Salmonella are said to be Gram-negative rod-shaped bacteria that are facultative and anaerobic in nature flagellated bacilli characterized by the H, O and Vi antigens in the family of Enterobacteriaceae (Wong et al., 2011; Klochko, 2016). The bacteria were discovered and isolated by Dr. Salmon, who was a renowned veterinary surgeon in the U.S., in 1885 after he was able to isolate the bacteria strain called choleraesuis or enterica from the intestines of an infected pig. As can be noted, the bacteria was named after its discoverer Salmon. According to Wong et al. (2011), the bacteria has also been named according to the places where it was isolated. For instance, there is a strain called Salmonella Indiana and another one called Salmonella London since they were isolated Indiana and London respectively.
As hinted out, the bacteria grow in hosts such as animals (pigs, birds among others), plants (food such as vegetables and fruits) and are eventually ingested by the human when they consume contaminated food or sometimes water. Once into the body of a human, the bacteria replicate in the liver or spleen and causes disease through affecting the intestine. The common symptoms of an ill person are diarrhea, fever, nausea or even vomiting, and abdominal cramps depending on the clinical manifestation (enteric fever from S. Typhi species; gastroenteritis from S. Typhimurium and S. Enteritidis; and other non-typhoidal salmonellosis) the disease has on the victim (Rabsch, 2002).
Notably, the infections resulting from Salmonella is a major public health problem on a global level. For this reason, the infections are a source of major negative economic impacts since the disease requires a great investment in the surveillance investigation, treatment, and prevention (Wong et al., 2011). According to the CDC, there have been 19,119 outbreaks in the U.S. between 1998 and 2015 where 373,531 illnesses have been reported resulting in 337 deaths and 14,681 hospitalization cases (Food Tool | CDC, 2017). With some noticeable been the Turkey Burgers outbreak in Hadar in 2011, Newport outbreak in 2014 resulting from cucumbers, an outbreak in 2014 involving nut butter in Braenderup among others.
Characteristics and classification of Salmonella
The genus of the bacteria strain is Salmonella and is divided into two different species: S. bongori and S. enterica (Rao, Riccardi, & Birrer, 2004; Klochko, 2016). The latter species is the most dominant and has over 2000 strains. The classification of the strains is further done based on the range of hosts the specific strain attacks. As already mentioned, the strains are characterized by the antigens Vi, H and O. Among the two-major species, the S. enterica, subgroup 1 (enterica) is the most pathogenic to humans with the most common strain, which is responsible for typhoid fever, being Salmonella enterica ssp. Enterica, serovar typhi that is commonly known as Salmonella typhi.
Salmonella enterica has five most recent strain outbreaks mostly in the U.S.: Serotype Braenderup (NVSL-96-12528) whose outbreak was reported in Texas, Tennessee, Iowa and New Mexico in January 2014 and was linked to nut butter; serotype Typhimurium which is a strain that causes a typhoid-like disease in mice and is does not result in fatality in humans and lasts only up to 4 days (Rabsch, 2002); serotype Enteritidis (NVSL 94-13062) which has been a common strain is the U.S. the disease affects chicken and can be transmitted to humans through consumption of infected chicken meat (Salmonella information, 2017); serotype Heidelberg/Sheldon (3347-1) whose outbreak was reported between 2013 and 2014 sourced from 3 California establishments that dealt with chicken products; and serotype Hadar that was isolated from 12 commercial turkey farms litter and waste samples in Hadar.
The Salmonella bacteria strains are said to require an optimal temperature of 37oC or 98.6oF for optimal growth which means that their functionality is easily disrupted at extremely high or low temperatures. As such, the bacteria fail to grow in refrigerated conditions even though they do not die off. Moreover, under extreme temperatures reaching between 57oC and 60oC or 134oF and 140oF, the bacteria easily die off, and such temperature can be used to kill the bacteria (Rao, Riccardi, & Birrer, 2004).
Infectious dose and disease characteristics
The bacteria affect the human when he or she consumes infected food products or through direct contact with infected persons, animals or their environment. In this case, the infection the bacteria are ingested through consumption of carrier food. The food passes through to the intestines where the bacteria bind to the walls of the intestines from where they penetrate through to the blood and gets deposited in the liver and the spleen for growth and multiplication. After the growth period, the bacteria can now cause infection within the body especially in the intestine. Excess (or unabsorbed) bacteria may be passed through defecation to the external environment and survive long enough until it gets to the next host (plant, animal or person) in poorly sanitized environments. According to (Salmonella information, 2017), there are approximately 16 million occurrences of typhoid fever, 3 million deaths as a result of Salmonella, and about 1.3 billion occurrences of gastroenteritis.
Contamination in production plants
The bacteria have been linked to contamination of a wide range of animals such as poultry, dairy products, eggs, vegetables and fresh fruits or even water. For instance, the Salmonella bacteria has been found in fruits and vegetables such as lettuce, melon, unpasteurized orange juice, alfalfa sprout, apple, tomato, parsley, cilantro, celery and mango (Wong et al., 2011). Consumption of such contaminated foods anywhere within the food chain has led to major outbreaks of the disease on a global scale.
