Search

The present work studies the effect of acid shock on cell viability of Bacillus cereus and Bacillus weihenstephanensis. Experiments were performed by means of flow cytometry (FCM) combined with fluorescent labelling. Propidium iodide, for membrane evaluation, and carboxyfluorescein diacetate, for esterase activity were used for differential staining of both strains. pH values of 3.4, 3.8 and 4.2 were selected on the basis of previous screening tests. FCM analysis showed to clearly discriminate between different populations, viable and damage cells, leading to successful assessment of acid shock effect on B. cereus and B. weihenstephanensis vegetative cell viability. Cellular viability decreased at lower pH values and longer exposure times, being B. weihenstephanensis more acid resistant than B. cereus. Reliability of FCM analysis for detection of B. cereus and B. weihenstephanensis vegetative cells was compared with classical viable plate count techniques (VPC). Although FCM and VPC data were not directly correlated and classical analytical methods show higher reliability, FCM analysis provides high-speed information at real time on damage at single cell level, whereas VPC only gives an indication of cells able to grow at a certain time. These data could help to establish more accurately the potential risk associated to foods or processing conditions where pH is involved, taking damage into consideration.

An active cardboard tray coated with a water-based emulsion including encapsulated (in β−cyclodextrins (CDs)) essential oils (EOs) was used to study the quality changes of cherry tomatoes (flow-packaged using a macroperforated film) during storage up to 24 days at 8 °C. Commercial polyethylene and non-active cardboard trays were used as control materials. Firmness of samples was well maintained for 24 days, while decay incidence was reduced from 8% to

Application of essential oils (EOs) in vapour phase under vacuum conditions may increase the effectiveness of EOs to sanitize plant products. Different treatments with EOs vapor were studied in vitro and in vivo against saprophyte microflora and pathogens at laboratory, pilot plant and industrial scales. Vapor of carvacrol combinations with oregano and peppermint essential oils (80:20 v:v) showed the best microbial reductions up to 1.5 log units for mesophiles ( in vitro at laboratory scale), 0.9–3.9 log units for several isolated molds ( in vitro at pilot plant scale) and approximately 1 log unit for Listeria innocua ( in vivo at pilot plant scale). Particularly, carvacrol:oregano and carvacrol:peppermint vapors showed the highest mold log reductions (>3.3) for Botrytis cinerea , Alternaria alternata and Aspergillus niger while Fusarium spp. and Penicillium spp. showed lower reductions. EOs vapors concentrations lower than 0.5 mg L −1 did not achieve mold reductions higher than 0.2 log units while EOs vapors concentrations ≥10 mg L −1 did not increase the antifungal activity. Fresh cilantro treated at industrial scale with vapor of carvacrol:peppermint (less off-flavors than carvacrol:oregano) (80:20 v:v) at 8 mg L −1 under vacuum (adapting a vacuum cooling system) showed 0.8, 1.8 and 1.2 lower mesophiles, psychrophiles and enterobacteria log units, respectively, than untreated samples after 12 d at 2 °C of storage. Yeast and mold growth in cilantro was low throughout the studied period at storage temperature of 2 °C. Yellowing and browning of cilantro treated at 8 mg L −1 was also minimal, compared to the rest of treatments, after 12 d at 2 °C. In conclusion, the application of carvacrol:peppermint (80:20 v:v) vapor at 8 mg L −1 under vacuum in an industrial installation may extend the shelf life of fresh cilantro further than 12 d at 2 °C ensuring its microbial and visual quality.

Strains belonging to Bacillus cereus Group include six different species, among which are Bacillus thuringiensis, Bacillus weihenstephanensis, and Bacillus cereus sensu stricto, a causative agent of food poisoning. Sequence of the panC-housekeeping gene is used for B. cereus Group affiliation to seven major phylogenetic groups (I-VII) with different ecological niches and variations in thermal growth range and spore heat resistance of B. cereus Group microorganisms varies among phylogenetic groups. We assigned a selection of B. cereus sensu stricto strains related to food poisoning from the Spanish cultivar Collection (Valencia) to Group IV strains based on panC gene sequence. Thermal inactivation assays revealed variability of spore heat resistance within these Group IV strains. Adequate food sanitizing treatments therefore require fast and reliable identification of particular strains. In the present study, feasibility of genotyping via high-resolution melting (HRM) analysis was examined. HRM analysis of amplified polymorphic 16S-23 intergenic spacer region (ISR) region proved to be discriminatory for B. cereus sensu stricto strain typing, while two other polymorphic regions within the bacterial rRNA operon allowed differentiation between Bacillus species, demonstrating its applicability for discrimination on the species and strain level within B. cereus Group.

