Abstract
Zymomonas mobilis is an ethanologenic, facultatively anaerobic alpha-proteobacterium, known for its inhibitory effect on the growth of a wide variety of microorganisms. This property might be interesting for the design of novel antimicrobials, yet it has negative implications for biotechnology, as it hinders the use of Z. mobilis as a producer microorganism in cocultivation. So far, the chemical nature of its inhibitory compound(s) has not been established. In the present study, we demonstrate that the putative inhibitor is a low-molecular-weight (below 3 kDa), thermostable compound, resistant to protease treatment, which is synthesized under aerobic conditions in Z. mobilis strains via the active respiratory chain. It is also synthesized by aerated nongrowing, glucose-consuming cells in the presence of chloramphenicol, thus ruling out its bacteriocin-like peptide nature. The inhibitory activity is pH-dependent and strongly correlated with the accumulation of propionate and acetate in the culture medium. Although, in Z. mobilis, the synthesis pathways of these acids still need to be identified, the acid production depends on respiration, and is much less pronounced in the non-respiring mutant strain, which shows low inhibitory activity. We conclude that propionate and acetate play a central role in the antimicrobial effects of Z. mobilis, which itself is known to bear high resistance to organic acids.
Highlights
Z. mobilis strains were screened for their antimicrobial activity by detecting the inhibition zones in the agar overlays with growing E. coli DH5α cells
We found that the antimicrobial activity was generated by Zymomonas respiratory catabolism and was independent of cell growth and protein biosynthesis
The Z. mobilis antimicrobial activity is much weaker under anaerobic conditions or in the non-respiring mutant, where ethanologenesis proceeds at its highest rate
Summary
The Krebs cycle and the pentose phosphate pathway are incomplete, and the glyoxylate shunt is absent [3,4] This bacterium possesses an active aerobic respiratory chain with oxygen consumption rates comparable to those of Escherichia coli or Corynebacterium glutamicum, albeit with atypically low degree of energy coupling [5–7]. In order to fully exploit its biotechnological potential, metabolic engineering aims to broaden the substrate and product spectra of Z. mobilis beyond the mere bioethanol synthesis with glucose, fructose, or sucrose-containing substrates [8–10]. This implies the design of novel bioprocesses, involving cocultivation with other producer microorganisms
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