| A Brown and Brenn gram stain was performed on one of the tissue biopsy specimens. Organisms were seen as shown in the image. Based on the history and the appearance of the bacteria, the most likely identification is: | View Page |
| Shown in the image is a close-in view of the colony growth after 48 hours incubation. What are the possible presumptive identifications suggested by the colonies observed? (Choose all that apply) | View Page |
| Listeria motility Listeria monocytogenes is optimally motile at 25C; and is non-motile at 35-37C. Motility may be directly assessed by observing bacterial cells with a tumbling motion in a direct mount preparation. In soft motility agar, the identification can be made by observing for a thin, umbrella-like lateral extension of growth from the stab line (noted by arrows in the image). | View Page |
| What test(s) which may be performed to establish a presumptive differential identification between group B streptococci and L. monocytogenes? | View Page |
| Shown in theimage are three tubes: (1) motility agar (note subsurface flare shown by arrows); (2) esculin hydrolysis (+), and (3) VP (+). The reactions illustrated here are sufficient to rule out Erysipelothrix rhusiopathiae. | View Page |
| Review 2 Low JC. Donachie W.: A review of Listeria monocytogenes and listeriosis. Veterinary Journal. 153:9-29, 1997 Following the initial isolation and description in 1926, Listeria monocytogenes has been shown to be of world-wide prevalence and is associated with serious disease in a wide variety of animals, including man. Our knowledge of this bacterial pathogen and the various forms of listeriosis that it causes has until recently been extremely limited, but recent advances in taxonomy, isolation methods, bacterial typing, molecular biology and cell biology have extended our knowledge. It is an exquisitely adaptable environmental bacterium capable of existing both as an animal pathogen and plant saprophyte with a powerful array of regulated virulence factors. Most cases of listeriosis arise from the ingestion of contaminated food and in the UK the disease is particularly common in ruminants fed on silage. Although a number of forms of listeriosis are easily recognized, such as encephalitis, abortion and septicaemia, the epidemiological aspects and pathogenesis of infection in ruminants remain poorly understood. The invasion of peripheral nerve cells and rapid entry into the brain is postulated as a unique characteristic of its virulence, but relevant and practical disease models are still required to investigate this phenomenon. | View Page |
| Review 2 Low JC. Donachie W.: A review of Listeria monocytogenes and listeriosis. Veterinary Journal. 153:9-29, 1997 Following the initial isolation and description in 1926, Listeria monocytogenes has been shown to be of world-wide prevalence and is associated with serious disease in a wide variety of animals, including man. Our knowledge of this bacterial pathogen and the various forms of listeriosis that it causes has until recently been extremely limited, but recent advances in taxonomy, isolation methods, bacterial typing, molecular biology and cell biology have extended our knowledge. It is an exquisitely adaptable environmental bacterium capable of existing both as an animal pathogen and plant saprophyte with a powerful array of regulated virulence factors. Most cases of listeriosis arise from the ingestion of contaminated food and in the UK the disease is particularly common in ruminants fed on silage. Although a number of forms of listeriosis are easily recognized, such as encephalitis, abortion and septicaemia, the epidemiological aspects and pathogenesis of infection in ruminants remain poorly understood. The invasion of peripheral nerve cells and rapid entry into the brain is postulated as a unique characteristic of its virulence, but relevant and practical disease models are still required to investigate this phenomenon. | View Page |
| Which of the following is NOT related to the virulence of Listeria monocytogenes? | View Page |
| A characteristic of the virulence of L. monocytogenes is its unique ability to invade peripheral nerves and rapidly enter into the brain. | View Page |
| Review 1 Rocourt J. Jacquet C. Reilly A.: Epidemiology of human listeriosis and seafoods. International Journal of Food Microbiology. 62:197-209, 2000 While rarely diagnosed prior to 1960, more than 10,000 cases of listeriosis were recorded in the medical literature between 1960 and 1982, and thousands more have been reported annually world-wide. This widespread increase in reporting is most likely due to demographic trends and changes in food production, processing and storage, especially the extended cold food chain and the ability of Listeria monocytogenes to grow at low temperatures L. monocytogenes is a bacterium responsible for opportunistic infections, preferentially affecting individuals whose immune system is perturbed, including pregnant women, newborns, people over 65 years, immunocompromised patients, such as cancer victims, transplant recipients, people on hemodialysis and AIDS patients. Thus, the increasing lifespan and medical progress allowing immunodeficient individuals to survive, partially explains the increasing incidence of listeriosis. Moreover, L. monocytogenes is ubiquitous and can grow at temperatures as low as 0 degrees C. At this temperature growth is very slow. The expansion of the agro-food industry, the widespread use of systems of cold storage and changes in consumers demands have led to a large increase in the pool of Listeria that can cause food-borne infections. | View Page |
| Review 3 Rouquette C. Berche P. The pathogenesis of infection by Listeria monocytogenes Microbiologia. 12:245-58, 1996 Listeria monocytogenes is a Gram-positive bacterium responsible for severe infections in human and a large variety of animal species. It is a facultative intracellular pathogen which invades macrophages and most tissue cells of infected hosts where it can proliferate. The molecular basis of this intracellular parasitism has been to a large extent elucidated. The virulence factors, including internalin, listeriolysin O, phospholipases and a bacterial surface protein, ActA, are encoded by chromosomal genes organized in operons. Following internalisation into host cells, the bacteria escape from the phagosomal compartment and enter the cytoplasm. They then spread from cell to cell by a process involving actin polymerisation. In infected hosts, the bacteria cross the intestinal wall at Peyer's patches to invade the mesenteric lymph nodes and the blood. The main target organ is the liver, where the bacteria multiply inside hepatocytes. Early recruitment of polymorphonuclear cells lead to hepatocyte lysis, and thereby bacterial release This causes prolonged septicaemia, particularly in immunocompromised hosts, thus exposing the placenta and brain to infection. The prognosis of listeriosis depends on the severity of meningoencephalitis, due to the elective location of foci of infection in the brain stem (rhombencephalitis). Despite bactericidal antibiotic therapy, the overall mortality is still high (25 to 30%). | View Page |
