Listeria – Most virulent food borne pathogen

Listeria is one of the most virulent foods borne pathogen; about 20 to 30% of deaths due to food poisoning are from Listeria Infection, these deaths exceeds more than Salmonella and Botulism.  L. monocytogenes is a bacterium often found in soil, sewage, silage, dust, and water as well as in the feces of animals and humans. It is most commonly transmitted to humans by foods such as processed cold meats and unpasteurized dairy products. Food-borne listeriosis can cause fever, severe headache, diarrhea, and serious infections that are especially dangerous for pregnant women, newborns, the elderly, and people with weakened immune systems.

Historical Background of Listeria

Before 1920’s L. Monocytogenes was first described by EGD Murray .It was based on the observation that sudden onset of   death of   six rabbit was due monocytogenes. After1920s major research work and publications were carried on the Listeria Monocytogenes. In 1940, Harvey Pine changed the genus name to Listeria. In 1952, in East Germany it was established that Listeria Monocytogenes causes Sepsis and meningitis in neonates. In 1981 it was widely recognized as a causative agent of food illness. An outbreak of Listerosis in Hailfene Nova Scotia has resulted in death of 18 pregnant women out of 41 pregnant women who ate coleslaw contaminated cabbage which was treated with raw sheep manure. After this incident epidemiologist recognized it as an important causative factor in food industry.

Morphology of Bacteria of Listeria

Listeria Monocytogenes is gram-positive facultative anaerobes agent in developing the disease Listerosis. Listeria belongs to

  • Division Firmcutes
  • Class Bacilli
  • Order Bacillales
  • Family Listeriacease
  • Genus Listeria
  • Specie L.Monocytogenes

It is Catalase positive and oxidase negative, produces β hemolysin, which causes destruction of Red blood cells. There are six species of Listeria, in which Listeria Monocytogenes is the most virulent and consistent with human being which is named after Joseph Lister.

Route of Infection of Listeria

L.Monocytogenes is associated with food, as raw milk, pasteurized fluid milk, cheese, ice cream, raw vegetable, fermented raw meat, and cooked poultry meat, raw and smoked fish. L.Monocytogenes has the ability to grow at 00 C degree temperatures and can multiply at refrigerated temperature of 40C.

Infection Cycle of Listeria

Intestinal epithelium is the prime site, invaded by bacteria by zip mechanism. Listeria  organism undergoes the  process of  internalization produces a chemical known as  listercolysin O which determine/gauge the  virulence  due  to which the  bacteria avoid vacuolization and  allows the bacterium to escape  in the cytoplasm of the host cell. The bacteria proliferate by the process of paracytophase which spread from one cell to another cell directly thus escaping immune detection and destruction process.

Pathogenesis of Listeria

Listeria Monocytogenes causes invasive and non invasive disease respectively.

  • Listeria (Invasive): which include Septicemia, meningitis, meningoencephalitis, encephalitis, corneal ulcer, pneumonia; intrauterine or cervical infection in pregnant women which may lead to spontaneous abortion or still birth, Surviving Neonate may suffer from pyogenic granulomas which may be distributed over the whole body. Neonate may suffer from physical retardation. Invasive symptoms may be preceded with influenza like symptoms including persistent fever.
  • Listeria (Non-invasive): Infection may be associated with Gastrointestinal symptoms ,such nausea ,vomiting, and diarrhea. Gastrointestinal symptoms may range  from one week to three weeks L.Monocytogenes is unique among  gram-positive  that secretes endo-toxin .

Laboratory Diagnosis of Listeria

  • Anton test: A test used in identification of Listeria Monocytogenes, instillation of culture in conjunctival sac, of rabbit results in severe kertoconjuctivitis within 24 hours.
  • Characteristic of Listeria.  It grows on culture media Mueller Hinton agar. Identification is enhanced if primary culture is done on agar containing sheep blood, small zone of hemolysis can be observed around and under the colonies. Isolation can be enhanced if tissue is kept at 40C for some days before inoculations into bacteriologic media.

Listeria has to be differentiated from coryneform a bacterium which is facultative anaerobe, catalase positive and motile. It produces acid but no gas in varieties of Carbohydrate, mobility is observed at room temperature. Hemolysin production is primary finding.

Risk Factors of Listeria

Older patients, Pregnant Women, People with weakened immune system, organ transplant and patients who are receiving drugs ,people with certain diseases such as HIVAIDs, Cancers, end stage renal disease, Liver disease ,Alcohol, Diabetes Mellitus are prone  to Listerosis.

Prevention of Listeria

  • The main prevention is through the promotion of safe handling,  cooking and  consumption, raw food, vegetable should be thoroughly washed and  cooked properly, as well as reheating left over or ready to eat food should be consume only if it  is properly  steaming hot.
  • Prevention is advice in high risk groups such as pregnant women and immune -compromised patients. Avoid unpasteurized pates and food such as cheese like feta. Listerosis is a serious infection usually caused by eating food contaminated with bacterium Listeria Monocytogenes which is an important PublicHealth problem in USA. It primary affect the older, pregnant women, and new born and adult with weak immune system.

Treatment of Listeria

  • General Treatment: An antibiotic Ampicllian is considered antibiotic of choice Gentamycin is added to have synergistic effect. Trimethoprium-sulphamethoxoxazole has been effective in patients who are allergic to Penicillin.
  • Food Industry : Listeria phase P 100 has  been proposed to be used as food  additive to control Listeria  Monocytogenes ,which is used for  food  safety and  suitable for treatment  of  bacterium.
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