UNIVERSITY OF COLORADO
SCHOOL OF MEDICINE
DEPARTMENT OF PATHOLOGY
RESIDENCY TRAINING PROGRAM
MICROBIOLOGY
Case Studies 01/16/2007
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Case Study Ia: Wound Infection
The colonies shown in the
photograph of the surface of
a 5% sheep blood agar plate
were recovered from a postoperative 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 coagulasenegative Staphylococcus
species or Micrococcus species.
What additional tests might be
performed to establish a more
definitive identification?
ANSWER
Case Study Ia: Wound Infection
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.
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
DIAGNOSIS:
Etiologic Agent:
Wound Infection
Micrococcus luteus
Case Study Ia: Wound Infection
Steps to the Identification of
Micrococcus species
SUMMARY
Colonies entire, convex,
smooth with, lemon-yellow
or orange-yellow pigment
Gram-Positive cocci in
tetrads
Bacitracin susceptible
Furazolidone resistant
MICROCOCCUS SP.
QUIT
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.
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