Staphylococcal infection

Staphylococcal infection may occur as a local or systemic infection. Staphylococci are one of the most important human bacterial pathogens, causing many diseases and the colonization of the skin and mucous membranes. They are also isolated from many animal species. Often the name reflects the location of the species most frequently found, therefore some species names refer to the niches they colonize, such as S. epidermidis, S. capitis or S. auricularis.

A few words about staphylococci

Staphylococci are gram-positive cocci, most often arranged in irregular clusters (Greek staphyle = clusters). They can also appear in various other forms, such as single cells, splits, and even chains, especially in preparations made directly from clinical material. These bacteria are relative anaerobes, usually catalase positive, do not spore and do not move. They may have an envelope.

Staphylococci belong to the family Staphylococcusin which they form a genus of Staphylococcus. At present, 32 species of staphylococci are known, of which 16 have been isolated from human infections. The most common are:

  1. S. aureus,
  2. S. epidermidis,
  3. S. haemolyticus,
  4. S. saprophyticus,
  5. St. lugdunensis

For diagnosis, staphylococci are classified based on the ability to produce the enzyme coagulase, into coagulase positive and coagulase negative species (coagulase-negative Staphylococci – CNS). Of the coagulase-positive species, infection in humans is caused by Staphylococcus aureus (the most dangerous for humans are S. aureus and coagulase-negative Staphylococcus lugdunensis). Other species, especially Staphylococcus epidermidis, which is the most common, and Staphylococcus saprophyticus, do not pose a great threat and most often cause infections in people who are predisposed to them. The first is responsible primarily for infections related to the presence of biomaterials (e.g. catheters or cardiac prostheses) – and these are most often hospital-acquired infections, and the second – for urinary tract infections, especially in women during the reproductive period – these are infections acquired outside the hospital .

How are staphylococci built?

The structure of staphylococci is typical of gram-positive bacteria. Peptidoglycan and teichic acids are the main components of the cell wall. It is they who give stiffness and shape to staphylococcal cells. S. aureus strains have ribitoloteichic acid, while S. epidermidis – most often glyceroloteichic acid. Many strains of S. aureus have a polysaccharide shell. There are 11 types of it. In the S. aureus cell wall there are many adhesin proteins that bind proteins of the host extracellular matrix, as well as the protein A characteristic of S. aureus that binds to the Fc fragment of immunoglobulins.

Staphylococcal infections – epidemiology

The main reservoir of S. aureus is man, and the most frequently inhabited place is the nasal vestibule. There are two types of nasal carriers believed to be:

  1. S. aureus: the first – permanent and concerning 20-30 percent. people;
  2. the second – transitional, covering 50-60 percent, in 20-30 percent carriage is not observed.

In addition, S. aureus is often localized in the nasopharynx, the area of ​​the anus, on the skin adjacent to the head hair and around the jaw. Increased carriage of staphylococcus may be observed in patients with type 1 diabetes, in people undergoing hemodialysis and peritoneal dialysis, HIV-positive, in people taking drugs intravenously.

Epidemiological data are mostly related to nosocomial infections, now more broadly defined as health care associated infections (HCAI).

Who is at risk of staphylococcal infection?

The increased risk of staphylococcal infection concerns patients after surgical procedures (infection of the operated site), with burn wounds, with vascular catheters and central lines, with implanted biomaterials, as well as patients undergoing long-term antibiotic therapy. The appearance of CA-MRSA, however, drew attention to the possibility of outbreaks in the community outside hospital, e.g. within the family or in a sports team.

Staphylococcal reservoirs in a hospital are both patients and staff, especially those with staphylococcal skin lesions or infections, as well as chronic carriers. Transmission occurs mainly through the hands of the staff. Infection by airborne droplets is much less frequent and is primarily important in burn wards. Infection can also come from patients with staphylococcal pneumonia.

