Teachable moment in classrooms:

  1. microbiology – Legionella bacterium
  2. lymphatic and immune system chapter – development and role of macrophages in immune defense
  3. respiratory system chapter – cells that make up the alveoli
  4. cellular basis of life chapter – functions of lysosomes

The news item:  Recently the following article appeared online:

3 dead in Legionnaire’s disease outbreak at New York assisted living facility

Since the discovery of Legionella bacteria at the Albany facility, 20 people have been hospitalized and three of those who tested positive have died.

The article states that at the time of the writing of the report 3 people have already died from Legionnaire’s disease in an assisted living facility, and that the infection likely spread to residents by them inhaling mist contaminated by the Legionella bacterium.

So, Why Do I Care??  While the name “Legionnaire’s disease” make it sound like it has an uncommon occurrence, but this bacterial infection, and the pneumonia it causes, is responsible for over 10,000 yearly hospitalizations in the USA. The bacterium specifically disables macrophages, so understanding how this happens may help us design pharmaceuticals or other interventions to help infected people, and to apply this knowledge to other disorders where macrophages have a role.

Plain English, Please!!! First, let’s talk about how macrophages are involved in the defense of our lungs. We find resident macrophages in the lumen of the cup-shaped, microscopic alveoli of the lungs. Macrophages there internalize, phagocytose, microorganisms that invaded the alveoli. The internalization brings the microbes into microscopic bubbles called phagosomes, and once that phagosome fuse with a lysosome full of acid and digestive enzymes, the microbes will be digested into their molecular components. To picture a macrophage in action, imagine a vacuum cleaner where the microbes are “internalized” into a vacuum cleaner bag (the phagosome) which would be merging with a zip-lock bag full of acid and digestive enzymes. Digesting viruses, bacteria, fungi keep the alveoli free of harmful microbes.

Second, let’s talk about how Legionella bacterium infects macrophages. Once inside the phagosome, the Legionella bacteria delay the fusion with the lysosome, and that delay gives enough time for the bacteria to multiply. Once the phagosome fuses with the lysosome, the bacteria neutralize the acidic pH, and avoid destruction. Then hundreds of new Legionella bacteria are exocytosed; imagine a vacuum cleaner that is pushing out from itself the bag.

Third, let’s talk about how Legionella disables macrophages. While multiplying inside the phagosome the bacteria release chemicals that slow down the synthesis of proteins by the host macrophage. Normally macrophages use proteins called cytokines to stimulate neighboring macrophages to speed up phagocytosis. The reduced level of protein synthesis in the Legionella-infected macrophages slows down the production of both cytokines and the digestive enzymes used in lysosomes. The combined effect is that the macrophage activity never speeds up because of the reduced level of cytokines, and the digestion in the lysosomes is slower, leaving the bacteria unharmed. Large swarms of newly-released bacteria attract the white blood cells called neutrophils, and their cytokines cause the inflammation (pneumonia) and fever of the Legionnaire’s disease.

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