Abstract
Comparison of enumeration of egg shell bacterial load by a conventional direct plating method and a neoteric filtering method was evaluated. Egg shells were inoculated with approximately 10³ S. typhimurium organisms. Initial experiments evaluated the amount of colony forming units recovered from 50ml of sterile Phosphate Buffered Solution (PBS) inoculated with a 1ml dilution (ranges 10⁻⁶-10⁻⁸ inoculum dilution). This experiment was replicated three times, with a recovery dilution in the second and third trials. The filter method recovered significantly more bacteria than the direct plate method. Experiments were then conducted in order to determine if these two methods would be comparable when using commercial table eggs. Primary experiments of this type used autoclaved eggs inoculated with approximately 10³ S. typhimurium (marker strain). The eggs were allowed to remain in the inoculum for five minutes, aseptically removed, and incubated for thirty minutes, after which the eggs were rinsed in 50ml of sterile PBS (rinsate procedure), 1ml was plated using conventional methods and the remainder of the rinsate was filtered. This experiment was replicated six times. The final experiment conducted was to determine the sensitivity of the filter to the plating method. Using farm fresh chicken and turkey eggs, approximately 7.85-9.70log₁₀CFU/ml (ranges 10⁻⁵-10⁻⁸ inoculum dilution) S. typhimurium organisms were inoculated. Chicken egg rinsate was plated in 1ml triplicate samples and 1, 5, and 9ml duplicate samples were filtered. Turkey egg trials plated triplicate 1ml samples of the rinsate and a 10⁻¹ dilution of the rinsate and filtered 1, 5, and 9ml samples of the rinsate and 1ml triplicate samples of the diluted rinsate. All samples in all three experiments were placed on Brilliant Green Agar containing 20mg/ml novobiocin and 25mg/ml naladixic acid and were incubated for 24 hours at 37C. The filter method recovered significantly higher results than the direct plate method of the same inoculum dilution. However, between the rinsate of turkey eggs and its dilution, no significant difference was found.
Reynolds, Paige Lea (2002). Comparison of direct plating versus filtering of egg shells inoculated with Salmonella typhimurium. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -2002 -THESIS -R49.