Millions of people die as a direct result of bacterial infections – especially when the bacteria have become resistant to the antibiotics we use to treat them. It is this drug resistance that makes them particularly deadly.
And the latest data is not encouraging. The emergence of several drug-resistant bacterial species is proving to be a serious problem for healthcare systems around the world, especially for low- and middle-income countries.
According to the World Health Organization (WHO), bacterial antimicrobial resistance was estimated to be directly responsible for 1.27 million deaths worldwide in 2019, and could be linked to as many as 4.95 million deaths.
Priority resistant bacteria that can cause serious illness include: mycobacterium tuberculosisthe bacterium that causes tuberculosis, and high-burden resistant pathogens such as Salmonella, Shigella, Pseudomonas aeruginosaAnd Staphylococcus aureus.
Difference between antibiotic resistance and antimicrobial resistance
In this article we will focus on resistance to antibiotics. But in a broader sense, you may have heard the term antimicrobial resistance (AMR).
AMR affects all kinds of microbes – small living things or organisms – including bacteria, but also parasites, viruses and fungi, which have adapted to resist common medical treatments.
Simply put, the drugs no longer work as well as they used to, and we’re struggling to find new ones to treat diseases. Even common infections like urinary tract infections (UTIs) can become fatal if left untreated.
How the World Health Organization considers bacteria dangerous
The WHO monitors and ranks bacteria based on various data:
The bacteria are given a score for each metric and then ranked. The 2024 list includes twenty 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 enters the nervous system.
Klebsiella pneumoniae can become a so-called ‘superbug’ in hospitals, spreading rapidly and becoming resistant to most available drug treatments.
It is specifically resistant to carbapenem, an ‘antibiotic of last resort’ – it is 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 in klebsiella bacteria, Escherichia coli (E.coli) bacteria are commonly found in the intestines of human and non-human animals. They are also found in the environment, food and water.
Most species E.coli are harmless, but some can cause illnesses including diarrhea, urinary tract infections, pneumonia and sepsis. E. Coli concentrations in the Seine in Paris 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 diseases, such as gonorrhea. E.coli is also resistant to carbapenem.
3. Acinetobacter baumannii
In 2012, researchers described Acinetobacter baumannii as an “emerging opportunistic bacterial pathogen” associated with hospital-acquired infections (HAIs).
It was found that long-term hospitalized patients with compromised immune systems, or patients who stayed in the hospital for more than 90 days, were at 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 will kill 1.25 million people in 2023, including 161,000 people with HIV.
The WHO said: “TB is likely to return to being the world’s leading cause of death from a single infectious agent, after three years in which it was replaced by the coronavirus disease (COVID-19).”
Mycobacterium tuberculosis is resistant to rifampicin – an antibiotic used to treat mycobacterial infections, such as tuberculosis and leprosy, or M. leprae.
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5. Salmonella Typhi
Salmonella Typhi causes typhoid fever, a life-threatening disease. It mainly affects people living in regions with poor sanitation and unsafe water and food sources – such as parts of Asia, Africa and Latin America.
The U.S. Centers for Disease Control and Prevention estimates that there are approximately 9 million cases of typhoid worldwide each year. Salmonella Typhi is resistant to fluoroquinolone, 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 types Shigella: Shigella sonnei, Shigella flexneri, Shigella boydii, Shigella dysentery. Shigella bacteria cause diarrhea, stomach pain and fever.
They spread through contaminated food and water, as well as during sexual activity with a sick person. They are fluoroquinolone resistant.
SHogela Species resistant to almost all classes of antimicrobials are increasing in prevalence and becoming globally dominant. The greatest burden of disease occurs in low- and middle-income countries with poor sanitation.
7. Enterococcus faecium
Enterococcus faecium lives in the intestinal flora, also called the microbiome. It can lead to serious illness in people with diabetes or chronic kidney disease.
Enterococci, can cause infections such as urinary tract infections and nervous system infections if it gets into parts of the body outside the intestines.
Enterococci are resistant to vancomycin, an antibiotic also used to treat infections caused by staphylococci bacteria, which are 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 pose specific risks in immunocompromised patients. Despite being moved from ‘critical’ to ‘high’ priority, it remains a major problem due to its drug resistance profile.
Edited by: Fred Schwaller
Select sources:
2024 WHO List of Bacterial Priority Pathogens (WHO BPPL), World Health Organization
Global burden of bacterial antimicrobial resistance 1990-2021: a systematic analysis with predictions to 2050. Lancet, 2024. DOI: 10.1016/S0140-6736(24)01867-1
Global burden of bacterial antimicrobial resistance 1990-2021: a systematic analysis with predictions to 2050, Lancet, 2024. DOI: German Center for Infection Research, glossary
National (US) Human Genome Research Institute, Glossary