8 most dangerous antibiotic-resistant bacteria in the world

Millions of people die as a direct result of bacterial infections — especially when the bacteria have developed a resistance to the antibiotics we use to treat them. It's this drug resistance that makes them particularly deadly. 

And the latest data is not encouraging. The emergence of several drug-resistant bacteria species is proving a serious problem for health systems around the world, especially for low-and-middle income countries.

It's estimated that bacterial antimicrobial resistance was directly responsible for 1.27 million global deaths in 2019, and could be associated with as many as 4.95 million deaths, according to the the World Health Organization (WHO).

Priority drug-resistant bacteria which can cause serious illness include mycobacterium tuberculosis, the bacteria that causes tuberculosis, and high-burden resistant pathogens such as Salmonella, Shigella, Pseudomonas aeruginosa, and Staphylococcus aureus.

Difference between antibiotic resistance and antimicrobial resistance

In this article, we'll focus on antibiotic resistance. But more broadly, you may have heard the term antimicrobial resistance (AMR).

AMR refers to all kinds of microbes — tiny living things, or organisms — including bacteria but also parasites, viruses and fungi, which have adapted to resist common medical treatments.

Put simply: the drugs don't work as well as they used to, and we're struggling to find new ones to treat illnesses. Even common ones like urinary tract infections (UTIs) can turn fatal when left untreated.

How the World Health Organization ranks bacteria as dangerous

The WHO monitors and ranks bacteria according to various data:

• Mortality rates
• Incidence (number of infections)
• Non-fatal health burden
• Trend of resistance
• Transmissibility
• Preventability
• Treatability
• Pipeline (new drugs in development) 

The bacteria get a score for each metric and then get ranked. The 2024 list features two dozen priority pathogens. Here are the top eight.

Top 8 bacterial pathogens

1. Klebsiella pneumoniae

Klebsiella is a type of bacteria found in the intestines and human feces. Klebsiella pneumoniae can cause pneumonia, bloodstream infections, wound or surgical site infections and meningitis if it gets into the nervous system. 

Klebsiella pneumoniae can become a so-called "superbug" in hospitals, spreading fast and becoming resistant to most available drug treatments.

It is specifically resistant to carbapenem, an "antibiotic of last resort" — it's used when all other treatments for multidrug-resistant pathogens have failed. Klebsiella pneumoniae is also resistant to a third-generation antibiotic called cephalosporin.

2. Escherichia coli (E. coli)

As with klebsiella bacteria, Escherichia coli (E. coli) bacteria are usually found in the intestines of human and non-human animals. They are also found in the environment, food and water.

Most kinds of E. coli are harmless, but some can cause illnesses, including diarrhea, urinary tract infections, pneumonia and sepsis. E.Coli concentrations in Paris' Seine River came under scrutiny during the 2024 Summer Olympics in France. 

E. coli is resistant to third-generation cephalosporin — a commonly prescribed antibiotic, which is also used to treat sexually transmitted infections, such as gonorrhea. E. coli is also resistant to carbapenem.

3. Acinetobacter baumannii

Back in 2012, researchers described Acinetobacter baumannii as an "emerging opportunistic bacterial pathogen" associated with hospital-acquired infections (HAIs).

Long-term hospital patients with a compromised immune system, or those in hospital for more than 90 days, were identified as having a high risk of contracting an infection. Acinetobacter baumannii is carbapenem-resistant.

4. Mycobacterium tuberculosis

Mycobacterium tuberculosis (TB) causes tuberculosis, a potentially fatal, bacterial infection of the lungs.

TB killed 1.25 million people, including 161,000 people with HIV, in 2023.

The WHO said "TB has probably returned to being the world's leading cause of death from a single infectious agent, following three years in which it was replaced by coronavirus disease (COVID-19)."

Mycobacterium tuberculosis is resistant to rifampicin — an antibiotic used to treat mycobacterial infections, like TB and leprosy, or M. leprae.

5. Salmonella Typhi

Salmonella Typhi causes typhoid fever, a life-threatening illness. It mostly affects people living in regions with poor sanitation and unsafe water and food sources — such as parts of Asia, Africa and Latin America.

The US Centers for Disease Control and Prevention estimates there are about 9 million cases of typhoid worldwide every year. Salmonella Typhi is resistant to flouroquinolone — a broad-spectrum antibiotic, with a range of side effects, which have led the European Medicines Agency to restrict its use.

6. Shigella species

There are four species of Shigella: Shigella sonnei, Shigella flexneri, Shigella boydii, Shigella dysenteriae. Shigella bacteria cause diarrhea, stomach pain and fever.

They spread via contaminated food and water, but also during sexual activity with a sick person. They are fluoroquinolone-resistant.

Shigella species that are resistant to almost all antimicrobial classes are increasing in prevalence and becoming globally dominant. The greatest burden of disease is in low- and middle-income countries with poor sanitation.

7. Enterococcus faecium

Enterococcus faecium lives in the gut flora, also known as microbiome. It can lead to serious illness for people with diabetes or a chronic kidney condition.

Enterococcus, can cause infections such as UTIs and infections of the nervous system, if it enters parts of the body outside the gut.

Enterococcus are resistant to vancomycin — an antibiotic, which is also used to treat infections caused by staphylococci bacteria, which is also drug-resistant.

8. Pseudomonas aeruginosa

Pseudomonas aeruginosa leads to infections in the blood, lungs, urinary tract and other parts of the body, often after surgery in hospitals. Pseudomonas aeruginosa bacteria are multidrug-resistant (MDR), including the common antibiotic, carbapenems.

Pseudomonas aeruginosa infections carry specific risks in immunocompromised patients. Despite being moved from "critical" to "high" priority, it remains a significant concern due to its drug resistance profile.

Edited by: Fred Schwaller

Select sources:

2024 WHO Bacterial Priority Pathogens List (WHO BPPL), World Health Organization https://www.who.int/publications/i/item/9789240093461

Global burden of bacterial antimicrobial resistance 1990-2021: a systematic analysis with forecasts to 2050. Lancet, 2024. DOI: 10.1016/S0140-6736(24)01867-1

Global burden of bacterial antimicrobial resistance 1990-2021: a systematic analysis with forecasts to 2050, Lancet, 2024. DOI: German Center for Infection Research, glossary https://www.dzif.de/en/glossar

National (US) Human Genome Research Institute, glossary https://www.genome.gov/genetics-glossary