Recovery of Bacteria Following Heat Treatment

Abstract

Heat stress has been used as a method of killing bacteria for many years, and is the basis for federal regulations promulgated to reduce pathogens in biosolids (40 CFR 503). However, recent studies have suggested that heat stressed organisms may be able to reactivate and re-grow. The purpose of this study was to examine bacterial response to heat stress over time and to evaluate the ability of bacteria to recover and grow. Washed Cultures of E. coli were placed into bottles containing sterile buffer and buffer amended with 1% nutrient broth. The bottles were then subjected to heat at 55°C for 4, 6, and 24 hours. Contents of bottles were assayed over 10 days for growth on non-selective agar plates using the spread plate method along with the multiple tube fermentation test (MPN). All procedures were negative for growth immediately after heat stress; however in the samples heated for 4 hrs and 6 hrs, recovery of E. coli was seen in buffer with nutrient broth after 24 hrs. Recovery was also seen in buffer alone after 72 hrs. Samples heated for 24 hrs did not show the ability to recover.

Biosolids and Sludge Management

“…In air dried and rewetted biosolids,Rouch et al (2011) found Clostridium perfringens to be an overly conservative indicator. The study also found survival, but not regrowth, of E. coli or Water Environment Research, Volume 84, Number 10-Copyright © 2012 Water Environment Federation Salmonella in rewetted biosolids (15-20% moisture content), after being seeded with these species, indicating a degree of safety of stored biosolids upon rewetting by rain.The recovery capacity of bacteria following heat stress was noted byBoczek et al (2011) including enumeration in samples that did not have nutrients in the broth, which was more indicative of recovery rather than regrowth. The study also mentioned the development of a modification to EPA Method 1680 using microtiter-well assay to improve recoveries.…”