1 Institut für Medizinische Mikrobiologie und Immunologie, Pharmazeutische Mikrobiologie, Meckenheimer Allee 168, University of Bonn, D-53115 Bonn; 2 Institut für Pharmazeutische Biologie und Mikrobiologie, Bundesstrasse 45, University of Hamburg, D-20146 Hamburg, Germany
Received 2 April 2002; returned 8 August 2002; revised 30 December 2002; accepted 13 January 2003
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Abstract |
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Introduction |
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Unusual Serratia spp. include six species, namely Serratia ficaria, Serratia fonticola, Serratia odorifera, Serratia plymuthica, Serratia rubidaea and Serratia entomophila. With the exception of the latter, all serratiae have been isolated from human clinical specimens,1,4 and virulence-associated properties have also been found in Serratia spp. other than S. marcescens or members of the S. liquefaciens group.58 S. ficaria was first characterized in 1979 as an important part of the fig tree ecosystem.9 Although it had been isolated from clinical samples in several instances2,1017 its definite role as a human pathogen was only established in two studies.2,10 S. ficaria predominantly causes gallbladder empyemas and sepsis in areas that cultivate fig trees and its frequency to bring about biliary infections in Mediterranean France has been estimated to be 0.7%.2 In contrast to S. ficaria, S. fonticola is widely distributed in nature and has been predominantly isolated from water,1,18 but also from soil and sewage,18 molluscs,19 birds20 and clinical samples, mainly from wounds and the respiratory tract.21,22 The natural reservoir of S. odorifera is unknown; most strains have been isolated from clinical specimens and food.23 S. odorifera was described in 1978 by Grimont et al.23 and includes two biovars (biovars 1 and 2).24 Cook & Lopez24 described S. odorifera biovar 1 as an emerging pathogen, because several strains have been shown to cause sepsis with clinically significant morbidity and high mortality in neonates and patients with underlying conditions.2429 In contrast to biovar 1, human infections due to S. odorifera biovar 2 have only been reported in rare instances.30 S. plymuthica has frequently been isolated from the rhizosphere of various plants,31,32 but it is likely that its reservoir also comprises water and small mammals.1 Clinically, it is regarded as a significant pathogen,31 and a variety of infections including osteomyelitis, peritonitis, pneumonia, sepsis and wound infections have been attributed to this microorganism.3239 The natural reservoir of S. rubidaea, consisting of the subspecies S. rubidaea subsp. burdigalensis, S. rubidaea subsp. rubidaea and S. rubidaea subsp. colindalensis,1 is predominantly plants, in particular coconuts,1,40,41 where it can attain high cell densities.41 Human infection due to S. rubidaea is regarded to be associated with the consumption of contaminated coconuts or vegetable salads, as several studies have shown a high degree of salads contaminated with S. rubidaea.1 S. rubidaea has been shown to cause sepsis and other infections in outpatients and hospitalizated patients.1,42 In a recent study, S. rubidaea subsp. burdigalensis was shown to be an invasive pathogen.43 S. entomophila causes a fatal disease in a New Zealand grass grub species.44 It was disregarded in our examinations because it has been isolated from either insects or the environment, but not from clinical sources or animals other than insects.44
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Materials and methods |
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A total of 104 serratiae comprising S. ficaria (n = 15), S. fonticola (n = 18), S. odorifera (n = 16), S. plymuthica (n = 32) and S. rubidaea (n = 23) were examined. The strains had been isolated from human clinical specimens, animals, plants and from environmental sources. An overview of the origins of the strains tested is shown in Table 1. S. ficaria ATCC 33105T, S. fonticola ATCC 29844T, S. odorifera ATCC 33077T, S. plymuthica ATCC 183T, S. rubidaea ATCC 27593T and Escherichia coli ATCC 25922 (DSM 1103) served as reference and control strains for antibiotic susceptibility testing. The type strains of S. ficaria, S. fonticola, S. odorifera, S. plymuthica and S. rubidaea as well as S. marcescens CCUG 6, S. liquefaciens CCUG 5159 and Salmonella enterica subsp. enterica ATCC 13311T (DSM 5569) served as reference and control strains for ß-lactamase activity tests. S. marcescens CCUG 6 and S. liquefaciens CCUG 5159 were kindly provided by E. Falsen (Göteborg, Sweden); E. coli DSM 1103 and S. enterica DSM 5569 were derived from the German Culture Collection of Microoganisms and Cell Cultures (Braunschweig, Germany).
