Multi drug-resistant organisms (MDROs) are defined as microorganisms, predominantly bacteria, that are resistant to ≥3 classes of antibiotics (e.g. 3rd generation cephalosporins, amino glycosides, quinolones, carbapenems etc.). These highly resistant organisms deserve special attention in healthcare facilities. Certain GNB, including those producing extended spectrum beta-lactamases (ESBLs) and others that are resistant to multiple classes of antimicrobial agents, are of particular concern. In addition to Escherichia coli and Klebsiella pneumoniae, these include strains of Acinetobacter baumannii resistant to most antimicrobial agents, or all except imipenem, and organisms such as Stenotrophomonas maltophilia, Burkholderia cepacia and Ralstonia pickettii, that are intrinsically resistant to broad-spectrum antimicrobial agents.

Vancomycin-resistant enterococci (VRE) are considered as the second most important Health Associated Infection (HAI) in many hospitals. Screening is required for preventing and controlling the nosocomial spread of VRE, which commonly occurs through contact with a contaminated environment or contact directly or indirectly with a person who is infected or colonized with this organism. VRE rates are significantly increased when patients are required to undergo invasive procedures, have prolonged hospital stays, have indwelling urinary catheters or central intravenous lines or are immunocompromised.

Carbapenemase-producing organisms (CPOs) and carbapenem-resistant Enterobacterales (CRE) are resistant to the carbapenem class of antibiotics. Patients with CRO/CPO infections have significantly worse outcomes than patients with susceptible infections. Organisms may become   resistant because they produce carbapenemase enzymes (e.g., KPC, NDM, OXA, VIM, IMP) that make carbapenems ineffective. Carbapenemase genes can be transferred between different kinds of bacteria and lead to the spread of antibiotic resistance.