In a patient with a lower respiratory tract infection whose sputum Gram stain shows numerous Gram‑negative diplococci, what is the most likely organism and what is the recommended initial antibiotic regimen?

Gram-Negative Diplococci in Sputum: Organism Identification and Antibiotic Management

Most Likely Organism

The most likely organism is Moraxella catarrhalis, a gram-negative diplococcus that is a significant respiratory pathogen in adults with lower respiratory tract infections, particularly in those with underlying chronic lung disease. 1, 2, 3

• M. catarrhalis appears as gram-negative diplococci on Gram stain and is morphologically indistinguishable from Neisseria species, though it can occasionally appear gram-positive (in 17% of cases) 2, 3
• This organism is responsible for a significant proportion of bronchopulmonary infections in adults and causes acute exacerbations in patients with COPD or chronic bronchitis 1, 2, 3
• The organism is frequently isolated in pure culture (46.6% of cases) or in combination with Streptococcus pneumoniae (23%) or Haemophilus influenzae (21%) 3

Critical Antibiotic Resistance Pattern

Beta-lactamase production is present in 77-86% of M. catarrhalis isolates, making plain penicillins and ampicillin ineffective. 4, 5, 3

• Resistance rates are highest to cotrimoxazole (82.5%), penicillin (77.7%), and ampicillin (71.4%) 4
• Multidrug resistance to ≥3 antimicrobials occurs in approximately 35% of isolates 4

Recommended Initial Antibiotic Regimen

Amoxicillin-clavulanate 875/125 mg orally twice daily for 5-7 days is the first-line treatment for lower respiratory tract infections caused by M. catarrhalis. 6, 7, 5

Rationale for This Choice:

• The clavulanate component neutralizes beta-lactamase production present in most M. catarrhalis strains 6, 7, 5
• This regimen provides reliable coverage of the three most common COPD/bronchitis pathogens: M. catarrhalis, H. influenzae, and S. pneumoniae 6, 7
• Clinical trials demonstrate sputum clearance of M. catarrhalis within 3 days of treatment 5
• Susceptibility to cefotaxime (87.3%) and other beta-lactamase-stable agents remains high 4

Alternative Regimens

For Penicillin-Allergic Patients:

• Levofloxacin 500 mg orally once daily for 5-7 days 6, 7
• Moxifloxacin 400 mg orally once daily for 5 days 6, 7
• Both fluoroquinolones achieve excellent bronchial secretion concentrations and cover typical COPD pathogens 6

Additional Effective Options:

• Cefotaxime or other third-generation cephalosporins (87.3% susceptibility) 4
• Tetracycline/doxycycline (85.7% susceptibility) 4
• Ciprofloxacin (84.1% susceptibility) 4
• Erythromycin (80.9% susceptibility), though macrolide resistance in S. pneumoniae limits this choice 4

Clinical Indications for Treatment

Antibiotics should be initiated when the patient presents with increased sputum purulence PLUS increased dyspnea and/or increased sputum volume (Anthonisen Type I or Type II criteria). 6, 7

• Purulent green sputum is 94.4% sensitive and 77% specific for high bacterial load ≥10^7 CFU/mL 7, 8
• Severe exacerbations requiring mechanical ventilation warrant immediate antibiotic therapy 6, 7

When to Obtain Sputum Culture

Obtain sputum culture before initiating antibiotics in the following situations: 6, 8

• Severe exacerbation requiring hospitalization
• FEV₁ <30-50% predicted (severe COPD)
• Recent antibiotic use (within 3 months) or >4 courses per year
• Prolonged disease course or treatment failure
• Prior isolation of Pseudomonas aeruginosa

Common Pitfalls to Avoid

• Do not use plain amoxicillin or ampicillin due to high beta-lactamase production rates (71-77% resistance) and increased relapse rates 6, 4, 2
• Do not dismiss M. catarrhalis as a contaminant when isolated from quality sputum in symptomatic patients with underlying lung disease 7, 3
• Do not confuse with Neisseria gonorrhoeae on Gram stain—clinical context (respiratory vs. genitourinary) is essential 2
• Beware of gram-positive appearance on Gram stain in 17% of cases, which may lead to misidentification 3
• Avoid treating colonization without clinical symptoms—positive culture alone without increased dyspnea, sputum volume, or purulence does not warrant antibiotics 8

Expected Clinical Response

• Fever should resolve within 2-3 days of initiating appropriate antibiotic therapy 1, 7
• Sputum cultures should clear within 3 days of treatment 5
• Patients should return if fever persists beyond 48 hours 1, 7

Adjunctive Therapy

• Short-acting bronchodilators (β₂-agonists with or without anticholinergics) should be used concomitantly 6, 7
• Systemic corticosteroids (prednisone 40 mg orally daily for 5 days) improve outcomes in COPD exacerbations and should be added 6, 7