Recovery of spores of Clostridium botulinum in yeast extract agar and pork infusion agar after heat treatment

Meta-analysis of D-values of proteolytic Clostridium botulinum and its surrogate strain Clostridium sporogenes PA 3679

Foodborne botulism is a serious disease resulting from ingestion of preformed Clostridium botulinum neurotoxin in foodstuff. Since the 19th century, the heat resistance of this spore forming bacteria has been extensively studied in order to guarantee the public health associated with low acidic, ambient stable products. The most largely used heat resistance parameters in thermal settings of such products are the D121.1°C values (time required to have a 10-fold decrease of the spore count, at 121.1°C) and the z-values (temperature increase to have a 10-fold decrease of D-values). To determine D121.1°C and z-values of proteolytic C. botulinum and its nontoxigenic surrogate strain C. sporogenes PA3679, a dataset of 911 D-values was collected from 38 scientific studies. Within a meta-analysis framework, a mixed-effect linear model was developed with the log D-value (min) as response and the heat treatment temperature as explicative variable. The studies (38), the C. botulinum strains (11), and the heat treatment media (liquid media and various food matrices, split into nine categories in total) were considered as co-variables having a random effect. The species (C. botulinum and C. sporogenes) and the pH (five categories) were considered as co-variables having a fixed effect. Overall, the model gave satisfactory results with a residual standard deviation of 0.22. The heat resistance of proteolytic C. botulinum was found significantly lower than the C. sporogenes PA 3679 one: the mean D-values at the reference temperature of 121.1°C, in liquid media and pH neutral, were estimated to 0.19 and 1.28min for C. botulinum and C. sporogenes, respectively. On the other hand, the mean z-values of the two species were similar: 11.3 and 11.1°C for C. botulinum and C. sporogenes, respectively. These results will be applied to thermal settings of low-acid ambient stable products.

The effect of recovery conditions on the apparent heat resistance of Bacillus cereus spores

The effect of recovery media and incubation temperature on the apparent heat resistance of three ATCC strains (4342, 7004 and 9818) of Bacillus cereus spores were studied. Nutrient Agar (NA), Tryptic Soy Agar (TSA), Plate Count Agar (PCA) and Milk Agar (MA) as the media and temperatures in the range of 15-40 degrees C were used to recover heated spores. Higher counts of heat injured spores were obtained on PCA and NA. The optimum subculture temperature was about 5 degrees C below the optimum temperature for unheated spores. No significant differences in heat resistance were observed with the different recovery conditions except for strains 4342 and 9818 when MA was used as plating medium. Large differences in D-values were found among the strains (D100 = 0.28 min for 7004; D100 = 0.99 min for 4342; D100 = 4.57 min for 9818). The 7004 strain showed a sub-population with a greater heat resistance. The z values obtained for the three strains studied under the different recovery conditions were similar (7.64 degrees C +/- 0.25).

Selective enumeration of spores of Clostridium species in dried foods

The suitability of a variety of media and procedures for the enumeration of sulphite-reducing clostridia in food was investigated. The most suitable procedure was pasteurization of the 1/10 macerate for at least 1 min at 80 degrees C; followed by culture at 30 degrees C for up to 3 d in a sulphite-based, differential reinforced clostridium medium, without bicarbonate or lactate but with an increased iron concentration, and sulphite and iron added after sterilization. Black sulphite-reducing colonies were finally tested for sensitivity to metronidazole and confirmation of their failure to grow on agar slopes under aerobic conditions.

Effect of thermal treatments in oils on bacterial spore survival

The heat resistance of Bacillus cereus F4165/75, Clostridium sporogenes PA 3679 and Cl. botulinum 62A spores suspended in buffer (pH 7.2), olive oil and a commercial oil (a mixture of rapeseed oil and soy oil) was investigated. Linear survivor curves were obtained with B. cereus spores in the three menstrua and with 62A and PA 3679 spores suspended in buffer. However, the inactivation kinetics of the clostridial spores suspended in oils were concave upward with a characteristic tailing-off for 62A spores suspended in olive oil. These deviations from the semi-log model could not be ascribed to a heterogeneity in heat resistance of the spore population or to the variation of aw during heating. Spore resistance to heat increased in the order: buffer much less than commercial oil less than olive oil. The greater heat resistance of oil-suspended spores was ascribed to the low aw (0.479 and 0.492 for commercial oil and olive oil, respectively) and to the composition of the oils. The difference in z values (ca 28 degrees C in oils and 10 degrees-12 degrees C in buffer) suggested that the mechanism of inactivation differs for spores suspended in lipids and in aqueous systems. The thermodynamic data were consistent with this hypothesis.