Nine of the Most Harmful Bacteria to Humans

Although not all bacteria are deadly, some dangerous bacteria can really harm your body. They can also provoke different illnesses if not detected and treated in time. 

Reviewed and approved by the doctor Maricela Jiménez López.

Written by Francisco María García

Last update: 04 July, 2023

At the beginning of the year, the World Health Organization (WHO) published a list of the most harmful bacteria to humans. Among them are the ones that took the most human lives.

It’s important to know what they are, and the symptoms and associated illnesses they cause in order to maintain your own health and that of your family. In fact, the majority cause common diseases that put lives at risk if left untreated.

The list also signals the necessity that exists for medical laboratories to find new antibiotics to combat each threat in an effective way.

1. Acinetobacter baumannii

This is perhaps the most resistant microorganism around today. As a matter of fact, thousands of people die each year as a result of infection by this bacterium. It has multiple mechanisms of resistance to the drugs that are supplied for its treatment.

According to multiple investigations, the bacterium has the ability to synthesize a large number of enzymes that inhibit antibiotics. It also generates changes in the proteins on its surface and organelles to prevent the action of drugs. As if that were not enough, it has different expulsion pumps to eliminate the antibiotics that enter its cytoplasm.

• The associated illness usually starts with an aggressive variety of pneumonia accompanied by a severe infection in the urinary tract. However, the bacteria can also cause wound infections and abscesses in many organs.
• Acinetobacter baumannii has a critical priority level. For this reason, laboratories have already been urged to focus on discovering better medicines to combat it.

2. Pseudomonas aeruginosa

The attacks of this pathogenic agent are extremely varied. Therefore, it’s sometimes difficult to detect.

• This dangerous bacteria is associated with immunocompromised patients. For example, those with AIDS or cystic fibrosis. A pseudomonas aeruginosa infection is also associated with people who are hospitalized, mechanically ventilated, or severely burned.
• The bacterium can affect multiple organs, depending on its portal of entry. Among the most frequently affected structures are the skin, lungs, urinary tract, ears, and heart valves.

Its danger lies in the inability of people with immunodeficiency to fight the infection, which worsens their clinical condition. It also causes pulmonary complications and severe infections in people who are already compromised by other viruses.

3. Enterobacteriaceae

Enterobacteriaceae is a family of microorganisms that cause diseases that basically affect the digestive and excretory systems (the colon, stomach, and intestines).

• This group of dangerous bacteria is responsible for childhood gastroenteritis, salmonella, typhoid fever, and bacillary dysentery. They can also cause other conditions such as severe urinary tract infections when they colonize other tissues.
• In the past, it claimed a high number of lives during episodes of the plague.
• Many of these dangerous bacteria have a strong resistance to penicillin.

The Enterobacteriaceae family includes more than 60 different genera, around 25 of which are clinically relevant to humans, according to a publication in Clinical and Microbiology Reviews. Among the most relevant genera, Escherichia, Salmonella, Shiguella, Proteus, Klebsiella, Serratia, and Enterobacter stand out.

4. Enterococcus faecium

This bacterium can live in the human intestine without damaging it, but under certain conditions, it can become a harmful agent.

Enterococcus faecium is responsible for neonatal meningitis, an infection that causes headache, stiff neck, photophobia, and fever. Fortunately, it’s an extremely rare etiological agent. In fact, it only represents between 0.3 and 4 percent of all neonatal meningitis, according to a publication in the journal, Enfermedades Infecciosas y Microbiología Clínica.

• It currently has a high priority level but isn’t paramount.
• It’s resistant to certain antibiotics.
• It’s capable of generating multiple conditions. For example, urinary tract infections, bacteremia, endocarditis, intra-abdominal, pelvic, or skin infections.

5. Staphylococcus aureus

Some bacteria can trigger infections and diseases in various organs of the human body. This is the case with Staphylococcus aureus. It can cause problems in the skin, respiratory, and digestive systems.

Among the symptoms caused by this microorganism, the following stand out:

• Sepsis
• Cellulitis
• Meningitis
• Pneumonia
• Conjunctivitis

In addition, the vast majority of infections that occur in hospital centers are attributed to this bacteria. In fact, a study published in the journal, Medicina Clínica, found that up to 20 percent of infections acquired in intensive care units involve Staphylococcus aureus.

6. Helicobacter pylorus

Helicobacter pylorus is a bacterium that lives exclusively in the stomach and generally infects the gastric mucosa, causing inflammation. Additionally, it can cause the following diseases:

• Gastritis
• Lymphomas
• Septic ulcers
• Colon irritation

In some cases, people become infected with this bacteria without developing any symptoms throughout their whole life. It’s transmitted through saliva, feces, and dental plaque.

Helicobacter pylori are one of the most dangerous bacteria for humans. This is due to their ability to adapt to hostile environments. In addition, according to the Revista Colombiana de Gastroenterología, the bacteria is resistant to multiple antibiotics such as amoxicillin, metronidazole, clarithromycin, furazolidone, levofloxacin, and tetracyclines.

7. Campylobacter

This is probably the largest transmitter of food poisoning worldwide. Among its common symptoms are:

• Fever
• Diarrhea
• Vomiting
• Abdominal pain

In some cases, this bacteria can cause arthritis or Guillain-Barré syndrome. While it can be dealt with by conventional antibiotics, it’s also one of the most common and dangerous kinds of bacteria.

It can be transmitted by ingestion of contaminated food or water and by contact with carrier animals.

8. Salmonella

Salmonella is a genus that includes many bacteria belonging to the family Enterobacteriaceae, which cause salmonella infection. These bacteria are also capable of producing other conditions such as typhoid fever, bacteremia, or suppurative infections.

• The incubation period ranges from a week to almost a month.
• Symptoms range from fever, vomiting, and diarrhea to an upset stomach.

Once incubated, the symptoms could last up to two months, in the worst-case scenario. The bacteria can survive for long periods of time in dairy products such as butter and milk, as well as in chocolate.

9. Neisseria gonorrhea

Some dangerous bacteria harmful to humans are also spread through sexual contact. This is the case with Neisseria gonorrhoeae infection.

• As its name indicates, this microbe is the cause of gonorrhea.
• Simple antibiotics such as ampicillin and penicillin can counteract it when it’s discovered in time.

However, the real danger of Neisseria gonorrhoeae infection lies when it’s not discovered in time. That’s because the bacteria can enter the female reproductive system and cause pelvic inflammatory disease. This can affect fertility.

Gonorrhea during pregnancy also represents a danger, despite being rare. Multiple studies associate infection with premature rupture of the membranes, premature delivery, low birth weight, and contamination of the baby during delivery.

Make sure to get regular check-ups

Make sure you have an annual (at least) check-up to see how your health is and if there’s anything you need to improve.

Always maintain good lifestyle habits and, in case of any persistent symptoms and discomfort, consult your doctor as soon as possible. Indeed, early detection and treatment of any disease, including those caused by the most dangerous bacteria are vital.

Finally, remember that you should avoid self-medication, as this may only cause complications. In fact, if you want to take any medication, it’s best to consult your doctor.