A 54-year-old man comes to the emergency department due to fever, chills, and malaise over the past day. His other medical problems include recently diagnosed Hodgkin disease. Temperature is 39.4 C (103 F), blood pressure is 102/61 mm Hg, and pulse is 114/min. The patient appears ill. Blood cultures grow motile gram-positive rods that produce a very narrow zone of beta hemolysis on blood agar. The bacteria are also found to grow well at refrigerated temperature. Which of the following processes is most important for eliminating these bacteria from the body?

Question

A 54-year-old man comes to the emergency department due to fever, chills, and malaise over the past day.  His other medical problems include recently diagnosed Hodgkin disease.  Temperature is 39.4 C (103 F), blood pressure is 102/61 mm Hg, and pulse is 114/min.  The patient appears ill.  Blood cultures grow motile gram-positive rods that produce a very narrow zone of beta hemolysis on blood agar.  The bacteria are also found to grow well at refrigerated temperature.  Which of the following processes is most important for eliminating these bacteria from the body?

Quizplus · Accepted Answer

11ec160b_383f_0c48_9249_f917cd5cae1b_MD0001_00 Listeria monocytogenes is a facultative intracellular, gram-positive rod that produces a very narrow zone of beta hemolysis on blood agar (similar to group B Streptococcus [Streptococcus agalactiae]). Listeria is immotile at body temperature but demonstrates tumbling motility at room temperature. The pathogen is also distinguished by an ability to multiply at refrigerated temperatures (cold enrichment), which is one reason Listeria is an important cause of foodborne illness (eg, refrigerated soft cheese, meats, unpasteurized milk). The elimination of intracellular pathogens is largely reliant on the cell-mediated immune response as bacteria within cells are protected from circulating immune factors (eg, antibodies, complement). Cells infected by intracellular bacteria display bacterial protein fragments on their surface, which triggers cell lysis by cytotoxic T cells. This releases the organism from the protected intracellular space and exposes it to phagocytosis by macrophages. Although intracellular pathogens can often replicate within inactivated macrophages due to virulence factors that prevent phagolysosome destruction, cytotoxic T-cells release interferon gamma to activate macrophages leading to the formation of fully mature (acidified) phagolysosomes capable of destroying the bacteria. As the elimination of Listeria is reliant on the cell mediated immune response, patients with impaired cell-mediated immunity are at greatest risk for invasive Listeria infections (eg, sepsis, meningitis), particularly those at the extremes of age (ie, neonates, elderly) or patients who are immunosuppressed (eg, Hodgkin disease, AIDS) or pregnant. (Choice B) Eosinophils are a component of the innate immune system and are tasked primarily with the eradication of parasitic infections (eg, helminths). Eosinophils play a minor role in the phagocytosis and destruction of intracellular bacteria. (Choice C) The humoral immune system secretes immunoglobulins and is largely tasked with eliminating extracellular infections such as Streptococci and Staphylococci. This system is not able to effectively target intracellular bacteria as immunoglobulins cannot access the intracellular space. (Choice D) Mast cells contribute to defense against parasites and play an important role in allergic responses, particularly immediate hypersensitivity reactions; they do not confer immunity to intracellular pathogens. (Choice E) The complement system is part of the innate immune system and aids in the opsonization and destruction of bacteria. The terminal complement components (eg, C5b-C9) create a membrane attack complex to lyse bacteria and are particularly important against Neisseria infections. Educational objective: Listeria monocytogenes is a facultative, intracellular, gram-positive rod that produces a very narrow zone of beta hemolysis on blood agar, shows tumbling motility at room temperature, and can grow at refrigerated temperatures. Intracellular pathogens such as Listeria are largely eliminated by the cell-mediated immune response (eg, T-cells, macrophages, cytokines). Patients with impaired cell-mediated immunity are at risk for invasive Listeria infections. __________ References: Innate and adaptive immune responses to Listeria monocytogenes: a short overview. Page(s): PMC2042024 (http://www.ncbi.nlm.nih.gov/pubmed/2042024)

A 54-Year-Old Man Comes to the Emergency Department Due to Fever