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Understand  –  Antibiotic resistance

Multidrug-resistant bacteria

Some bacteria are resistant to many different antibiotics; they are multidrug-resistant. Multidrug-resistant bacteria can be difficult to treat and facilitates spread of antibiotic resistance.

Multidrug-resistant bacteria

When a single bacterium is resistant to more than one antibiotic it is said to be multidrug-resistant. This can occur in two distinct ways.

  • A bacterium can have several different resistance genes, each providing resistance to a particular antibiotic. Accumulation of resistance genes often takes place on small DNA-pieces called plasmids that can be transferred between bacteria in a single event. Read more under Plasmids and co-selection.
  • The other possibility is that a single resistance mechanism gives resistance to more than one antibiotic. For example, one resistance strategy bacteria use is to pump the antibiotic out of the cell. Sometimes such pumps can recognize many different molecules, including different types of antibiotics. That is, the bacteria use a single pump to pump out several different antibiotics. This is also called cross-resistance.

Why multidrug-resistant bacteria are problematic:

  • Infections with multidrug-resistant bacteria are hard to treat since few or even no treatment options remain. In some cases health care providers have to use antibiotics that are more toxic for the patient.
  • Multidrug-resistance facilitates spread of antibiotic resistance. When multidrug-resistance plasmids are transferred to other bacteria, these become resistant to many antibiotics at once. In environments where bacteria are continuously exposed to antibiotics, like in hospitals or some large production animal farms, multidrug-resistance may be favorable and therefore selected and spread further.
  • Multidrug-resistance complicates efforts to reduce resistance. When many different antibiotics select for the same resistant bacteria or plasmids, reducing use of one type of antibiotic is not enough to reduce resistance to that antibiotic.

Multidrug-resistant bacteria are increasing

There is an increasing prevalence of pathogenic multidrug-resistant bacteria globally. An example is ESBL (extended spectrum beta lactamase)-producing Gram-negative bacteria like E. coli and Klebsiella pneumoniae. ESBLs are enzymes that destroy many clinically important antibiotics. Infections with bacteria expressing ESBLs are hard to treat and are becoming increasingly common. A worrisome trend is that more and more people around the world are asymptomatic carriers of ESBL-producing bacteria. For example, more than 50% of the community populations in some parts of Southeast Asia are colonized with ESBL-producing bacteria according to recent estimates . Numbers are increasing also in other parts of the world. This puts many at risk for future antibiotic-resistant infections. Read more in Why should I care? – Risks for the individual and society.

Selected Resources

Resource Description
Antibiotic Resistant Bacteria: 10 of the Worst An overview of 10 dangerous and often antibiotic resistant bacteria – the diseases they cause, characteristics and resistance levels (short infographics from Nesta).
Nesta. Antibiotic Resistant Bacteria: 10 of the Worst [Internet]. 2014 [cited 2014 Oct 2]. Available from:
Woerther P-L, Burdet C, Chachaty E, Andremont A. Trends in human fecal carriage of extended-spectrum β-lactamases in the community: toward the globalization of CTX-M. Clin Microbiol Rev [Internet]. 2013 Oct;26(4):744–58. Available from: