The Genus Serratia

226 F. Grimont and P.A.D. Grimont CHAPTER 3.3.11
latter hemolysis pattern was produced only by S.
marcescens biogroup A4.
The cell-bound hemolysin has been studied
and direct contact between erythrocytes and S.
marcescens cells has been demonstrated (Braun
et al., 1985). The lysis of erythrocytes requires
actively metabolizing bacteria and does not need
calcium ions. A 7.3-kilobase-pair chromosomal
fragment encoding the hemolytic activity has
been cloned in Escherichia coli and sequenced
(Poole et al., 1988). Two open reading frames
designated shlA and shlB have been observed.
The hemolysin protein (molecular weight
165,056) is encoded by shlA, and the product of
shlB somehow activates shlA. Protein shlA integrates
into the erythrocyte membrane and causes
osmotic lysis through channel formation (Schiebel
and Braun, 1989). When an E. coli strain
carrying the Serratia hemolysin gene was compared
to the isogenic strain without such a gene
in an experimental rat model (10 7 CFU/ml
injected via urethra into the bladder), the
strain carrying the hemolysin gene colonized the
urinary tract more and led to a stronger inflammatory
response compared to the hemolysinnegative
strain (Marre et al., 1989).
A capsulated Serratia strain injected in the
peritoneal cavity of the mouse multiplied and
killed the mouse whereas an uncapsulated variant
was avirulent (Ohshima et al., 1984).
Traub (1983) has shown that antibodies
directed against O antigens of challenge strains
afford passive protection in mice. The predominance
of the O6 and O14 serotypes of S. marcescens
in infections, the surface localization of
LPS and its masking of other antigens suggests
that LPS is a prime candidate for exploitation as
a protective antigen in a vaccine against S.
marcescens (Jessop, 1985).
Traub and Fukushima (1979a) classified S.
marcescens strains into three categories with
respect to their serum susceptibility. In solutions
with 80% (vol/vol) of fresh serum, 88% of strains
were found to be “delayed serum-sensitive” i.e.,
they were killed after a few hours exposure; 6%
of strains were “promptly serum-sensitive” i.e.,
they were killed within a matter of minutes; and
6% of strains resisted an overnight exposure to
serum. The delayed serum-sensitive strains were
shown to be killed via the activation of the alternative
pathway of the human complement system
since killing was unaffected by inhibition of
the classical pathway (Traub and Kleber, 1976).
Conversely, promptly serum-sensitive strains
were killed in a delayed fashion when the classical
pathway was inhibited (Traub and Kleber,
1976). Depletion of fresh human serum of C3
with hydrazine hydrate completely abolished the
bacterial activity against both promptly serumsensitive
and delayed serum-sensitive strains
(Traub and Fukushima, 1979a). Complement is
of prime importance with respect to efficient
opsonization and subsequent phagocytic killing
by human peripheral blood granulocytes (Traub,
1982). In an intraperitoneal mouse model, treatment
of animals with cyclophosphamide (generating
a leukopenia) or zymosan (depletion of
complement) did not increase susceptibility to S.
marcescens. A combination of both treatments
was necessary to significantly raise susceptibility
to S. marcescens (Traub et al., 1983).
Crude culture filtrates have been shown to
produce dermal hemorrhage after intradermal
inoculation (Liu, 1961), corneal damage (Kreger
and Griffin, 1975), and endophthalmitis after
intravitreal injection (Salceda et al., 1973). These
are due to one or more extracellular proteases
produced by S. marcescens. Up to four proteases
were purified (Lyerly and Kreger, 1979; Matsumoto
et al., 1984)—two metalloproteases of 56
and 60 KDa and two thiol proteases of 73 KDa.
These proteases play a prominent role in
the pathogenesis of experimental pneumonia
(Lyerly and Kreger, 1983) and keratitis (Lyerly
et al., 1981). Most studies focused on the 56-KDa
protease. Protease(s) were shown to cause: 1)
liquefactive necrosis of the cornea (Kamata et
al., 1985); 2) inactivation of five major proteinase
inhibitors including α1-proteinase inhibitor
(Virca et al., 1982), α 2-macroglobulin, Cl inhibitor,
α 2-antiplasmin, and antithrombin III (Molla
et al., 1989), which participate in regulating various
cascade mechanisms (fibrinolysis and clotting
cascade, complement system, inflammatory
response); 3) degradation of immunoglobulins G
and A, fibronectin, and other serum proteins
(Molla et al., 1986; Traub and Bauer, 1985); 4) a
potent toxic effect of fibroblasts that was mediated
by internalization of a proteinase-α2-macroglobulin
complex via the α 2-macroglobulin
receptor, followed by regeneration of the proteinase
activity in the cell (Maeda et al., 1987);
5) activation of the Hageman factor-kallikrein
system generating kinin, which leads to
enhanced vascular permeability (Matsumoto et
al., 1984); and 6) cleavage of human C3 and C5
to yield leukotactic fragments (Ward et al.,
1973).
Although resistance plasmids probably play
no role in the virulence of S. marcescens in experimental
models (Traub et al., 1983), multiple
drug resistance may affect the course and prognosis
of infections.
Non-pigmented strains of S. marcescens are
generally more resistant to antibiotics than
pigmented strains because they often harbor
resistance plasmids. Environmental strains of S.
marcescens or strains isolated before the antibiotic
era are resistant to colistin, cephalothin,
ampicillin (low level of resistance), tetracycline,