Related Experiment Videos
Evaluation of thin agar layer method for recovery of acid-injured foodborne pathogens
1Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-1600, USA.
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.
Insights
The thin agar layer (TAL) method effectively enumerates acid-injured foodborne pathogens. This convenient, one-step technique shows comparable recovery rates to tryptic soy agar (TSA) and outperforms selective media.
Area of Science:
- Food Microbiology
- Food Safety
- Bacteriology
Background:
- Acidic conditions can injure foodborne pathogens, affecting their enumeration.
- Selective media are commonly used but may inhibit injured cell recovery.
- The thin agar layer (TAL) method offers a potential improvement for recovering stressed microorganisms.
Purpose of the Study:
- To evaluate the efficacy of the thin agar layer (TAL) method for enumerating acid-injured foodborne pathogens.
- To compare the recovery rates of TAL with nonselective tryptic soy agar (TSA) and selective media.
- To assess the TAL method's performance for various foodborne pathogens including Escherichia coli O157:H7, Salmonella Typhimurium, Staphylococcus aureus, Yersinia enterocolitica, and Listeria monocytogenes.
Main Methods:
- Foodborne pathogens were injured using 2% acetic acid for 1, 2, or 4 minutes.
- Enumeration was performed using the thin agar layer (TAL) method, nonselective tryptic soy agar (TSA), and pathogen-specific selective media.
- Recovery rates were statistically compared between the different methods.
Main Results:
- The TAL method showed no significant difference compared to TSA for enumerating acid-injured Escherichia coli O157:H7, Salmonella Typhimurium, Staphylococcus aureus, and Yersinia enterocolitica.
- Both TAL and TSA recovered significantly higher numbers of these pathogens than selective media.
- For Listeria monocytogenes, no significant difference was observed among TSA, TAL, and selective media, though selective media yielded fewer cells.
Conclusions:
- The thin agar layer (TAL) method is a convenient and effective one-step procedure for recovering acid-injured foodborne pathogens.
- TAL and TSA demonstrate superior recovery of acid-injured bacteria compared to traditional selective media.
- The TAL method provides a valuable alternative for accurate enumeration of stressed foodborne pathogens.