Staphylococcus aureus

Staphylococci are roundish, non-mobile, grape-like arranged, gram-positive, non-sporeforming, catalase-positive, optionally anaerobic spherical bacteria with a diameter of 0.5-1.5 µm. They are immobile and are thus arranged in pairs or in irregular (grape-like) clusters. Optimum temperature for growth and multiplication is 30-37 °C. Staphylococcus aureus (S.aureus) has the greatest medical importance of the known staphylococcal species. S. aureus is the most important representative of the coagulase-positive staphylococci. As a colonizer of the skin and the mucous membranes of the oropharynx in humans and warm-blooded vertebrates, the pathogen plays a major role as a source of infection. Invasive S. aureus diseases can occur as superficial, deep and systemic infections. Furthermore, S. aureus can lead to food intoxications under certain conditions.

Local and systemic infections caused by S. aureus (they initially affect the skin and its appendages).

Localized infections

• Superficial folliculitis (folliculitis simplex)
• Boils
• Carbuncle
• Pyoderma
• Acute paronychia
• Empyemes
• Wound infections
• otitis media
• Sinusitis
• Purulent parotitis
• Mastoiditis, (secondary)
• Foreign body infections

Systemic infections caused by S. aureus

• Meningitis
• Osteomyelitis
• Endocarditis
• Sepsis
• Pyomyositis.
• Pneumonia caused by S. aureus can occur in the wake of an influenza A virus infection. They can also occur as nosocomial pneumonia in ventilated patients.

Toxin-mediated diseases

• Staphylococcal scalded skin syndrome (SSSS): The exfoliative toxins (ETA, ETB, ETC) produced by certain strains of S. aureus cause staphylogenic toxic epidermal necrolysis (TEN; synonym: staphylococcal scalded skin syndrome, SSSS).
• Toxic shock synd rome (TSS): TSS is based on the superantigenic effect of the Toxic shock syndrome toxin (TSST-1), but also on the toxic effect of Enterotoxin B or Enterotoxin C (also superantigens).
• Food Intoxications (Staphylococcal Enteritis, Staphylococcal Enterocolitis): They are caused by the ingestion of enterotoxins produced by S. aureus in contaminated food before ingestion.

Each strain of S. aureus has its own repertoire of virulence factors and thus has its own pathogenicity. The following virulence factors define the pathogenicity of S. aureus:

• Coagulase: extracellular enzyme which is important for the separation of pathogenic and less pathogenic species in practice. The enzyme binds to prothrombin in serum and activates the formation of fibrin from fibrinogen.
• Clumping factor: the enzyme causes fibrin to form from plasma proteins, thus enveloping the bacteria in the body's own material.

Other virulence factors of S. aureus:

• Penicillinase: special betalactamase which destroys benzylpenicillin (penicillin G), ampicillin and ureidopenicillin by splitting the betalactam ring and renders therapy ineffective. Oxacillin or methicillin, cephalosporins, penems and oxalactams are penicillinase-stable.
• Polysaccharide capsule: some strains form mucus capsules: which protects against phagocytosis. However, this phagocytosis is quickly lost under culture conditions.
• Protein A: almost all strains have a protein structure on their surface with protein A to which immunoglobulins bind with their Fc fragment. This "reverse" binding causes the bacterium to evade phagocytosis, as the Fc fragment is no longer available as a receptor for macrophages. This property can be used in laboratory diagnostics to identify S. aureus.
• Fibronectin binding protein: The bacteria are coated with endogenous fibronectin.
• Collagen binding protein: The bacteria are coated with the body's own collagen.
• Intercellular adhesin: Almost all staphylococci, including S. aureus strains, can produce intercellular adhesin from linear poly-N-acetylglucosamine. Such mucus substances are the basis for biofilm formation; microcolonies can grow within the mucus layer and are safe from the body's own defence.
• Extracellular adhesion protein (Eap). The protein binds to ICAM1 receptors of endothelial cells and thus hinders the binding of leukocytes. This inhibits the marginalisation and also the migration of the defence cells to the site of infection.
• Staphylokinase/Fibrinolysin: Through fibrinolysin formation S. aureus can dissolve a self-generated fibrin clot. While at the beginning of a S. aureus invasion into the human body, fibrin precipitation protects the pathogen, S. aureus can dissolve the fibrin barrier after appropriate multiplication in order to spread further in the tissue. It also neutralizes the antimicrobial effect of defensins.
• Hyaluronidase: With this depolymeridase, the pathogen can spread in the tissue by dissolving the intercellular substances.
• Haemolysins: S. aureus can produce four different haemolysins (alpha-, beta-, gamma- and delta-haemolysin), which lead not only to the dissolution of erythrocytes but also of parenchymal cells.
• Leukocidin: An important pore-forming, phage-encoded toxin that destroys macrophages and granulocytes. Strains that possess the lukF/lukS gene for this pantoin-valentine toxin (PVL) are highly pathogenic because they cause progressive wound infections and also abscessing pneumonia, even in young adults, by destroying the non-specific cellular defence. They are often also methicillin-resistant (MRSA).

