Luminometric and differential scanning calorimetry (DSC) studies on heat- and radiation inactivation of Bacillus subtilis luxAB spores.

A bioluminescent derivative of Bacillus subtilis containing a plasmid encoding a luxAB fusion under control of a vegetative promoter and gives bioluminescence upon addition of an exogenous long-chain aldehyde has been used as test organism. Its spore populations have been produced and their heat- and radiation survival curves established. Heat-sensitization effect of pre-irradiation of spores was proven not only by colony counting but also with differential scanning calorimetry. Under a linearly programmed temperature increase, the heat destruction of spores surviving 2.5 kGy gamma irradiation resulted in at a few centigrade lower temperature than that of untreated spores. Heat denaturation endotherms in the DSC-thermogram of irradiated spores were shifted to lower temperatures as well. Comparative turbidimetric, luminometric and phase-contrast microscopic studies of untreated, heat-treated and irradiated spore populations showed that the kinetics of germination and the light emission during germination of radiation-inactivated spores were the same as those of untreated spores, revealing that the pre-formed luciferase enzyme packaged into the spores during sporulation remained intact after an irradiation dose causing 90% decrease in number of colony forming spores. Therefore, in contrast to heat-treated spores, the initial bioluminescence reading upon germination of irradiated spores does not reflect the viable count of their population.

Growth of untreated and radiation-damaged Listeria as affected by environmental factors.

The growth of untreated Listeria monocytogenes 4ab No. 10 and that of the surviving fraction of its population treated with 0.8 kGy gamma rays was investigated in a microtitreplate system at incubation temperatures between 3 degrees C and 35 degrees C in Tryptic Phosphate Broth (TPB) or Brain Heart Infusion Broth (BHIB) media containing NaCl between 0.25 to 16.75% (w/v), and acidified with citrate-phosphate buffers to pH values between 4.63 and 7.06. The initial viable count was 3 x 10(3)/ml. Time periods to visible growth were recorded. Radiation survivors showed increased salt- and pH-sensitivities and increased minimum temperature for growth in TPB-based media. Adverse effects of sub-optimal environmental factors (reduced water activity, pH and temperature) and radiation injury were much less pronounced in BHIB-based media. Polynomial equations describing the combined effects of hydrogen ion and salt concentrations on the detectable growth at 30 degrees C were constructed for quantitative assessment of interactions. The results demonstrate that combining environmental stresses with low-dose irradiation can control growth of L. monocytogenes.

Interaction of ionising radiation and acidulants on the growth of the microflora of a vacuum-packaged chilled meat product.

Microbiological effects of gamma irradiation dose of 2 kGy, with and without reduction of pH to 5.3-5.2, have been investigated with a vacuum-packaged, minced meat product prepared from pork and beef with spices and cereal fillings. Either glucono-delta-lactone or ascorbic acid were used as acidulants. Experimental batches were stored at 0-2 degrees C for 4 weeks. Effect of temperature abuse condition was also studied by transferring packages for one week to 10 degrees C after 2-week holding at 0-2 degrees C. The irradiation caused two decimal reduction of the aerobic viable cell count determined after incubation at room temperature and four decimal reduction in the Enterobacteriaceae count. Lactic acid bacteria appeared to be more radiation resistant and became the dominant component of the microflora during storage. Combination of pH-reduction and irradiation prevented growth of Enterobacteriaceae even at 10 degrees C incubation.