Methicillin-resistant Staphylococcus aureus (MRSA) infection is caused by a strain of staph bacteria that's become resistant to the antibiotics commonly used to treat ordinary staph infections.

Most MRSA infections occur in people who've been in hospitals or other health care settings, such as nursing homes and dialysis centers. When it occurs in these settings, it's known as health care-associated MRSA (HA-MRSA). HA-MRSA infections typically are associated with invasive procedures or devices, such as surgeries, intravenous tubing or artificial joints.

Another type of MRSA infection has occurred in the wider community — among healthy people. This form, community-associated MRSA (CA-MRSA), often begins as a painful skin boil. It's spread by skin-to-skin contact. At-risk populations include groups such as high school wrestlers, child care workers and people who live in crowded conditions.

Vancomycin resistant enterococcus (VRE) infection is caused by a type of bacteria (enterococcus) that has acquired resistance to the commonly used intravenous antibiotic Vancomycin.

VRE infections typically occur in the hospital or healthcare setting, and have been associated with institutional outbreaks. The bacteria is also capable of colonizing patients without causing symptoms. Colonization is typically considered necessary, but not sufficient for symptomatic infection to develop. Some hospitals will isolate patients with VRE in order to prevent transmission.

Resistance to 3rd generation cephalosporins is commonly due to production of extended spectrum beta-lactamases (ESBL), and is increasingly being seen in E. coli and K. pneumoniae species.

Organisms resistant to 3rd Generation Cephalosporins, including ESBL organisms, are a growing cause of disease in hospital and the community, and often require treatment with non-beta-lactam antibiotics. Much like with VRE, these organisms can both cause symptomatic disease as well as asymptomatic colonization of the gastrointestinal tract.

Carbapenem resistant enterobacteriaceae (CRE) are bacteria that contain genetic mechanisms of resistance (often highly mobile) that confer resistance to almost all beta-lactams, including the broad-spectrum class of carbapenems.

Although established globally, certain mechanisms of carbapenem resistance are showing rapid dissemination from regions of high antimicrobial use/resistance. Examples of these include OXA and NDM-1 beta-lactamases. Like ESBL organisms and VRE, CRE can cause symptomatic disease and asymptomatic colonization of the gut.

Typically non-beta-lactam antibiotics are used for treatment of these organisms.