Staphylococcus aureus causes a wide range of clinical infections as it is a major human pathogen. It is a leading cause of bacteremia, infective endocarditis, osteoarticular, skin and soft tissue, pleuropulmonary, and device-related infections. S. aureus infections will likely remain both common and serious.

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What is staphylococcus aureus bacteremia?

Primary clinical foci or sources of infection are vascular catheter-related infections, SSTIs, pleuropulmonary infections, osteoarticular infections, and IE. SAB can be classified as “complicated” or “uncomplicated.” Predictors of complicated SAB were community acquisition, positive follow-up blood cultures at 48 to 96 h, persistent fever at 72 hours, and skin findings suggesting an acute systemic infection (petechiae, vasculitis, infarcts, ecchymoses, or pustules)

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Infective endocarditis

S. aureus is the most common cause of IE. A major factor is its propensity to cause severe disease and its frequent antibiotic resistance.

The probability of developing S. aureus prosthetic valve IE is highest within the first 12 months after valve replacement surgery and is likely associated with the incomplete endothelialisation of the prosthetic valve after placement and also ongoing health care contact.

Clinical manifestations

Patient characteristics:

1. Injection drug use
2. Healthcare-associated infections
3. A shorter duration of symptoms prior to diagnosis
4. Persistent bacteremia
5. Presence of a presumed intravascular device source
6. Stroke
7. Diabetes mellitus
8. Left-sided valvular disease is more common than right-sided disease
9. The mitral valve is more commonly involved than the aortic valve
10. Complications for S. aureus IE are common, particularly for left-sided IE, in which embolism of the systemic circulation and heart failure frequently occurs.
11. Clinical manifestations suggesting S. aureus prosthetic valve IE are persistent fever and persistent SAB

Other clinical findings in S. aureus prosthetic valve IE are:

1. Peripheral emboli
2. Splenomegaly
3. New regurgitant murmurs

Diagnosis

1. Diagnosis of S. aureus IE is generally established by the application of modified Duke criteria, which incorporate a combination of factors, including history and physical exam, blood culture results, and echocardiography results.
2. In a minority of cases, standard blood or tissue culture results will not detect S. aureus, hence Real-time PCR (RT-PCR) targeting 16S rRNA genes may be a useful adjunct for the microbiological diagnosis of endocarditis.

Management

• Antimicrobial therapy

1. All patients with S. aureus IE require prolonged i.v. antibiotics.
2. Recent guidelines have recognised daptomycin as an option for treatment

• Surgery

1. Early surgery reduces the risk of subsequent embolic events in an RCT for patients with native valve IE and large vegetations or severe valvular disease
2. For patients with intracerebral haemorrhage, there is consensus that surgery should be delayed by at least 1 month.
3. For those patients with ischemic stroke, surgery does not need to be delayed if there are indications for surgery.

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Skin and soft tissue infections

S. aureus causes a variety of SSTIs, ranging from benign (e.g., impetigo and uncomplicated cellulitis) to immediately life-threatening conditions. It is the most common pathogen isolated from surgical site infections (SSIs), cutaneous abscesses, and purulent cellulitis.

Clinical features

1. Impetigo is the most common bacterial skin infection in children which presents as bullous or papular lesions that progress to crusted lesions
2. Cutaneous abscess
3. Nonpurulent cellulitis may be seen in a minority of cases
4. Necrotising fasciitis
5. Pyomyositis can occur with both MSSA and MRSA
6. A particularly devastating SSI is mediastinitis complicating median sternotomy for cardiac surgery. S. aureus is the most common cause of postoperative mediastinitis

Treatment

1. Topical antibiotics, including mupirocin, fusidic acid, and retapamulin
2. Trimethoprim and Sulfamethoxazole course
3. Cephalosporin combinations
4. Current IDSA MRSA treatment guidelines recommend vancomycin, linezolid, daptomycin, telavancin, or ceftaroline for patients hospitalised with a severe purulent SSTI
5. In the case of a nonpurulent SSTI, a -lactam antibiotic is recommended for mild or moderate infection, whereas vancomycin is recommended as part of empirical therapy for severe nonpurulent SSTI

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Osteoarticular infections

S. aureus is the most common pathogen in all three major classes of osteoarticular infection, namely, osteomyelitis (OM), native joint septic arthritis and prosthetic joint infection (PJI).

1. Osteomyelitis

Osteomyelitis is an infection of bone resulting in its inflammatory destruction, bone necrosis, and new bone formation.