Stochastic models can be useful to predict the risk of foodborne illness. The presence of Bacillus cereus in liquid egg can pose a serious hazard to the food industry, since a mild heat treatment cannot guarantee its complete inactivation. However, most of the information available in the scientific literature is deterministic, including growth of B. cereus. In this paper, a stochastic approach to evaluate growth of B. cereus cells influenced by different stresses (presence of nisin and lysozyme separately or in combination) was performed, using an individual-based approach of growth through OD measurements. Lag phase duration was derived from the growth curves obtained. From results obtained, histograms of the lag phase were generated and distributions were fitted. Normal and Weibull distributions were ranked as the bestfit distributions in experiments performed at 25 °C. At 16 °C, lag values (obtained in presence of combinations of both antimicrobials) were also fitted by a Gamma distribution. Predictions were compared with growth curves obtained in liquid egg exposed to mild heat, nisin and/or lysozyme to assess their validity.

The different strains of Bacillus cereus can grow at temperatures covering a very diverse range. Some B. cereus strains can grow in chilled food and consequently cause food poisoning. We have identified a new sensor/regulator mechanism involved in low-temperature B. cereus growth. Construction of a mutant of this two-component system enabled us to show that this system, called CasKR, is required for growth at the minimal temperature ( T min ). CasKR was also involved in optimal cold growth above T min and in cell survival below T min . Microscopic observation showed that CasKR plays a key role in cell shape during cold growth. Introducing the casKR genes in a Δ casKR mutant restored its ability to grow at T min . Although it was first identified in the ATCC 14579 model strain, this mechanism has been conserved in most strains of the B. cereus group. We show that the role of CasKR in cold growth is similar in other B. cereus sensu lato strains with different growth temperature ranges, including psychrotolerant strains.

[SPA]Los microorganismos pertenecientes al grupo Bacillus cereus poseen importancia económica y para la salud pública. De entre todos ellos, Bacillus cereus sensu stricto es el agente causal de enfermedades gastrointestinales y Bacillus weihenstephanensis es una especie psicrótrofa capaz de crecer a temperaturas entre 4 y 7ºC. Además, ambos son responsables de pérdidas económicas relevantes derivadas del deterioro de alimentos. Debido a su habilidad para formar endosporos altamente resistentes, estas especies son capaces de persistir en ambientes de producción de alimentos, contaminando aquellos productos que han sufrido un tratamiento térmico moderado. Además, se han aislado cepas psicrótrofas y algunas mesófílas a partir de alimentos refrigerados. En este contexto y con el fin de contribuir a la mejora de los sistemas de conservación de alimentos, lo que asegura la calidad microbiológica de los mismos, el objetivo principal de la presente tesis ha sido el estudio del impacto de factores ambientales relevantes, comúnmente denominados como barreras, en la supervivencia y el crecimiento de diferentes especies del grupo B. cereus. Así mismo, se han realizado estudios centrados en la evaluación de la idoneidad de métodos de identificación rápida de dichos microorganismos y de la fiabilidad de técnicas que permitan estudios de viabilidad bacteriana a nivel de células individuales. Los alimentos vegetales listos para consumir son altamente demandados en los países industrializados ya que, debido a su conservación mediante tratamientos térmicos moderados, mantienen una alta calidad sensorial y nutricional. Sin embargo, este procesado facilita la persistencia de los microorganismos esporulados y se han aislado diversas cepas de B. cereus a partir de estos productos.[ENG] Bacillus cereus group species range high in economical and medical importance. Among all of them, Bacillus cereus sensu stricto is the causative agent of gastrointestinal diseases and Bacillus weihenstephanensis is a psychrotrophic species able to grow in the range of 4-7ºC. Both species are also recognized as spoilage bacteria causing economic losses. Due to their ability to form highly resistant spores, these microorganisms may persist in the food environment and contaminate mildly heat treated products. Moreover, psychrotrophic and some mesophilic strains have been isolated from chilled foodstuffs. On this ground, to contribute to improve food preservation systems that ensure microbiological quality, the major aim of the present thesis was to study the impact of relevant environmental factors, commonly referred to as hurdles, on the ability to survive and grow of different species of B. cereus group and to assess novel methods for fast and reliable identification of these microorganisms as well as to test the feasibility of techniques used in viability studies at single cell level. Ready to eat vegetable foods are widely consumed in industrialized countries due to their tasty, healthy and wholesome attributes. However, B. cereus strains have been previously isolated from these mildly preserved foods., Universidad Politécnica de Cartagena