Listeriosis: an important public health concern in the United States
Listeriosis is a fatal foodborne opportunistic bacterial infection caused by a genus of gram positive bacteria called Listeria. Listeria is found in soil, water and in some animals including cattle and poultry. Individuals with weakened immunity system especially T cell mediated immunity such as pregnant women and older people are at highest risk of listeriosis. These bacteria have 6 species among which Listeria monocytogenes is the disease causing one in humans. Listeria bacteria do not form spores, they don't have capsules, and they are motile at 10 to 25 degrees Centigrade. Listeriosis occurs by ingestion of contaminated raw and industrially processed food that contains L. monocytogenes pathogen.These bacteria can survive the food processing technology, can tolerate high salt concentration and low pH and are able to multiply in refrigeration temperature which makes this pathogen a serious threat to food industry.
As listeriosis is a foodborne infection the primary site of entry of the bacteria is the gastrointestinal tract of the host and then the liver and the CNS. L. monocytogenes is intracellular parasite that can survive in macrophages to invade non phagocytic cells such as epithelial cells, hepatocytes and endothelial cells where they undergo intracellular proliferation [1]. The hemolysin/hly gene is the virulence factor that leads the Listeria bacteria for intracellular survival. Other virulence factors are also involved in listeria pathogenesis such as cell to cell adhesion protein internlanin (In1A & In1B) for epithelial cell invasion and ActA for cell to cell spread. Internlanin binds with the host cell membrane receptors E cadherin (transmembrane glycoprotein), C1q complement fraction receptor, ECM (Heparin Sulfate Proteoglycan/HSPG) and MET receptor for hepatocyte growth factor (HGF) [2]. After phagocytosis the bacterial phagolysosome listeriolysin O (cholesterol dependent pore forming toxins/CDTX) enter the host cytoplasm and multiply there. In the cytoplasm bacterial surface protein Act A initiate actin formation through which listeria spread cell to cell which results in infection. Studies suggest that iron metal plays an important role in regulation of virulence gene expression in L. monocytogenes [3]. Recent research also identifies an auxiliary protein secretion system (SecA2) that secretes autolytic enzymes that promotes pathogenesis in L. monocytogenes [4].
Consumption of raw fruit, vegetables and meat, unpasteurized milk and milk products, smoked seafood, processed ready to eat food may result in listeriosis. CDC estimates that approximately 1600 illnesses and 260 deaths due to listeriosis occur annually in the United States. In 2011 147 illnesses, 33 deaths and one miscarriage occurred due to consumption of contaminated cantaloupe from a single farm [5]. Symptoms of listeriosis include fever, chill, severe body and headaches, diarrhea, vomiting and influenza type symptoms, which can be treated by antibiotics in most of the cases but may be life-threatening in some cases [6].
