Death of enterohemorrhagic Escherichia coli O157:H7 in real mayonnaise and reduced-calorie mayonnaise dressing as influenced by initial population and storage temperature.

This study was undertaken to determine the survivability of low-density populations (10(0) and 10(2) CFU/g) of enterohemorrhagic Escherichia coli O157:H7 inoculated into real mayonnaise and reduced-calorie mayonnaise dressing and stored at 20 and 30 degrees C, temperatures within the range used for normal commercial mayonnaise distribution and storage. Inactivation patterns at 5 degrees C and inactivation of high-inoculum populations (10(6) CFU/g) were also determined. The pathogen did not grow in either mayonnaise formulation, regardless of the inoculum level or storage temperature. Increases in storage temperature from 5 to 20 degrees C and from 20 to 30 degrees C resulted in dramatic increases in the rate of inactivation. Populations of E. coli O157:H7 in the reduced-calorie and real formulations inoculated with a population of 0.23 to 0.29 log10 CFU/g and held at 30 degrees C were reduced to undetectable levels within 1 and 2 days, respectively; viable cells were not detected after 1 day at 20 degrees C. In mayonnaise containing an initial population of 2.23 log10 CFU/g, viable cells were not detected after 4 days at 30 degrees C or 7 days at 20 degrees C; tolerance was greater in real mayonnaise than in reduced-calorie mayonnaise dressing stored at 5 degrees C. The tolerance of E. coli O157:H7 inoculated at the highest population density (6.23 log 10 CFU/g) was less in reduced-calorie mayonnaise dressing than in real mayonnaise at all storage temperatures. In reduced-calorie mayonnaise dressing and real mayonnaise initially containing 2.23 log10 CFU/g, levels were undetectable after 28 and 58 days at 5 degrees C, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of modified-atmosphere storage on the growth of uninjured and heat-injured Aeromonas hydrophila

The growth of uninjured and heat-injured Aeromonas hydrophila incubated at 5 degrees C (22 days) and 30 degrees C (31 h) under air, N2, and CO2 was investigated. At 30 degrees C, the growth patterns of cells on brain heart infusion agar incubated under air and N2 were similar, although slight differences in the lengths of the lag phases and the final populations were detected. The lag phases of cells incubated under air and N2 were substantially longer at 5 degrees C than at 30 degrees C. The population of uninjured A. hydrophila incubated at 5 degrees C under air and N2 remained constant, whereas the number of injured cells declined before the exponential growth phase. Growth at 5 degrees C was enhanced when uninjured and heat-injured A. hydrophila were incubated under N2. At 30 degrees C, cells incubated under CO2 exhibited noticeably longer lag phases and lower growth rates than did cells incubated under air and N2. The viable populations of uninjured and heat-injured cells incubated at 5 degrees C under CO2 declined steadily throughout incubation.     

Tetrapod phylogeny inferred from 18S and 28S ribosomal RNA sequences and a review of the evidence for amniote relationships [published erratum appears in Mol Biol Evol 1991 May;8(3):398]

The 18S ribosomal RNAs of 21 tetrapods were sequenced and aligned with five published tetrapod sequences. When the coelacanth was used as an outgroup, Lissamphibia (living amphibians) and Amniota (amniotes) were found to be statistically significant monophyletic groups. Although little resolution was obtained among the lissamphibian taxa, the amniote sequences support a sister-group relationship between birds and mammals. Portions of the 28S ribosomal RNA (rRNA) molecule in 11 tetrapods also were sequenced, although the phylogenetic results were inconclusive. In contrast to previous studies, deletion or down- weighting of base-paired sites were found to have little effect on phylogenetic relationships. Molecular evidence for amniote relationships is reviewed, showing that three genes (beta-hemoglobin, myoglobin, and 18S rRNA) unambiguously support a bird-mammal relationship, compared with one gene (histone H2B) that favors a bird- crocodilian clade. Separate analyses of four other genes (alpha- crystallin A, alpha-hemoglobin, insulin, and 28S rRNA) and a combined analysis of all sequence data are inconclusive, in that different groups are defined in different analyses and none are strongly supported. It is suggested that until sequences become available from a broader array of taxa, the molecular evidence is best evaluated at the level of individual genes, with emphasis placed on those studies with the greatest number of taxa and sites. When this is done, a bird-mammal relationship is most strongly supported. When regarded in combination with the morphological evidence for this association, it must be considered at least as plausible as a bird-crocodilian relationship.      

Salmonella Survival on Pecans as Influenced by Processing and Storage Conditions

Survival of Salmonella senftenberg 775W, S. anatum, and S. typhimurium during exposure to currently practiced, as well as abusive, pecan processing and storage conditions was studied. Thermal treatments normally carried out during the processing of pecans are inadequate to consistently destory salmonellae in highly contaminated inshell nuts. Pecan nut packing tissue was toxic to salmonellae, thus affording some protection against high initial contamination and subsequent survival of the organisms. Examinations of inoculated inshell pecans stored at -18, -7, 5, and 21 C for up to 32 weeks revealed that the extent of survival was inversely correlated to the storage temperature. S. senftenberg 775W and S. anatum were not detectable on inshell nuts after 16 weeks of storage at 21 C. Little decrease in viable population of the three species was noted on inoculated pecan halves stored at -18, -7, and 5 C for 32 weeks. Due to organoleptic quality deterioration in pecan nutmeats at elevated temperatures, sterilization methods other than thermal treatment appear to be required for the elimination of viable salmonellae from pecan nuts.     

