**5. The occurrence of** *L. monocytogenes* **in RTE foods and food contact surfaces in food service facilities**

RTE foods have gained considerable popularity in many developing and developed countries because of their perceived better flavor, affordability and accessibility [28]. However, numerous *L. monocytogenes* outbreaks have been associated with RTE foods [29]. The prevalence of *L. monocytogenes* in RTE food is a major concern relating to food safety because RTE foods are consumed without further processing (cooking) or washing at home (**Table 2**). It is for this reason that stringent microbiological guidelines need to be formulated and followed to ensure that processors produce RTE food that is safe [30]. Implicated RTE foods include RTE deli meats, raw milk and other raw milk dairy products (soft cheese) (**Table 1**). Between 1999 and 2011, 73% of all food-borne outbreaks of listeriosis that occurred in the United Kingdom (UK) were attributed to the consumption of sandwiches [31].

The presence of *L. monocytogenes* in RTE food is attributed to contamination during production, distribution or storage [32]. *L. monocytogenes* contamination in various food factory environments has been reported at nearly all stages of processing ([5] and Rodrigues et al. [33]). When compared with other food-borne pathogens such as *Staphylococcus aureus*, *E. coli* 015:H7, and *Salmonella* and *Shigella* species, *L. monocytogenes* has been found to be most prevalent on food contact surfaces in food service establishments [34]. Owing to its ability to grow in contaminated food during storage at refrigeration temperature, *L. monocytogenes* has


**Table 2.** Listeria in food: foods that are susceptible to contamination by *Listeria monocytogenes* [57, 74].

*L. monocytogenes* can survive a low pH of 5.5 through a phenomenon known as the acid tolerance response (ATR), which causes cells to be more resistant in adverse acidic conditions [23]. The bacterium, which is notable for its persistence in food-manufacturing environments, is relatively salt-tolerant and is able to grow at refrigerator temperature, and is therefore often associated with the consumption of raw or minimally processed and often chilled RTE foods (e.g., soft and semi-soft cheese and smoked fish products), which are consumed without fur-

*L. monocytogenes* is able to attach to food processing surfaces and multiply to form biofilms in inaccessible locations in processing facilities [9]. Biofilms protect the bacterium against antimicrobial action, enabling it to colonize food processing equipment, conveyor belts, pipes, floors and drainage systems and to persist for months or even years, cross-contaminating different surfaces in food processing plants [26]. The formation of biofilms on various food contact surfaces by *L. monocytogenes* makes it extremely difficult to control this pathogen effectively, especially in processing plants where inadequate cleaning has been carried out [27].

**5. The occurrence of** *L. monocytogenes* **in RTE foods and food contact** 

RTE foods have gained considerable popularity in many developing and developed countries because of their perceived better flavor, affordability and accessibility [28]. However, numerous *L. monocytogenes* outbreaks have been associated with RTE foods [29]. The prevalence of *L. monocytogenes* in RTE food is a major concern relating to food safety because RTE foods are consumed without further processing (cooking) or washing at home (**Table 2**). It is for this reason that stringent microbiological guidelines need to be formulated and followed to ensure that processors produce RTE food that is safe [30]. Implicated RTE foods include RTE deli meats, raw milk and other raw milk dairy products (soft cheese) (**Table 1**). Between 1999 and 2011, 73% of all food-borne outbreaks of listeriosis that occurred in the United Kingdom

The presence of *L. monocytogenes* in RTE food is attributed to contamination during production, distribution or storage [32]. *L. monocytogenes* contamination in various food factory environments has been reported at nearly all stages of processing ([5] and Rodrigues et al. [33]). When compared with other food-borne pathogens such as *Staphylococcus aureus*, *E. coli* 015:H7, and *Salmonella* and *Shigella* species, *L. monocytogenes* has been found to be most prevalent on food contact surfaces in food service establishments [34]. Owing to its ability to grow in contaminated food during storage at refrigeration temperature, *L. monocytogenes* has

**4. The occurrence of** *L. monocytogenes* **in the food processing** 

ther processing [24, 25].

**surfaces in food service facilities**

(UK) were attributed to the consumption of sandwiches [31].

**environment**

74 Listeria Monocytogenes

been found in raw and processed RTE foods that required low temperature storage [35]. The high volume of food products such as meat, vegetables, dairy products and fruits that pass through the cold chain in food service establishments could contribute to the high incidence of *L. monocytogenes* in RTE food and on food contact surfaces [34, 36].

Inadequate cleaning procedures and hygiene practices can promote the formation of biofilms on food contact surfaces in food service establishments, thereby increasing the chances of *L. monocytogenes* cross-contamination within food service facilities [37]. Because biofilms are able to resist most sanitisers and disinfectants used, cross-contamination by *L. monocytogenes* poses a serious food safety risk in food service establishments, including domestic kitchens [38]. The ease with which *L. monocytogenes* is able to adhere to food contact surfaces and form biofilms increases the likelihood of its persisting on food contact surfaces, and hence crosscontaminating the final food products [39, 40]. The presence of food debris on food contact surfaces encourages the formation of *L. monocytogenes* biofilms [41].
