Controlling toxin production (lecithinase) by the food borne pathogen bacillus cereus isolated from some fridge-keptvegetables

In the present study,Bacillus cereus previously isolated from local 7daysfridge – kept green pepper and lettuce showed the ability to produce thehighest visible growth, in corn millet broth, after 24hrs incubation at 37 ºC. Thereafter, the growth decreased gradually by increasing the incubation period up to 120 hrs. However, the reverse occurred with the amount of secreted lecithinase enzyme (toxin) where secretion started after 24hrs incubation and then significantly increased gradually to reach the highest amount after 96 hrs incubation. No trend could be observed between incubation temperature and both bacterial growth and toxin production, however the temperature 37 ºC was optimum for both whereas 28 ºC was the minimum. The results revealed the ability of the isolate to grow at a wide pH range extending from 2.6 to 10.6. Adjusting the pH of the corn millet growth medium at acidity (2.6-5.0) or neutrality was followed by low viable colony count, however at alkalinity (8.0-10.5) the count was high. Concerning lecithinase production, the acidic (2.6 – 5.0) or neutral pHs (7.0) was not suitable for toxin production where it stopped completely but the isolate began to produce toxin at alkaline pHs starting from pH 8.6 to 10.6. A little amount of toxin was produced by B. cereus in presence of lemon juice in the growth medium while it was completely ceased in presence of acetic acid,a phenomenon which can help in controlling toxin secretion by such contaminating bacteria.