It can mean one of the following things:

1. Unnecessary use – where an antimicrobial is not indicated and there is no health benefit for the patient (e.g. treatment of an upper respiratory infection caused by a virus or an antibiotic is not recommended)
2. Inappropriate use – where timing, antimicrobial choice, dose, route, frequency of administration or duration of treatment is incorrect. For example:
   • delayed administration in a critically ill patient,
   • choice of an antibiotic with an unnecessarily broad or too narrow spectrum 
   • drug-bug mismatch
   • iv route when oral can be used
   • dose is too high or too low compared to what is indicated for that patient
   • duration is too long or too short
   • duration is > 24 hours for surgical prophylaxis (except where guidelines endorse longer duration)
   • treatment is not streamlined or changed when microbiological culture data become available
   • prescription of agent for patients with a known allergy to the agent
   • drug-drug interaction

The 3 main outcomes are:     

1. Development of resistant organisms, Clostridium difficile infections
2. Patient harm such as treatment failure, adverse drug events, and increased mortality
3. Increased healthcare and societal costs

They usually use one of the following mechanisms:

1. Producing destructive enzymes to neutralise antibiotics
2. Modifying antimicrobial targets by mutation, so that drugs cannot recognize them
3. Creating bypasses that allow bacteria to function without the enzymes targeted by antibiotics
4. Preventing antibiotics from entering by creating a “biofilm” or otherwise reducing permeability
5. Removing antimicrobial agents by pumping them out (efflux)

There are several definitions of AS. Here are a few:

• “Organisational or healthcare system-wide approach to promoting and monitoring
  judicious use of antimicrobials to preserve their future effectiveness”
• “The optimal selection, dosage, and duration of antimicrobial treatment that results
  in the best clinical outcome for the treatment or prevention of infection, with
  minimal toxicity to the patient and minimal impact on subsequent resistance”
• „The right antibiotic for the right patient, at the right time, with the right dose, and
  the right route, causing the least harm to the patient and future patients“ (we like this one the most :) )
  

Several studies have found that implementing Antimicrobial stewardship in a hospital setting will have the following benefits:

1. Improve patient outcomes - improve infection cure rates, reduce surgical infection rates and reduce morbidity and mortality 
2. Improve patient safety and minimize unintended consequences of antimicrobials - reduce antimicrobial consumption, without increasing mortality or infection-related readmissions
3. Reduce resistance rates
4. Reduce healthcare costs without impacting the quality of care