Chapter 15, Problem 2CCS

The Stealth Invader

Tim is often seen walking around campus, hanging out at the coffee shop, laughing with his friends, and, as he puts it, “investing time with the ladies.” Tim started smoking in high school and has never tried to kick the habit. He jokes about the “smoker’s cough” that has punctuated his conversations over the summer and start of the fall semester. Recently, the cough has been getting worse, his throat is scratchy, his head hurts, and he is tired all the time. Tim wonders if he has some long-lasting flu, though he hasn’t had a fever. He decides not to waste time with a doctor’s visit, but the cough starts keeping him up at night. In October, after a month of progressively worse coughing, he’s had enough; he heads to the campus clinic right after his morning classes.

The clinic physician notes the persistence and worsening of Tim’s cough and confirms that Tim has no fever. To rule out a bacterial infection, she orders a routine sputum culture (a test of the material coughed up from Tim’s lungs), which comes back negative. Tim is sent home with the advice to quit smoking.

A week later, he’s back, coughing nonstop, short of breath, sweaty, and aching. A different doctor orders a chest X-ray exam and several blood tests. The X-ray film reveals fluid in Tim’s lungs, and real-time PCR confirms an infection with Mycoplasma pneumoniae, which is an atypical Gram-positive bacterium. Regular sputum microscopy doesn't detect this pathogen, which lacks cell walls and doesn’t stain well.

Mycoplasma pneumoniae infects and disrupts the mucous membranes of the lungs, invading and disrupting the epithelium. Pneumonia caused by M. pneumoniae is called “walking pneumonia” because the symptoms are often milder and come and go, making this disease different from streptococcal pneumonia. Most patients get better on their own, but not Tim. Smoke has compromised the lungs’ innate immunity. A course of an antimicrobial drug-azithromycin-clears Tim’s lungs of the bacterium within a week. Tim decides to quit smoking.

1. 1. Which Toll-like receptors (TLRs) might have been involved in Tim’s innate immune response to Mycoplasma?
2. 2. Why didn’t Tim’s naturally occurring interferons help clear the infection?
3. 3. What structures and chemicals normally fend off lung infections?
4. 4. Why didn’t Tim’s innate lung defenses operate properly?