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Identification to the genus level was performed using a commercial identification system for Enterobacteriaceae (Micronaut-E; Merlin-Diagnostika, Bornheim, Germany), according to the instructions of the manufacturer. To assure a reliable species assignment, additional conventional carbon utilization tests were performed according to the recommendations of Grimont & Grimont,1 using adonitol (ADO), D-arabitol (D-ARA), L-arabitol (L-ARA), dulcitol (DUL) and palatinose (PAL; all Fluka Chemie, Buchs, Switzerland) as carbon sources. Carbon utilization tests were performed as described recently, using an incubation temperature of 37°C.45
Antibiotics and antibiotic susceptibility testing
Antibiotic susceptibility tests were performed according to the German standard (DIN), applying a microdilution procedure in IsoSensitest broth (Oxoid, Basingstoke, UK) for all the strains and in cation-adjusted MuellerHinton broth (CAMHB; Difco Laboratories, Detroit, MI, USA) for seven representative strains each (seven strains/species). The MICs were determined with a photometer for microtitre plates (see above) after inoculation of antibiotic-containing microtitre plates (Merlin-Diagnostika) with 100 µL of appropriate bacterial suspension (3 x 105 to 7 x 105 cfu/mL) and incubation for 22 h at 36 ± 1°C. All antibiotics were kindly provided by Merlin-Diagnostika who produced the antibiotic-containing plates.
Evaluation of natural antibiotic susceptibility
Plotting the MIC of a particular antibiotic for one species against the number of strains found with the respective MIC usually results in a bimodal distribution. Generally, one peak with relatively low MICs represents the natural population and one peak with higher MICs represents the strains with acquired (secondary) resistance. Analysis of the MIC distribution of all strains of one species for each antibiotic permitted determination of the biological thresholds, which limit the natural population at high MICs but not those strains with secondary resistance. Whether the MICs of the natural population were above or below the breakpoints of the standards, which assess the clinical susceptibility, was investigated. When the natural population was sensitive or intermediate according to the cited standard, it was described as naturally sensitive or naturally intermediate, respectively. When the natural population was clinically resistant, it was described as naturally resistant. The method has been described in detail previously.46,47 In this study, breakpoints according to the German standard (DIN) were applied.48 For antibiotics for which DIN clinical assessment criteria do not exist, breakpoints according to French,49 Swedish50 or NCCLS criteria, valid for Enterobacteriaceae,51 Neisseria gonorrhoeae52 and staphylococci,53 were employed. Breakpoints for apramycin, ribostamycin, lividomycin A and biapenem were used as published recently.54
ß-Lactamase activity and induction assay
ß-Lactamase activities and induction of five representative strains of each unusual Serratia sp. and of S. marcescens CCUG 6, S. liquefaciens CCUG 5159 (positive controls) and S. enterica ATCC 13311 (negative control) were tested as follows: an overnight (18 h) culture grown on IsoSensitest agar (Oxoid) at 37°C was used to prepare a saline suspension with an OD640 of 0.25 (Hitachi 15020 Spectralphotometer; Colora, Germany). The suspension (1 mL) was added to each of two 100 mL Erlenmeyer flasks, each containing 24 mL of IsoSensitest broth, which had been pre-warmed to 37°C. These cultures were incubated at 37°C in an incubator shaker (New Brunswick Scientific Co., Inc., Edison, NJ, USA) with shaking at 100 rpm until the growth had achieved an OD640 of 0.2 ± 0.02. Imipenem was then added to one of the cultures to a final concentration of 0.5 mg/L. No inducer was added to the other culture, which served as a control. Incubation of both cultures was allowed to continue in an incubator shaker with shaking for 2 h. Two millilitres from each culture was withdrawn and used to measure the absorbance and for a viable cell count by spread plates; a further 15 mL was centrifuged (15 min, 4000g) and the pellet was resuspended in 5 mL of 0.1 M phosphate buffer, pH 7.0. After re-centrifugation (15 min, 4000g), the pellet was resuspended in 1.5 mL of phosphate buffer and frozen overnight. After defrosting at room temperature, sonication on ice (Sonifier B12; Danbury, Schwäbish Gmünd, Germany) and centrifugation for 20 min at 20 000g yielded a crude supernatant for ß-lactamase assays. ß-Lactamase activity was quantified as described by Peter et al.55 with nitrocefin as the substrate.56 The protein content of each sample was determined by the method of Lowry et al.,57 with bovine serum albumin as the standard.
