Bacteria

Morphology

Cytology:
Slender to medium-width rods staining pink to red on Gram stain; singly, in pairs, or in loose aggregates, extracellular and/or within neutrophils. Direct smear morphology allows only a preliminary morphologic categorization and is not reliable for species-level identification; possible morphotype classification includes Enterobacterales (medium-width rods), Pseudomonas (slender, elongated rods; exception: mucoid variant), Haemophilus/Bacteroides (small coccobacilli), with overall accuracy ~76 %. (2)

Histology:
In tissue sections, bacterial rods may be inconspicuous on HE; tissue Gram stains can improve detection with low sensitivity but high specificity on FFPE sections;  for skin sections ~43 % and ~98 % (3); a negative result does not exclude infection.

Clinical notes

Clinically important gram-negative bacilli especially in respiratory and urinary tract infections. In high-quality respiratory specimens, numerous and predominant gram-negative bacilli support the diagnosis of gram-negative pneumonia. In the urinary tract, Enterobacterales predominate, with most common Escherichia coli.
Klebsiella pneumoniae is the leading cause of pyogenic liver abscess in Asia and increasingly worldwide; hypervirulent strains (hvKp, serotypes K1/K2) can cause invasive disease in otherwise healthy individuals, often associated with diabetes mellitus. Metastatic complications include endophthalmitis (~5-7 %), meningitis, and septic pulmonary emboli (6).

See also: Klebsiella granulomatis / Granuloma inquinale (Donovanosis)

Ancillary Testing

Correlation with microbiology essential - culture, organism identification, MALDI-TOF MS, antimicrobial susceptibility testing; String test in Klebsiella spp., detection of virulence factors, if applicable

Additional Information

Interpretation should always take specimen quality and the inflammatory background into account. Bacteria associated with leukocytes support a host inflammatory response, but this finding alone does not prove true infection in every case. Mucoid variants are discussed separately.

Thicker FFPE sections (6 µm vs. 4 µm) may improve organism detection (4 [limited data, single small study]).