Combined Effects of Water Activity, Solute, and Temperature on the Growth of Vibrio parahaemolyticus

Vibrio parahaemolyticus was grown at 36 C in tryptic soy broth (pH 7.8) containing added levels of NaCl ranging from 0.5 to 7.9% (wt/wt). The fastest generation time was 16.4 min in tryptic soy broth containing 2.9% NaCl (TSBS) which corresponded to a water activity (a(w)) of 0.992 (+/-0.005). Tryptic soy broth containing lower or higher levels of NaCl resulted in higher or lower a(w), respectively, and slower generation times. Growth was measured turbidimetrically at 36 C in TSBS containing added amounts of NaCl, KCl, glucose, sucrose, glycerol, or propylene glycol. The solutes used to reduce a(w) to comparable levels resulted in extended lag times of varied magnitude, dissimilar growth rates, and different cell numbers. Reduction of a(w) with glycerol was less inhibitory to growth than similar a(w) reductions with NaCl and KCl. Sucrose, glucose, and propylene glycol generally had the greatest effect on extending the lag times of V. parahaemolyticus when the addition of these solutes was made to establish similar a(w) levels lower than 0.992. Minimal a(w) for growth at 15, 21, 29, and 36 +/- 0.2 C for each of four strains of V. parahaemolyticus was tested in TSBS containing added solutes. Reduced a(w) was generally most tolerable at 29 C, whereas higher minimal a(w) for growth was required at 15 C. Solutes added to TSBS to achieve reduction in a(w), minimal a(w) for growth after 20 days, and incubation temperatures were as follows: glycerol, 0.937, 29 C; KCl, 0.945, 29 C; NaCl, 0.948, 29 C; sucrose, 0.957, 29 and 36 C; glucose, 0.983, 21 C; and propylene glycol, 0.986, 29 C. Each of the four strains tested responded similarly to investigative conditions. It appears that minimal a(w) for growth of V. parahaemolyticus depends upon the solute used to control a(w).     

Morphological Alterations in Bacillus megaterium as Produced by Aflatoxin B1

Morphologically abnormal cells were produced by Bacillus megaterium NRRL B-1368 in response to aflatoxin B(1). Filamentous forms were characterized by early granulation and unusually large and numerous deposits of poly-beta-hydroxybutyric acid within the cells. Pantoyl lactone was without effect as a reversing agent for the observed inhibition of cell septum formation. B. megaterium cells and spores produced on toxic (3.8 mug of aflatoxin B(1) per ml) and nontoxic Trypticase Soy Broth and Trypticase Soy Agar (TSA) were observed by using phase contrast and electron microscopy. Transfer of aberrant forms to nontoxic TSA yielded macrocolonies with daughter cells morphologically indistinguishable from untreated cells. Agar slide cultures of filamentous cells transferred to nontoxic TSA indicated that normal cells were formed. Electron photomicrographs showed a decreased number of mesosomes in filamentous cells as compared to control cells. There were no observable morphological differences in spores formed on toxic or nontoxic TSA.     

Evaluation of Chemicals for Restricting Colony Spreading by a Xerophilic Mold, Eurotium amstelodami, on Dichloran-18% Glycerol Agar

Twenty chemicals were screened for their effectiveness in restricting colony spreading by four strains of a xerophilic mold, Eurotium amstelodami, on dichloran-18% glycerol agar. Triton X-100, Triton X-301, Tergitol NP-7, and Tergitol 15-S-3 (each at 200 μg/ml) and 1,000 μg of sodium deoxycholate, 1 μg of iprodione, 0.1 μg of propiconazole, and 0.01 μg of Maxim per ml were judged to be most effective for restricting the rate of colony spreading.     

Comparison of media and methods for detecting and enumerating Listeria monocytogenes in refrigerated cabbage.

Direct plating, selective enrichment, and cold enrichment followed by secondary selective enrichment procedures were compared for detecting and enumerating Listeria monocytogenes in chopped cabbage stored at 5 degrees C for up to 64 days. Addition of Fe3+ to solid media enhanced detection of the organism. Cold enrichment (5 degrees C) in nutrient broth and brain heart infusion broth followed by secondary enrichment (48 h, 30 degrees C) in Trypticase soy-yeast extract-antibiotic broth and thiocyanate-nalidixic acid broth and plating on selective agar media (Doyle and Schoeni selective enrichment agar [minus acriflavin hydrochloride, supplemented with 5 micrograms of Fe3+/ml] and McBride Listeria agar) resulted in the detection of highest populations.     

Influence of modified-atmosphere storage on the growth of uninjured and heat-injured Aeromonas hydrophila.

The growth of uninjured and heat-injured Aeromonas hydrophila incubated at 5 degrees C (22 days) and 30 degrees C (31 h) under air, N2, and CO2 was investigated. At 30 degrees C, the growth patterns of cells on brain heart infusion agar incubated under air and N2 were similar, although slight differences in the lengths of the lag phases and the final populations were detected. The lag phases of cells incubated under air and N2 were substantially longer at 5 degrees C than at 30 degrees C. The population of uninjured A. hydrophila incubated at 5 degrees C under air and N2 remained constant, whereas the number of injured cells declined before the exponential growth phase. Growth at 5 degrees C was enhanced when uninjured and heat-injured A. hydrophila were incubated under N2. At 30 degrees C, cells incubated under CO2 exhibited noticeably longer lag phases and lower growth rates than did cells incubated under air and N2. The viable populations of uninjured and heat-injured cells incubated at 5 degrees C under CO2 declined steadily throughout incubation.