UNIVERSITY OF COLORADO

1
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
2
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 dome-shaped. Hemolysis is absent. A
musty odor of unpeeled potatoes was detected.
3
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
4
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 20-E 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.
5
Abbreviated Identification of Serratia odorifera
Smooth, gray-white, entire, convex colonies on
sheep blood agar. Odor of unpeeled potatoes
Pink colonies on MacConkey agarno 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)
6
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 DNAse
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.
7
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.
BACK