• Exfoliatin toxins: Biochemically, 2 proteins can be distinguished (exfoliatin A and B). It is an epidermolytic toxin produced relatively rarely (about 5%) by S. aureus strains. A serine protease which cleaves the desmosomes between the cells in the stratum granulosum of the skin so that blisters form, so-called Staphylococcal scalded skin syndromes (SSSS).

• Enterotoxins: 5 enterotoxins (A-E) can be detected. Only a few strains of S. aureus (approx. 5 %) can form one or more of these enterotoxins. These enterotoxins are heat stable, so that they are an extremely important factor in food hygiene (food poisoning!). The most frequent sources of poisoning are milk and egg products in all variations as well as pork.
• Toxic Shock Syndrome Toxin (TSST): TSST-1 is produced by only about 1% of the S. aureus strains. It acts like a "superantigen", i.e. many lymphocytes are stimulated to produce cytokines, regardless of their antigen specificity. These lead to the picture of toxic shock syndrome (TSS).
• Penicillin-resistant strains: Most strains of S. aureus form a penicillinase, a beta-lactamase and are therefore resistant to beta-lactam antibiotics, but not to flucloxacillin, a semi-synthetic derivative. The so-called MRSA (methicillin-resistant S. aureus) strains have acquired the ability to form a slightly modified penicillin binding protein 2 (PBP2a) by acquiring the mecA gene (more rarely also the mecC gene). Betalactams (with the exception of ceftarolin and ceftobiprole) can no longer bind to this protein and are therefore no longer effective.

Approximately 30% of all people permanently harbour S. aureus on the skin or on the mucous membranes (nasal mucosa). About 30 % of all people are passagger colonized.

Carrier rates are higher in people who are frequently exposed to S. aureus, such as health care workers, patients with larger wounds, patients with tracheotomies or recumbent catheters, dialysis patients, diabetics, patients with atopic diathesis (especially atopic dermatitis), patients in need of care and i.v. drug addicts.

Methicillin-resistant S. aureus (MRSA) plays a special role as a nosocomial pathogen, triggering epidemics, especially in intensive care units. Such strains, which usually also carry many virulence factors, are particularly common in hospitals: ha- (hospital acquired) MRSA. However, such strains also occur in the population: ca- (community acquired) MRSA. Especially people who have contact with animals (especially with pigs) harbour the so-called la- (livestock acquired) MRSA. In addition to beta-lactam resistance, such MRSA often have a parallel resistance to many other antibiotics, especially quinolones, so that they are also called "multiresistant S. aureus".

In case of intoxication with orally ingested staphylococcal toxins, the incubation period is a few hours (about 2-6 hours), in case of infection 4-10 days. In persons with colonization, endogenous infection can develop even months after initial colonization. The diseases caused by S. aureus including MRSA can be divided into localized or generalized pyogenic infections and toxin-mediated diseases (see below staphylococcal infections):

For S. aureus as an infectious agent, humans are the main reservoir. Staphylococci are relatively resistant to the lysozyme in the secretions (sweat, sebum). But animals can also be affected. In humans, the nasopharyngeal cavity is preferentially populated. From here, the opportunistic pathogen can be spread via hand contact, directly via droplet emission or indirectly via dust and cause nosocomial infections.

S. aureus can contaminate hydrophobic surfaces such as plastic materials and stainless steel alloys. There is a clear risk of infection of catheters and shunts as well as joint replacement and stabilisation measures. MRSA is just as virulent with regard to invasive infections as S. aureus in general. Only due to delays in adequate therapy is the infection more lethal, e.g. in sepsis.

In in-patient facilities, the transmission occurs in most cases through the hands of e.g. nursing and medical staff. In the case of nasal colonisation, the pathogen can spread from the nasal vestibule, the actual reservoir for S. aureus, to other areas of the skin (including the hands, axilla, perineal region) and mucous membranes (e.g. throat). The main predisposing factors for S. aureus infections are: diabetes mellitus, need for dialysis, presence of foreign bodies (plastic materials such as venous catheters, urethral catheters, tracheostoma, metal alloys such as joint replacements), injuries to the skin as an external barrier, immunosuppression or certain infections, e.g. with influenza A viruses.

Certain strains of MRSA have a special ability to spread epidemically. This ability to spread, known as "epidemic virulence", characterises a complex behaviour of S. aureus strains, which is determined by factors of the strains themselves (resistance, endowment with pathogenicity factors; so-called "intrinsic virulence") and factors of their environment (hygienic and antibacterial measures). The degree of ability to spread also determines whether individual diseases or outbreaks occur.