The Waldvogel classification system describes three types of OM:

• Hematogenous OM
• Contiguous-focus OM (from adjacent structures such as joint spaces or soft tissues or from trauma or surgery with direct implantation of organisms)
• OM with vascular insufficiency (most commonly in patients with diabetes or peripheral vascular disease and generally involving the foot)

Management- Prolonged courses of antibiotics: 6-8 weeks of therapy

2. Septic arthritis

1. It is most commonly monoarticular, but 10% of cases are polyarticular which occurs mainly in the context of SAB.
2. The knee is the most commonly involved joint in acute septic arthritis, comprising 50% of cases, followed by the hip and then the shoulder.
3. Septic arthritis involving the pubic symphysis or the sacroiliac joint accounts for 5% of cases and can be difficult to diagnose.
4. Sternoclavicular septic arthritis is strongly associated with IDUs but can occur in other settings.
5. Both joint pain and swelling are present in 80% of cases at presentation, but fever is present in only 30 to 50% of cases.
6. Arthrocentesis is the definitive diagnostic test for septic arthritis.

Management

• Most experts agree that one or more episodes of drainage of the joint space are urgently required in all cases
• Followed by a minimum of 3 to 4 weeks of antistaphylococcal antibiotic treatment, the initial 2 weeks of which should be intravenously administered.

3. Osteoarticular infections in children

Acute hematogenous OM in children presents with fever and malaise, local pain, and point tenderness and most typically involves the metaphysis of the tibia or the femur, leading to limping or an inability to run.

The pain is usually poorly localised but becomes more focal over time. The hallmark of pain is its constant nature. Overlying redness and swelling are often present, which can create diagnostic confusion.

Management

• Antistaphylococcal penicillin or cephalosporin is recommended
• Clindamycin
• Vancomycin

4. Prosthetic joint infection

Early PJIs (presenting within 30 days of implantation) generally present as deep wound infections. The patient is usually acutely ill, with fever and joint inflammation and effusion. Clinical evidence of wound infection within the postoperative period is that the strongest risk factor for early PJI

Management

• PJI has been traditionally treated with 2-stage joint replacement, with resultant cure rates of 90%.
• Over the past 2 decades, the debridement and implant retention (DAIR) procedure has been increasingly practised and has led to acceptable cure rates
• Zimmerli and colleagues proposed an algorithm for the selection of a surgical treatment strategy for patients with a PJI, which suggests that DAIR should be considered only if the patient meets all of the following criteria:

1. 3 weeks since the onset of symptoms
2. A stable implant
3. Good soft tissue envelope
4. An organism that is susceptible to rifampin and/or quinolones.

• Other patients should undergo one- or two-stage replacement or, for those who are unfit for any surgery, attempted long-term antibiotic suppression.
• There are two main approaches to antibiotic treatment in patients with staphylococcal PJI treated with DAIR:

1. 6 weeks of i.v. vancomycin (for MRSA or coagulase-negative staphylococci) or antistaphylococcal penicillin (for MSSA), following adequate debridement
2. Shorter courses of i.v. vancomycin or antistaphylococcal penicillin (2 to 6 weeks) along with 3 to 6 months of oral rifampin-based combination therapy (rifampin combined with a second oral agent, most commonly ciprofloxacin or fusidic acid)

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Other prosthetic device infections

S. aureus is particularly proliferated at infecting foreign bodies within the human host.

1. Cardiac device infections
2. Intravascular catheter infections
3. Infections involving other prosthetic devices
4. Infection of ventricular shunts
5. S. aureus infection of penile implants

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Pleuropulmonary infections

S. aureus is an important cause of pneumonia. It was initially implicated as a devastating respiratory complication of influenza during the 1918 pandemic. It thereafter remained an infrequent but well-documented cause of community-acquired pneumonia (CAP).

S. aureus has had a more predominant role in hospitalised patients with respiratory infections and has been implicated in each of the three other major subsets of pneumonia:

• Hospital-acquired pneumonia (HAP)
• Ventilator-associated pneumonia (VAP)
• Healthcare-associated pneumonia (HCAP)

It is also a common pathogen in patients with cystic fibrosis.

Management

1. Empirical antibiotics targeting MRSA in high-risk patients with pneumonia
2. In the setting of known MRSA pneumonia, ATS/IDSA guidelines recommend vancomycin, linezolid, or clindamycin

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Other staphylococcal syndromes

• Epidural Abscess
• Meningitis
• Toxic Shock Syndrome
• Urinary Tract Infection
• Septic Thrombophlebitis

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Disclaimer- The views and opinions expressed in this article are those of the author and do not necessarily reflect the official policy or position of M3 India.

About the author of this article: Dr Neetu Jha is an MDS practising as an Endodontist and Esthetic Dentist from Pune.