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Questions and Answers
Which scenario would present the greatest challenge to definitively establishing a causal relationship between a microbe and a specific disease using Koch's postulates?
Which scenario would present the greatest challenge to definitively establishing a causal relationship between a microbe and a specific disease using Koch's postulates?
- A virus can be isolated from diseased animals and grown in pure culture, but introducing the cultured virus into healthy animals does not reliably cause disease.
- A bacterium is consistently isolated from diseased animals, introducing the bacterium into a healthy animal causes disease, and the bacterium is re-isolated, but the bacterium cannot be grown in pure culture. (correct)
- A bacterium is consistently isolated from diseased animals, grows readily in pure culture, and causes disease in a healthy animal but cannot be re-isolated.
- Specific bacterial species live asymptomatically in the blood of healthy animals; however, these species are isolated from diseased animals exhibiting bacteremia.
A researcher is investigating a novel bacterial species suspected of causing a specific disease in livestock. Application of Koch's postulates reveals that the bacteria can be isolated from diseased animals and grown in pure culture. Inoculation of the cultured bacteria into healthy animals consistently induces the disease. However, attempts to re-isolate the bacteria from these experimentally infected animals are unsuccessful. What is the MOST likely explanation for these findings?
A researcher is investigating a novel bacterial species suspected of causing a specific disease in livestock. Application of Koch's postulates reveals that the bacteria can be isolated from diseased animals and grown in pure culture. Inoculation of the cultured bacteria into healthy animals consistently induces the disease. However, attempts to re-isolate the bacteria from these experimentally infected animals are unsuccessful. What is the MOST likely explanation for these findings?
- The bacteria undergo genetic changes _in vivo_ that prevent their subsequent isolation using the same methods. (correct)
- The animal model used is not truly susceptible to the disease, leading to an atypical immune response.
- The initial isolation procedure was flawed, leading to the erroneous identification of the causative agent.
- The disease is likely multifactorial, requiring other co-factors not present in the experimentally infected animals.
Which of the following bacterial mechanisms directly facilitates the establishment of a microcolony and initial biofilm formation on host tissues?
Which of the following bacterial mechanisms directly facilitates the establishment of a microcolony and initial biofilm formation on host tissues?
- Production of toxins that damage host cells and promote bacterial spread.
- Evasion of phagocytosis through the expression of a polysaccharide capsule.
- Expression of surface adhesion molecules and pili to promote adherence. (correct)
- Secretion of collagenases that degrade the extracellular matrix.
A researcher discovers a new bacterial species that secretes a novel enzymatic factor. In vitro studies reveal that this factor degrades fibronectin, a key component of the extracellular matrix. How might this enzymatic activity contribute to the pathogenicity of the bacteria?
A researcher discovers a new bacterial species that secretes a novel enzymatic factor. In vitro studies reveal that this factor degrades fibronectin, a key component of the extracellular matrix. How might this enzymatic activity contribute to the pathogenicity of the bacteria?
Which of the following strategies employed by intracellular bacterial pathogens DOES NOT represent a mechanism for evading destruction following phagocytosis by a host cell?
Which of the following strategies employed by intracellular bacterial pathogens DOES NOT represent a mechanism for evading destruction following phagocytosis by a host cell?
An investigation of a novel bacterial pathogen finds that it survives within macrophages by preventing the fusion of phagosomes with lysosomes. Which cellular process would be MOST directly affected by this mechanism?
An investigation of a novel bacterial pathogen finds that it survives within macrophages by preventing the fusion of phagosomes with lysosomes. Which cellular process would be MOST directly affected by this mechanism?
A research team is studying a bacterial pathogen that replicates within the cytoplasm of host cells. They observe that the bacteria induce the formation of actin tails, which propel them through the cytoplasm and into neighboring cells. What selective advantage does this mechanism MOST likely provide to the pathogen?
A research team is studying a bacterial pathogen that replicates within the cytoplasm of host cells. They observe that the bacteria induce the formation of actin tails, which propel them through the cytoplasm and into neighboring cells. What selective advantage does this mechanism MOST likely provide to the pathogen?
A bacterium is isolated from a patient exhibiting signs of severe tissue damage. Further analysis reveals that the bacterium produces a toxin that functions as a protease, cleaving proteins present in the extracellular space. What is the MOST likely mechanism by which this toxin contributes to the observed tissue damage?
A bacterium is isolated from a patient exhibiting signs of severe tissue damage. Further analysis reveals that the bacterium produces a toxin that functions as a protease, cleaving proteins present in the extracellular space. What is the MOST likely mechanism by which this toxin contributes to the observed tissue damage?
A novel bacterial strain is found to express a surface protein that binds specifically to a receptor present on epithelial cells lining the respiratory tract. This interaction triggers the internalization of the bacteria into the epithelial cells via endocytosis. What is the MOST likely role of this surface protein in the pathogenesis of the bacterial strain?
A novel bacterial strain is found to express a surface protein that binds specifically to a receptor present on epithelial cells lining the respiratory tract. This interaction triggers the internalization of the bacteria into the epithelial cells via endocytosis. What is the MOST likely role of this surface protein in the pathogenesis of the bacterial strain?
