title: Spray method for recovery of heat-injured Salmonella Typhimurium and Listeria monocytogenes.
abstract: Selective agar is inadequate for supporting recovery of injured cells. During risk assessment of certain foods, both injured and noninjured cells must be enumerated. In this study, a new method (agar spray method) for recovering sublethally heat-injured microorganisms was developed and used for recovery of heat-injured Salmonella Typhimurium and Listeria monocytogenes. Molten selective agar was applied as an overlay to presolidified nonselective tryptic soy agar (TSA) by spray application. Heat-injured cells (55°C for 10 min in 0.1% peptone water or 55°C for 15 min in sterilized skim milk) were inoculated directly onto solidified TSA. After a 2-h incubation period for cell repair, selective agar was applied to the TSA surface with a sprayer, and the plates were incubated. The recovery rate for heat-injured Salmonella Typhimurium and L. monocytogenes with the spray method was compared with the corresponding rates associated with TSA alone, selective media alone, and the conventional overlay method (selective agar poured on top of resuscitated cells grown on TSA and incubated for 2 h). No significant differences (P > 0.05) were found in pathogen recovery obtained with TSA, the overlay method, and the spray method. However, a lower recovery rate (P < 0.05) was obtained for isolation of injured cells on selective media. Overall, these results indicate that the agar spray method is an acceptable alternative to the conventional overlay method and is a simpler and more convenient approach to recovery and detection of injured cells.
authors.affiliation: Department of Food and Animal Biotechnology, Center for Agricultural Biomaterials, and Research Institute for Agriculture and Life Sciences, Seoul National University, Seoul 151-921, Republic of Korea.
authors.forename: Kyeong-Hwan
authors.initials: KH
authors.lastname: Back
authors.forename: Sang-Oh
authors.initials: SO
authors.lastname: Kim
authors.forename: Ki-Hwan
authors.initials: KH
authors.lastname: Park
authors.forename: Myung-Sub
authors.initials: MS
authors.lastname: Chung
authors.forename: Dong-Hyun
authors.initials: DH
authors.lastname: Kang
mesh_terms: D000362:Agar; D001431:Bacteriological Techniques; D015169:Colony Count, Microbial / Q000379:methods*; D005511:Food Handling / Q000379:methods*; D005516:Food Microbiology; D006358:Hot Temperature; D006801:Humans; D008089:Listeria monocytogenes / Q000302:isolation & purification*; D018570:Risk Assessment; D012486:Salmonella typhimurium / Q000302:isolation & purification*
chemical_list: D000362:Agar