Characterization of chocolate milk spoilage patterns.

Standard plate counts (SPC) and psychrotrophic plate counts (PPC) from chocolate milk samples were compared with those of unflavored milk samples plated within 24 h of processing and at 7, 10, and 14 days of storage at 6 degrees C using matched samples collected over four time periods from four milk-processing plants. Bacterial numbers within 24 h of processing were not significantly different in unflavored and in chocolate milk samples (P > 0.001), with SPC less than 1,000 CFU/ml and PPC below 10 CFU/ml for both types of products. SPC and PPC were higher in chocolate milk samples than in unflavored milk samples collected from all four plants after 14 days of storage (P < 0.001). To examine the effects of chocolate milk components on bacterial numbers, SPC for days 0, 7, 14, and 21 were monitored in samples of experimentally prepared unflavored milk, milk with chocolate powder and sucrose (chocolate milk), milk with sucrose only, and milk containing chocolate powder only. At days 14 and 21, SPC were higher in both chocolate milk and in milk with chocolate powder only, than in either the unflavored milk or milk with sucrose only (P < 0.001). These findings suggest that the addition of chocolate powder to milk can contribute to a greater relative increase in bacterial numbers in pasteurized chocolate milk than in identically processed unflavored milk at 14 days postprocessing.

Nisin resistance in Listeria monocytogenes ATCC 700302 is a complex phenotype.

Nisin resistance in Listeria monocytogenes ATCC 700302 is a complex phenotype involving alterations in both the cytoplasmic membrane and the cell wall and a requirement for divalent cations. In addition to a lower ratio of C15 to C17 fatty acids than in the wild-type strain (A. S. Mazzotta and T.J. Montville, J. Appl. Microbiol. 82: 32-38, 1997), this nisin-resistant (Nisr) strain contained significantly more zwitterionic phosphatidylethanolamine and less anionic phosphatidylglycerol and cardiolipin. The extraction of cardiolipin was enhanced by a penicillin-lysozyme step to disrupt the cell wall. This study is the first to quantify the phosphatidylethanolamine component of the L. monocytogenes cytoplasmic membrane. While these cytoplasmic membrane changes were induced by nisin, the Nisr strain also showed altered sensitivities to cell wall-acting compounds, even when grown in the absence of nisin, suggesting a constitutive alteration in the strain's cell wall. A model which integrates the roles of the cell membrane, cell wall, and divalent cations is presented. Finally, nisin resistance in L. monocytogenes ATCC 700302 conferred cross-resistance to the class IIa bacteriocin pediocin PA-1 and the class IV leuconocin S.

Nisin Resistance in Clostridium botulinum Spores and Vegetative Cells.

The frequencies at which vegetative cells and spores of Clostridium botulinum strains 56A, 62A, 17409A, 25763A, 213B, B-aphis, and 169B formed colonies on agar media containing 0, 10(sup2), 10(sup3), and 10(sup4) IU of nisin per ml at 30(deg)C were determined. Strain 56A had the highest frequencies of nisin resistance, while strains 62A, 169B, and B-aphis had the lowest. For most strains, spores were more resistant than vegetative cells. One exposure to nisin was sufficient to generate stable nisin-resistant isolates in some strains. Stepwise exposure to increasing concentrations of nisin generated stable resistant isolates from all strains. Spores produced from nisin-resistant isolates maintained their nisin resistance. The frequency of spontaneous nisin resistance was reduced considerably by lowering the pH of the media and adding 3% NaCl. Nisin-resistant isolates of strains 56A and 169B also had increased resistance to pediocin PA1, bavaricin MN, plantaricin BN, and leuconocin S.

Inhibition of Listeria monocytogenes by Lactobacillus bavaricus MN in beef systems at refrigeration temperatures.

The ability of Lactobacillus bavaricus, a meat isolate, to inhibit the growth of three Listeria monocytogenes strains was examined in three beef systems: beef cubes, beef cubes in gravy, and beef cubes in gravy containing glucose. The beef was minimally heat treated, inoculated with L. bavaricus at 10(5) or 10(3) CFU/g and L. monocytogenes at 10(2) CFU/g, vacuum sealed, and stored at 4 or 10 degrees C. The meat samples were monitored for microbial growth, pH, and bacteriocin production. The pathogen was inhibited by L. bavaricus MN. At 4 degrees C, L. monocytogenes was inhibited or killed depending on the initial inoculum level of L. bavaricus. At 10 degrees C, at least a 10-fold reduction of the pathogen occurred, except in the beef without gravy. This system showed a transient inhibition of the pathogen during the first week of storage followed by growth to control levels by the end of the incubation period. Bacteriocin was detected in the samples, and inhibition could not be attributed to acidification. Low refrigeration temperatures significantly (P < or = 0.05) enhanced L. monocytogenes inhibition. Moreover, the addition of glucose-containing gravy and the higher inoculum level of L. bavaricus were significantly (P < or = 0.05) more effective in reducing L. monocytogenes populations in most of the systems studied.