Predominance of Distinct Listeria Innocua and Listeria Monocytogenes in Recurrent Contamination Events at Dairy Processing Facilities

Abstract

The genus Listeria now comprises up to now 21 recognized species and six subspecies, with L. monocytogenes and L. innocua as the most prevalent sensu stricto associated species. Reports focusing on the challenges in Listeria detection and confirmation are available, especially from food-associated environmental samples. L. innocua is more prevalent in the food processing environment (FPE) than L. monocytogenes and has been shown to have a growth advantage in selective enrichment and agar media. Until now, the adaptive nature of L. innocua in FPEs has not been fully elucidated and potential persistence in the FPE has not been observed. Therefore, the aim of this study is to characterize L. innocua (n = 139) and L. monocytogenes (n = 81) isolated from FPEs and cheese products collected at five dairy processing facilities (A–E) at geno- and phenotypic levels. Biochemical profiling was conducted for all L. monocytogenes and the majority of L. innocua (n = 124) isolates and included a rhamnose positive reaction. L. monocytogenes isolates were most frequently confirmed as PCR-serogroups 1/2a, 3a (95%). Pulsed-field gel electrophoresis (PFGE)-typing, applying the restriction enzymes AscI, revealed 33 distinct Listeria PFGE profiles with a Simpson’s Index of Diversity of 0.75. Multi-locus sequence typing (MLST) resulted in 27 STs with seven new L. innocua local STs (ST1595 to ST1601). L. innocua ST1597 and ST603 and L. monocytogenes ST121 and ST14 were the most abundant genotypes in dairy processing facilities A–E over time. Either SSI-1 (ST14) or SSI-2 (ST121, all L. innocua) were present in successfully FPE-adapted strains. We identified housekeeping genes common in Listeria isolates and L. monocytogenes genetic lineage III. Wherever there are long-term contamination events of L. monocytogenes and other Listeria species, subtyping methods are helpful tools to identify niches of high risk.

Monitoring by a Sensitive Liquid-Based Sampling Strategy Reveals a Considerable Reduction of Listeria monocytogenes in Smeared Cheese Production over 10 Years of Testing in Austria

“…Since the occurrence of L. monocytogenes contamination was similar for both categories (Table 2), we confirmed that heat treatment of milk had little impact on the presence of L. monocytogenes in the smears and that, in the majority of our observations, cheese is more likely to become contaminated after coagulation [18,35,49].…”

“…by ISO enrichment methods, PCR-based species differentiation should be performed on typical Listeria colonies isolated on selective agar [ 32 , 33 ]. Persistence of L. innocua was shown to occur more frequently than persistence of L. monocytogenes and is, therefore, seen as an indicator of inadequate hygiene [ 34 , 35 ].…”

The Saprophytic Lifestyle of Listeria monocytogenes and Entry Into the Food-Processing Environment

“…The most internationally recognized scheme was proposed by Ragon et al (2008), an adaptation of the initially proposed scheme by Salcedo et al (2003). This typing method is still currently used (Kaszoni-Rückerl et al, 2020;Stessl et al, 2021).…”

What are the recognized species of the genus Listeria?

“…In recent times, investigators have reported different numbers of species for the genus; hence, it is important to clarify the number of species available now. A recent article [1] reported that the genus Listeria comprises up to 21 species. The authors relied on the information in the List of Prokaryotic Names with Standing in Nomenclature at the ‘Bacterio.net’ website (http://www.bacterio.net/listeria.html) to conclude 21 species.…”