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Shigella spp (Shigellosis)

Shigella is a species of enteric bacteria that causes disease in humans and other primates. Shigella is gram-negative rods that are non-motile and non-spore forming.
The bacteria are primarily a human disease but has been found in some primates. Shigella are facultative anaerobes, similar to enterics such as E. coli (Brayan, Mckiley, & Mixon, 1971).

Epidemiology​

Shigella transmission can occur through direct person-to-person spread or from contaminated food and water. The minimal infectious dose can be transmitted directly from contaminated fingers, since intermediate bacterial replication is not required to achieve the low infectious dose.
In developed countries, most cases are transmitted by faecal-oral spread from people with symptomatic infection. In developing countries, both faecal-oral spread and contamination of common food and water supplies are important mechanisms of transmission symptom.

Pathogenesis​

Shigella attaches to and penetrate intestinal cell walls of the small intestines by producing toxins that may promote the diarrhoea characteristic of the disease. The Shiga toxin enables the bacteria to penetrate the epithelial lining of the intestines, leading to a breakdown of the lining and haemorrhage.
Shigella also have adhesins that promote binding to epithelial cell surfaces and invasion plasmid antigens that allow the bacteria to enter target cells, thus increasing its virulence (CDC, 2011).

Symptoms​

They include abdominal pain, cramps, diarrhoea, fever, vomiting, blood, pus or mucus in stools. Mild infections cause low-grade fever (38 – 38.9°C) and watery diarrhoea 1 to 2 days after people ingest the bacteria.

Abdominal cramps and a frequent urge to defecate are common with more severe infections. Children, particularly young children, are most likely to have severe complications of high fever (41°C) sometimes with delirium. Severe dehydration with weigh loss is also a symptom.

Detection of toxin​
Shigella infection is diagnosed through testing of a stool sample. First a stool sample must be obtained from the potentially infected person, and then the sample is placed on a medium to encourage the growth of bacteria.
If and when there is growth, the bacteria are identified, usually by looking at the growth under a microscope (Ibid).

Control and prevention​
Shigella is heat-sensitive and will be killed by thorough heating (over 70°C). Raw or undercooked foods and cross-contamination, when cooked material comes into contact with raw produce or contaminated materials (cutting boards), are the main causes of infection.
Proper cooking and hygienic food handling thus can prevent Shigella infections to a large extend. There is currently no vaccine for Shigellosis prevention, but there is current research that appears promising.
The most effective method for prevention is frequent and vigorous hand washing with warm, soapy water and ensuring clean drinking water sources and proper sewage disposal in developing nations (Malangu, 2016).
Occurrence is worldwide and has higher prevalence in developing countries. Shigellosis is a major cause of diarrhoea in infants and children under the age of 5 years and accounts for 5-15% of diarrhoeal diseases cases seen at treatment centres.

References​

1. Brayan, F., Mckiley, T., & Mixon, B. (1971). Use of Time Temperature in Detecting the Responsible Vehicles and Contributing Factors of Foodborne Disease Outbreaks. Journal of Milk Food Technology, 34, 576-582.
2. CDC. (2011). Estimates of Foodborne Illnesses in the United States. New York: Centers of Disease Control.
3. Malangu, N. (2016). Risk Factors and Outcomes of Food Poisoning in Africa. Intech Open.

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Escherichia coli

Escherichia coli belongs to the Enterobacteriaceae family. They are gram negative rods up to 3um in length, ferment glucose and wide range of sugars. Produce pink colonies on McConkey agar. Hemolytic activity on blood agar is a character of certain strains of E. coli.

It is motile with peritrichous flagella and often fimbriate (Jay, 2000). E. coli 0157:H7 is an important serotype and seems to be predominate in most areas. The strains producing verotoxin are shiga-like toxin (SLT) which produces diarrhoea in humans and animals.
Source of infection​Contamination of food by human and animal faeces. The organism can persist in manure, water trough and other farm location. The association of E. coli 0157:H7 with raw meat, under cooked ground beef and raw milk lead to investigation of the role of cattle as a reservoir of the pathogens (Buchanan & Doyle, 1997).
Pathogenesis​Enterohemorrhagic E. coli (EHEC) strain may produce one or more types of cytotoxins which are collectively referred as shiga-like toxins (SLTs) since they are antigenically and functionally similar to shiga toxin produced by Shigella dysenterica.

However, new terminology has been applied, and what was SLT is now Stx. All Stxs consists of a single enzymatically active A subunits and multiple B subunits. Stx-sensitive cells possess the toxin receptor, globotriaosyceramide (Gb3), and sodium butyrate appears to play a role in sensitizing cells to Stxs.

Once toxins bind toGb3, internalization follows with transport to the trans-Golgi network. Inside the host cells, the A subunits bind to and release and adenine residue that inhibits protein synthesis. The B subunits form pentamers in association with a single A subunit and thus are responsible for the binding of the toxin to the neutral glycolipid receptors (Ibid).
Symptoms​The incubation period is 72-120 hours. The clinical signs initially may be diarrhoea in a few days. However, there is no fever. The symptoms of E. coli septicaemia are mainly referable to bacteraemia, end toxaemia and the effect of bacteria localization in a variety of tissue spaces throughout the body (Quinlan, 2013).
Detection of toxin​Laboratory diagnoses involve culturing the food on McConkey agar or sorbitol. Strains can be identified by serotyping using specific antisera. Stxs can be detected by ELISA and gene coding can be detected by DNA hybridization techniques. Sorbitol McConkey agar is recommended for isolation of E. coli 0157:H7 from food and faeces samples.
Control and Prevention​The prevention of food borne illnesses caused by E. coli can be prevented by the same method as prevention of other food borne illness caused by bacteria. Food should be properly cooked since the organism is heat sensitive.

Occurrence is worldwide with most incidences in developing countries. Case fatality ratio for EPEC, ETEC, EIEC infections in industrialized countries <0.1%, for EHEC infection about 2%.

Case fatality ratio of E. coli infections in infants and children much higher in developing countries. Children and the elderly are particularly vulnerable and may suffer more severely. Most cases of EHEC infections are reported in summer.
References​

1. Buchanan, R., & Doyle, M. (1997). Food Borne Disease: Significance oof E. coli O157:H7 and other Enterohaemorrhagic E. coli. Food Technology, 5, 69-76.
2. Jay, J. (2000). Modern Food Microbiology (6th ed.). Gaithersburg, Maryland: Aspen Publications.
3. Quinlan, J. J. (2013). Foodborne Illness Incidence Rates and Food Safety Risks for Populations of Low Socioeconomic Status and Minority Race/Ethnicity: A Review of the Literature. International Journal of Environmental Research and Public Health, 10, 3634-52.