Evaluation of Thin Agar Layer Method for Recovery of Acid-Injured Foodborne Pathogens

Abstract

The thin agar layer (TAL) method of Kang and Fung was used to enumerate acid-injured foodborne pathogens. This method involves overlaying 14 ml of nonselective medium (tryptic soy agar [TSA]) onto a prepoured and solidified pathogen-specific, selective medium in a petri dish. After surface plating, injured cells resuscitated and grew on TSA during the first few hours of incubation; then, the selective agents from the selective medium diffused to the top layer, interacted with the recovered microorganisms, and started to produce typical reactions. Foodborne pathogens were exposed to 2% acetic acid for 1, 2, or 4 min, and the recovery rate with the TAL method was compared with the rate of TSA and pathogen-specific, selective media. No significant difference occurred between TSA and TAL (P > 0.05) for enumeration of acid-injured Escherichia coli O157:H7, Salmonella Typhimurium, Staphylococcus aureus, and Yersinia enterocolitica, and both recovered significantly higher numbers than the selective medium for each respective pathogen (P < 0.05). For recovery of acid-injured Listeria monocytogenes, no difference (P > 0.05) occurred among TSA, TAL, and selective media. However, fewer cells were recovered in the selective media. The TAL method is a one-step, convenient procedure for recovery of acid-injured cells.

Inactivation of Salmonella enterica cells in Spanish potato omelette by high hydrostatic pressure treatments

“…During the first hours of incubation, injured salmonellae repaired their cell damage and started to grow in the TSA while the selective agents from XLD diffused gradually to the TSA top layer part. This procedure yielded slightly lower but not statistically different counts compared to TSA for sublethal heat injury and also improved the recovery of acid-injured salomonellae (Kang & Fung, 2000;Wu, Fung, Kang, & Thompson, 2001). In our study with salmonellae pressurized at 300 MPa, the differences between TAL method and TSA counts were not statistically significant.…”

Antibacterial effects of American cranberry (Vaccinium macrocarpon) concentrate on foodborne pathogens

“…The TAL plates were made by overlaying a total of 14 ml of tryptic soy agar (TSA, Difco), onto a prepoured and solidified selective medium in a petri dish (100 Â 15 mm) Wu, Fung, Kang, & Thompson, 2001). The pathogen-specific agar was MacConkey sorbitol agar (MSA, Difco) for E. coli O157:H7, modified oxford agar (MOX, Difco) for L. monocytogenes, xylose lysine desoxycholate agar (XLD, Difco) for S. Typhimurium, and Baird-Parker agar (BP, Difco) for S. aureus.…”

Assessing the growth and recovery of Salmonella Enteritidis SE86 after sodium dichloroisocyanurate exposure

“…In order to quantify microorganisms present in a diluted sample, an appropriate diluent should not promote bacterial multiplication or death, should be ease to prepare, should present low cost, and ideally, should be able to recover stressed cells (Wu et al , 2001; Wu, 2008). Based on these appointments and according to our results, The most appropriate diluent was SaS, because it did not cause decreasing in the NaDCC exposed cell numbers and did not sustain the multiplication of not exposed cells.…”