UNIVERSITY OF COLORADO
SCHOOL OF MEDICINE
DEPARTMENT OF PATHOLOGY
RESIDENCY TRAINING PROGRAM
MICROBIOLOGY
V: Enterobacteriaceae:
Non-Lactose Fermenters
Case Study 5b:
“Thigh Abscess with Septicemia”
Abstracted from a CACMLE Teleconference with
Frank Kocka, Ph. D.
Cook County Hospital, Chicago, Illinois
Thigh Abscess with Septicemia
The colonies illustrated in this
photograph were recovered after 24
hours incubation at 35o C from a
thigh abscess of an infant on day
120 after premature delivery,
complicating a femoral artery line.
Prior medical history revealed
previous episodes of E. coli and
Staphylococcus aureus septicemia
related to indwelling lines.
The colonies illustrated here are
large, off-white, entire, and domeshaped. Hemolysis is absent. A
musty odor of unpeeled potatoes
was detected.
Thigh Abscess with Septicemia
Photograph of the appearance of the
colonies of this isolate growing on
the surface of MacConkey agar after
24 hours incubation at 35o C.
The colonies are large, entire,
smooth, and dome-shaped. Note the
distinct light rose-pink pigmentation;
however, note in particular that the
pigment is confined to the colonies
and does not leach out into the
surrounding agar.
What might this colony pigmentation signify?
What Genus within the Family Enterobacteriaceae is most likely?
What biochemical characteristics help in making the identification?
ANSWERS
Thigh Abscess with Septicemia
The reactions observed on a
Microtiter Neg ID Panel Type 2
plate inoculated with a
suspension of this isolate
produced a biocode number
of 77604356, coding to
“Enterobacter aerogenes”.
The positive DNAse and
negative ornithine reactions,
however, are inconsistent with
this species. Using an API 20E strip, the biocode of
5247753 coded out to an
identification of:
Serratia odorifera
This case illustrates the need to
recognize characteristics that may
indicate when the identification provided
by a given system cannot be correct. In
such cases, additional tests or an
alternate system may be required.
Abbreviated Identification of Serratia odorifera
Smooth, gray-white, entire, convex colonies on
sheep blood agar. Odor of unpeeled potatoes
Pink colonies on MacConkey agar—no leaching
into agar — weak or late lactose fermenter
DNAse test Positive
Urea Negative
Citrate +
Lysine
Positive
Ornithine
Negative
Serratia odorifera, Biogroup 2
(Confirm with biotype profile)
Serratia odorifera: RECAPITULATION
An initial clue to the presumptive identification of Serratia odorifera
is the detection of the musty, unpeeled potato odor emanating from
mature colonies. The observation of a rose-pink pigment confined
to colonies growing on MacConkey agar may also be an initial clue.
A positive DNA’se reaction points to Serratia species.
Serratia odorifera is an uncommon cause of opportunistic human
infections. Serratia marcescens is the Serratia species most
commonly recovered from clinical specimens. However, the lack of
the characteristic odor and the negative ornithine reaction rules
against this identification. Demonstrating the inability to produce
acid from sucrose would also be in favor of Serratia odorifera.
In this case, the source of the infection was most likely nosocomial,
with extension into the site of the catheter wound from the adjacent
skin where the organism undoubtedly resided as a commensal.
ANSWERS TO QUESTIONS
The pigment confined to the
colonies, without leaching into the
adjacent agar, indicates that
indiginous pigment has been
produced and lactose has not been
fermented. Among the
Enterobacteriaceae, the genus
Serratia, particularly, S. marcescens
is singled out.
Serratia species are unique among
the Enterobacteriaceae in producing
three hydrolytic enzymes: lipase,
gelatinase, and DNAse.
Illustrated in this photograph is the
surface of DNA agar on which are
growing the colonies of this isolate
after 24 hours incubation at 35o C.
Note the distinct sunset-red
pigmentation of the agar adjacent to
the growth of the colonies along the
streak line.
This pigment change indicates
hydrolysis of the DNA under the action
of DNAse, with clearing and a pH shift
in the toluidine O blue indicator in the
medium. This positive reaction points
to an identification of Serratia species.
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