ß-Lactamase detection by SDSPAGE
The separation of ß-lactamases by SDSPAGE enables classification (Ambler ß-lactamase categorization) and an estimation of the molecular weight of the enzymes expressed. ß-Lactamase crude extracts of the type strains of S. ficaria, S. fonticola, S. odorifera, S. plymuthica and S. rubidaea were separated in 13% acrylamide, 0.35% bisacrylamide, 0.1% SDS gels (Roth, Karlsruhe, Germany). After electrophoresis and 1 h incubation in 2.5% Triton X-100 (Fluka, Steinheim, Germany), ß-lactamase bands were visualized by staining with 1 mM nitrocefin solution. ß-Lactamases with known molecular weights [AmpC enzymes from S. marcescens CCUG 6 (mol. wt 37.0 kDa) and Enterobacter cloacae 11-14 (mol. wt 39.2 kDa), class A ß-lactamase TEM-1 from E. coli JM 83 pBR322 (mol. wt 28.9 kDa)] were used for comparison.
AmpC gene detection by PCR
The type strains of S. ficaria, S. fonticola, S. odorifera, S. plymuthica and S. rubidaea and S. marcescens CCUG 6 were examined by PCR for the presence of ampC. Degenerated ampC primer pairs used for the detection of genes encoding chromosomally and plasmid-encoded AmpC enzymes in Enterobacteriaceae species were applied58 (and K. J. Sherwood, unpublished results). The primer pair used to amplify a fragment of ampC was P1 (GGATTCCGGGTATGGCSGTNGC) and P4 (TCCCAGCCTARYCCCTGRTACAT). PCR was conducted under standard conditions; template DNA was obtained as described by Nakajima et al.,59 but without initial boiling and final centrifugation. PCR products were visualized on 1% agarose (Roth) gels.
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Results |
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The Micronaut-E identification system allowed a secure identification of all submitted Serratia strains to the genus level. Although the database of the Micronaut-E system includes all species tested in this study with one exception (S. fonticola), it is not capable of identifying unusual serratiae to the species level (data not shown). Additional carbon utilization tests allowed a reliable species identification of all strains. The characteristic phenotypic patterns were: ADO+, D-ARA+, L-ARA+, DUL, PAL+ for S. ficaria; ADO+, D-ARA+, L-ARA+, DUL+, PAL+ for S. fonticola; ADO+, D-ARA, L-ARA+, DUL, PAL for S. odorifera; ADO, D-ARA, L-ARA, DUL, PAL+ for S. plymuthica; and ADO+, D-ARA+, L-ARA, DUL, PAL+ for S. rubidaea. The patterns were identical to those published by Grimont & Grimont.1
Antibiotic susceptibility, natural sensitivity and resistance
The MIC distributions for all the Serratia strains tested are presented in Table 2. The natural antimicrobial resistances of the tested serratiae as well as a comparison with the respective data of S. marcescens and members of the S. liquefaciens group taken from our recent study4 are summarized in Table 3. With few exceptions, species-related differences in natural susceptibility were seen in each antibiotic group but not all differences affected clinical assessment criteria (Tables 2 and 3). Despite differences in susceptibility to some agents, all species tested were uniformally naturally resistant to penicillin G, oxacillin, cefazolin (some strains of S. rubidaea were naturally intermediate), cefuroxime, all tested macrolides (some strains of S. plymuthica were naturally intermediate to azithromycin), lincosamides, streptogramins, glycopeptides, fusidic acid and rifampicin (Table 2). Uniform natural sensitivity was seen to several aminoglycosides (amikacin, gentamicin, netilmicin, tobramycin, neomycin, apramycin and lividomycin A), piperacillin, piperacillin/tazobactam, some cephalosporins (cefixime, cefoperazone, cefotaxime, ceftibuten, ceftriaxone, ceftazidime and cefepime), carbapenems, aztreonam, all fluoroquinolones (but not pipemidic acid) and to the antifolates. Major species-related differences in natural susceptibility affecting clinical assessment criteria were seen with tetracyclines, some aminoglycosides (streptomycin, kanamycin, spectinomycin and ribostamycin), aminopenicillins, ticarcillin, cefaclor, loracarbef, cefoxitin, pipemidic acid, chloramphenicol, nitrofurantoin and fosfomycin. S. ficaria was naturally intermediate and resistant to tetracyclines, aminopenicillins (in the presence and absence of ß-lactamase inhibitors) and loracarbef, and