Bacteroides fragilis (Fig.20-3), the most important of all anaerobes because of its frequency of occurrence in clinical infection and its resistance to antimicrobial agents, is a Gram-negative bacillus with rounded ends 0.5 to 0.8 μm in diameter and 1.5 to 4.5 μm long. Most strains are encapsulated. Vacuolization or irregular staining is common, particularly in broth media. Some pleomorphism also may be seen. By electron microscopy, the ultrastructure of B fragilis is similar to that of other Gram-negative bacteria. The guanine-plus-cytosine content is 42 percent. Prevotella melaninogenica and Porphyromonas asaccharolytica are short to coccoid Gram- negative rods; they produce a distinctive pigment (brown to black), which is a heme derivative that colors the colony (Figs.20-4 and 20-5). Many strains of P melaninogenica require vitamin K, or similar compounds, as well as heme.