Staphylococcus aureus

Staphylococci are gram-positive bacteria which appear in clusters on Gram stain.

A nasal commensal in almost half of the general population.

A major cause of bloodstream infection in both the community and healthcare settings and has significant morbidity and mortality.

Can cause a range of infections including: cellulitis, abscesses, osteomyelitis, pneumonia, bacteremia, and endocarditis.

The main site of colonization is the anterior nares although additional sites such as the axilla and groin are increasingly being recognized.

Spread of the bacteria between individuals occurs primarily via direct contact, which is particularly important in a healthcare environment.

Can propagate through fomites and airborne routes.

Possesses several virulence factors including cytotoxins, enzymes, and super antigens that contribute to both direct and indirect effects of infection.

Has structural components and secretes products that can efficiently target human tissues to evade host defense mechanisms.

Is adaptable and robust and has the ability to survive in a variety of environments.

Has a vast number of surface proteins and capsular polysaccharides that facillitate adhesion, colonization of tissues, immune evasion, and ability to form biofilms.

Can protect itself against host defense systems and antibiotics.

Has a propensity to infect and destroy normal, healthy tissues.

Uses hemoglobin from red blood cells as it source of iron essential for survival and virulence.

A commensal bacterium that colonizes the skin and mucosa surfaces including the nares, pharynx and vagina.

Approximately 25-40% of people are colonized.

Carriers are divided into persistent and intermittent types.

Patients on hemodialysis, intravenous drug abusers, patients with HIV, patients undergoing surgery, and patients with diabetes have a higher rates of carriage.

Males and elderly also at increased risk for infections.

Nasal carriers and bacterial burden are risk factors for future infection.

CDC estimates about one third of general population carries staphylococcal organisms in their nares, usually without evidence of disease.

About 2% of the general population are methicillin-resistant staphylococcal aureus (MRSA) carriers.

Disease caused by a breach in the mucosa barrier or skin allowing multiplication of bacteria in subcutaneous tissues with skin and soft tissue infections.

Virulent pathogen with ability to cause metastatic foci of disease.

Both methicillin-resistant and methicillin-susceptible strains account for more healthcare associated infections than any of the pathogen.

Staphylococcal aureus is the most common form of ventilator associated pneumonia and surgical site infections iand is the second most common cause of central catheter associated bloodstream infections.

Can infect any organ system.

Associated with skin and wound infections, bloodstream infections, pneumonia, osteomyelitis, endocarditis, lung abscess and pyomyositis.

S. aureus strains also produce enzymes and exotoxins that likely cause or increase the severity of certain diseases. 

 

Such diseases include food poisoning, septic shock, toxic shock syndrome, and scalded skin syndrome.

May be associated with catheter related infections, with local site infections, thrombophlebitis, catheter tunnel infections and central venous catheter related bloodstream infections.

Often colonizes the anterior nares, particularly among type I diabetics, drug abusers and patients on hemodialysis.

Nasal colonization is a major risk factor fro subsequent invasive infection.

Patients with inflammatory skin conditions and atopic dermatitis commonly are colonized by this organism.

In patients with atopic dermatitis S. aureus is colonized in 60 to 90% of patients.

Nasal carriers of high numbers of Staphylococcus aureus organisms increase health associated infectionswith this agent 3 to 6 times the risk noncarriersor low-level carriers.

Greater than 80% of healthcare associated staphylococcal aureus infections or endogenous (Wertheim HF).

In Europe greater than 20% of patients admitted to nonsurgical wards are nasal carriers and 80% of bacteremic infections in carriers attributable to anterior nares colonization.

Mupirocin applied intranasally is an effective agent to decolonize and prevent invasion of staphylococcal aureus infections in patients on long-term dialysis.

ICU decolonization involving nasal mucopirocin and daily chlorhexidine gluconate bathing, reduced MRSA clinical cultures by 37% and all- cause blood streams infections by 44% in 43 hospitals randomized REDUCE MRSA trial.

Mupirocin nasal ointment in randomized trials has not reduced surgical site infections in orthopedic and general surgery patients, although it may be effective in preventing health-care associated infections and carriers of this organism (Perl TM, Kalmeijer MD).

In a randomized controlled double blind study utilizing PCR to identify nasal carriers of S.aureus and using mupirocin ointment to the nostrils and chlorhexidine gluconate soap to the skin the number of surgical site infections with his agent can be reduced (Bode LGM).

The size of the effect of decolonization suggests a reduction in the risk of hospital associated Staphylococcal aureus infections by almost 60%(Bode LGM).

