Multidrug-resistant bacteria

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

Multidrug-resistant bacteria

When a bacterium is resistant to at least one antibiotic in three (or more) different antibiotic classes it is said to be multidrug-resistant. This can happen in two separate 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 bacterial 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 have more side effects 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 help select for the same resistant bacteria or plasmids – reducing the use of one type of antibiotic will not be 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 carrying ESBL-producing bacteria in their intestines, as a part of their normal microbiome. Globally, there has been an overall increase from low numbers in the beginning of the 2000s to more than 25% in 2016-2020. However, there is a large variation between different regions, and in some places more than 30% of the population carry such bacteria. 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