A prototype automated system using fluorescent antibody (FA) was evaluated for rapid detection of salmonellae in foods. Samples were enriched in selenite cystine and tetrathionate broths. After incubation, both were transferred into fresh selenite cystine for a 4-h "post-enrichment" to dilute possible background fluorescence from product. These cultures were then analyzed automatically, and results were compared with those obtained by the methods of the Association of Official Analytical Chemists (AOAC). Initially, 167 samples of milk powder, dried yeast, and imported frog legs were examined. The AOAC and automated FA methods correlated well with all samples but frog legs. Difficulty with the latter was caused by procedural and mechanical problems coupled with high numbers of competing microorganisms in post-enrichment cultures. Modification of procedure and partial redesign of equipment corrected these difficulties, and excellent correlation was obtained with another 116 frog leg samples. All 89 AOAC-confirmed positives were also detected by the automated FA method, and there were only 4% false FA positives. The system shows potential for screening products for salmonellae; however, all positives should be confirmed by manual biochemical and serological methods.
Over the past 2 years in our laboratory, Hektoen enteric (HE) agar was evaluated as a medium for the detection of salmonellae in foods and feeds. HE agar was used in conjunction with brilliant green, bismuth sulfite, and Salmonella-Shigella agars, according to AOAC methods. More than 2000 samples representing over 40 different products were tested and the cultural results on all media were compared. HE agar was more efficient than the other media for recovery of salmonellae in contaminated samples and showed a lower number of false positives. HE agar is a highly selective medium for Salmonella species and we recommend that it be evaluated for incorporation into the AOAC methods.
Standard methods agar (SMA) and letheen agar (essentially SMA plus lecithin and Tween 80) were compared for bacterial growth and ability to neutralize cosmetic preservatives. Potato dextrose and malt extract agars (each prepared with and without lecithin and Tween 80) were compared with letheen agar and SMA in similar studies with fungi. Twelve bacterial strains, representing 8 species, and 2 fungal species were used as inocula. Plate counts of bacterial cultures (no preservatives present) ranged from 0 to 50% higher on letheen agar than on SMA except for 3 strains of Staphylococcus, which were 8-29% lower. Fungal counts were about the same on all media. Cosmetics (10 g) representing 4 preservative systems (hexachlorophene, benzoin, formaldehyde, and parabens) were inoculated with diluted cultures. Counts at 10−1 and 10−2 dilutions were typically 10-200% higher on letheen agar; however, in one case (benzoin, S. aureus, 10−1) the count was 400 on SMA vs 20 000 on letheen agar. Although differences in fungal counts were not as great, letheen agar partially neutralized the preservatives’ action. Results show that product dilution does not sufficiently reduce the effects of preservative carryover and neutralizers should be incorporated into plating media for this purpose.
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