster acidification stagnated within 24 hours, likely due to draining of intracellular NADH that coincides with substantial loss of culturability. Conversely, without manganese, non-growing cells persisted to acidify for weeks, albeit at a reduced rate, but maintaining redox balance and culturability. Strikingly, despite being unculturable, α-keto acid-derived aldehydes continued to accumulate in cells incubated in the presence of manganese, whereas without manganese cells predominantly formed the corresponding alcohols. This is most likely reflecting NADH availability for the alcohol dehydrogenase-catalyzed conversion. Overall, manganese influences the lactococcal acidification rate, and flavor formation capacity in a redox dependent manner. These are important industrial traits especially during cheese ripening, where cells are in a non-growing, often unculturable state.