What is the spectrum of first‑line antibiotics for common adult infections such as sinusitis, otitis media, community‑acquired pneumonia, urinary‑tract infection, skin and soft‑tissue infection, intra‑abdominal infection, and acute bacterial meningitis?

Antibiotic Spectrum for Common Adult Infections

Acute Sinusitis

For acute purulent maxillary sinusitis, use amoxicillin-clavulanate, second-generation cephalosporins (cefuroxime-axetil), or third-generation cephalosporins (cefpodoxime-proxetil, cefotiam-hexetil) as first-line therapy, with treatment duration of 7-10 days. 1

• Pristinamycin is an alternative for patients with beta-lactam allergy 1
• Reserve respiratory fluoroquinolones (levofloxacin, moxifloxacin) for frontal, fronto-ethmoidal, or sphenoidal sinusitis where complications are more likely, or for first-line treatment failure 1
• Avoid ampicillin-sulbactam due to high E. coli resistance rates 1

Common pitfall: Do not use antibiotics for diffuse, bilateral nasal symptoms with serous discharge in an epidemic context—these are likely viral and require only symptomatic treatment 1

Otitis Media

First-line treatment for acute otitis media is amoxicillin, with erythromycin-sulfisoxazole or TMP-SMZ reserved for penicillin-allergic patients. 2

• Switch to beta-lactamase-stable agents (amoxicillin-clavulanate) if no response occurs within 48-72 hours 2
• For suspected penicillin-resistant pneumococcus, use high-dose amoxicillin with or without clavulanate, or clindamycin 2
• Antibiotics are not indicated for initial treatment of otitis media with effusion 2

Community-Acquired Pneumonia (CAP)

Outpatient Management (No Comorbidities)

For previously healthy adults without cardiopulmonary disease, prescribe either a macrolide (azithromycin 500 mg day 1, then 250 mg daily for 4 days; or clarithromycin 250-500 mg twice daily) or doxycycline 100 mg twice daily for a minimum of 5 days. 1, 3

• These regimens provide excellent coverage for S. pneumoniae, Mycoplasma pneumoniae, Chlamydia pneumoniae, and Haemophilus influenzae 3
• Extend therapy beyond 5 days only if clinical stability criteria are not met (resolution of vital sign abnormalities, ability to eat, normal mentation) 1

Outpatient Management (With Comorbidities)

For patients with cardiopulmonary disease (COPD, heart failure) or risk factors for drug-resistant S. pneumoniae, combine a beta-lactam (high-dose amoxicillin, amoxicillin-clavulanate, or cefuroxime) with either a macrolide or doxycycline. 3

• This combination addresses both typical bacterial pathogens and atypical organisms 3
• Alternatively, use respiratory fluoroquinolone monotherapy (levofloxacin, moxifloxacin) 1

Hospitalized Patients (Medical Ward)

For hospitalized CAP patients on medical wards, use intravenous second-generation cephalosporin (cefuroxime 750-1500 mg every 8 hours) or third-generation cephalosporin (ceftriaxone 1 g daily or cefotaxime 1 g every 8 hours) plus a macrolide. 1

• Alternative: IV benzyl penicillin 1-4 million units every 2-4 hours or IV amoxicillin 1 g every 6 hours in areas with low beta-lactamase-producing H. influenzae 1
• Assess response at day 2-3 by monitoring fever resolution and lack of pulmonary infiltrate progression 1

ICU Patients

For severe CAP requiring ICU admission, use a second- or third-generation cephalosporin (cefotaxime) combined with either a respiratory fluoroquinolone (levofloxacin, ciprofloxacin) or a macrolide (erythromycin 1 g every 6 hours IV). 1

• For pulmonary abscess, cavitated pneumonia, or suspected aspiration, use IV amoxicillin-clavulanate 2 g every 6 hours 1
• Consider adding rifampicin 600 mg every 12 hours +/- clindamycin 600 mg every 8 hours for severe cases 1

Critical limitation: Do not use doxycycline as monotherapy in severe CAP requiring ICU admission, areas with high pneumococcal resistance to tetracyclines, or when Pseudomonas aeruginosa is suspected 3

Urinary Tract Infections

Uncomplicated Cystitis (Women)

For uncomplicated bacterial cystitis in women, prescribe nitrofurantoin for 5 days, trimethoprim-sulfamethoxazole (TMP-SMZ) for 3 days, or fosfomycin as a single dose. 1

