Effect of Substrate Characteristics on the Growth and Sporulation of Two Biocontrol Microorganisms during Solid State Cultivation

Ga Young Lee,Wenqi Li,Ulalo M Chirwa,Jian Shi

doi:10.3390/fermentation6030069

Abstract

Biocontrol agents are a group of naturally occurring organisms capable of interrupting the lifespan and suppressing the propagation of disease organisms. The use of biocontrol agents offers an environment-friendly and sustainable solution to the synthetic agrochemicals. In this study, we investigated parboiled rice and millets as substrates for spore production of two model biocontrol microorganisms (Bacillus pumilus and Streptomyces griseus) under solid state cultivation (SSC) conditions. The effects of cultivation parameters such as initial moisture content, water activity, and cultivation time on microbial growth and spore production were studied. Furthermore, texture profile analysis was performed to test the stress and strain curve and the hardness and stickiness of the substrates. The greatest spore production occurred at 50% moisture content with millets as a substrate, yielding a count of 1.34 × 108 spores/g-wet-substrate enumerated with plate count analysis and 1.70 × 108 events/g-wet-substrate using flow cytometry analysis. Substrate texture profile was highly correlative to the initial moisture content and substrate type and all proved to be essential process variables in controlling the bacterial growth and sporulation during SSC processes.

Highlights

Soybeans are the foremost oilseed in the United States, recording 90 percent of U.S oilseed yield.In the year of 2018, soybeans with ~89.2 million planted acres, account for the second-most-planted field crop in the U.S after corn [1]
In order to achieve maximum microbial growth and spore production, this study aims to investigate the effect of substrate type and initial moisture content on sporulation as the most important factors [28,29]
Qualitative observations on the microbial growth were made every 24 h over a period of 120 h to determine the suitable culturing temperature and pH for the two strains on agar plates cultured at 25 ± 2 ◦ C and 28 ± 2 ◦ C, each at three pH levels of 5.5, 6.0, and 7.0

Summary

Introduction

Soybeans are the foremost oilseed in the United States, recording 90 percent of U.S oilseed yield.In the year of 2018, soybeans with ~89.2 million planted acres, account for the second-most-planted field crop in the U.S after corn [1]. Symptoms first present themselves as small brownish-red dots, or lesions, that appear on the upper side of leaves, with raised pustules forming on the underside [5] These pustules eventually burst open and release spores, causing the infected leaves to fall off and leading to a premature shutdown of the plant. While several other methods of management and disease control have been suggested, such as better cultural practices, genetic resistance research, and fungal vaccines, the use of foliar fungicides is still currently the most effective method of managing the disease Classes such as chlorothalonil and azoxystrobin have been shown to have the most significant impact, but continued research in other fungicides is being conducted [2,6,7]

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