Question

An 18-year-old female incurred a deep penetrating injury of the dorsum of her right foot when a kitchen knife fell from a platter she was carrying while going barefoot. The initial injury partially resolved; however, three days later the foot began to swell, become red, and painful. A deep subcutaneous abscess developed, with a central sinus tract from which a cloudy, serous fluid exuded. A gram stain from the wound showed many gram positive cocci in clusters. Colonies isolated from the specimen were described as entire, convex, and smooth, with a slight yellow pigmentation. Beta hemolysis was observed.

Answer 1

Ans: As given in the question that the bacterial strain is gram positive and coccus in shape. From this we can take Staphylococcus and Streptococcus under observation as these two are gram positive and coccus in shape. Now for Streptococcus, they divide to occur in a chain form because cell divison in streptococci occurs along a single axis, so as they grow, they tend to form chain like structure and it is given that we are observing clusters of cocci so we can leave behind Straptococcus.

Now we only left with Staphylococcus and they indeed form grape like clusters. Also it is given that colonies isolated were entire convex and smooth with slight yellow pigmentation and when plated on blood agar plates beta hemolysis occur. These properties are shown by Staphylococcus aureus, their colonies are  Convex, smooth, yellow-pigmented colonies with entire margin. S. aureus appears as grape-like clusters when viewed through a microscope, and has large, round, golden-yellow colonies, often with beta hemolysis, when grown on blood agar plates.

So from all this we can conclude that our bacteria infecting at the site of wound is Staphylococcus aureus