Staphylococci bacteria are involved in a variety of diseases and are often responsible for hospital-acquired infections. They naturally live in the skin flora and mucosa of humans and animals. They can also be found in the environment (in water, soil, air, food, and other objects). Treating infection is difficult because many strains – between 20 and 50%, according to hospital data – show multiple resistance to antibiotics.





Staphylococcus aureus

Staphylococcus aureus, also known as “golden staph”, is the most common Staphylococcus strain in human and animal pathology. Along with Escherichia coli bacteria, it shares the dubious distinction of being the leading cause of nosocomial infections (infections contracted in hospital). S. aureus is also the second most common cause of food poisoning in France, after Salmonella.



S. aureus can cause suppurative (pus-forming) or toxin-producing infections, which are very difficult to treat because most strains have now developed multiple resistance to antibiotics.


In humans

In cases of food poisoning, bacterial strains multiply in food, releasing toxic substances known as enterotoxins that cause violent, repeated vomiting, often accompanied by diarrhea. Patients usually recover in one or two days without after-effects.


S. aureus can also lead to various skin infections, including furuncles (boils), folliculitis, paronychia, impetigo, and breast abscesses in breast-feeding women. Mucosal infections are also common and can affect the eyes (conjunctivitis), ears (otitis), genital region (endometritis or salpingitis), or airways (pneumonia or pleurisy).


All these skin and mucosal infections may develop complications and result in septicemia. In such cases, progression may be fulminant and acute, involving a range of secondary regions (including the heart valves, bones, joints, kidneys, or brain).


Staphylococcal toxic shock syndrome (rare but often fatal), and its minor form, staphylococcal scarlet fever, are caused by strains that produce the staphylococcal toxic shock syndrome toxin (TSST-1) or enterotoxins. Extensive exfoliative dermatitis, and its minor form, localized bullous impetigo (also known as staphylococcal scalded skin syndrome (SSSS)), are caused by exfoliatin-producing strains. Serum antibodies can block these highly immunogenic toxins (superantigens).


In animals

Staphylococcus aureus, together with related species such as Staphylococcus intermedius and Staphylococcus hyicus, are major pathogens for animals: furunculosis in dogs (S. intermedius) is often recurrent and can be difficult to treat. Mastitis in cattle, sheep, and goats (usually S. aureus), sheep abscess disease, also known as Morel’s disease (S. aureus subsp. anaerobius), and exudative dermatitis in piglets (S. hyicus) can have a severe economic impact.


Infections on the rise

Around 30% of healthy humans are carriers of S. aureus and present no symptoms. Infection can occur in hospitals 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 a weakened immune system) and also the growing number of invasive procedures that damage the skin/mucosal barrier, 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 patient isolation and constant compliance with hygiene regulations. 


Staphylococci in the skin and mucosal flora


Potentially pathogenic bacteria

Unlike Staphylococcus aureus, “white” or “coagulase-negative” staphylococci, mainly Staphylococcus epidermidis (70%), are naturally found in the skin and mucosal flora in humans.


But these staphylococci can become pathogenic in specific 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 or heart valve prostheses, probes, catheters, etc.). These implants can become contaminated by strains from the skin and mucosal flora of the patient or hospital staff.


These bacteria, which only become pathogenic in specific conditions, are known as “opportunistic” bacteria.



These opportunistic strains can lead to severe infections (such as septicemia, endocarditis, pyelonephritis, meningitis, and osteomyelitis), most of which are nosocomial (hospital-acquired). As with strains of Staphylococcus aureus, white staphylococci strains isolated in hospitals often exhibit multiple resistance to antibiotics (50 to 70% of strains).


Staphylococcus saprophyticus differs from other staphylococci in many respects. It causes acute cystitis, mainly in young women with no particular immune weaknesses. After Escherichia coli, Staphylococcus saprophyticus is the second leading bacterial cause of urinary infections.


Treatment and prevention

In hospital environments, strict 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 has led to a shortage of treatment options, although vaccine approaches are currently being studied.


At the Institut Pasteur

In the Biology of Gram-Positive Pathogens Unit, led by Patrick Trieu-Cuot, the Signaling and Pathogenesis of Staphylococci team, headed by Tarek Msadek, is developing several areas of research:

- bacterial responses to environmental variations and their role in Staphylococcus aureus pathogenesis and interactions with the host;
- the impact of variations in the bacterial envelope on host–pathogen interactions, the formation of biofilms, and the innate immune response;
- the characterization of resistance to antibiotics and antimicrobial peptides.


Illustration – Copyright Institut Pasteur
Caption – Staphylococcus aureus (“golden staph”) in scanning electron microscopy.


October 2013



The Institut Pasteur teams


Research units

> Biology of Gram-Positive Pathogens Unit
led by Patrick Trieu-Cuot