The direct enumeration of Listeria monocytogenes in a food with a low background microflora

Concentrated yoghurt (labneh) with an inherently low count of non-lactic acid bacteria was inoculated with various levels of Listeria monocytogenes, and direct plating onto Listeria Selective Agar or PALCAM Agar proved effective for monitoring high counts. At levels of < 10 cfu/g, an MPN count using UVM I/Fraser Broth was more reliable, and a ‘presumptive’ count, taking the colour change in Fraser Broth from yellow to black as being ‘positive’, had a failure rate of < 2%. Given that such low counts are below the likely infective dose for humans, it is suggested that the direct MPN count could be employed for routine quality control examinations.