A novel strategy to differentiate Listeria spp. and other gram-positive foodborne pathogens in the selective enrichment step using modified PALCAM broth
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Publication Details
Author list: Sangadkit W., Saranak J., Thipayarat A.
Publisher: Faculty of Food Science and Technology
Publication year: 2016
Journal: International Food Research Journal (1985-4668)
Volume number: 23
Issue number: 4
Start page: 1773
End page: 1781
Number of pages: 9
ISSN: 1985-4668
Languages: English-Great Britain (EN-GB)
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Abstract
This research explored alternative rapid screening strategies of Listeria spp. detection by the occurrence of blackened broth as a result of the esculin-ferric ammonium citrate reaction. The presence of Enterococcus faecalis in industrial food sample scan mislead the presumptive results of Listeria spp.; hence, incorporation of sugar fermentability tests was exploited to distinguish Listeria spp. from other Listeria competitors. Various sugar alternatives were used as carbon sources and phenol red was added to indicate pH changes in the modified PALCAM broth (mPB). Listeria monocytogenes and other Gram-negative competitors (i.e., Escherichia coli and Salmonella Anatum) and Gram-positive non-Listeria bacteria (i.e., Staphylococcus aureus and E. faecalis) were cultured in these modified substrates and incubated at 37ฐC for 24 h. Viable cell count and broth color were monitored using the micro scale cultivation technique and spectrophotometer at discrete wavelengths from 340 to 650 nm, respectively. The results showed that the indicative signals of broth color changes were detected as early as 8-12 h after incubation. All tested strains grew very well in these mPBs maintaining high viable cell counts (106-107 CFU/ml). The esculin activity was very effective in differentiating Gram-positive from Gram-negative cultures at 550 nm. As Gram-positive bacteria, E. faecalis produced similar blackened broth similar to what Listeria monocytogenes did with the esculinbased broth. However, the use of other sugar supplements (i.e., sorbitol, sucrose, or xylose) improved the optical differentiation of L. monocytogenes from E. faecalis. The growth of all tested Gram-negative bacteria was inhibited by strong nature of mPB selective agents and other Gram-positive strains were void of esculin activity. The combined sugar fermentability and esculin activity were able to presumptively screen the presence of L. monocytogenes in industrial food samples and environmental swabs.
Keywords
Esculin hydrolysis, Foodborne pathogens, Sugar Fermentation
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