A MICROBIOLOGICAL SURVEY AND CHARACTERIZATION

In our study, two dairy compost heaps and one vegetable scrap compost heap were constructed on two research farms in Clemson, SC. Samples were taken from each heap from different locations at 8 elected intervals and analyzed for enterococci and VRE counts by spread plating on Bile Esculin agar (BEA) and BEA containing 6 g/ml of vancomycin, respectively. The initial populations of enterococci and VRE in compost were in the range of 6.46-7.43 and 5.36-6.70 log CFU/g, respectively. After 30 days of active composting the average VRE populations declined ca. 4.6, 4.07, 3.97 and 1.6 logs at the top, center, bottom and surface locations of the heaps, respectively, whereas the enterococci populations declined for 4.36, 3.54, 3.10, and 2.28 logs, respectively. During the two dairy compost trials, the temperatures at the top, center, and bottom locations were in excess of 55¡C for 14 - 18, 12, and 8 days, respectively, whereas temperatures of the surface samples never exceeded 32¡C. During thermophilic composting, microbial activity slightly reduced the pH, there after it began to rise slightly due to the production of ammonia. The moisture content inside the heaps changed very little during active composting in any of the 3 trials. Both enterococci and VRE isolates obtained from compost trials were further characterized for genus and species determination, minimum inhibitory concentration (MIC), and heat tolerance screening. Our results revealed that the genus of all 88 VRE isolates was enterococci and that no VRE were Enterococcus faecalis or Enterococcus faecium. However, 9 and 6% of enterococci isolates from Trial 1 were tested as E. faecalis and E. faecium, respectively. For those VRE isolates taken from composting, MIC testing revealed that 85 out of 88 suspected VRE isolates were resistant to \u3e 256 g/ml of vancomycin and all 88 isolates were resistant to \u3e 64 g/ml of teicoplanin. Eighty-three (94%) of VRE isolates were confirmed to contain the vanA gene for vancomycin resistance as well as 4 (11%) enterococci isolates from Trial 1. The D-values of selected VRE isolates (n=8) from Trial 1 compost heaps were in the range of 9.7-17.73 min at 60C, 4.73-12.57 min at 65C, and 1.59-4.44 min at 70C. The z-values for those VRE isolates ranged from 11.92-17.87¼C. The results revealed that the isolates from day 30 had the highest D-values while isolates from day 60 had the highest z-value, as compared with isolates from day 0 which had the lowest D- and z-values. Box-PCR analysis of VRE isolates (n=12) from Trial 1 revealed that 42% were identical and appeared on compost days 7, 14, and 60, suggesting that this specific strain was able to adapt to the elevated temperatures in compost by developing heat resistance. Our studies demonstrated that VRE may become inactivated in compost if conditions are optimal and that some enterococci may develop heat resistance during the thermophilic phase of composting. This is important because it suggests that improperly composted manures may serve as a means for the spread of VRE on food products intended for human consumption