Staphylococcus aureus is the most common Staphylococcus strain in human and animal pathology. Along with Escherichia coli, it shares the dubious distinction of being the leading cause of nosocomial infections (infections contracted in hospitals). In France, S. aureus also tops the list of bacteria responsible for food poisoning.
Diseases and susceptibility factors
Staphylococcus aureus can infect all organs. It is important to note that 30 to 50% of the population are healthy carriers of Staphylococcus, i.e. the bacteria are present on the skin or in the external mucous membranes, especialy nasal, with no detrimental symptoms. This type of carriage is natural since Staphylococcus is part of the bacterial flora of the skin and mucous membranes. Symptomatic infections (i.e. those resulting in disease) can occur in various cases:
- When the skin barrier is broken (wounds). The bacterium can then develop within the lesion leading to a skin infection whose degree of severity depends upon the bacterial strain causing the infection, the delay in applying treatment, and the patient’s general state of health. These infections are often suppurative (pus-forming), as a result of bacterial proliferation. Among these diseases, superficial or deep folliculitis, boils and carbuncles are the most common. Some Staphylococcus strains, producing toxins such as exfoliatin, can cause specific skin diseases such as scalded skin syndrome or bullous impetigo in young children.
- During surgical procedures. As staphylococci are very widespread, internal infections can develop. In a great many cases, the bacterium originates from the patient’s own flora and develops its pathogenicity within the patient's body. Disease development will depend on the organ infected, and can lead to osteomyelitis (inflammation and infection of the bones), and various visceral infections, such as endocarditis (inflammation of the heart), pneumonia (lung infection) or pyelonephritis (kidney infection). Once again, these diseases can be exacerbated due to the production of specific toxins by the bacteria, whose damaging effect on the tissues of the infected host will aggravate the clinical profile (necrotizing pneumonia due to the Panton-Valentine toxin or staphylococcal toxic shock syndrome due to the TSST-1 toxin).
- During diseases or treatments that lead to a weakened immune system, and in individuals with diabetes (combination of lowered immune defenses and high blood sugar levels).
Although superficial and localized Staphylococcus infections are often benign, they must be taken into account as blood-borne spread of the bacterium can lead to secondary infections.
Staphylococcal food poisoning is not a progressive infection but is caused by food that has been in contact with a Staphylococcus strain producing enterotoxins. These toxins can be found in raw or cooked food (they are heat resistant), causing vomiting, diarrhea and stomach cramps following ingestion. These symptoms generally disappear several hours later without the need for any treatment.
Staphylococci are classified according to their ability to induce blood clotting. Staphylococcus aureus is a coagulase-positive Staphylococcus. Although S. aureus is the strain involved in the most serious human diseases, other staphylococci that are coagulase-negative can also cause infections. These are opportunistic pathogens requiring a point of entry into the body and a weakened immune system to cause infections. They are generally considered less dangerous than S. aureus. This is particularly the case of Staphylococcus epidermidis, a human commensal bacterium and part of the mucocutaneous flora of virtually the whole population. This Staphylococcus can nevertheless become pathogenic under certain circumstances, for example in immunodeficient patients (AIDS patients, those undergoing radiotherapy or chemotherapy, or newborn babies) or when foreign materials are inserted into the body (joint prostheses, prosthetic heart valves, probes, catheters, etc.). The implanted material can then become contaminated by strains from the mucocutaneous flora of the patient or hospital staff. This type of infection is mainly nosocomial (hospital-acquired) or iatrogenic (resulting from a medical procedure).
Among the staphylococci responsible for human infections, there is also Staphylococcus saprophyticus, associated with acute cystitis, primarily in young women. It is the second leading cause of bacterial urinary infections after Escherichia coli.
Although Staphylococcus aureus can also be associated with animal diseases (mastitis in cows, ewes and goats), some staphylococci are more specific to animals. The most common of these are Staphylococcus intermedius (causing canine furunculosis) and Staphylococcus hyicus (leading to exudative dermatitis in pigs). These bacteria rarely cause human infections, but S. intermedius can be spread to humans through bites (infected wounds) and lead to bacteremia (bacteria in the blood). Cases of human bacteremia due to S. hyicus have also been observed, mainly through contact with pig farms.
Infections on the rise
Around 30 to 50% of humans are healthy carriers of S. aureus with no detrimental symptoms. In hospitals, infections can occur in patients with weakened immune systems, or if there is a breach in the skin/mucosal barrier, allowing strains carried by patients or staff to enter the body. The rising number of staphylococcal infections can be linked to the increasing number of immunodeficient patients (with lowered immune defenses) as well as the growing number of invasive procedures rupturing the skin/mucosal barriers, such as surgery or the insertion or catheters, probes, and prostheses.
Hygiene as a first line of defense
The only way to limit the spread and persistence of hospital-acquired strains is through patient isolation and strict implementation of hygiene measures.
Treatment and prevention
In hospital environments, stringent hygiene and isolation measures are vital to contain the spread of these bacteria. Antibiotics are currently the preferred treatment, particularly in the early stages of infection. However, the recent emergence of vancomycin-resistant strains suggests the likelihood of a shortage of treatment options, although several vaccine approaches are currently being studied. Methicillin-resistant strains of staphylococci, or MRSA, are among the most common antibiotic-resistant bacteria in the hospital, accounting for around 27% of Staphylococcus aureus infections (National survey on the prevalence of nosocomial infections and anti-infection treatments in healthcare institutions, France, May-June 2017). The use of viruses that specifically infect and destroy bacteria, called bacteriophages, is used in certain countries such as Russia or Georgia to fight Staphylococcus aureus infections.
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