**8. Methods commonly used for the identification of** *L. monocytogenes* **in foods**

#### **8.1. Culture methods**

*L. monocytogenes* can be isolated from contaminated samples by subjecting them to pre-enrichment. This entails mixing samples with enrichment media such as *Listeria* Enrichment Broth (Sigma), after which the enrichment samples can be cultured on *L. monocytogenes*-specific agar plates such as Listeria Mono Differential Agar (Sigma). Isolation can be performed using various other media and procedures [50]. Thereafter, pure cultures of *L. monocytogenes* to be used for downstream identification and characterization analysis can be prepared by isolating individual colonies from agar plates [51]. The culture-based methods are often used in combination with immunoassay- or molecular PCR-based methods for accurate detection of *L. monocytogenes* in food samples [52].

#### **8.2. Immunoassay**

During immunoassay, monoclonal antibodies specific to *L. monocytogenes* can be incorporated into various techniques for identification. Immunoassay tests usually have high specificity and are fast and easy to use, but do not permit identification to species level. Another disadvantage of this method is that the presence of a low number of listeria cells in a sample can give rise to a false positive [53]. Various variants of immunoassays are available, including sandwich-type enzyme-linked immunosorbent assay (S-ELISA) [54], nanoparticle immunoassay [55], and enzyme-linked fluorescent assay (ELFA) [56].

#### **8.3. PCR-related methods**

**7. Legislation relating to the occurrence of** *Listeria monocytogenes* **in** 

detected in 25 g of food in which the growth of *L. monocytogenes* is likely to occur [48].

shelf life, in up to 5 collected food samples [49].

*monocytogenes* **in foods**

**8.1. Culture methods**

**8.2. Immunoassay**

**8. Methods commonly used for the identification of** *L.* 

The Food Safety Standard of Ireland has prescribed the following in relation to *L. monocytogenes*: *L. monocytogenes* should be absent in 25 g of RTE food destined for infant consumption or for serving as a special food for medical purposes in up to 10 collected food samples. Similarly, in the case of RTE foods that are able to support the growth of *L. monocytogenes*: *L. monocytogenes* should be absent in 25 g of RTE food following production or should not exceed 100 cfu per gram of food placed on the market during its shelf life, in up to 5 collected food samples. Lastly, in the case of RTE foods that are not able to support the growth of *L. monocytogenes*: *L. monocytogenes* should not exceed 100 cfu per gram of food placed on the market during its

*L. monocytogenes* can be isolated from contaminated samples by subjecting them to pre-enrichment. This entails mixing samples with enrichment media such as *Listeria* Enrichment Broth (Sigma), after which the enrichment samples can be cultured on *L. monocytogenes*-specific agar plates such as Listeria Mono Differential Agar (Sigma). Isolation can be performed using various other media and procedures [50]. Thereafter, pure cultures of *L. monocytogenes* to be used for downstream identification and characterization analysis can be prepared by isolating individual colonies from agar plates [51]. The culture-based methods are often used in combination with immunoassay- or molecular PCR-based methods for accurate detection of *L. monocytogenes* in food samples [52].

During immunoassay, monoclonal antibodies specific to *L. monocytogenes* can be incorporated into various techniques for identification. Immunoassay tests usually have high specificity

Most food legislation stipulates the microbial criteria for food-borne bacteria such as *L. monocytogenes* or their toxins and metabolites in specific foods. These criteria often prescribe the acceptable levels of these bacteria or their toxins in food products available on the market [46]. Most foods that support the growth of *L. monocytogenes* should be the focus of risk management efforts. Countries such as Germany, the Netherlands and France have set a tolerance level of 100 colony forming units (cfu) of *L. monocytogenes* per gram of food at the time of consumption while others, such as the USA and Italy, require a total absence of *L. monocytogenes* in 25 g of food [47]. The new criteria for *L. monocytogenes* in RTE food gazetted by Food Standards Australia-New Zealand on 31 July 2014 prescribe two sets of criteria for *L. monocytogenes* for application based on whether the growth of the bacterium does or does not occur inherently in a particular RTE food. These criteria include fewer than 100 cfu of *L. monocytogenes* per gram of food in which the growth of *L. monocytogenes* is not likely to occur, and that *L. monocytogenes* should not be

**foods**

76 Listeria Monocytogenes

PCR-based techniques involve the amplification of a specific gene segment of *L. monocytogenes* such as HlyA-, Iap-, PrfA and SsrA using specific primers followed by monitoring of the amplified segment using agarose gel electrophoresis or other detection techniques such as SYBR Green [57]. Similarly, the 16S rRNA genes of *L. monocytogenes* can be amplified, sequenced, and searched against existing databases for identification [52]. The disadvantage of PCR-based techniques is related to the costs associated with the purchase of the instrument and reagent, as well as the expertise required to conduct the experiments [58].