uniformally naturally resistant to cefaclor and cefoxitin (concomitant with high-level cefazolin and cefuroxime resistance). S. fonticola was unique in being naturally sensitive and intermediate to tetracyline, and naturally resistant to ticarcillin and amoxicillin (but sensitive or intermediate to aminopenicillins in the presence of ß-lactamase inhibitors). S. odorifera and S. rubidaea were the species least susceptible to quinolones, resulting in several S. rubidaea strains with natural resistance to pipemidic acid and a few strains with intermediate susceptibility to some fluoroquinolones (Table 2). Both species were also the least susceptible to tetracyclines, chloramphenicol, streptomycin and spectinomycin, resulting in natural resistance to tetracycline and chloramphenicol for all the strains and natural resistance to spectinomycin for some strains of the respective natural populations. In addition, S. rubidaea was the species least susceptible to ribostamycin and antifolates. S. plymuthica was naturally intermediate to tetracycline and was the species most susceptible to quinolones, macrolides and lincosamides. Interestingly, strains of S. odorifera, S. plymuthica and S. rubidaea that were sensitive to amoxicillin were naturally resistant or intermediate to cefazolin and cefuroxime (Table 2).
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For most of the antibiotics tested, there were either no or only minor differences in susceptibility depending on the medium (one doubling dilution step at the maximum). For all the species, the MICs of macrolides and tetracyclines were generally one to two doubling dilution steps and the MICs of fosfomycin one to four (depending on the species and the individual strain) doubling dilutions higher in IsoSensitest broth than in CAMHB (data not shown). This led to an altered clinical assessment of the respective natural populations of some species to some of these antibiotics (Tables 2 and 3). For example, in CAMHB all Serratia spp. tested were naturally susceptible to fosfomycin. Apart from fosfomycin, species-related medium dependencies in susceptibility testing were found with some ß-lactams, in particular certain cephalosporins (data not shown). In IsoSensitest broth, some amoxicillin-sensitive and -intermediate strains of S. ficaria, S. plymuthica and S. rubidaea were generally more susceptible to cefaclor, cefazolin and loracarbef (one to two doubling dilution steps) than in CAMHB (the MICs of aminopenicillins were similar in both media.) The MICs of cefixime, cefdinir, ceftibuten, cefotiam, cefuroxime and ceftriaxone for some amoxicillin-sensitive or -intermediate strains of S. rubidaea were also one to two doubling dilution steps higher in CAMHB than in IsoSensitest broth.
Quality assurance
The MICs of all antibiotics were reproducible for S. ficaria ATCC 33105T, S. fonticola ATCC 29844T, S. odorifera ATCC 33077T, S. plymuthica ATCC 183T, S. rubidaea ATCC 27593T and E. coli ATCC 25922. For certain ß-lactams the MIC ranges were broader for reference strains of Serratia than for E. coli ATCC 25922. With few exceptions, the MICs for E. coli ATCC 25922 in IsoSensitest broth and CAMHB were within the control limits for susceptibility testing according to DIN and NCCLS criteria (data not shown).
ß-Lactamase activity and induction assay
All serratiae examined produced ß-lactamase(s). The ß-lactamases of S. ficaria, S. fonticola and S. odorifera strains were inducible, whereas ß-lactamase activities in S. rubidaea and in four of five strains of S. plymuthica after induction were similar to those measured without an inducer. Ranges of specific ß-lactamase activity (mmol/L·mg) were as follows: 0.170.54 (not induced) versus 1.619 (induced) for S. ficaria [induction factor (IF) 6>100]; 0.552.34 versus 2.945 for S. fonticola (IF 432); 0.781.0 versus 5.08.5 for S. odorifera (IF 69); 0.662.0 versus 0.622.8 for S. rubidaea (IF 0.91.4); 1.01.2 versus 1.21.6 for four of five strains of S. plymuthica (IF 1.21.4); and 0.87 versus 2.4 (IF 3) for S. plymuthica LMG 6823. As expected, ß-lactamases of S. marcescens CCUG 6 and S. liquefaciens CCUG 5159 were inducible [specific activities were 0.34 versus 48 mmol/L·mg (IF > 100) and 1.5 versus 4.4 mmol/L·mg (IF 3), respectively]; S. enterica ATCC 13311 expressed no ß-lactamase.
