UNIVERSITY OF COLORADO SCHOOL OF MEDICINE DEPARTMENT OF PATHOLOGY RESIDENCY TRAINING PROGRAM MICROBIOLOGY Case Studies 01/16/2007 Note: Move to the next slide by either clicking on the background screen or on the right arrow button below. Go to previous page by clicking the back button. On each answer page, click the “back” button to return.

The colonies shown in the photograph of the surface of a 5% sheep blood agar plate were recovered from a post- operative wound culture. Based on the colony morphology, what presumptive bacterial species identifications might be made? Can Staphylococcus aureus be ruled out? ANSWER Case Study Ia: Wound Infection

Illustrated in the photograph is the gram stain appearance of a smear prepared from one of the isolated colonies seen in the previous screen. The observation of relatively large gram-positive cells arranged in distinct tetrads would support the colony appearance of coagulase- negative Staphylococcus species or Micrococcus species. What additional tests might be performed to establish a more definitive identification? ANSWER Case Study Ia: Wound Infection

In view of the colony morphology and the microscopic features as previously observed, the resistance to furazolidone and susceptibility to bacitracin, what species identification is most likely? IS THIS ISOLATE CLINICALLY SIGNIFICANT? ANSWER As illustrated in the photograph, the unknown isolate is susceptible to 0.04 ug of bacitracin ("A") disk, but resistant to the 100 ug/ml furazolidone ("FX") disk. Case Study Ia: Wound Infection

DIAGNOSIS: Wound Infection Etiologic Agent: Micrococcus luteus

SUMMARY Colonies entire, convex, smooth with, lemon-yellow or orange-yellow pigment Gram-Positive cocci in tetrads Bacitracin susceptible Furazolidone resistant MICROCOCCUS SP. QUIT Case Study Ia: Wound Infection Steps to the Identification of Micrococcus species

These colonies are circular, entire, convex, and smooth. A distinct lemon-yellow pigment is observed. Hemolysis is absent. Although a presumptive identification of Staphylococcus aureus might be made, the observation of a lemon-yellow rather than a golden yellow pigment might suggest the possibility of a Micrococcus species. A gram stain should be performed to further investigate this possibility. BACK

The formation of tetrads suggests either a Staphylococcus species other than S. aureus, or Micrococcus species. Several tests are available to separate Staphylococcus from Micrococcus species. Most commonly, the differential resistance to furazolidone and bacitracin (“A” disk) is performed, as illustrated in the following screen. Micrococcus sp. also utilize carbohydrates oxidatively and produce acid only in the OF open tube; Staphylococcus species are fermentative. Micrococcus species also are resistant to the action of lysozyme and will grow in lysozyme broth. BACK

This isolate was further identified as Micrococcus luteus. The recovery of Micrococcus species from clinical specimens is usually considered to be of minimal significance. In the case presented, a repeat culture should be procured if the infection persists, as the Micrococcus luteus most likely represents contamination by a skin commensal. BACK