Acidified nitrite inhibits proliferation of Listeria monocytogenes — Transcriptional analysis of a preservation method

• Nitrite inhibited L. monocytogenes growth in short-ripened spreadable raw sausages.
• Nitrite inhibited proliferation of L. monocytogenes in vitro.
• The inhibitory effect of nitrite was strongest in combination with acidification.
• Acidified nitrite caused massive transcriptional changes in L. monocytogenes.
• Acidified nitrite induced a general stress response in L. monocytogenes.

Sodium nitrite (NaNO2) is added as a preservative during raw meat processing such as raw sausage production to inhibit growth of pathogenic bacteria. In the present study it was shown in challenge assays that the addition of sodium nitrite indeed inhibited growth and survival of Listeria monocytogenes in short-ripened spreadable raw sausages. Furthermore, in vitro growth analyses were performed, which took into account combinations of various parameters of the raw sausage ripening process like temperature, oxygen availability, pH, NaCl concentration, and absence or presence of NaNO2. Data based on 300 growth conditions revealed that the inhibitory effect of nitrite was most prominent in combination with acidification, a combination that is also achieved during short-ripened spreadable raw sausage production. At pH 6.0 and below, L. monocytogenes was unable to replicate in the presence of 200 mg/l NaNO2. During the adaptation of L. monocytogenes to acidified nitrite stress (pH 6.0, 200 mg/l NaNO2) in comparison to acid exposure only (pH 6.0, 0 mg/l NaNO2), a massive transcriptional adaptation was observed using microarray analyses. In total, 202 genes were up-regulated and 204 genes were down-regulated. In accordance with growth inhibition, a down-regulation of genes encoding for proteins which are involved in central cellular processes, like cell wall/membrane/envelope biogenesis, translation and ribosomal structure and biogenesis, transcription, and replication, recombination and repair, was observed. Among the up-regulated genes the most prominent group belonged to poorly characterized genes. A considerable fraction of the up-regulated genes has been shown previously to be up-regulated intracellularly in macrophages, after exposure to acid shock or to be part of the SigB regulon. These data indicate that the adaptation to acidified nitrite partly overlaps with the adaptation to stress conditions being present during host colonization.