Slide 2. Gangrene of the leg caused by Clostridium perfringenes
This aspirate shows many gram-positive bacilli, some with square ends. Because Clostridium perfringenes produces a potent phospholipase that lyses neutrophils, few neutrophils are present on the slide. The size of C. perfringenes is usually uniform, and many have square ends. C. perfringenes can decolorize readily to appear gram variable or gram negative.

Slide 3. Osteomyelitis caused by the Streptococcus anginosus group
This bone aspirate contains a few neutrophils and gram-positive cocci singly, in pairs, and in chains. Organisms are often sparse in bone aspirates. Gram-negative bacteria can be particularly difficult to detect within the reddish background material.

Slide 4. Osteomyelitis caused by Corynebacterium species
This sternal aspirate contains only a few neutrophils and many small gram-positive bacilli that resemble diphtheroids. Because of their method of division, corynebacteria tend to form sharp angles with one another, many of the configurations are said to resemble “Chinese characters.”

Slide 5. Septic arthritis caused by Streptococcus pyogenes
In this aspirate of a knee joint are chains of intracellular gram-positive cocci. S. pyogenes cannot be differentiated from other chaining streptococci by its appearance on Gram stain. Microaerophilic streptococci, for example, could look like this.

Slide 6. Moraxella osloensis conjunctivitis
Material swabbed from an eye contains many short, fat gram-negative bacilli. Their structure and the source of the specimen suggest that Moraxella caused the infection. Many organisms are paired, and since they may also be nearly circular, they may resemble Neisseria.