S. aureus infections with both MSSA and MRSA can occur as single infections or outbreaks. In the case of an outbreak, the most important thing is to apply an epidemiological inquiry as soon as possible, which should be supported by molecular typing of strains from the outbreak. The greatest epidemic risk of spreading staphylococcal infection exists in the wards:

  1. neonatal,
  2. intensive care,
  3. treatment,
  4. burns.

There is also a wide reservoir of MRCNS strains in the hospital and widespread carriage, especially on the skin of patients.

Staphylococcus aureu

S. aureus, in addition to multiple virulence factors predisposing to the pathogenicity of this microorganism, developed many mechanisms of resistance to antibacterial drugs, which made it one of the most dangerous human pathogens.

What factors determine staphylococcal infection?

Staphylococcus aureus has a large variety of cellular and extracellular stimuli that are responsible for its invasiveness and causing lesions. Their activity depends on the growth phase, environmental factors and the site of infection.

A certain relationship was observed between the presence / expression of a specific virulence stimulus and the location of the infection. In the first stage of infection, they are very important surface proteins, which are responsible for the adhesion of the microorganism to the host tissues, collectively referred to as MSCRAMM (microbial surface components recognizing adhesive matrix molecules) proteins. These proteins have the ability to bind collagen, laminin, fibronectin, fibrinogen, thrombospondin, sialoproteins and vitronectin. Thanks to these proteins, colonization occurs, for example, in the area where the skin is broken, as well as bones and biomaterials, which are quickly coated with serum proteins or extracellular matrix.

Important adhesin surface proteins are also the so-called clumping factor A and B. They bind to fibrinogen and clot it non-enzymatically, which is used in the routine diagnosis of staphylococci. The polysaccharide coating plays an important role in the process of colonization of epithelia. Of the 11 isolated capsular types, types 5 and 8 are particularly common in the cause of severe sepsis. The coat, like the A protein found in the cell wall, is antiphagocytic in nature. Protein A is able to bind to the Fc fragment of immunoglobulins, especially IgG, but also IgA and IgM, blocking the classical pathway of complement activation and inhibiting phagocytosis.

Golden staphylococcus secretes a large amount of cytolytic toxins (e.g. haemolysin, leukocidin) and enzymes (coagulase, proteinases), which facilitate the spread of this bacterium in the infected organism. They cause tissue damage.

A special group are superantigenic toxins responsible for specific diseases (see below). These include exfoliathins (ETA, ETB and ETD), also called epidermolytic toxins, toxic shock toxin (TSST) and enterotoxins. An important mechanism of pathogenicity in staphylococci, especially important in S. epidermidis, is the ability to form a biofilm.

Factors of pathogenicity are regulated by various systems, the best known of which is agr. This system regulates the expression of virulence genes depending on the population density (quorum-sensing), allowing the expression of factors responsible for adhesion to the host cells in the logarithmic growth phase of staphylococci, while in the stationary phase – expression of extracellular toxins. The agrB, agrC and agrD genes polymorphism defines four staphylococcal allelic groups (from I to IV). Determining which polymorphic group of the agr locus the studied strain belongs to is one of the methods of molecular typing of Staphylococcus aureus isolates, that is, determining the degree of similarity of isolates at the DNA level, which enables the determination of the source and its propagation paths. Other regulatory systems have also been described, such as, for example, sae, arlS, srrAB, sar, RAP, and sigmaB. Some of the virulence factors are located on the so-called Staphylococcus aureus pathogenicity islands (SaPl), which as mobile elements give staphylococcal cells great plasticity and variability.

Diseases caused by staphylococcus

Staphylococcal infections may be local or systemic (invasive infections). In addition, there are characteristic diseases caused by staphylococcal toxins. Staphylococcal infections can be acquired both in-hospital and out-of-hospital.

