Lecithinase-producing bacteria in commercial and home-made foods: Evaluation of toxic properties and identification of potent producers

Abstract

The aim of this study was to determine bacterial contamination in 28 refrigerated and unrefrigerated, commercial and home-made food items in Al Madinah Al Munawarah, Saudi Arabia. Of the 60 bacterial strains isolated, 53 were recovered from food samples refrigerated for 7 days and 7 from fresh, unrefrigerated food. The isolated bacteria were screened for lecithinase enzyme (toxin) production by the cup plate clearing zone technique. All isolates were positive on the basis of their ability to grow aerobically on egg yolk agar, giving black colonies as a characteristic feature of lecithinase production and opaque zones with diameters of 4–27mm. Isolates were categorized into low, moderate and high lecithinase producers. Potent lecithinase-producing strains isolated from refrigerated green pepper (PS1) and lettuce (LS1) had a remarkably large zone measuring 27±1.9mm. All isolates also had additional toxic properties, including caseinase and haemolytic activities. Genotypic characterization by amplified ribosomal DNA restriction analysis showed that strains PS1 and LS1 were from same genus. Phenotypic characterization with biochemical tests and the Phoenix identification system suggested their affiliation to the Bacillus group. 16S rDNA sequence analysis of both strains showed them to be Bacillus cereus, with 99% sequence similarity to B. cereus strain J8B-67.