This is an extract of a report by a 3rd-year food science student, Jiahui Tan, at RMIT University in 2008. While salmonellae and Campylobacter species, mainly C. jejuni, are more often thought of as poultry- product-borne pathogens, due to the ubiquitous occurrence of literiae, especially L. monocytogenes, listeriosis can be an egg-borne disease.
Listeria is a Gram-positive, non-spore forming rod with a total of six species recognized. However, only Listeria monocytogenes is an important human pathogen. (Adams & Moss, 2006) Other species of this genus includes L. ivanovii, L. murrayi, L. innocua, L.seeligeri and L. gravi. (Jay et. al., 2005) L. innocua has also been frequently used as a biological indicator for L. monocytogenes because of its similar response to physical, chemical or thermal treatments. (Kamat & Nair, 1996)
According to Adams & Moss (2006), L. monocytogenes is ubiquitous in the environment and grows over a large range of temperature from 0 to 42 oC with an optimum between 30 and 35 oC. Swaminathan & Gerner-Smidt (2007) also described L . monocytogenes as a foodborne pathogen that is resistant to acid and capable of growing at low temperature. This means that it may grow in refrigerated food which poses a risk to food that are originally stored in chilled temperature to extend shelf life. The ability to grow over a wide temperature range, in the acidic environment, as well as in the absence or in very low amounts of O2, suggest that L. monocytogenes is able to multiply in many environment which is potentially dangerous in the processing of food. (Gómez et. al., 2005)
It is well established that Listeria are widely distributed in nature and any fresh product may harbour L. monocytogenes. (Jay et. al., 2005) These range from raw milk (Carlos et. al, 2001), soft cheeses (Guerra et. al, 2001), fish (Farber, 1991), meat products (Grau & Vanderline, 1992), poultry (Vitas & Garcia-Jalon, 2004), raw eggs (Leasor & Foegeding, 1989; Brackett & Beuchat, 1991), vegetables (Koseki & Isobe, 2005) and ready-to-eat-food (Vitas & Garcia-Jalon, 2004). For the association of cheeses, Adams & Moss (2006) mentioned that the survival of L. monocytogenes is due to the ripening process. It was reported that this pathogen survives poorly in unripened soft cheeses such as cottage cheese, but well in products such as Camembert and Brie.
In spite of high incidence of L. monocytogenes in poultry, little information is available on incidence in raw egg. (Laird et. al., 1991) Although there was no documented cases of clinical illness cause by L. monocytogenes for the consumption of egg or egg products, findings suggest that a possible public health problem could develop. (Moore & Madden, 1992). Calderon-Miranda et. al., (1999) suggest that an increase in handling during manufacturer of breaking of shell eggs and adjusting of solid content may enhance the growth and survival of L. monocytogenes.
L. monocytogenes has been implicated as the causative agent of both epidemic and sporadic foodborne illness known as listeriosis. (Carlos et. al., 2001). The symptoms in human are not well defined as there are a unique set of symptoms for the disease depending on the state of host. (Jay et. al., 2005) A range of clinical conditions for the disease can vary from mild, flu-like illness (Adams & Moss, 2006) to manifestation; including septicaemia and meningitis (Vitas et. al., 2004).
Jay et. al. (2005) mentioned that the infectious dose for listeriosis is not well established but may be low in susceptible individuals. According to Adam & Moss (2006), the prevalence of L. monocytogenes isolated from samples are found to be as low as 102cfu/g. The first outbreak of listeriosis by food occurred in the Maritime Provinces of Canada in 1981. (Adams & Moss, 1996). It involved 41 cases in all with 28.6% mortality rate in adult cases. Rouquete & Berche (1996) reports similar data with an approximate 20% mortality with an increase of up to 75% in high risk groups such as pregnant women, neonates, and immunocompromised adults. Forum data obtained from Swaminathan & Gerner-Smidt (2007) reported that the death rate was 44% for an outbreak due to coleslaw in 1981 at Nova Scotia, Canada. These evidence demonstrated that listeriosis is a serious disease and has a very high mortality rate. NNDSS (2008) reported an increase of listeriosis from 1994, with 35 cases, to 2006 with 61 cases for Australia, of which Victoria reported the second highest occurrence of 13 cases. The rise is almost a 75% increase over that period and hence the probably of occurrence and severity indicates a high risk of listeriosis.
Naghmouchiet. al. (2007) also mentioned that this organism is ranked first among food-borne pathogenic bacteria in terms of death rate, far ahead of Campylobacter, Salmonella and Escherichia coli O157:H7. Therefore, measures to prevent of this foodborne pathogen in the future is important for a production of safe foods for the consumer, as its low presence can cause a huge impact to the individual’s health.
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