Notably, the vector animals such as rats, birds, and flies carry the bacteria and spread it through their faeces (Wong et al., 2011). The bacteria can survive in the environment for a long time, and thus they are directly transmitted to moving animals such as swine, chickens, and cows. Eventually, the vectors and the bacteria find their way to food products within the firm and the food production equipment and products. If uncontrolled before the produced food gets to the consumers, the contaminated food and food products are consumed and thus cause a direct risk to the consumer which creates high chances of propagating an outbreak.
Salmonella spp. in acidified foods and mode of resistance
Considering the low pH within the hosts stomach due to the presence of HCl, gastric juice (the antibacterial fluid in the body) and other chemicals, the Salmonella bacteria have special adaptations to withstand the acidic conditions. Alvarez-Ordonez et al. (2011) link the survival of Salmonella bacteria to its nucleophilic nature and its ability to induce an acid tolerance response (ATR) which entails the capacity to undergo adaptive responses that help moderate the pH. Specifically, the bacteria can create a low internal pH by inducing the expression of amino acid decarboxylase systems; synthesizing acid shock proteins; and modification of the membrane FA composition by reducing the membrane fluidity by regulating cadaverine and agmatine in the cell. Notably, through the same mechanism that enables the bacteria to withstand acid conditions in the stomach of the host, the bacteria employ the same process of lowering the internal pH to survive in acidified foods (Alvarez-Ordonez et al., 2011). As a result, the bacteria are able to self-induce an acid-resistance within acidified food products.
Reduction times of common food pathogens
In a research by Breidt, Hayes, and McFeeters (2007), the reduction times for food pathogens was determined. Specifically, the study was hinged on testing the 5-Log Reduction times for food pathogens in acidified cucumbers during storage at 10 and 25oC. Notably, this research was based on the concerns raised by the increased outbreaks of acid-resistant foodborne pathogens such as Escherichia coli O157: H7, Listeria monocytogenes, and Salmonella enterica in acid foods such as orange juice and apple cedar that usually have pH values lower than 4.0. Based on such concerns, the researchers made a proven argument that even acidified vegetable foods with pH values range between 3.3 and 4.6 posed a risk to food safety. For that reason, the researchers set out to establish and demonstrate how thermal treatments are needed to achieve a 5-Log Reduction in the acidified vegetable products. However, the researchers do not forget to mention the fact that heat treatment of such acidified food products with a pH value of less than 3.3 would result in poor product quality.
For their study, Breidt, Hayes, and McFeeters (2007), the researchers used acetic acid with pH of 3.3 or less as the primary acidulant whereby they maintained the vegetable product samples at a minimum equilibrated temperature of 10oC. The holding times for the reduction of the Escherichia coli O157: H7, Listeria monocytogenes, and Salmonella enterica strains within the vegetable samples were then determined at these conditions. Eventually, the researchers were able to establish that the Escherichia coli O157: H7 was the most resistant microorganism even within the acidified conditions and the preset temperature. In this case, Escherichia coli O157: H7 took up to 5.7 days to achieve a 5-Log reduction in cell numbers. On the other hand, Listeria monocytogenes took 0.5 days for a 5-Log Reduction while Salmonella enterica took 2.1 days to achieve the same. Notably, all these tests were carried out at 10oC. At 25oC, Escherichia coli O157: H7 was seen to have a 5-Log Reduction time of up to 1.4 days. For the other two strains, the researchers did not provide the reduction times since they concluded from statistical predictions that their values were significantly less than that of E. coli.
Bredit et al. (2013) conducted similar research in 2013 titled Determination of 5-Log Reduction Times for Escherichia coli O157: H7, Salmonella enterica, or Listeria monocytogenes in Acidified Foods with pH 3.5 or 3.8. The study was also aimed to establish that high temperatures can be used to destroy acid-resistant vegetative pathogenic and spoilage bacteria (Bredit et al., 2013). Dressings and mayonnaises whose pH values were 3.5, or higher were acidified with 1.5- 2.5% or higher acetic acid are taken to cause lethality. Based on the question whether there is a 5-Log Reduction of Escherichia coli O157: H7, Listeria monocytogenes, and Salmonella enterica strains by the use of preservatives or acetic acids or both as food additives without necessarily adding in post-packaging pasteurization contents. In a bid to answer the question, the study was conducted by determining the 5-Log Reduction times of the three strains of bacteria at 10oC using pickling brines medium with a range of benzoic and acetic acid combinations maintained at pH values ranged between 3.5 and 3.8. Notably, 15 varying acid-pH combinations were used, and all gave significantly higher Escherichia coli O157: H7 acid resistance than times than they did for both Listeria monocytogenes and Salmonella enterica strains. Eventually, the results found indicate that the addition of benzoic acid added to the efficacy of achieving shorter 5-Log Reduction times in the target pathogens. This was after holding times of less than or equal to 4 days achieved a 5-Log Reduction for vegetative pathogens at pH 3.8 with 2.5:0.1% (acetic: benzoic) acids and at pH 3.5 with 2.5% acetic only. In conclusion, the study deemed ben...
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