What are antibiotic resistant bacteria?

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When bacteria are continually exposed to small amounts of antibiotics they often develop immunity to them. It’s a case of "survival of the fittest," with the strongest bacteria, that are least susceptible to a specific antibiotic, surviving, adapting and multiplying. These are called "resistant bacteria" because they have adapted to the point where antibiotics are no longer an effective means of killing them.  As a result, some antibiotics have lost their effectiveness against specific infectious diseases. The Centers for Disease Control estimates that each year in the US almost 2 million people acquire bacterial infections in hospitals, 70% of which are resistant to at least one commonly used antibiotic.

There is the danger that the ignorant man may easily underdose himself and, by exposing his microbes to non-lethal quantities of the drug, make them resistant.
— Sir Alexander Fleming

An example of antibiotic-resistant bacteria is Staphylococcus aureus, a bacterium that is the most common cause of staph infections, and can also cause pneumonia, meningitis, toxic shock, skin abscesses, heart valve infections and other serious and deadly medical conditions.  Many strains of S. aureus are now resistant to the antibiotics oxacillin, penicillin and amoxicillin. Some strains of the bacteria have begun developing resistance to newer drugs like methicillin and vancomycin, powerful drugs once considered ‘last resort' antibiotics for only the most serious infections.  The threat of prolonged illness or death from an S. aureus infection has increased as it has become more resistant and fewer drugs are able to effectively control or eliminate it. According to the Centers for Disease Control (CDC), 80,000 invasive infections occur every year in the United States that are attributed to Methicillin-resistant Staphylococcus aureus (MRSA). Based on direct medical costs of $27,083 to $34,900 per patient (includes non-surgical infections) and CDC estimates of 80,000 patients hospitalized with MRSA in 2014, annual costs to treat hospitalized patients would be between $2.2 billion and $2.8 billion for the U.S. alone. MRSA is pictured above.