Pectic Enzyme Activities of Bacteria Associated with Rotted Onions (Allium cepa)

The aerobic bacteria associated with soft rot in onions (Allium cepa) were isolated and identified as a Vibrio sp., Micrococcus epidermidis, Pseudomonas cepacia, an Acinetobacter sp., a Xanthomonas sp., Bacillus polymyxa, and Bacillus megaterium. With the cup-plate assay method, no pectin hydrolase could be detected from any of these isolates when they were cultured in pectin medium, but lyase and pectinesterases were detectable. Onion tissue cultures showed pectin hydrolase activity for P. cepacia and B. polymyxa and lyase and pectinesterase activities for all of the isolates, usually at higher levels of activity than those of the pectin medium culture filtrates. In both culture media, Vibrio sp. showed the highest lyase and pectinesterase activities. In the viscometric test, all of the isolates achieved at least a 50% decrease in viscosity for lyase enzyme, with M. epidermidis and Vibrio sp. recording viscosity decreases as high as 83%. The ability to cause soft rot in onion bulbs was demonstrated by P. cepacia and Xanthomonas sp. Benzoic acid at a concentration of 0.8 mg/ml caused total suppression of enzyme production, whereas sodium benzoate at this concentration reduced pectinesterase production by 71% and lyase production by 72%. The possible use of these preservatives in the control of soft rot in onions is noted.