Deck 23: Pathogens and Infection

Question 1

Our adult bodies harbor about 10 times more microbial cells than human cells.

Answer 1

Salmonella typhimurium is a gram-negative bacterium, which causes gastroenteritis in humans. These bacteria enter the epithelial cells (non-invasive) by inducing membrane ruffling. This disrupts the balance between absorption and secretion leading to diarrhea and typhoid-like fever. S. typhimurium delivers bacterial effector proteins such as SptP (type III secreted protein) and SopE (Salmonella outer protein) into the cells of the host by using the type III secretion system. Rac, an actin-organizing GTP (guanosine triphosphate) binding protein is directly activated by the SopE. This leads to an active reorganization of the actin cytoskeleton. This reorganization induces ruffling in the host cell membrane, allowing for uptake of the bacterium. This effect is short-lived and the reversal is mediated by another type III secretion target SptP. This protein is a GTPase (GTP hydrolyzing) activating protein for Rac. This turns off the Rac protein after stimulation by SopE, thus suppressing membrane ruffling after an invasion. These proteins are simultaneously introduced into the host cell, but the effects of SopE and SptP are temporarily staggered. This can be achieved if the proteins require an activating factor or a conformational alteration triggered only under certain conditions for their activity. The protein SptP could have regulatory domains, which may lead to the activation of the protein only residing after some time in the host cell.

Question 2

0 The intracellular bacterial pathogen Salmonella typhimurium, which causes gastroenteritis, injects effector proteins to promote its invasion into nonphagocytic host cells by the trigger mechanism. S. typhimurium first stim- ulates membrane ruffling to promote invasion, and then suppresses membrane ruffling once invasion is complete. This behavior is mediated in part by injection of two effec- tor proteins: which promotes membrane ruffling and invasion, and SptP, which blocks the effects of Both effector proteins target the monomeric GTPase, Rac, which in its active form promotes membrane ruffling. How do you suppose and SptP affect Rac activity? How do you suppose the effects of and SptP are staggered in time if they are injected simultaneously?

Answer 2

Cholera is a diarrheal disease caused due to the infection by the organism Vibrio cholera. It is usually spread by water. Before microorganisms were identified as the causative organisms of diseases, many researchers hypothesized the relations between disease outbreaks and basic public amenities like water supply and sanitation. It was initially believed that cholera was caused from "miasmas" in the bad air (1901). John Snow, widely regarded as the father of molecular epidemiology investigated cholera outbreaks in London. He developed a theory which contradicted the then popular theory of "miasmas" and "bad air". He suggested that the disease was most probably spreading through the water, by observing the patterns of cholera outbreaks and their proximity to public water supply systems. He noted that most deaths and incidences of cholera were localized to few water supply tanks and hypothesized that it was spreading through water that is contaminated by sewage. Many scientists couldn't find anything suspicious in the water supplied, as the infections were caused by microorganisms. Since these organisms were invisible to the naked eye, they remained undiscovered as agents of diseases and in general for decades. It was only after the discovery of the microscope that these organisms were identified and studied.

Question 3

1 John Snow is widely regarded as the father of mod- ern epidemiology. Most famously, he investigated an out- break of cholera in London in 1854 that killed more than 600 victims before it was finished. Snow recorded where the victims lived, and plotted the data on a map, along with the locations of the water pumps that served as the source of water for the public (Figure Q23-2). He concluded that the disease was most likely spread in the water, although he could find nothing suspicious-looking in it. His conclu- sion ran counter to the then-current belief that cholera was from "miasmas" in bad air. Very few believed his theory during the next 50 years, with the "bad air" theory persist- ing until at least 1901. What do you suppose Snow saw in the data that led him to his conclusion? Why do you think most scientists remained skeptical for so long?

