Bacteria

Morphology

Clinically Significant Gram-Negative Diplococci

Cytology: 
Kidney bean-shaped Gram-negative diplococci, typically arranged in pairs with adjacent sides flattened, occasionally forming short chains. Size varies by species: Neisseria gonorrhoeae and N. meningitidis (0.6-1.0 μm), Moraxella catarrhalis (0.8-1.2 μm, often larger and more pleomorphic). Frequently adherent to or within neutrophils in purulent exudate, particularly characteristic of N. gonorrhoeae in urogenital specimens. Background often shows abundant mucus and mixed flora in respiratory samples. Intracellular location in neutrophils supports pathogenic significance but is not species-specific.

Histology:
Non-specific acute inflammation with neutrophilic infiltration. Organisms rarely visible on routine H&E staining. In meningitis cases (N. meningitidis), may see acute purulent meningitis with neutrophilic infiltration of subarachnoid space. In gonorrheal urethritis, acute inflammatory infiltrate with neutrophilic exudate.

Differential Diagnosis:

• Haemophilus influenzae - pleomorphic Gram-negative coccobacilli, smaller, more rod-like
• Acinetobacter spp. - Gram-negative coccobacilli, often "sticky", may appear diplococcal
• Intracellular Gram-negative rods - Legionella (may be poorly visible on Gram stain, silver stain helpful), Chlamydia (Giemsa may show intracytoplasmic inclusions)
• Budding yeast mimicking diplococci - larger, may show budding
• Stained mucin granules or cellular debris
• Commensal Neisseria species (non-sterile specimens): N. sicca, N. mucosa, N. perflava - morphologically identical

Clinical notes

N. gonorrhoeae: Urethritis, cervicitis, proctitis, pharyngitis, disseminated gonococcal infection, pelvic inflammatory disease
N. meningitidis: Meningitis, meningococcemia, pneumonia, arthritis, nasopharyngeal carriage
M. catarrhalis: Acute exacerbation of COPD, bronchitis, community-acquired pneumonia, post-viral bacterial superinfection, otitis media, sinusitis

Ancillary Testing

Bacterial culture with susceptibility testing; Moraxella catarrhalis: smooth, opaque colonies on blood or chocolate agar; oxidase positive. Neisseria meningitidis in airway samples is uncommon; grows on chocolate agar with CO₂, oxidase positive, utilizes glucose and maltose. For M. catarrhalis, β-lactamase detection. Blood cultures if systemic features present.

Additional Information

Specimen quality: High squamous cell content may suggest oropharyngeal contamination. Many Neisseria species are oral commensals and may contaminate non-sterile specimens. Clinical correlation is essential for distinguishing pathogenic from commensal species, particularly in respiratory specimens, where M. catarrhalis must be differentiated from commensal Neisseria spp.

Intracellular diplococci in neutrophils support infection but require species confirmation through molecular methods or comprehensive biochemical testing. Selective media alone cannot reliably distinguish pathogenic from commensal species.