Consider a large outbreak of hepatitis A that occurred in Pennsylvania in 2003. Investigators found almost all of the case-patients had eaten at a particular restaurant during the 2–6 weeks (i.e., the typical incubation period for hepatitis A) before onset of illness. While the investigators were able to narrow down their hypotheses to the restaurant and were able to exclude the food preparers and servers as the source, they did not know which particular food may have been contaminated. What type of study (retrospective cohort or case control) should the investigators use to determine which ingredient was contaminated?
First, let's recall the differences between cohort and case control studies. Retrospective cohort studies enroll participants based on exposure status, while case control studies enroll participants based on disease or non-disease status. Retrospective cohort studies are especially useful when you have a small, well defined population to interview, such as everyone who attended a wedding or conference. Case control studies work well when the disease is more rare or in a larger, less well defined population. In this instance, the number of people who ate at a given restaurant over a 2-6 week period is almost certainly in the thousands. It would not be feasible to track down or interview everyone who was "exposed" to (ate at) this restaurant in this time period, as in a retrospective cohort study. Therefore, a case control study will be the simpler choice for this investigation. The cases will be those who have been identified as patients, and the controls will be a small selection of people who ate at the restaurant during the relevant time period but who did not become ill with hepatitis A. You will ask them about which of the menu items they ate and calculate Odds Ratios.
Remplir les phrases: Si tu te lèves tôt demain, tu ______ (être) à l'heure pour ton examen. Si j'étais riche, je m' ______________ (acheter) un nouveau sac.
This exercise tests your knowledge of "si" clauses. Let's first make sure we remember the rules of "si". Si + present --> futur Si + imparfait --> conditionnel In the first sentence, look at the verbs. We have "Si tu te lèves...", which is in present tense. Therefore, the other verb in the sentence must be in what tense? Future tense, correct. How do you conjugate être in the future tense, tu form? It would be seras. So the answer is : Si tu te lèves tôt demain, tu __seras__ à l'heure pour ton examen. Now look at the verbs in the next sentence. We have "Si j'étais riche...". What tense is the verb in? Imparfait. So the next verb must be in the conditionnel. How do you conjugate acheter in the conditionnel, je form? It would be achèterais. So the answer is : Si j'étais riche, je m' ___achèterais____ un nouveau sac.
Clostridium difficile is a species of gram-positive bacteria that can colonize the human digestive tract and cause severe diarrhea. C. difficile is an obligate anaerobe. In the lab, you take a sterile needle, touch it to a sample of C. difficile, and stab the needle several inches deep into a tube of semi-solid agar. Describe the pattern of C. difficile growth in the test tube several days later under optimum lab growth conditions.
The most important clue here is that C. difficile is an obligate anaerobe. What does this mean? First, remember that anaerobe means without oxygen, so the bacteria prefers to grow without oxygen. Next, remember that obligate means "obligated" or "must", so the bacteria can only grow in environments where there is no oxygen. Now consider what area of the test tube would have the least oxygen. The area toward the top of the tube is closest to the air, and would have the most oxygen. Therefore, the area toward the bottom of the tube would have almost no oxygen and bacteria would be seen growing there. Because C. difficile is an obligate anaerobe, there would be visible growth only at the bottom of the tube.