A few weeks ago I posted a cover of Science showing lovely little golden balls -- the bacterium Staphylococcus aureus revealed by the scanning electron microscope, a nasty antibiotic-resistant microorganism. As bad luck would have it, only a week later, someone dear to me became infected by this very pathogen.
To tell the truth, the planet belongs to bacteria. They were here before us. They outnumber all other organisms put together. A significant part of our body weight is bacteria, mostly harmless, some beneficial. Harmless, beneficial or deadly, they have only one objective: exploiting our bodies for their own purpose.
The body's first line of defense is the outer walls and moats: the skin, with its impregnable barrier of keratin, and the mucus membranes. Other exterior membranes are flushed with fluids: saliva, tears, and nasal secretions. The skin and the lower intestinal tract harbor populations of benign bacteria that do battle for the body the way pacified tribes on the marches fought for the Roman Empire.
Despite these defenses, microbes sometimes overwhelm the outer defenses by force of sheer numbers. Or they slip in quietly by an unguarded gate. They are masters of deceit and disguise.
Once the enemy has penetrated the outer membranes, more sophisticated defense systems swing into action. The presence of an alien microorganism triggers chemical alarms that cause white blood cells to move to the site of the intrusion. The white blood cells do their best to engulf the enemy the way an amoeba engulfs its prey.
Most effective of all the body's defenses are the lymphocytes, the agents of the immune response. Lymphocytes are small, round, non-dividing cells that are always on the alert. At any time there are as many as 2 trillion lymphocytes patrolling the human body. The huge number is crucial: Lymphocytes are very specific about what intruders they can recognize. Each lymphocyte is trained by evolution to respond to a particular alien. Recognition of a foreign body causes lymphocytes to become active and start dividing. The offspring cells produce huge numbers of antibodies.
The body is protected by a stupendous array of traps, triggers, walls, moats, and chemical alarms. Some of the body's cells act as patrols, sentries, infantry, and artillery to defend the integrity of the larger society. The defense system never rests. And all of this goes on without our awareness -- unless and until something goes wrong.
Then the defense mechanism of last resort comes into play: the brain.
We invented antibiotics, and temporarily gained the upper hand. But we have been profligate in their use, especially in agriculture. The little golden balls have evolved defenses, as we might have expected. It's us against them.