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I'm Not Dead Yet: Bacterial Tales from the Crypt and Survival after Heat Treatment

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Heat stress has been used as a method of killing bacteria for many years, and is one approach promulgated by federal regulations to reduce pathogens in biosolids (40 CFR 503). However, recent studies have suggested that heat stressed organisms may be able to recover and re-grow after thermal treatment. 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 hr. Contents of bottles were assayed over 10 days for growth on non-selective agar plates using the spread plate method. All plates were negative for growth immediately after heat stress, however in the samples heated for 4 hr. and 6 hr., recovery of E. coli was seen in buffer with nutrient broth after 24 hr. Recovery was also seen in buffer alone after 72 hr. Samples heated for 24 hr. were able to recover only in the presence of nutrients. Further work is being conducted using lower initial densities of E. coli to assess the difference between recovery and re-growth. A similar study was performed using a biosolids sample from a full-scale process that utilized pre-pasteurization followed by mesophilic digestion. Samples after digestion had no measurable densities of E. coli, however, after storage of the samples, detectable densities of E. coli were measured over a period of several days.
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Keywords: VBNC; biosolids; heat stress to bacteria; reactivation; regrowth; thermophilic treatment

Document Type: Research Article

Publication date: 01 January 2010

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