Abstract
Gram-negative Pseudomonas and Gram-positive Bacillus are the most common spoilage bacteria in raw and pasteurized milk, respectively. In previous studies, nitrogen (N2) gas flushing treatments of raw and pasteurized milk at cold chain-temperatures inhibited bacterial spoilage and highlighted different susceptibilities to the N2 treatment with the exclusion of certain bacterial types. Here, we investigated the effects of pure N2 gas flushing on representative strains of these genera grown in mono- or co-cultures at 15 and 25°C. Bacillus weihenstephanensis, a frequent inhabitant of fluid dairy products, is represented by the genome-sequenced KBAB4 strain. Among Pseudomonas, P. tolaasii LMG 2342T and strain C1, a raw milk psychrotroph, were selected. The N2 gas flushing treatment revealed: (1) temperature-dependent responses; (2) inhibition of the growth of both pseudomonads; (3) emergence of small colony variants (SCVs) for B. weihenstephanensis strain KBAB4 at 15°C induced by the N2 treatment or when grown in co-culture with Pseudomonas strains; (4) N2 gas flushing modulates (suppressed or stimulated) bacterial antagonistic reactions in co-cultures; (5) most importantly, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses revealed that at 25°C the majority of the KBAB4 cells were killed by pure N2 gas flushing. This observation constitutes the first evidence that N2 gas flushing has bactericidal effects.
Highlights
The challenges presently faced by food production systems require the urgent development of additional steps to alleviate food-borne pathogens and to tackle food spoilage
The N2 gas flushing treatment revealed: (1) temperature-dependent responses; (2) inhibition of the growth of both pseudomonads; (3) emergence of small colony variants (SCVs) for B. weihenstephanensis strain KBAB4 at 15◦C induced by the N2 treatment or when grown in co-culture with Pseudomonas strains; (4) N2 gas flushing modulates bacterial antagonistic reactions in co-cultures; (5) most importantly, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses revealed that at 25◦C the majority of the KBAB4 cells were killed by pure N2 gas flushing
The trends in antibiotic resistance levels observed in psychrotrophic bacterial populations over time during the cold storage of raw milk suggest that bacterial growth should be prevented by supplementary means (Munsch-Alatossava and Alatossava, 2007; Munsch-Alatossava et al, 2012a,b)
Summary
The challenges presently faced by food production systems require the urgent development of additional steps to alleviate food-borne pathogens and to tackle food spoilage. Numerous studies point to the vulnerability of the cold chain for raw or pasteurized milk. Milk is colonized by numerous “psychrotrophic” or “psychrotolerant” bacterial types which can grow at refrigeration temperatures and induce spoilage (Cousin, 1982; Hayes and Boor, 2001). The trends in antibiotic resistance levels observed in psychrotrophic bacterial populations over time during the cold storage of raw milk suggest that bacterial growth should be prevented by supplementary means (Munsch-Alatossava and Alatossava, 2007; Munsch-Alatossava et al, 2012a,b). Many studies have examined complementary methods to cold storage to control bacterial growth in highly perishable fluid milk products (Murray et al, 1983; Ruas-Madieodo et al, 1996; Dechemi et al, 2005; Rajagopal et al, 2005)
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