Slide 2. Mixed infection
This Gram stain of exudate from a skin ulcer shows many gram-negative coccobacilli; short, plump gram-negative bacilli; and gram-positive cocci singly, in pairs, and in clusters. Because such a mixture of organisms commonly colonizes specimens from skin ulcers, bacterial cultures are not usually diagnostically worthwhile in the absence of
cellulites, abscess, or suspicion of cutaneous diphtheria. Only in these circumstances are the bacteria in the ulcer likely to be pathogenic.

Slide 3. Mixed infection
This specimen, from a necrotizing penile infection, includes many gram-negative coccobacilli, gram-negative rods longer than the coccobacilli, gram-positive cocci, and gram-positive bacilli. To identify any organism confidently is difficult. Cultures grew Escherichia coli (the larger gram-negative bacilli), Bacteroides species (the gram-negative coccobacilli), and Clostridium perfringenes (the gram-positive bacilli).

Slide 4. Corynebacterium diphtheriae
Material swabbed from a skin ulcer includes gram-negative coccobacilli and a moderate number of gram-positive bacilli, many of which meet at acute angles, a configuration characteristic of “diphtheroids.” The culture grew mixed cutaneous flora and Corynebacterium diphtheriae.

(Labeled view)