#### **9. Prevention and control of** *Listeria monocytogenes* **in food systems**

The prevention and control of *L. monocytogenes* in RTE foods is paramount in protecting consumers against listeriosis. In a document entitled "Guidelines on the application of general principles of food hygiene to the control of *L. monocytogenes* in foods" the World Health Organization has provided guidelines that can be followed to minimize the likelihood of the occurrence of *L. monocytogenes* in RTE foods. According to the [59], food safety measures need to be carried out at different levels of a food production environment, and must include:


While the food industry is taking numerous measures to protect foods from *Listeria*, consumers of RTE food, especially those belonging to the vulnerable groups, must take suitable


most food processing industries are taking measures to protect foods from listeria, consumers of RTE food, especially those belonging to vulnerable groups, must take suitable precautions during the handling of food to prevent the growth of *L. monocytogenes* and contamination of food by this organism. Continuous identification of those groups of consumers vulnerable to listeriosis and food safety education directed at them specifically will increase their awareness of the need for safer food handling practices such as hand washing and safe storage of RTE

Contamination, Prevention and Control of *Listeria monocytogenes* in Food Processing and Food…

http://dx.doi.org/10.5772/intechopen.76132

79

I would like to acknowledge my wife, Wendy Tabit, for reading this manuscript and making

I declare that I have no conflict of interest regarding the publication of this research chapter.

[1] Madden RH, Hutchison M, Jordan k, Pennone V, Gundogdu O, Corcionivoschi N. Prevalence and persistence of *Listeria monocytogenes* in premises and products of small food

[2] Centre for Disease Control and Prevention (CDC). *Listeria* (listeriosis): Multistate Outbreak of Listeriosis Linked to Soft Raw Milk Cheese Made by Vulto Creamery (Final Update) [Internet]. 2017. Available from: https://www.cdc.gov/listeria/outbreaks/soft-

[3] Roberts AJ, Wiedmann M. Pathogen, host and environmental factors contributing to the pathogenesis of listeriosis. Cellular and Molecular Life Sciences. 2003;**60**(5):904-918 [4] Beno SM, Stasiewicz MJ, Andrus AD, Ralyea RD, Kent DJ, Martin NH, Wiedmann M, Boor KJ. Development and validation of pathogen environmental monitoring programs for small cheese processing facilities. Journal of Food Protection. 2016;**79**(12):2095-2106

business operators in Northern Ireland. Food Control. 2018;**87**:70-78

cheese-03-17/index.html [Accessed: February 12, 2018]

food in an effort to prevent listeriosis.

**Acknowledgements**

**Conflict of interest**

**Author details**

Frederick Tawi Tabit

**References**

Address all correspondence to: tabitft@uinisa.ac.za

University of South Africa, Johannesburg, South Africa

suggestions.

**Table 3.** Listeria in food: Advice to people vulnerable to listeriosis [75].

precautions during the handling of food in their households to prevent the growth and contamination of food by *L. monocytogenes* (**Table 3**).

#### **10. Consumer awareness of listeriosis**

Continuous provision of food safety education to consumers through various channels such as social media increases consumer awareness of the need for safer food handling practices such as hand washing and safe storage of RTE food [60]. The food standard agency of the UK has identified and targeted consumers who are at risk of contracting listeriosis. Vulnerable people, many of whom live and obtain their food independently include those with various forms of cancer, diabetes, alcoholism and diseases of the kidneys, liver, cardiovascular system (e.g., heart disease), digestive system (e.g., Crohn's disease) and musculoskeletal/connective tissue system (e.g., lupus) [61]. Even though most consumers of food sold by street vendors may not have confidence in the safety of RTE foods sold on the street, this often does not affect their preference for such foods because of their affordability, availability and convenience [28].

#### **11. Conclusions**

RTE foods have gained considerable popularity in many developing and developed countries because of their perceived better flavor, affordability and convenience. Consumers will continue to consume RTE foods despite their association with *L. monocytogenes* outbreaks. While most food processing industries are taking measures to protect foods from listeria, consumers of RTE food, especially those belonging to vulnerable groups, must take suitable precautions during the handling of food to prevent the growth of *L. monocytogenes* and contamination of food by this organism. Continuous identification of those groups of consumers vulnerable to listeriosis and food safety education directed at them specifically will increase their awareness of the need for safer food handling practices such as hand washing and safe storage of RTE food in an effort to prevent listeriosis.
