Methicillin-resistant Staphylococcus aureus
Methicillin-resistant Staphylococcus aureus (MRSA) is a group of gram-positive bacteria that are genetically distinct from other strains of Staphylococcus aureus. MRSA is responsible for several difficult-to-treat infections in humans. It caused more than 100,000 deaths worldwide attributable to antimicrobial resistance in 2019.
Colorized scanning electron micrograph of a human neutrophil ingesting MRSA
A selective and differential chromogenic medium for the qualitative direct detection of MRSA
MRSA resistance to oxacillin being tested. The top S. aureus isolate is a control that is not resistant to oxacillin; the other three isolates are MRSA-positive.
Mueller–Hinton agar showing MRSA resistant to an oxacillin disk
Antimicrobial resistance (AMR) occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. All classes of microbes can evolve resistance where the drugs are no longer effective. Fungi evolve antifungal resistance, viruses evolve antiviral resistance, protozoa evolve antiprotozoal resistance, and bacteria evolve antibiotic resistance. Together all of these come under the umbrella of antimicrobial resistance. Microbes resistant to multiple antimicrobials are called multidrug resistant (MDR) and are sometimes referred to as superbugs. Although antimicrobial resistance is a naturally occurring process, it is often the result of improper usage of the drugs and management of the infections.
Antibiotic resistance tests: Bacteria are streaked on dishes with white disks, each impregnated with a different antibiotic. Clear rings, such as those on the left, show that bacteria have not grown—indicating that these bacteria are not resistant. The bacteria on the right are fully resistant to three of seven and partially resistant to two of seven antibiotics tested.
A CDC infographic on how antibiotic resistance (a major type of antimicrobial resistance) happens and spreads
A CDC infographic on how antibiotic resistance spreads through farm animals
A number of mechanisms used by common antibiotics to deal with bacteria and ways by which bacteria become resistant to them
