Application of MALDI-TOF Mass Spectrometry for Accurate Identification of Listeria monocytogenes in Dairy Products from Egypt

Listeria monocytogenes is a Gram-positive foodborne bacterium that poses a major risk to public health, because of its persistence in processing environments and its tolerance to common sanitation measures. Dairy products, especially unpasteurized milk, soft cheese, and ice cream are recognized as key sources of listeriosis infection. This study investigated the occurrence of L. monocytogenes in dairy products obtained from retail markets in Beni-Suef Governorate, Egypt, and evaluated the efficiency of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for precise bacterial identification. A total of 75 samples comprising raw milk (n=25), Kareish cheese (n=25), and soft ice cream (n=25) were analyzed using conventional cultural methods followed by MALDI-TOF MS confirmation. Four presumptive Listeria isolates were recovered on selective Oxford agar. However, MALDI-TOF MS analysis confirmed only three isolates (75%) as L. monocytogenes, while one isolate (25%) was identified as Aneurinibacillus aneurinilyticus, demonstrating a false-positive result of cultural methods. The overall prevalence of confirmed L. monocytogenes was 4.0% (3/75), with product-specific rates of 4.0% in raw milk, 8.0% in Kareish cheese, and 0.0% in ice cream. These outcomes emphasize the improved reliability of MALDI-TOF MS compared with conventional biochemical identification and its potential for fast, cost-efficient, and accurate confirmation at the species level. The results support integrating MALDI-TOF MS into national food safety monitoring programs to enhance the detection and control of listeriosis in Egypt.