Use of a fluorescent viability stain to assess lethal and sublethal injury in food-borne bacteria exposed to high-intensity pulsed electric fields.
OBJECTIVE
To apply scanning electron microscopy, image analysis and a fluorescent viability stain to assess lethal and sublethal in food-borne bacteria exposed to high-intensity pulsed electric fields (PEF).


RESULTS
A rapid cellular staining method using the fluorescent redox probes 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) and 4',6-diamidino-2-phylindole was used for enumerating actively respiring cells of Listeria mononcytogenes, Bacillus cereus and Escherichia coli. This respiratory staining (RS) approach provided good agreement with the conventional plate count agar method for enumerating untreated and high-intensity PEF-treated bacteria suspended in 0.1% (w/v) peptone water. However, test organisms subjected to similar levels of lethality by heating at 56 degrees C resulted in ca 3-log-unit difference in surviving cell numbers ml(-1) when enumerated by these different viability indicators. PEF-treated bacteria were markedly altered at the cellular level when examined by scanning electron microscopy.


CONCLUSIONS
While PEF-treatment did not produce sublethally injured cells (P < 0.05), substantial subpopulations of test bacteria rendered incapable of forming colonies by heating may remain metabolically active.


CONCLUSIONS
The fluorescent staining method offers interesting perspectives on assessing established and novel microbial inactivation methods. Use of this approach may also provide a better understanding of the mechanisms involved in microbial inactivation induced by PEF.
Department of Bioscience, University of Strathclyde, Glasgow, UK.
S
S
Yaqub
J G
JG
Anderson
S J
SJ
MacGregor
N J
NJ
Rowan
D001409:Bacillus cereus / Q000254:growth & development*; D004558:Electric Stimulation; D004926:Escherichia coli / Q000254:growth & development*; D005456:Fluorescent Dyes; D005516:Food Microbiology; D005519:Food Preservation / Q000379:methods; D007211:Indoles; D008089:Listeria monocytogenes / Q000254:growth & development*; D008855:Microscopy, Electron, Scanning; D013778:Tetrazolium Salts
D005456:Fluorescent Dyes; D007211:Indoles; D013778:Tetrazolium Salts; C007293:DAPI