The Genus Serratia

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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,
CHAPTER 3.3.11 The Genus Serratia 227 and nitrofurantoin and are usually susceptible to carbenicillin, third-generation cephalosporins, chloramphenicol, streptomycin, kanamycin, gentamicin, tobramycin, amikacin, trimethoprim/ sulfamethoxazole, fosfomycin, nalidixic acid, and other quinolones (P.A.D. Grimont, unpublished observations). Clinical strains harboring plasmids often show additional resistance to various antibiotics (Farrar, 1980; Hedges, 1980), with a single plasmid being able to confer resistance to one to eleven antibiotics (Hedges, 1980; Olexy et al., 1982). Mutants resistant to nalidixic acid are often encountered in urology wards, and these mutants are often resistant to other quinolones. The antibiotics most often active against nosocomial strains of S. marcescens are amikacin, moxalactam, and cefotaxime. However, some strains may produce enzymes inactivating amikacin (Farrar, 1980) or may overproduce a class C βlactamase (low-level resistance to cefotaxime and some other third-generation cephalosporins), which may combine with decreased permeability to β-lactams (high resistance to cefotaxime and some other third-generation cephalosporins) (Hechler et al., 1989). Compounds Produced by Serratia Pigments Prodigiosin, a nondiffusible red pigment, is a secondary metabolite formed by the enzymatic condensation of 2-methyl-3-amylpyrrole (MAP) and 4-methoxy-2,2′-bipyrrole-5-carboxyaldehyde (MBC), leading to a tripyrrole derivative, 2methyl-3-amyl-6-methoxyprodigiosene(Williams and Qadri, 1980). Little is known about the biosynthetic pathways of MAP or MBC, except that a proline molecule is incorporated intact into one of the pyrrole groups of MBC (Williams and Qadri, 1980). Pigment synthesis require air, probably molecular oxygen. In the genus Serratia, prodigiosin is only produced by strains of S. marcescens, S. plymuthica, and S. rubidaea. In S. marcescens, prodigiosin is produced by biogroups A1 and A2/6 and never by biogroups A3, A4, A5/8, or TCT (Grimont, P. A D., 1977). Nonpigmented strains of biogroups A1 or A2/6 are often blocked in the synthesis of either MAP or MBC (Grimont, P. A. D., 1977; Williams and Qadri, 1980). Strains in the nonpigmented biogroups are likely to lack the condensing enzyme (Ding and Williams, 1983). Some genes encoding prodigiosin biosynthesis were cloned by Dauenhauer et al. 1984 and expressed in Escherichia coli. The clones obtained had acquired the ability to condense MAP with MBC or to produce MAP in addition to the condensing enzyme. Strains belonging to the ubiquitous biotype A4 can synthesize a pink diffusible pigment called pyrimine (Grimont and Grimont, 1984; Williams and Qadri, 1980). Pyrimine, or ferrorosamine A, is a ferrous complex of L-2(2-pyridyl)-D′-pyrroline-5-carboxylic acid, a secondary metabolite also known to be produced by Erwinia rhapontici (Feistner et al., 1983). A yellow diffusible pigment, 2-hydroxy-5carboxymethylmuconic acid semialdehyde, is produced from the meta cleavage of 3,4dihydroxyphenylacetic acid (3,4-DHP) by the enzyme 3,4-DHP 2,3-dioxygenase (Trias et al., 1988). This enzyme is induced by tyrosine in all S. marcescens strains. However, presently only S. marcescens strains of biotype A8a, which have lost the ability to grow on aromatic compounds, can produce the yellow pigment. An uncolored muconic acid, β-cis-cis-carboxymuconic acid, can be produced by the ortho cleavage of 3,4dihydroxybenzoate. Biosurfactants Both pigmented and some nonpigmented strains of Serratia marcescens produce a biosurfactant that can act as a wetting agent. This wetting agent is produced in large amounts at 30°C but not at 37°C during the stationary phase of growth. Three aminolipids, designated W1 to W3 and having the wetting activity, were separated by thin-layer chromatography (Matsuyama et al., 1986). W1 was identified as serratamolide, a cyclodepsipeptide earlier discovered by Wasserman et al. 1961. Fatty Acids The major fatty acids components in whole-cell methanolysates of Serratia species are 3-hydroxytetradecanoic, n-hexadecanoic, hexadecanoic, and octadecanoic (not separated from octadecadienoic) acids. These contribute 50–80% of the component fatty acids in each strain. Significant quantities are also contributed by ntetradecanoic acid (Bergan et al., 1983). Flavors Alkyl-methoxypyrazines are responsible for the potatolike odor produced by all strains of S. odorifera and S. ficaria and some strains of S. rubidaea. The major alkyl-methoxypyrazine produced by these Serratia strains is 3-isopropyl-2methoxy-5-methylpyrazine. Minor compounds include 3-sec-butyl-2-methoxy-5(6)-methylpyrazine and 3-isobutyl-2-methoxy-6-methylpyrazine (Gallois and Grimont, 1985). Production of these three molecules by other bacterial species has never been observed. The food industry uses pyrazines to improve the flavor of some products.
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The Genus Serratia

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