Effect of dairy product environment on the growth of Bacillus cereus

Abstract

pH is one of the most important parameters to manage bacterial replication in foodstuffs. In this study, the ability of 2 Bacillus cereus strains, 1 clinical human isolate (GPe2) and 1 isolate from a dairy product (D43), were investigated for in vitro growth at different pH values (from 3.5 to 7.5) at 2 temperatures (15 and 37°C), showing their ability to grow from 5.5 to 7.5 and from 5.0 to 7.5, respectively. The ability of spores of these 2 microorganisms to germinate in different typologies of dairy products (unflavored yogurt, Taleggio cheese, mascarpone cheese, and raw and pasteurized milk) was also investigated by inoculating the spores and maintaining the products at 15°C. No growth was observed in yogurt, likely due to the combined effect of low pH (<5) and the presence of natural microflora. An inhibitory action of the natural microflora on the growth of B. cereus was also hypothesized for Taleggio cheese and raw milk, as these substrates were characterized by a high natural lactic acid bacteria population and permissive pH values (5.8/6.8 in Taleggio cheese, >7 in raw milk). In pasteurized milk and mascarpone cheese, where pH was not restrictive for B. cereus growth and where no significant natural microflora was present, growth occurred rapidly up to loads close to 7 log cfu/g.

Identification and Pathogenic Potential of Bacillus cereus Strains Isolated from a Dairy Processing Plant Producing PDO Taleggio Cheese

“…In this light, B. cereus contamination of the brining vat could be considered as non-critical. In fact, differently from other bacterial species [29,30], B. cereus shows difficulties in replication on the PDO Taleggio cheese surface with the increasing ripening period [7].…”

“…In particular, Bacillus cereus is recognized as a common contaminant of dairy products with frequent isolation from raw and pasteurized milk [1][2][3], as well as from final products [4,5]. This microorganism was also proven able to replicate in some dairy products such as ricotta and mascarpone cheese [6,7].…”

The Bacillus cereus Food Infection as Multifactorial Process

“…The first step for this is profound knowledge about survival and growth of B. cereus in the different food matrices, which depend mainly on pH and a w (water activity) values, processing and storage temperatures, oxygen availability, and the presence of microflora, but also on their production of bacteriocins, diacetyl, carbon dioxide, hydrogen peroxide, ethanol, or on further food additives [132][133][134][135][136][137][138][139][140][141][142][143][144][145]. While vegetative B. cereus cells can mainly be eliminated by mild heat treatment [146], spores are able to survive high temperatures, such as pasteurization or spray drying of milk [147].…”

Potential growth of Listeria monocytogenes in Italian mozzarella cheese as affected by microbiological and chemical-physical environment

“…This microflora showed an evident growth during all the challenge tests performed at the different temperatures, reaching values above 7 log cfu/g. The antagonistic activity of natural microflora, and in particular of lactic acid bacteria, toward L. monocytogenes has been described in several dairy products, showing a potential biopreservative role (Favaro et al, 2015;Tirloni et al, 2017b) due to nutritional competition and the production of active compounds such as bacteriocins, H 2 O 2 , organic acids, bacteriocin-like substances, dyacetil, carbon dioxide, reuterin, and ethanol. Such antagonistic activity was not shown in the substrate and temperature conditions considered in this study, as L. monocytogenes grew faster compared with streptococci and lactobacilli.…”