ß-Lactamase detection by SDSPAGE
The ß-lactamases of the type strains of all tested Serratia spp. showed distinct bands estimating molecular weights between 37 and 40 kDa, characteristic for the expression of AmpC enzymes (Figure 1). In addition, a further ß-lactamase band estimating a mol. wt <30 kDa, indicating the expression of a class A enzyme, was found in S. fonticola ATCC 29844T and S. rubidaea ATCC 27593T.
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PCR amplification products for ampC were obtained for all the strains tested (data not shown). PCR fragments ranged in size from 750 to 800 bp, as was to be expected based on previous experiments with AmpC genes.58
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Discussion |
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Natural antibiotic susceptibility patterns of this study showed that unusual Serratia spp. are naturally resistant to numerous, in part structurally unrelated antimicrobial agents, indicating a range of different mechanisms of resistance. Because some phenotypic resistance patterns depended on the species, it is obvious that the respective underlying mechanisms must be species specific.
Natural susceptibility patterns to ß-lactams indicated that all Serratia spp. examined are likely to produce chromosomally encoded ß-lactamases, which are specific for the species. Generally, phenotypic analysis of MIC susceptibility patterns of appropriate key ß-lactams gives evidence about the ß-lactamase type expressed, which in Enterobactericeae belongs to either class A or class C (AmpC). Examinations concerning chromosomally encoded ß-lactamases of Serratia spp. have been focused on S. marcescens, which produces a well-characterized AmpC ß-lactamase.60,61 In S. marcescens and many other Enterobacteriaceae, chromosomal AmpC expression is reflected by resistance or decreased susceptibilities to amoxicillin, the same susceptibilities to aminopenicillins in the presence of clavulanic acid as to amoxicillin alone, natural resistances to narrow-spectrum cephalosporins (e.g. cefaclor, loracarbef and cefazolin) and natural ticarcillin sensitivity.3,4 Surprisingly, in this study it was shown that numerous strains of S. odorifera, S. plymuthica and S. rubidaea were naturally sensitive to amoxicillin, amoxicillin/clavulanate and ticarcillin but resistant to narrow-spectrum cephalosporins including cefuroxime (Tables 2 and 3). To our knowledge, this phenotypic pattern is unique among Enterobacteriaceae species and points, together with species-related differences in susceptibility to ceftibuten and cefoxitin, to the expression of unusual species-specific AmpC enzymes. Sensitivity to amoxicillin but resistance to narrow-spectrum cephalosporins has only been reported for some strains of S. plymuthica,31,32 but research in the underlying mechanisms was not performed either for S. odorifera, S. plymuthica or S. rubidaea. In contrast to the ß-lactamases of these species, the enzyme of S. ficaria seems to be a more common AmpC ß-lactamase, indicated by the natural resistance of several strains to amoxicillin and amoxicillin/clavulanate, and the species-associated high-level resistance to cefazolin and cefoxitin (Table 2). Studies dealing with the naturally occurring ß-lactamase of S. ficaria have not been published. The broad variety of different ß-lactamases within unusual serratiae is completed by S. fonticola. With respect to Serratia spp., the unique ß-lactam susceptibility pattern of S. fonticola, showing natural resistance to amoxicillin, ticarcillin and several cephalosporins, but natural sensitivity to amoxicillin/clavulanate, points to the expression of a chromosomally encoded class A ß-lactamase with an enhanced cephalosporinase activity.
