Lab 1: Blood Cultures

This week in lab we were all given a case study along with a blood culture bottle in order to determine what type of infection was present.  My case was based on a 25 year old woman who was admitted to the hospital for treatment of acute leukemia.  On admission she presented with fever and pain originating from a gingival lesion.  Blood cultures were negative on admission.  She started chemotherapy and developed neutropenia and on day 7 of aplasia, she developed fever associated with gingival pain and pus at the entry site of the central line. 

My first course of action was to make a gram stain of the blood culture. (shown below)

Gram Stain of specimen

Because the gram stain showed gram positive cocci in clusters I plated the specimen on SBA and incubated it overnight at 35°C.  The next day I examined the plate and found small, creamy white, opaque, colonies. (shown below)



 Growth on Sheep Blood Agar



Based on the gram stain and colony morphology my preliminary ID was Staphylococcus epidermidis.  I then performed a catalase test and a staphaurex test.  The catalase test was positive confirming the Staphylococcus genus.  The staphaurex test was negative as well as the tube coagulase test which rules out S. aureus.  Trehalose and mannitol fermentation tests were also performed and were both negative which further confirms the preliminary ID.  A crystal GP was done to confirm S. epidermidis.

S. epidermidis is normal flora on the skin and is common in infections with prosthetic devices such as catheters and shunts.  The oral mucosa is also considered to be a portal of entry.  The bacteria most likely infected her oral mucosa which caused the gingival pain.  After the chemotherapy compromised her immune system the bacteria spread into the blood causing sepsis.