The Genus Serratia

232 F. Grimont and P.A.D. Grimont CHAPTER 3.3.11
Gas Production
Gas production differentiates Serratia marcescens
from S. liquefaciens—S. proteamaculans
better when glucose agar is used than when a
liquid medium with a Durham inverted tube is
used (Grimont et al., 1977b). Glucose agar consists
of nutrient agar (meat extract [Liebig], 3 g;
yeast extract [Difco], 10 g; agar [BBL], 15 g; distilled
water to 1 liter; pH 7.4) supplemented with
1% (wt/vol) glucose. Dispense into tubes (160 ×
10 mm). Autoclave 20 min at 120°C, cool at 50°C,
and inoculate before the agar sets. Tubes are
examined for bubbles of gas for up to 3 days.
β-Xylosidase
The method of Brisou et al. (1972) for demonstrating
β-xylosidase can conveniently be
adapted for use with sterile microculture plates.
Aqueous (1% wt/vol) p-nitrophenyl-β-xyloside
is dispensed into the wells in 0.05-ml amounts,
followed by 0.05 ml of a fresh bacterial suspension
in 0.25 M phosphate buffer, pH 7. Plates are
examined for a yellow color after 24 h.
Identification of Serratia at the Genus Level
Members of the genus Serratia share the characteristics
defining the family Enterobacteriaceae.
Only occasionally can a nitrate-negative strain
be isolated. The properties that best define the
genus Serratia are listed in Table 3. Although
lipase activity on tributyrin or corn oil is listed,
S. odorifera strains are only weakly lipolytic. A
weak urease activity, lack of motility, or presence
of a capsule are occasionally observed. Serratiae
are clearly differentiated from Klebsiella spp.,
Enterobacter aerogenes, and E. cloacae by production
of gelatinase, lipase, DNase, and by
growth on caprate or caprylate as sole carbon
source. Some soft-rot Erwinia spp. produce
extracellular proteinase and DNase, but these
strains are pectinolytic and lack glucose and gluconate
dehydrogenases (Grimont et al., 1977b).
Identification of Serratia Species
The characteristics best allowing identification of
each Serratia species are given in Tables 4 and 5.
Carbon source utilization tests are invaluable for
unambiguous identification. Indole production
by S. odorifera is not reproducibly observed
when peptone water is used. A defined medium
containing trytophane (e.g., “urée-indole”
medium, Diagnostics Pasteur, Marnes-la-
Coquette, France; or the indole test in API 20E
strips) gives consistently positive results with this
species. With the use of classical tests (Ewing,
1986), S. plymuthica may be confused either with
Table 3. Characteristics defining the genus Serratia. a
Positive characteristics of all species of the genus Serratia b
Motile rods
Growth at 20°C in 1 day
Growth at pH 9
Growth with 4% NaCl
Growth on minimal medium without addition of growth
factor
Acid from maltose
Acid from mannitol
Acid from salicin
Acid from trehalose
ONPG c hydrolyzed
Glucose oxidized to gluconate d
Gluconate oxidized to 2-ketogluconate
Carbon source utilization tests: N-acetylglucosamine, cisaconitate,
4-aminobutyrate, citrate, fructose, galactose,
galacturonate, gluconate, glucose, glucuronate, glycerol,
m-inositol, 2-ketogluconate, L-malate, maltose, mannitol,
mannose, putrescine, ribose, trehalose
Additional positive characteristics of Serratia species other
than S. fonticola b
Voges-Proskauer test (Richard)
Lipase (tributyrin, corn oil)
Growth on caprylate-thallous agar
DNase
Proteinase(s)
Negative characteristics of all species of the genus Serratia b
Urease (Ferguson)
H2S from thiosulfate
Phenylalanine/tryptophane deaminase
Polygalacturonidase
Amylase (4-day reading)
β-Glucuronidase
Carbon source utilization tests: 5-aminovalerate, mcoumarate,
ethanolamine, glutarate, histamine, L-sorbose,
tryptamine
Sodium ion requirement
Anaerobic growth with KClO3
Additional negative characteristics of Serratia species other
than S. fonticola
Acid from dulcitol
Carbon source utilization tests: dulcitol, 3-phenylpropionate
a
Taken from Grimont et al., 1977b, and unpublished
observations.
b
More than 90% of the isolates of each species give the same
reaction.
c
ONPG, ortho-nitrophenyl-galactoside.
In the presence of pyrroloquinoline quinone.
S. proteamaculans, S. liquefaciens, S. rubidaea,
or Enterobacter agglomerans. In the past, S.
plymuthica had been identified as “atypical S.
liquefaciens” (negative for lysine and ornithine
decarboxylase) or atypical S. rubidaea (sorbitol
positive). The species S. odorifera can also be
recognized in a number of “rhamnose-positive S.
liquefaciens” strains. Carbon source utilization