No abstract

Shredded cabbage was inoculated with Listeria monocytogenes Scott A cells and stored in normal air or a modified (70% carbon dioxide and 30% nitrogen) atmosphere at 5 and 25°C. Under the normal atmosphere at 25°C, colony counts increased by 2 logs within 2 d of storage but then decreased to undetectable levels within 6 d of storage. In the modified atmosphere at 25°C, numbers also decreased to undetectable levels within 6 d, but with a less marked initial increase and a decline that was more rapid than in the unmodified atmosphere. In the cold (5°C), the counts increased gradually, but only by about 1 log, in both atmospheres. In the normal atmosphere at 5°C, however, colony counts decreased sharply after 13 d of storage. Reductions in colony counts coincided with decreases in cabbage pH and development of spoilage. The increased level of carbon dioxide was ineffective in controlling L. monocytogenes at 5°C. At 25°C cabbage spoilage was rapid and colony counts declined under both atmospheres of storage.

A collaborative study was conducted in 15 laboratories to evaluate 2 different techniques for enumerating Bacillus cereus in foods. A direct plating technique using mannitol-egg yolk-polymyxin agar and a most probable number (MPN) technique using trypticase-soy-polymyxin broth were compared for the enumeration of high and low populations of B. cereus in mashed potatoes. The collaborative results showed that the overall mean recovery obtained with the low population level was essentially the same by both techniques. However, the overall mean recovery was significantly higher by the direct plating technique at the high population level. A statistical evaluation of the data also showed that the direct plating technique had better repeatability and reproducibility than did the MFN technique at both the high and low population levels. These results suggest that the MPN technique is suitable for examining foods containing low populations of B. cereus, but that the direct plating technique is preferable for foods that contain a high population of this organism. The confirmatory technique used in the proposed method is reliable for presumptive identification of isolates as B. cereus. The method has been adopted as official first action.

The Bacteriological Analytical Manual (6th edition) specifies use of glucose-salt-teepol (GST) broth for detection of Vibrio vulnificus and other halophilic vibrios in seafood. Since teepol is no longer commercially available, this study compared five enrichment broths for their ability to recover V. vulnificus. Ten samples of seeded oysters were analyzed using a three-tube MPN and enriched in each of five broths in duplicate. Broth cultures were then streaked onto cellobiose-polymyxin B-colistin (CPC) agar and sodium dodecyl sulfate-polymyxin B-sucrose (SDS) agar plates. Average (± standard error) recovery (log MPN/g) from each broth was as follows: Alkaline-peptone-water (APW), 4.16 ± 0.20; Marine (MRN) broth, 3.63 ± 0.16; Horie's broth, 2.88 ± 0.17; Monsur's broth, 2.43 ± 0.16; and GST broth, 1.28 ± 0.28. APW and MRN broths yielded significantly (P<0.05) higher recovery than other broths by the Kruskal-Wallis nonparametric rank test. Vibrio vulnificus was isolated with higher frequency from CPC (81%) as compared with SDS (61%) agar plates. Background growth was minimal on CPC agar, facilitating selection of V. vulnificus colonies. Based on these results, APW enrichment broth and CPC isolation agar were more efficient for recovery of V. vulnificus from oysters than other broth and agar combinations