Search

Soft rot is a major problem encountered in potatoes during postharvest storage. The soft rot bacterium Erwinia carotovora was inhibited by a novel bacteriocin-like substance (BLS) produced by Bacillus licheniformis P40. The BLS caused a bactericidal effect on E. carotovora cells at 30 microg mL(-1). Transmission electron microscopy showed that BLS-treated cells presented wrinkled bacterial surfaces and shrinkage of the whole cell, indicating plasmolysis. Erwinia carotovora cells treated with BLS were analyzed by FTIR showing differences in the 1390 cm(-1) and 1250-1220 cm(-1) bands, corresponding to assignments of membrane lipids. BLS was effective in preventing E. carotovora spoilage on potato tubers, reducing the symptoms of soft rot at 240 microg mL(-1) and higher concentrations. Soft rot development was completely blocked at 3.7 mg mL(-1). This BLS showed potential to protect potato tubers during storage.

The antimicrobial effect of 5 naphthoquinones was tested against the phytopathogenic bacteria Erwinia carotovora. Disk diffusion tests and determination of minimal inhibitory concentrations (MIC) indicate that the compound naphthazarin (NTZ) has the best antibacterial activity among the naphthoquinones tested. Studies on the mode of action indicate the effect of NTZ was bactericidal at 10 microg/mL. When cultivation was done in the presence of sodium ascorbate, the restoration of E. carotovora growth was observed with 3 microg/mL NTZ, but not when a 10 microg/mL dose was used. The incubation of NTZ with bacterial suspension of E. carotovora resulted in important changes in the absorption spectra of this naphthoquinone, indicating that a redox reaction takes place. These results may suggest that NTZ induces an increase of reactive oxygen species that are toxic to the cell. The compound NTZ was also effective in preventing E. carotovora growth on potato tubers, inhibiting the soft rot development at a concentration of 2 mg/mL.

Antimicrobial substances were produced by Bacillus subtilis BS 107 in a defined medium and isolated from culture filtrate by precipitation at pH 2.5. Active fractions were extracted in ethyl acetate, acetone, and 80% ethanol and purified by thin-layer chromatography (TLC) on silica gel plates developed with an ethanol-water mixture (2:1, v/v). In each case, a band with a Rf of 0.75 formed an inhibitory zone when the TLC plates were placed in contact with agar seeded with test cultures of the Erwinia spp. The antibiotic was released into the culture medium during early stages of growth of Bacillus subtilis BS 107 but higher amounts were released in older cultures. The antibiotic was resistant to the action of nucleases, proteases, and lipase. It was stable when autoclaved twice for 35 min at 2 atm (1 atm = 101.325 kPa) in acidic, neutral, and alkaline solutions. It remained active over the pH range of 1-14 during 1 month of observation and exhibited no loss of antimicrobial activity when stored at 4 degrees C for over 1 year. Bacillus subtilis BS 107 showed activity in vitro and in vivo against Erwinia carotovora subsp. atroseptica and Erwinia carotovora subsp. carotovora, the causal agents of potato blackleg and tuber soft rot. The application of an antagonist or its antibiotic to cut potato tissues prevented or reduced symptoms of the diseases. The antibiotic was active in vitro against a broad spectrum of bacterial and fungal species.