An 8-Year Surveillance of the Diversity and Persistence of Listeria monocytogenes in a Chilled Food Processing Plant Analyzed by Amplified Fragment Length Polymorphism

Abstract

Contamination routes of Listeria monocytogenes were examined in a chilled food processing plant that produced readyto-eat and ready-to-reheat meals during an 8-year period by amplified fragment length polymorphism (AFLP) analysis. A total of 319 L. monocytogenes isolates were recovered from raw materials (n ϭ 18), the environment (n ϭ 77), equipment (n ϭ 193), and products (n ϭ 31), and 18 different AFLP types were identified, five of which were repeatedly found to be persistent types. The three compartments (I to III) of the plant showed markedly different contamination statuses. Compartment I, which produced cooked meals, was heavily contaminated with three persistent AFLP types. AFLP type A1 dominated, and it comprised 93% of the isolates of the compartment. Compartment II, which produced uncooked chilled food, was contaminated with four persistent and five nonpersistent AFLP types. The equipment of compartment III, which produced cooked ready-toreheat meals, was free of contamination. In compartments that produced cooked meals, the cleaning routines, product types, and lack of compartmentalization seemed to predispose production lines to persistent contamination. The most contaminated lines harbored L. monocytogenes in coolers, conveyors, and packing machines. Good compartmentalization limited the flow of L. monocytogenes into the postheat-treatment area and prevented the undesired movement of equipment and personnel, thus protecting the production lines from contamination. In compartment II, grated cheese was shown to cause product contamination. Therefore, special attention should be paid to continuous quality control of raw ingredients when uncooked ready-toeat foods are produced. In compartment II, reconstruction of the production line resulted in reduced prevalence rates of L. monocytogenes and elimination of two persistent AFLP types.During the past decade, consumer demand for foods requiring minimal preparation time has increased the readyto-eat and ready-to-reheat chilled food markets. For the chilled food processing industry, contamination of products with Listeria monocytogenes is a major safety concern because the bacterium is widely spread in the environment, can grow at refrigerated temperatures, and may cause persistent plant contamination (10,12,19,29). In addition, some chilled food products are consumed without further heating or without adequate reheating to destroy L. monocytogenes, and therefore, these products can serve as sources of human listeriosis in outbreaks and sporadic cases.A number of studies have shown that L. monocytogenes contamination of food products primarily results from postprocessing contamination from the equipment and the processing environment (3,5,19,24,26), although the initial contamination may be introduced to the production plant via raw materials (17). The contamination routes of L. monocytogenes in the processing environment have been examined in several studies targeting meat, poultry, and fish processing plants (3,7,29,31). However, despite the incr...

Characterization of Listeria monocytogenes Isolates from Human Listeriosis Cases in Italy

“…Consequently, a large proportion of the human listeriosis cases under investigation could be grouped into molecular subtype clusters, some of which could represent common-source outbreaks. Furthermore, the geographic scale of these clusters appeared in some cases to be limited to a single region but in others to be multiregional, according to previous reports documenting that human listeriosis clusters can be widely distributed geographically (17,22,23). Two clusters, IV and X, included strains isolated over periods of approximately 8 and 7 years, respectively.…”

“…The epidemiological picture is often intricate because of the increasingly wider scale of commercialization of many food products and the evolving food habits associated with demographic, social, and economic changes. Finally, further difficulties arise from the protracted period of incubation of the listeriosis agent and the possibly space-time-scattered distribution of cases (7,17,22,23).…”

“…The occurrence of two indistinguishable subtypes by chance only should be considered first as a possible explanation. However, considering that L. monocytogenes colonization can persist in food-processing plants for some years, the prolonged shelf lives of some food products, and the combined effects of these two factors, alternative hypotheses should not be discarded a priori (17). In any case, additional information apart from molecular subtyping data must be gathered before more-reliable epidemiological inferences can be drawn.…”

Genome Sequencing Identifies Two Nearly Unchanged Strains of Persistent Listeria monocytogenes Isolated at Two Different Fish Processing Plants Sampled 6 Years Apart

“…The bacterium is an intracellular human pathogen, and it also has a saprophytic life-style and can therefore be isolated from soil and decaying plant material (4). Although it can be present in raw food materials, the processing plant environment is typically the immediate source of L. monocytogenes contamination of food products (5)(6)(7)(8). Even though food processing equipment and facilities are cleaned frequently, some molecular subtypes of L. monocytogenes may persist in the food processing environment for many years (7)(8)(9).…”

Roles of Four Putative DEAD-Box RNA Helicase Genes in Growth of Listeria monocytogenes EGD-e under Heat, pH, Osmotic, Ethanol, and Oxidative Stress Conditions

“…monocytogenes is frequently found in raw and processed foods and in food-processing environments (3,16,27,32,39,56,57). In the food chain, the bacterium is exposed to various stresses through sanitation, disinfection, heating, cooling, and the addition of food additives and preservatives.…”