Answer 3

Influenza rates for 2.5-5 lakhs deaths worldwide every year. Influenza epidemic have been found to be seasonal occurring in the temperate latitudes of the Northern and Southern Hemispheres in their winters. In the tropics, there is significant influenza activity year round, with a peak in the rainy season. The influenza virus survival outside the human body has been correlated with humidity and the presence of UV radiation (from sunlight). Low humidity and absence or low amounts of UV radiation have been related to increased incidences of influenza. In the tropics, peaks of infections are seen during the rainy season. It the weather conditions like low temperature, reduced amounts of direct sunlight and low humidity. This allows for quicker transmission of the virus. An alternative theory proposed suggests that high levels of Vitamin D reduce the viral replication and transmission within the host. Vitamin D synthesis in the human body is aided by an exposure to sunlight and thus, reduced sunlight leads to lower levels of Vitamin D. This provides ideal conditions for virus growth. Thus, the seasonal patterns of influenza outbreaks in the temperate latitudes of Northern, tropics and Southern hemisphere can be attributed to humidity and the amount of sunlight received.

Question 4

2 Influenza epidemics account for 250,000 to 500,000 deaths globally each year. These epidemics are markedly seasonal, occurring in temperate climates in the northern and southern hemispheres during their respec- tive winters. By contrast, in the tropics, there is significant influenza activity year round, with a peak in the rainy sea- son (Figure Q23-3). Can you suggest some possible expla- nations for the patterns of influenza epidemics in temper- ate zones and the tropics?

Question 5

3 Several negative-strand viruses carry their genome as a set of discrete RNA segments. Examples include influ- enza virus (eight segments), Rift Valley fever virus (three segments), Hantavirus (three segments), and Lassa virus (two segments), to name a few. Why does segmentation of the genome provide a strong evolutionary advantage for these viruses?

Question 6

4 Avian influenza viruses readily infect birds, but are transmitted to humans very rarely. Similarly, human influenza viruses spread readily to other humans, but have never been detected in birds. The key to this speci- ficity lies in the viral capsid protein, hemagglutinin, which binds to sialic acid residues on cell-surface glycoproteins, triggering virus entry into the cell (Movie 23.8). Hemag- glutinin on human viruses recognizes sialic acid in a 2-6 linkage with galactose, whereas avian hemagglutinin rec- ognizes sialic acid in a 2-3 linkage with galactose. Humans make carbohydrate chains that have only the 2-6 linkage between sialic acid and galactose; birds make only the 2-3 linkage; but pigs make carbohydrate chains with both linkages. How does this situation make pigs ideal hosts for generating new strains of human influenza viruses?

Question 7

5 The majority of antibiotics used in the clinic are made as natural products by bacteria. Why do you suppose bacteria make the very agents we use to kill them?

Question 8

Which statements are true? explain why or why not.
Animals and plants use oxidation to extract energy from food molecules.

Question 9

6 In the early days of penicillin research, it was dis- covered that bacteria in the air could destroy the penicillin, a big problem for large-scale production of the drug. How do you suppose this occurs?

Question 10

Which statements are true? explain why or why not.
7 When the Oxford team of Ernst Chain and Nor- man Heatley had laboriously collected their first two grams of penicillin (probably no more than 2% pure!), Chain injected two normal mice with 1 g each of this preparation, and waited to see what would happen. The mice survived with no apparent ill effects. Their boss, Howard Florey, was furious at what he saw as a waste of good antibiotic. Why was this experiment important?

Question 11

Which statements are true? explain why or why not.
The microbiomes from healthy humans are all very similar.

Question 12

Which statements are true? explain why or why not.
Pathogens must enter host cells to cause disease.

Question 13

Viruses replicate their genomes in the nucleus of the host cell.

Question 14

You should not take antibiotics for diseases caused by viruses. Discuss the following problems.

Question 15

In order to survive and multiply, a successful pathogen must accomplish five tasks. Name them.
C. difficile infections are difficult to eradicate because the bacteria exist in two forms: a repli- cating, toxin-producing form and a spore form that is resis- tant to antibiotics. Fecal microbiota transplantation-the transfer of normal gut microbiota from a healthy individ- ual-can resolve >90% of recurrent infections, a much bet- ter cure rate than further antibiotic treatment alone. Why do you suppose microbiota transplantation is so effective?

Question 16

Clostridium difficile infection is the leading cause of hospital-associated gastrointestinal illness. It is typically treated with a course of antibiotics, but the infection recurs in about 20% of cases.

Question 17

What are the three general mechanisms for hori- zontal gene transfer?