Growth of gamma-proteobacteria in cucumber fermentation is prevented by lactobacilli and the cover brine ingredients

Authors: ROTHWELL, MADISON - North Carolina State University; ZHAI, YAWEN - North Carolina State University; PAGAN MEDINA, CHRISTIAN - Former ARS Employee; Perez Diaz, Ilenys

Submitted to: Microbiology Spectrum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2022
Publication Date: 5/1/2022
Citation: Rothwell, M.A., Zhai, Y., Pagan Medina, C., Perez Diaz, I.M. 2022. Growth of gamma-proteobacteria in cucumber fermentation is prevented by lactobacilli and the cover brine ingredients. Microbiology Spectrum. 10(3):e01031-21. https://doi.org/10.1128/spectrum.01031-21.
DOI: https://doi.org/10.1128/spectrum.01031-21

Interpretive Summary: While the abundance of specific gamma-proteobacteria species varies among vegetable type, several harbor Enterobacteriaceae and Pseudomonadaceae that benefit the plant system. It is documented that such bacterial populations decrease in density early in vegetable fermentations. Consequently, it is assumed that they do not contribute to the quality of finished products. This study explored the viability of gamma-proteobacteria in Cucumber Juice Medium, used as a model system, and cucumber fermentations, which are characterized by an extremely acidic endpoint pH at 3.23 ± 0.17. The data presented demonstrates that cucumbers provide the nutrients needed by gamma-proteobacteria to proliferate and reduce pH to 4.47 ± 0.12. Control of gamma-proteobacteria in fermentations depends on the cover brine constituents and the indigenous competing microbiota. This knowledge is of importance when developing guidelines for the safe fermentation of vegetables.

Technical Abstract: This study investigated the ability of gamma-proteobacteria, indigenous to fresh cucumber, to grow in the expressed fruit juice (CJM) and fermentation. It was hypothesized that fresh cucumbers can support prolific growth of gamma-proteobacteria but that the cover brine composition and acid production by the competing lactobacilli in the fermentation of the fruit act as inhibitory agents. The gamma-proteobacteria proliferated in CJM with an average maximum growth rate (µmax) of 0.3895 ± 0.0929 and doubling time (Td) of 1.885 ± 0.465/h. A significant difference was found between the gamma-proteobacteria µmax and Td relative to Lactiplantibacillus pentosus LA0445 (0.2319 ± 0.019; 2.89/h) and Levilactobacillus brevis 7.2.43 (0.221 ± 0.015; 3.35/h) but not Lactiplantibacillus plantarum 3.2.8 (0.412 ± 0.119; 1.87/h). While inoculation level insignificantly altered the µmax and Td of the bacteria tested; it impacted the length of lag and stationary phases for the lactobacilli. Unlike the lactobacilli, the gamma-proteobacteria were inhibited in CJM supplemented with a low salt fermentation cover brine containing calcium chloride, acetic acid and potassium sorbate. The gamma-proteobacteria, P. agglomerans, was unable to proliferate in cucumber fermentations brined with calcium chloride at a pH of 6.0 ± 0.1 and the population of Enterobacteriaceae was outcompeted by the lactobacilli within 36 h. Together these observations demonstrate that the prolific growth of gamma-proteobacteria in CJM is not replicated in cucumber fermentation. While the gamma-proteobacteria growth rate is faster that most lactobacilli in CJM, their growth in cucumber fermentation is prevented by the cover brine and the acid produced by the indigenous lactobacilli. Thus, the lactobacilli indigenous to cucumber and cover brine composition influence the safety and quality of fermented cucumbers.