• High resistance rates for TMP-SMZ and ciprofloxacin in many communities preclude their empiric use, particularly in patients recently exposed to these agents or at risk for ESBL-producing organisms 4
• Second-line options include oral cephalosporins (cephalexin, cefixime), fluoroquinolones, or amoxicillin-clavulanate 4

Uncomplicated Pyelonephritis

For uncomplicated pyelonephritis, use fluoroquinolones for 5-7 days or TMP-SMZ for 14 days based on antibiotic susceptibility. 1

• For ESBL-producing E. coli, oral options include nitrofurantoin, fosfomycin, pivmecillinam, amoxicillin-clavulanate, finafloxacin, or sitafloxacin 4
• Parenteral options for ESBL-producing organisms include piperacillin-tazobactam (for ESBL-E. coli only), carbapenems, ceftazidime-avibactam, or aminoglycosides 4

Skin and Soft Tissue Infections

Impetigo

For impetigo, use oral dicloxacillin 250 mg four times daily, cephalexin 250 mg four times daily, or topical mupirocin ointment three times daily for limited lesions. 1

• Alternatives include erythromycin 250 mg four times daily (though resistance may occur), clindamycin 300-400 mg three times daily, or amoxicillin-clavulanate 875/125 mg twice daily 1
• Treatment duration is approximately 7 days depending on clinical response 1

Methicillin-Susceptible S. aureus (MSSA) Infections

For MSSA skin and soft tissue infections, use oral dicloxacillin 500 mg four times daily or cephalexin 500 mg four times daily. 1

• Parenteral options include nafcillin or oxacillin 1-2 g every 4 hours IV, or cefazolin 1 g every 8 hours IV 1
• For penicillin-allergic patients (except immediate hypersensitivity), use cefazolin or cephalexin 1

Methicillin-Resistant S. aureus (MRSA) Infections

For MRSA skin and soft tissue infections, use vancomycin 30 mg/kg/day in 2 divided doses IV as the parenteral drug of choice. 1

• Oral alternatives include linezolid 600 mg twice daily, clindamycin 300-450 mg three times daily (if susceptible), doxycycline 100 mg twice daily, or TMP-SMZ 1-2 double-strength tablets twice daily 1
• Daptomycin 4 mg/kg every 24 hours IV is an alternative parenteral option 1

Nonpurulent Cellulitis

For nonpurulent cellulitis, use a 5- to 6-day course of antibiotics active against streptococci, particularly for patients able to self-monitor with close primary care follow-up. 1

• Beta-lactam antibiotics (penicillins G and V for classical erysipelas; cephalosporins group 1-2 or isoxazoyl penicillins for wound-related infections) represent first-line therapy 5
• These agents are efficacious, have well-defined adverse events, and are cost-effective 5

Intra-Abdominal Infections

Mild-to-Moderate Community-Acquired Infection

For mild-to-moderate community-acquired intra-abdominal infection, use single-agent therapy with ticarcillin-clavulanate, cefoxitin, ertapenem, moxifloxacin, or tigecycline, or combination therapy with metronidazole plus cefazolin, cefuroxime, ceftriaxone, cefotaxime, levofloxacin, or ciprofloxacin. 1

• Coverage must be active against enteric gram-negative aerobic and facultative bacilli and enteric gram-positive streptococci 1
• Obligate anaerobic coverage is required for distal small bowel, appendiceal, and colon-derived infections 1
• Avoid ampicillin-sulbactam due to high E. coli resistance rates 1
• Avoid cefotetan and clindamycin due to increasing Bacteroides fragilis resistance 1
• Empiric enterococcal coverage is not necessary 1

Common pitfall: Do not use aminoglycosides routinely in adults with community-acquired intra-abdominal infection due to availability of less toxic, equally effective agents 1

Higher-Severity or Health Care-Associated Infection

For severe community-acquired or health care-associated intra-abdominal infections, use single-agent carbapenem therapy (imipenem-cilastatin) or combination therapy with a third-generation cephalosporin, monobactam (aztreonam), or aminoglycoside plus clindamycin or metronidazole. 6

• These regimens provide broader anti-pseudomonal activity 1
• Avoid using these broader regimens for mild-to-moderate infections to reduce toxicity risk and prevent acquisition of resistant organisms 1

Acute Bacterial Meningitis

While the provided evidence does not contain specific guideline recommendations for acute bacterial meningitis empiric therapy, general medicine knowledge indicates that empiric therapy typically includes a third-generation cephalosporin (ceftriaxone or cefotaxime) plus vancomycin, with ampicillin added for patients >50 years or immunocompromised to cover Listeria monocytogenes. However, this should be confirmed with current meningitis-specific guidelines not provided in this evidence set.