Drug-resistant bacteria of various types are far less frequent than the strains from which they evolved, but infections are far harder to treat.

Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to all antibiotics in the penicillin family and related synthetics, although other drugs can still be used.

No antibiotics at all are fully effective against the following newly emerging strains of common bacteria:

K. pneumoniae

• Where it lives: It typically is found in the colon of humans and animals. It is widespread in nature.
• What it does: It can cause urinary tract infections and pneumonia in healthy people, and more serious infections in severely ill hospitalized patients.
• Of note: Besides kidney and lung infections, very ill people can get infections involving wounds, the bloodstream, abdomen, liver and gallbladder.
  

A. baumannii

• Where it lives: It can be found in soil and water. Up to 25 percent of healthy adults carry it.
• What it does: It largely infects the most severely ill hospitalized patients, but community-acquired cases are also reported. It can infect the lungs, urinary tract, bloodstream, brain, skin and muscle.
• Of note: Resistant strains have been reported causing infections in wounded soldiers in Iraq.
  

P. aeruginosa

• Where it lives: It is in soil and water, and on plants.
• What it does: It usually is a cause of health-care- associated infections. But community infections are getting more common, particularly among those needing frequent antibiotics.
• Of note: It can infect most parts of the body. Victims of burns and chemotherapy patients are often infected.