Mission Microbe
A microscope lets scientists see bacteria, but why can't a microscope alone prove that bacteria caused an illness?
Because scientists also need laboratory tests and evidence linking the bacteria to the illness.
Which is more useful to an epidemiologist: knowing what one patient ate or finding the food that many patients ate? Explain.
The food many patients ate because it identifies a common exposure.
Unlike bacteria, antibiotics do not work against me. I spread quickly through cruise ships, schools, and restaurants, causing vomiting and diarrhea. Who am I?
Norovirus
An investigator interviews 40 people after a picnic. 18 people ate potato salad; 17 people ate apple pie. Which food should investigators focus on first
The potato salad
Two people ate the same meal, but only one became sick. Name one scientific reason investigators would not immediately rule out the meal as the source.
People have different immune systems, ate different amounts, or one person may have had prior immunity or another exposure.
Scientists find bacteria on a cutting board and on raw chicken. What should they investigate next to determine if cross-contamination occurred?
Compare bacteria from the cutting board and chicken and determine whether they match.
Five patients became sick after eating at different restaurants. What information would investigators compare first?
Foods eaten, dates of illness, and common exposures.
I am used to make bread and help dough rise by producing carbon dioxide. Who am I?
Yeast (Saccharomyces cerevisiae)
Why is keeping hot foods hot and cold foods cold important?
It slows or prevents bacterial growth.
A patient remembers everything they ate except breakfast. Why is this missing information important?
Breakfast may contain the food responsible for the illness.
Why is growing bacteria on an agar plate more helpful than simply looking at it under a microscope?
It allows scientists to isolate, identify, and study bacterial growth patterns.
Why do epidemiologists interview many patients instead of just one?
Multiple interviews reveal patterns and common sources of exposure.
I am often linked to undercooked ground beef, raw spinach, and unpasteurized juices. Some strains can damage the kidneys and cause hemolytic uremic syndrome (HUS). Who am I?I'm commonly found in undercooked chicken.
Escherichia coli O157:H7 (STEC)
A family falls ill after eating potato salad left outside for 6 hours. Explain why this happened.
Bacteria multiplied because the food remained in the temperature danger zone too long.
Why do investigators collect food samples from homes and restaurants during an outbreak?
To test for bacteria and compare samples with patient illnesses.
A food sample tests positive for bacteria, but no one became sick. Explain two reasons why this might happen.
The bacteria may not have been harmful, there were too few bacteria to cause illness, or the food was not eaten.
Two foods appear linked to an outbreak. What additional evidence would help determine which food caused the illness?
Laboratory testing, traceback investigations, and more patient interviews.
I am a bacterium that can survive stomach acid and often spreads through contaminated poultry, raw milk, or untreated water. Although many people recover quickly, I have been linked to Guillain-Barré syndrome. Who am I?
Campylobacter jejuni
A cook washes vegetables but uses the same knife for raw chicken without cleaning it first. Explain what food safety mistake occurred.
Cross-contamination transferred harmful bacteria to the vegetables.
Three restaurants receive lettuce from the same farm, and customers at all three become sick. What does this suggest, and what should investigators do next?
The farm may be the contamination source; conduct a traceback investigation and test lettuce from the supplier.
During an outbreak investigation, one piece of evidence conflicts with all the others. Should investigators ignore it, collect more evidence, or change their conclusion? Defend your answer using scientific reasoning.
Collect additional evidence, evaluate reliability, avoid jumping to conclusions, and revise hypotheses only when supported by evidence.
Create an investigation plan for an outbreak that spreads across three different states. Explain how scientists from different agencies would work together to solve the case.
Discuss collaboration among CDC, FDA, state health departments, laboratories, interviews, traceback investigations, and data sharing.
A newly discovered microorganism survives freezing temperatures, forms spores, and contaminates vegetables. Design three laboratory tests that would help identify it and explain why each test is useful.
Examples include Gram stain, microscopy, culture characteristics, DNA sequencing, biochemical testing, and spore staining with justification.
A food company claims its product is safe because only a few people became sick. Evaluate this claim using scientific evidence and explain why investigators may still issue a recall.
Even a small number of illnesses can indicate contamination, especially with dangerous pathogens; recalls prevent additional illnesses.
After completing an investigation, explain how scientists decide whether they have enough evidence to close a case or continue collecting information.
Students should discuss the strength of evidence, consistency among findings, laboratory confirmation, traceback data, and remaining uncertainties.