Secretome Analysis of the Plant Biostimulant Bacteria Strains Bacillus subtilis (EB2004S) and Lactobacillus helveticus (EL2006H) in Response to pH Changes

It is well-known that there is a high frequency of plant-growth-promoting strains in Bacillus subtilis and that these can be effective under both stressful and stress-free conditions. There are very few studies of this activity in the case of Lactobacillus helveticus. In this study, the effects of pH on the secretome (proteins) in the cell-free supernatants of two bacterial strains were evaluated. The bacteria were cultured at pH 5, 7 and 8, and their secretome profiles were analyzed, with pH 7 (optimal growth pH) considered as the “control”. The results showed that acidity (lower pH 5) diminishes the detectable production of most of the secretome proteins, whereas alkalinity (higher pH 8) increases the detectable protein production. At pH 5, five (5) new proteins were produced by L. helveticus, including class A sortase, fucose-binding lectin II, MucBP-domain-containing protein, SLAP-domain-containing protein and hypothetical protein LHEJCM1006_11110, whereas for B. subtilis, four (4) types of proteins were uniquely produced (p ≤ 0.05), including helicase-exonuclease AddAB subunit AddB, 5-methyltetrahydropteroyltriglutamate-homocysteine S-methyltransferase, a cluster of ABC-F family ATP-binding-cassette-domain-containing proteins and a cluster of excinuclease ABC (subunit B). At pH 8, Bacillus subtilis produced 56 unique proteins. Many of the detected proteins were involved in metabolic processes, whereas the others had unknown functions. The unique and new proteins with known and unknown functions suggest potential the acclimatization of the microbes to pH stress.