Differentiation of Enterobacteriaceae, Pseudomonas aeruginosa, and Bacteroides and Haemophilus species in gram-stained direct smears.

The accuracy of examination of the Gram-stained direct smear to classify presumptively Gram-negative rods into three morphotype groups, that is, (a) Enteric bacteria, (b) Pseudomonas, and (c) Bacteroides or Haemophilus, was evaluated. Randomly selected clinical strains (4-9) each of Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Ps. aeruginosa, B. fragilis, and H. influenzae were used to produce peritonitis or subcutaneous abscesses in mice. A Gram-stained direct smear was prepared from exudate collected from each animal. The direct smears were examined to classify bacteria observed into one of the three morphotype groups. The percent accuracy was 82, 56, and 95, respectively, and 76 overall. The assumption was made that classification was based primarily on differences in length and width of the organisms. To test this hypothesis, we prepared scanning electron photomicrographs from each specimen of exudate and measured the lengths and widths of bacteria. Examination of the Gram-stained direct smear was more accurate for classification of enteric bacteria, H. influenzae, or B. fragilis. Electron microscopy was more accurate for classification of Ps. aeruginosa. The higher length-width radio should be helpful in recognizing Ps. aeruginosa in direct smears.