The mupirocin-chlorhexidine treatment was associated with a 3.4% surgical site infections compared to 7.7% in the placebo group and this effect was most pronounced for deep surgical site infections, it also reduced mean hosptial stay by 2 days(Bode LGM).

A meta-analysis assessing the use of nasal mupirocin in surgical patients who were carriers Staphylococcal aureus , the eradication of this bacteria reduce the rate of hospitalization associated infection by an estimated 45% (van Rijen M).

In a pooled analysis eight studies utilizing intranasal mupirocin was associated with a significant reduction in the infectious rate(van Rijen M).

Can colonize hospital workers and be the cause of hospital epidemics of infection.

Nasal carriers of Staphylococcus are likely to have extra nasal sites that are contaminated with the same strain and that such carriers are at increased risk for endogenous Staphyloccus aureus infections.

Nasal iodophor antiseptic does not meet criteria to consider noninferior to nasal mupirocin antibiotic for the outcome of staph aureus clinical cultures in adult ICU patients.

Disseminated foci of infection more common in community acquired bacteremia or in patients with sepsis without a known primary source.

Community acquired and hospital acquired staphylococci bacteremia are associated with mortality rates as high as 43%.

Associated with toxin mediated diseases which can be either directly or indirectly due to produced toxins.

Can be associated with this scalded skin syndrome and staphylococcal toxic shock syndrome caused by toxins elaborated directly by the organism.

May be the source of food poisoning due to ingestion of a preformed toxin and is characterized by a short incubation period of several hours prior to symptom onset.

Infrequently a cause of community acquired pneumonia.

Typically associated with influenza viral infections or influenza like illnesses.

Secondary pneumonia with S. aureus can be a complication of influenza with development of necrotizing pneumonia with a fatality rate of 33%.

About 57% of nosocomial isolates are resistant to methicillin (MRSA).

Predictors of bacteremia with methicillin resistant organisms include previous antibiotic exposure, history of hospital onset or exposure and the presence of decubitus ulcers.

Protected by antiphagocytic polysaccharide capsule.

Possesses catalase which inactivates hydrogen peroxide, a neutrophil mediator of bacterial death.

Infections commonly cause abscesses resulting in a low pH environment limiting host defenses.

Can elaborate toxin that manifest clinical disease.

Responsible for enterotoxin associated with food poisoning.

Toxins mediate scalded skin syndrome and toxic shock syndrome.

Scalded skin syndrome common process in infants and associated with exfoliation of the middle layer of the epidermis followed by the formation of sterile blisters.

Scalded skin syndrome usually resolves within one week without further sequelae.

Morbidity and mortality rates are low in the scalded skin syndrome.

Most staphylococci produce penicillinase and an increasing number are resistant to penicillinase-resistant penicillin analogues.

Causative agent in about 2% of community acquired pneumonia and 10% of nosocomial pneumonias with mortality of 30-80% in bacteremic associated disease.

Panton-Valentine leucocidin (PVL) is a virulence factor that modifies neutrophil membrane permeability and causes cell death.

Proinflammatory effects of PVL and lytic activity of neutrophils contributes to the development of necrosis.

Up to 70% of strains of MRSA carry PVL genes

PVL detected in fewer than 5% of nosocomial strains.

Anterior nares major site for colonization, but frequently recovered from skin and mucosal surfaces.

Nasal colonization usually precedes skin infections.

Patients with recurrent skin infections are usually nasal carriers of such organisms.

Bacteremia may be complicated by hematogenous seeding to different tissues, with the heart valves the major site of metastases of infection.

Bacteremia has a prevalence of infective endocarditis from 13-25%.

Sepsis associated in increased risk of infective endocarditis in patients with preexisting valvular disease, persistent fever, persistent bacteremia, community acquired disease, MRSA and with an unknown portal of entry of infection.

Annual incidence of Staphylococcus aureus bacteremia is 4.3-38.2 person-years in the US.

In a meta-analysis of Staph aureus bloodstream infections theere were no statistical differences in the risk of death comparing patients with S aureus exhibiting high vancomycin MIC versus those with low vancomycin MIC (Kalil, AC et al).

30 day all cause mortality for Staphylococcal aureus bacteremia is 20%, and methicillin-resistant is an independent risk factor for mortality.

Transesophageal Echocardiography is better then transthoracic echo choreography or physical exam and identifying infective endocarditis in patients with Staphylococcus aureus bacteremia.

Transesophageal echocardiography identifies infected endocarditis in approximately one fourth of the patients with S aureus bacteremia.