Rapid Estimation of Microbial Numbers in Fresh Ground Beef by Use of the Limulus Test

Abstract

From 153 samples of fresh ground beef, aerobic plate counts (APC) and counts of viable gram-negative bacteria (GNB) were determined along with ng of endotoxin/g by use of the Limulus amoebocyte lysate (LAL) test. When the 153 meats were grouped by one-half log cycle ranges beginning with samples that contained < log 4.00 GNB/g, they were placed into eight groups. Mean ng endotoxin for these groups increased from 14 for the five samples with GNB < log 4.00 to 52,340 for the 16 samples with log GNB from 7.00 to 7.49. The APC to GNB ratios for the 13 meats with log GNB between 4.00 – 4.49 was 21, but decreased steadily to 2.2 for the 16 with log GNB of 7.00 – 7.49/g, reflecting the progressive decrease in the proportions of gram-positive bacteria and yeasts as total viable counts increased. The mean number of viable GNB/ng of total endotoxin for 148 of these samples was 1900. To calculate the number of GNB in a sample, the latter value was multiplied by the LAL-determined endotoxin in the sample. When GNB/g was multiplied by the respective APC to GNB ratio values, the approximate APC/g was derived. Although the variance and confidence limits of some of the APC and GNB ratio values were wide, use of LAL to estimate APC in 1 h was found to be workable.