A scientist is investigating a bacterial pathogen that has developed resistance to multiple antibiotics. Through genomic analysis, they identify a novel gene encoding an efflux pump that actively transports antibiotics out of the bacterial cell. How does this efflux pump MOST directly contribute to the bacterium's pathogenicity?
A scientist is investigating a bacterial pathogen that has developed resistance to multiple antibiotics. Through genomic analysis, they identify a novel gene encoding an efflux pump that actively transports antibiotics out of the bacterial cell. How does this efflux pump MOST directly contribute to the bacterium's pathogenicity?
Flashcards
Koch's Postulates
Koch's Postulates
A set of criteria used to determine the causative agent of a disease, involving isolating the microbe from diseased animals, culturing it, inducing the disease in a susceptible animal, and re-isolating the microbe.
Pathogenicity
Pathogenicity
Properties that enable a microorganism to establish itself and replicate within a host.
Bacterial Adherence
Bacterial Adherence
Attachment to the surface of host cells, often facilitated by pili or surface adhesion molecules, allowing bacteria to evade host barriers and form microcolonies.
Bacterial Invasion
Bacterial Invasion
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Endocytosis in Pathogens
Endocytosis in Pathogens
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Phagosome Escape
Phagosome Escape
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Preventing Phagolysosome Fusion
Preventing Phagolysosome Fusion
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Tolerating Lysosome Fusion
Tolerating Lysosome Fusion
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Study Notes
- Isolating and identifying microorganisms from infected tissue aids in accurate diagnosis.
- Robert Koch, in the 19th century, emphasized isolating and confirming the disease's causative agent.
- Koch's postulates have been instrumental in determining the etiology of most infections.
- Identifying the causative agent becomes challenging if the organisms cannot be cultured in vitro.
Koch's Postulates
- The microorganism must be consistently found in diseased animals, but not in healthy ones.
- The microorganism must be isolated from a diseased animal and grown in a pure, uncontaminated culture.
- The isolated microorganism must cause the original disease when introduced into a susceptible animal.
- The microorganism should be able to be re-isolated from the experimentally infected animal.
Limitations of Koch's Postulates
- Some pathogenic organisms are found in healthy animals, only causing disease in a few.
- Certain species can live in blood without causing disease, resulting in asymptomatic bacteremia.
- Many pathogens, including most viruses, do not grow in nutrient media culture.
- Genotyping species of organisms is increasingly common, but it has limitations.
Pathogenicity
- Pathogenicity refers to the properties that enable a microorganism to establish itself (invasion) and replicate on or within a host.
- Mechanisms for pathogen invasion differ among pathogens.
Mechanisms for Bacterial Host Invasion
- Entry into the host is achieved by evading and avoiding host defenses, such as phagocytosis, acidic environments, and hydrolytic and proteolytic enzymes, through outer surface proteins like a polysaccharide capsule.
- Adherence to host cells is facilitated by structures such as pili (e.g., E. coli) or surface adhesion molecules to overcome barriers like mucus, leading to microcolony formation.
- Invasion involves bacterial enzymes, such as collagenases, to degrade the extracellular matrix, facilitating bacterial entry.
- Immunological evasion further propagates the organism and disease progression once bacteria enter host cells.
- Toxin release by invading bacteria or tissue damage from acute inflammation increases the spread and further invasion of other host cells.
Disease Progression or Resolution
- The effectiveness of host invasion and the associated pathogenicity determines disease progression or resolution.
Pathogens and Immune Evasion
- Pathogens are microorganisms that cause disease, including bacteria, viruses, or parasites.
- Many pathogens have developed strategies to evade the immune system.
Intracellular Pathogens
- Bacterial pathogens enter host cells through endocytosis.
- The pathogen induces the host cell to take it up via phagocytosis, ending up in a phagosome.
- Intracellular pathogens reside inside host cells as unwanted guests within the phagosome.
- Antibodies within phagocytic cells cannot penetrate the host cell, granting temporary protection.
- Intracellular pathogens have three options to avoid being killed by the fusion of the phagosome with the lysosome:
- Escape from the phagosome
- Modification of phagosomal membranes
- Tolerating lysosome fusion
- A single pathogenic species typically uses only one of these mechanisms.
Escape from the Phagosome
- Pathogens like Shigella and Listeria can escape from the phagosome before fusion occurs.
- These pathogens undergo unrestricted growth in the cytoplasm of the host cell.
- They form actin tails that allow them to move within the cell and into adjacent cells, continuing the infection.
- These microbes can spread from cell to cell without encountering the extracellular environment.
Modification of Phagosomal Membranes
- Pathogens like Salmonella escape intracellular death by modifying phagosomal membranes, preventing phagosome-lysosome fusion.
- Salmonella enterica serovar Typhi prevents phagolysosome fusion after invading intestinal cells.
- The bacteria reproduce inside macrophages, spreading through the circulatory system.
Tolerating Lysosome Fusion
- Bacteria such as Coxiella tolerate lysosome fusion.
- Coxiella bacteria reproduce in inclusion bodies within the acidic environment of the phagolysosome.
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