There is about a good chance that you have Staph - Staphylococcus aureus - on your body right now. In fact, it is estimated that 25-30% of us carry Staph on our skin or in our nose all the time. But a quarter of us are not sick, suffering from the symptoms of a Staph infection. What's the deal? It comes down to colonization vs. infection.
You are not who you think you are.
None of us savors the thought of billions of bacteria living on our skin, in our nose, and in our gut. We don't tend to be comfortable playing the role of home planet to these teeming microorganisms. But in fact the group of cells we call "me" is actually made up of 90% bacterial cells and only 10% human cells. Look down at your body you so confidently call your own: Their cells outnumber yours.
Ok, so you're not 90% bacteria. You can relax - those cells make up only 3% of your body mass. Human cells are huge compared to bacterial cells, so the 9:1 ratio is still won out by your body's cells. Nonetheless, we each carry about 1 trillion microorganisms around with us each day.
Good fences make good neighbors.
Bacteria like warm, dark, moist places, and so tend to reside in our nose, underarms, belly button, and other similar locations. They also live in our gut, helping us digest our food and keeping the rowdy bacteria strains under control. These habitats provide enough nutrients to keep a relatively small colony alive, not thriving and growing, just surviving.
Colonies of bacteria are made up of millions to billions of cells, and can be visible to the naked eye. Most of the bacteria that live on or in us is perfectly harmless (or even beneficial). If we are healthy and don't have any breaks in our skin, even if bacteria form colonies on our body, they don't lead to infection.
This is bacterial colonization: the presence of bacteria with no symptoms of infection.
Give them an inch...
This balance is disrupted if our protective barrier - skin - is disrupted. A break in the skin is like opening the doors to Walmart on Black Friday - the bacteria flood in and gorge on all those nutrients and thrive in the warmth and moisture. Their once-controlled colony multiplies and multiplies and becomes an infection that prevents tissue from healing, destroys healthy tissue, and in worst-case scenarios, can begin to travel to other parts of the body via the blood stream. This reproduction and spread is bacterial infection: clinical signs of infection or inflammation.
For most healthy individuals, applying an antiseptic to a cut or taking an antibiotic internally will destroy the infection and allow the tissue to heal and the body to recover its healthy balance of bacteria. It's a different story for someone in a hospital.
When colonies are dangerous
For individuals receiving treatment in a hospital, those colonies of bacteria present a great risk for contamination of wounds, incisions, or medical devices. Strict cleaning regiments for both the patient and the environment are critical to reducing the amount of colonization present (what specialists call the "bioburden").
In cases of MRSA or C. diff colonization, a patient will be put into "isolation" due to the antibiotic-resistance of MRSA and the overall resilience of C. diff. These precautions require visitors and medical staff alike to don gowns and gloves for all interactions. Even thought the patient may not be exhibiting symptoms of infection, the very presence of these colonies presents a danger to other patients.
In conclusion, we are all happily colonized by bacterial organisms. They cheerfully survive on and in our bodies in a delicate harmony - they help us digest, we provide them with nutrients, and life goes on. But those stealthy colonists have evolved into opportunistic experts, ready to amass into a destructive infection, given the chance.