Lab 3: Sputum Cultures

In lab this week we were given a case along with sputum cultures on sheep blood, chocolate and MacConkey agar.  My case stated that a 48 year old man who had a long history of alcoholism was admitted to the ICU with hypotension and GI bleeding.  He was intubated and remained ventilator depended for several weeks.  He developed fever and his chest radiograph showed an infiltrate and changes consistent with multiple small abscesses.   

I identified 2 isolates based on colony morphology.  The first isolate was greenish-gray with broken glass appearance and beta hemolytic on SBA and smelled like grapes.  On chocolate it formed large colonies with spreading margins.  On MacConkey it formed slight pink colonies meaning it is a non lactose fermenting gram negative rod.    The presumptive identity for isolate 1 was non lactose fermenting gram negative rod.   Based on the colony morphology I performed a oxidase test and spot indole test.  The oxidase was positive and the spot indole was negative confirming the isolate as Pseudomonas aeruginosa.

Pseudomonas aeruginosa on SBA

Pseudomonas aeruginosa on Mac

The second isolate was small, white and opaque on SBA and chocolate and did not grow on MacConkey. Because it didn’t grow on Mac I knew it was gram positive but I did a gram stain to determine the morphology.  The gram stain showed gram positive cocci in clusters so I presumptively identified the isolate as Staphylococcus.  A catalase test was positive and the staphaurex (coagulase) test was negative.  My final ID was coagulase negative Staphylococcus which is normal flora in the throat.



Gram stain showed gram positive cocci in clusters


 

Positive catalase test



P. aeruginosa is seen in hospital acquired aspiration pneumonia.  The patient was most likely immunocompromised because of his alcoholism and because he had been intubated for several weeks he most likely aspirated and inhaled gastric contents containing the bacteria into his lower respiratory tract.