Laboratory diagnostics: The basis of diagnostics is the detection of the pathogen. In order to obtain the result "MRSA", the species diagnosis S. aureus must always have been confirmed for the respective isolate and its resistance to oxacillin or cefoxitin must have been proven.

Species diagnosis for S. aureus (differentiation from coagulase-negative Staphylococcus spp. CNS):

• Phenotypic: Classical reference methods are the tests for coagulase (free enzyme, not to be confused with the clumping factor, specificity: 99,9%) and for heat-resistant DNase. As a quick agglutination test (originally only for the detection of the agglutination factor), various tests are available. Test kits are available on the market.
• Genotypic methods: The reference method for the diagnosis of staphylococcal species is the sequencing of 16S rRNA. The gene for heat shock protein 60 (hsp60) and for the beta-subunit of RNA polymerase (rpoB) also shows an inter-species polymorphism sufficient for differentiation of staphylococcal species. For the identification of S. aureus by PCR, the detection of a S. aureus-specific sequence missing from the CNS and the nuc gene (thermostable nuclease) have high specificity and sensitivity.
• Determination of resistance: Phenotypic methods for the determination of resistance: The reference method is the determination of the minimum inhibitory concentrations (MIC).
• Special attention is required for the detection of S. aureus with intermediate sensitivity to glycopeptides (GISA). As the GISA phenotype is unstable, only fresh primary cultures should be used for detection. An increased MIC for vancomycin (≥ 4 mg/l) and for teicoplanin (≥ 8 mg/l) are indications for the presence of GISA.
• Genotypic methods for resistance determination: The classical reference method is the PCR, which is not only established for mecA, but has also been established as a multiplex PCR for the detection of 8 additional resistance genes: The detection of mecA is also possible by real-time PCR.
• Molecular biological methods to diagnose species and resistance at the same time: Recently, test kits have become available as macroarrays based on multiplex PCR methods, which include species differentiation of S. aureus in addition to mecA gene detection.
• Typing of S. aureus: The SmaI macro restriction patterns ("pulse field gel electrophoresis") are considered the gold standard for typing with respect to discrimination ability. Multilocus sequence typing (MLST) is used to elucidate evolutionary relationships and to clearly assign clonal complexes and clonal lines. Sequence-based typing methods have the advantage of the unambiguous definition of types and thus the absolute comparability of the results, which can also be transmitted electronically with comparative ease.
• spa-typing: Significantly less effort and congruence with the results of MLST requires the one based on the polymorphism of the X-region of spa (encoding the protein A gene). Software and databases are available for fast and essential analysis. (www.seqnet.org).

For the treatment of infections with oxacillin-sensitive S. aureus, penicillinase-resistant penicillins (e.g. flucloxacillin) as well as 1st generation cephalosporins and inhibitor-protected penicillins are considered the agents of choice, in generalising infections combined with an aminoglycoside. Alternatives are combinations with rifampicin. For the treatment of soft tissue skin infections tigecycline and daptomycin (European approvals) have recently become available. For infections with MRSA and severe S. aureus infections in general, beta-lactam antibiotics should generally not be used.

Combinations of glycopeptides with rifampicin, with clindamycin or gentamicin (depending on the antibiogram) are indicated here.

Alternative: Fosfomycin and fusidic acid are available as further combination partners. Alternatively, linezolid from the oxazolidinone group of substances is available as monotherapy (oral or i.v. application possible).

Alternative: If necessary, the combination of rifampicin and cotrimoxazole is also suitable for the treatment of soft tissue skin infections.

Remediation of a MRSA colonisation: Standard procedure for the remediation of a nasal MRSA colonisation is the use of Mupirocin nasal ointment.

For the sanitation of an infestation of the throat or a colonization of the skin with MRSA, additional disinfectant mouthwashes or full body washes of the intact skin including the hair with antiseptic soaps and solutions with proven effectiveness are recommended.

To check the success of the treatment, control swabs (e.g. nose, throat, groin, perineal, if present wound, access to central venous catheters and original place of detection) should be taken at the earliest 3 days after completion of the remedial measures or after therapy.

The share of MRSA in S. aureus from hospital infections increased from 15 to > 20% between 1998 and 2004. 72% of all MRSA from Central Europe are resistant to erythromycin, 93.89% resistant to quinolones, 66% resistant to clindamycin. Due to the resistance mechanism, erythromycin-resistant S. aureus must always be considered potentially resistant to clindamycin and telithromycin (therefore these preparations are not therapeutic alternatives). Gentamicin resistance occurs in 17% of total S. aureus and 17% of MRSA; there is potential cross-resistance to amikazine and netilmicin. In MRSA from Germany, the frequencies of resistance to rifampicin are 2.0%, to fusidic acid sodium 4.6%, to trimethoprim/sulfonamide 3.6%, to mupirocin 1.7%. Resistance to linezolid was not found in 19,048 MRSA tested up to 2004 from Germany.

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