Which are the main diseases due to bacteria?

A substantial number of infections are caused by species of bacteria that are present on or in many people without producing disease.

The bacteria responsible for the most common conditions
The main diseases requiring notification

Very often, non-pathogenic bacteria become pathogenic when they end up in a place where they are not supposed to be, or when their numbers have increased unduly due to a weakening of the immune system. There are however also strains of bacteria that always cause disease, such as the enterohemorrhagic strains of Escherichia coli bacteria.

The bacteria responsible for the most common conditions

Streptococcus pneumoniae. Often referred to as “pneumococci”, these bacteria are normally present in the nose and pharynx of many people without causing any infection. It is therefore the healthy carriers in the population who transmit the bacteria to the elderly, children or other susceptible people, among whom serious disease may then occur. In Switzerland, pneumococci cause around one thousand serious infections (in the bloodstream or as meningitis) each year, as well as several thousand cases of pneumonia. These bacteria are also responsible for many self-resolving infections, including bronchitis and ear infections.

Staphylococcus aureus is present as part of skin flora in around one third of the human population, without causing disease. Depending on the strain and the immune system of the infected person, S. aureus can infect the skin and bone, and even the bloodstream. In hospitals, it is the most frequent cause of post-surgical infections. In animal husbandry, it can trigger various infections, particularly in the udders of dairy cows (mastitis).

Escherichia coli (E. coli) is an enterobacterium that lives in the intestines of humans and animals, usually without causing disease. E. coli is even useful and helpful. But these bacteria can cause infections if they go somewhere else in the body, such as the lower urinary tract, the abdomen or the brain in the case of new-borns. Certain pathogenic strains of E. coli (for example the toxin-producing O157:H7) are known to pass from animals to humans through food products. They can cause fever, nausea, vomiting, stomach cramps and diarrhoea, which can be fatal in some cases.

Klebsiella pneumoniae is another species of enterobacterium commonly present in the digestive tract of healthy humans and animals. It is nevertheless one of the most common culprits for causing hospital-associated infections as well as urinary tract and respiratory tract infections, especially pneumonia. In new-borns, Klebsiella pneumoniae can infect the bloodstream, leading to increased mortality rates. This type of bacteria also has the ability to easily acquire multiple forms of antibiotic resistance.

Also worth a mention are Acinetobacter baumannii and Pseudomonas aeruginosa, which are among the so-called hospital pathogens, because they mainly produce infections within hospitals and care home. Resistance of these two bacterial species to last-resort antibiotics such as carbapenems and polymyxins is increasing, nationally and globally.

The main diseases requiring notification

In Switzerland, a compulsory reporting system enables doctors and laboratories to declare certain infectious diseases to each canton’s Chief Medical Officer or to the Federal Office of Public Health (FOPH). This reporting allows early detection of potential public health problems, and therefore also early preventive measures if necessary. There are over 40 different diseases on the list of reportable diseases.

For bacterial infections that are reportable, the numbers of annually reported cases shown below are estimates based on the average of the years 2017 - 2019.

Bacterial agents transmitted by the respiratory route

Pneumococci: invasive diseases, around 900 reported cases per year
Tuberculosis: around 500 reported cases per year.
Legionella: around 500 reported cases per year.
Haemophilus influenzae (a type of bacteria that has nothing to do with the virus responsible for the flu): invasive diseases, around 100 reported cases per year.
Meningococci: invasive diseases, around 50 reported cases per year.

Bacterial agents transmitted by the oral route

Campylobacter: around 7,500 reported cases per year.
Salmonella: around 1,500 reported cases per year.
Infections with enterohemorrhagic E. coli : around 900 reported cases per year.
Shigella: around 200 reported cases per year.
Listeria: around 50 reported cases per year.
Typhoid and paratyphoid fever (S. typhi, S. paratyphi): around 20 reported cases per year.

Bacterial agents transmitted by the sexual or contaminated blood route