Local infections: They include mainly skin and subcutaneous tissue infections, which are characterized by the presence of purulent discharge. The most common are:

  1. impetigo (impetigo),
  2. inflammation of the hair follicles (folliculitis),
  3. czyraki (furunculus),
  4. furunculosis,
  5. abscesses (abscessus),
  6. inflammation of the sweat glands (hidradenitis suppurativa),
  7. inflammation of the breast gland (mastitis).
  8. less often it causes inflammation of the subcutaneous tissue (cellulitis), fasciitis and erysipelas.

Necrotizing fasciitis is considered to be a specific form of cellulitis and requires immediate surgical intervention and drainage, as the course of the infection can be electrifying and often fatal. The strains causing this disease often produce a leukocidin-like toxin (Panton-Valentine leucocidin – PVL), which is probably responsible for the rapid clinical picture. Surgical site infections, which are very often accompanied by general symptoms, also belong to this category of infections.

Staphylococcus often causes bacterial conjunctivitis. In addition, staphylococcus aureus causes a variety of infections of the upper respiratory tract, such as otitis media and sinusitis, especially chronic, and especially often the sphenoid sinus. It is isolated mainly from pharyngeal swabs, but is considered a carrier state requiring no treatment. S. aureus can cause peritonsillar abscesses which require surgical intervention.

Invasive infections

The most severe invasive infections, with high mortality, include sepsis and endocarditis, related to natural valves or related to the presence of vascular and cardiac prostheses, as well as various types of vascular catheters. In intravenous drug addicts, endocarditis most often affects the tricuspid valve and is often complicated by lung abscesses. Bacteria can enter the blood bed directly, most often during trauma, surgery or through continuity, from localized primary infections. Infection of the blood bed can lead to secondary blood-borne infection in several organs, mainly in:

  1. movement organ,
  2. kidneys,
  3. lungs
  4. central nervous system.

Mortality can be as high as 50 percent.

Pneumonia it may develop as a result of aspiration of the microorganism from the upper respiratory tract or through the bloodstream. Staphylococcal pneumonia is rare, but it is one of the most severe infections of this type. They are observed more often during epidemic periods of influenza, as well as in hospitalized patients, especially in mechanically ventilated patients. The influenza virus increases the frequency of S. aureus carriage in the throat and interferes with the function of the cilia, which impairs elimination of the staphylococcus. Hematogenic pneumonia usually concern patients with infected vascular catheters, and its frequent complication is lung abscess. In recent years, staphylococcal necrotizing pneumonia preceded by flu-like symptoms is very often diagnosed in children and young adults, possibly related to the secretion of leukocidin (PVL) by these microorganisms. These are often methicillin-resistant strains (CA-MRSA, see below).

Staphylococcus aureus is also an important etiological factor acute primary osteoarthritis in all age groups, except newborns. Infection can occur both through direct introduction of bacteria, e.g. in patients with limb fractures, after surgery, through continuity from infected tissues (e.g. diabetic foot, pressure ulcers), and also through blood from other parts of the body. In adults, the infection usually affects the bones of the spine, and in children, the epiphyses of the long bones.

In addition, staphylococcus is an important etiological factor of chronic inflammation of the bones. Arthritis infections are a particularly common staphylococcal infection. Treating these infections with antibiotics usually has no effect and requires removal of the prosthesis. S. aureus is also the most common cause of bursitis.

Staphylococcal infections in the central nervous system usually occur through the bloodstream, but also through contamination from infected tissues or as a result of direct microbial entry, e.g. as a result of neurosurgical procedures. Golden staphylococcus is an important etiological factor brain abscessesespecially as a result of embolism in patients with endocarditis (mitral or aortic valve). Meningitis can occur with a brain abscess or it can be the result of bacteremia. Subdural staphylococcal abscesses have been described as a consequence of inflammation of the cranial bones, and epidural spinal abscesses as accompanying inflammation of the bones of the spine. Moreover, staphylococcus aureus is the most common cause of purulent intracranial phlebitis, which occur as a complication of sinusitis, mastoiditis or facial subcutaneous infection.

Source: J. Cianciara, J. Juszczyk, Infectious and parasitic diseases; Czelej Publishing House

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