In an adult with suspected infectious aortitis, what empiric intravenous (IV) antibiotic regimen should be used to cover methicillin‑resistant Staphylococcus aureus (MRSA), Salmonella species, Gram‑negative bacilli, and streptococci until culture results are available?

Empiric Antibiotic Selection for Suspected Infectious Aortitis

For an adult with suspected infectious aortitis, initiate empiric IV vancomycin 15–20 mg/kg every 8–12 hours PLUS either ceftriaxone 2 g IV daily or cefepime 2 g IV every 8 hours immediately after obtaining blood cultures. This combination provides coverage against the most common pathogens—MRSA, Salmonella species, other Gram-negative bacilli, and streptococci—while awaiting culture results 1, 2.

Rationale for Empiric Coverage

The microbiology of infectious aortitis has evolved significantly. Historically dominated by streptococci in the pre-antibiotic era, current cases are most commonly caused by Salmonella species (approximately one-third of abdominal aortic infections) and Staphylococcus aureus 1, 3. Streptococcus pneumoniae and other streptococcal species remain important pathogens, particularly in thoracic aortitis associated with endocarditis 4, 3. Gram-negative bacilli beyond Salmonella also occur, necessitating broad initial coverage 1.

Specific Antibiotic Regimen Components

MRSA Coverage (Mandatory)

• Vancomycin 15–20 mg/kg IV every 8–12 hours is first-line for empiric MRSA coverage in serious infections, targeting trough concentrations of 15–20 mg/L 5, 2
• Daptomycin 6–10 mg/kg IV once daily is an acceptable alternative if vancomycin cannot be used, though higher doses (≥8 mg/kg) are preferred for serious S. aureus infections 5, 2
• Linezolid 600 mg IV every 12 hours is another alternative, though it should not be used for empirical therapy in bacteremic patients due to inferior outcomes compared to vancomycin 5

Gram-Negative and Salmonella Coverage (Mandatory)

• Ceftriaxone 2 g IV daily provides excellent coverage against Salmonella species and most Gram-negative bacilli, with good CNS penetration if needed 5
• Cefepime 2 g IV every 8 hours offers broader Gram-negative coverage including Pseudomonas aeruginosa, appropriate if healthcare-associated infection or multidrug-resistant organisms are suspected 5
• Piperacillin-tazobactam 3.375–4.5 g IV every 6 hours is an alternative providing broad Gram-negative and anaerobic coverage 5

Streptococcal Coverage (Inherent in Regimen)

• The recommended beta-lactams (ceftriaxone, cefepime) provide excellent streptococcal coverage, including S. pneumoniae 5, 4
• Vancomycin also covers streptococci, providing redundant coverage for these pathogens 5

Clinical Context and Risk Stratification

Infectious aortitis typically presents in men over 50 years with atherosclerosis, manifesting as fever, abdominal or back pain, palpable abdominal mass, and leukocytosis 1. Blood cultures are positive in most cases but may be negative initially 1, 4. The diagnosis should be suspected when CT imaging demonstrates a saccular aneurysm with inflammatory changes, periaortic soft tissue involvement, or periaortic hematoma 1, 4.

Atherosclerosis is the principal risk factor, creating arterial wall damage that facilitates bacterial attachment and subsequent aneurysm formation 3. Other risk factors include recent bacteremia, infective endocarditis, injection drug use, and immunocompromise 1, 3, 2.

Treatment Duration and Surgical Considerations

• Parenteral antibiotics should continue for at least 6 weeks, potentially longer, to ensure complete pathogen eradication and prevent recurrent infection 1, 6
• Surgical debridement and revascularization should be completed early because delay increases mortality risk from aneurysm rupture 1, 6
• Medical treatment alone carries high mortality, whereas combined surgical and antimicrobial treatment significantly improves survival 1, 6

Antibiotic Adjustment After Culture Results

Once culture and susceptibility results are available:

• For MSSA: switch vancomycin to cefazolin 2 g IV every 8 hours or nafcillin/oxacillin 2 g IV every 4–6 hours for superior efficacy 5, 2
• For MRSA: continue vancomycin or daptomycin based on MIC values and clinical response 5, 2
• For Salmonella species: continue ceftriaxone or adjust to fluoroquinolones (ciprofloxacin 400 mg IV every 12 hours) based on susceptibilities 1, 6
• For streptococci: penicillin G or ceftriaxone are preferred agents once identified 4, 3

Critical Pitfalls to Avoid

• Do not delay empiric antibiotics while awaiting imaging or culture results in patients with suspected infectious aortitis and sepsis, as mortality increases with delayed effective therapy 5, 1
• Do not use monotherapy for empiric coverage—the polymicrobial potential and high mortality mandate combination therapy until cultures narrow the spectrum 1, 3
• Do not omit MRSA coverage even if S. aureus seems unlikely, as it represents a leading cause of infectious aortitis with devastating consequences if untreated 1, 3, 2
• Do not rely on medical therapy alone—surgical intervention is essential for source control and definitive diagnosis 1, 6