To give an approach to the molecular basis of the susceptibility patterns, the ß-lactamases of representative strains of each species were characterized phenotypically and genotypically. PCR and SDSPAGE revealed that the type strains of the Serratia species examined possess ampC and also express these enzymes (Figure 1). Species specificity of these enzymes was not only reflected by MICs, but also by different apparent molecular weights of the ß-lactamase bands in the SDS gels (Figure 1), and by the data obtained from the ß-lactamase activity and induction assays. From these data it can be concluded that each Serratia sp. examined expresses its own naturally occurring AmpC ß-lactamase, which might be inducible (S. ficaria, S. fonticola, S. odorifera) or not inducible (S. rubidaea). It was of particular interest that ß-lactamase crude extracts from S. rubidaea and S. fonticola type strains revealed (as well as the observed AmpC bands) class A bands in SDS gels (Figure 1). In the case of S. rubidaea, class A enzymes might be plasmid-borne, since some strains of this species (including the type strain tested) were highly resistant to amoxicillin and ticarcillin (Table 2). Alternatively, it is possible that strains of this species possess genes for both chromosomally encoded AmpC and class A enzymes, but that expression of class A enzymes depends on the individual strain. In contrast to this species and to other unusual serratiae, studies dealing with ß-lactamases have been undertaken for S. fonticola. S. fonticola expresses a chromosomally encoded extended-spectrum class A ß-lactamase called SFO-1, with a pI of 8.1.62,63 SFO-1 was shown to be closely related to the recently described chromosomal class A ß-lactamase RAHN-1 of Rahnella aquatilis,64 but there is also a relatedness to the chromosomal class A enzymes of other Enterobacteriaceae as well as to some plasmid-mediated ß-lactamases such as MEN-1 and Toho-1.62,63 From the data of the present study it can be speculated that strains of S. fonticola express both SFO-1 and species-specific AmpC ß-lactamases. Concomitant expression of two naturally occurring ß-lactamases in Enterobacteriaceae is unusual but occurs in Yersinia enterocolitica47 and some other Yersinia spp.65 Assuming that S. rubidaea expresses no naturally occurring class A enzyme, S. fonticola might be the only human-affecting Serratia spp. producing a chromosomally encoded class A ß-lactamase, as a recent study pointed out that S. liquefaciens sensu stricto, S. proteamaculans and S. grimesii are likely to produce chromosomal AmpC ß-lactamases.4 This finding is in surprising agreement with the particular taxonomic position of S. fonticola within the genus Serratia: although it belongs clearly to the genus Serratia,19 DNADNA hybridization studies revealed a relatively low level of DNA relatedness to other Serratia species.66
Apart from the ß-lactams, there were significant species-related differences in natural susceptibility to several other antibiotics. It was shown that there are differences in susceptibility to tetracyclines, the molecular basis of which is unknown. The natural and acquired tetracycline resistance in Enterobacteriacecae species is mainly due to the expression of tetracycline-specific efflux (Tet) proteins.67 In addition, more or less non-specific multidrug transport systems and the outer membrane contribute to resistance to these antibiotics.68 It is likely that the natural low-level tetracycline resistance of S. ficaria, S. odorifera and S. rubidaea is attributed to a multidrug transport system rather than to a chromosomally encoded Tet protein expressed at low levels. In contrast to S. fonticola and S. plymuthica, these species showed decreased susceptibilities to chloramphenicol and to the quinolones tested, indicative of a multidrug efflux mechanism.68 Supporting this idea, the species-dependent levels of tetracycline resistance corresponded to the respective levels of the quinolone and chloramphenicol MICs (Table 2). Naturally occurring low-level resistance to tetracyclines combined with a decreased susceptibility to quinolones and chloramphenicol seems to be a typical feature of several Serratia spp. and is likely to be restricted to the genus Serratia, since this phenotype has been described recently in S. marcescens, but not in the S. liquefaciens group4 or in other Enterobacteriaceae.
Further interesting species-related differences in natural susceptibilty were seen with some aminoglycosides. Although there was no uniform natural resistance to any aminoglycoside, the decreased susceptibility of S. odorifera to streptomycin and spectinomycin and the decreased susceptibility of S. rubidaea to streptomycin, spectinomycin, kanamycin and ribostamycin indicates the presence of several species-specific aminoglycoside-modifying enzymes (AMEs) expressed at low levels. With respect to the underlying susceptibility patterns, it can be hypothesized that S. odorifera expresses a chromosomally encoded enzyme identical or similar to ANT (3''-I), which is the only described AME with an activity against streptomycin and spectinomycin and is widely distributed in Gram-negative bacteria.69 The phenotypic susceptibility pattern of S. rubidaea is difficult to explain. It is possible that strains of these species express an ANT (3''-I) enzyme in addition to a further naturally occurring AME. However, in contrast to our data, all known AMEs with activity against kanamycin and ribostamycin are also active against neomycin.69 An assumed novel AME should therefore have no activity against neomycin or should be active against all four substrates, provided that ANT (3''-I) is not present in S. rubidaea. Data similar to the results of this study were found in our recent study with respect to S. marcescens.4 The decreased susceptibility of S. marcescens to amikacin, kanamycin, netilmicin and tobramycin without any obvious resistance was in agreement with the features of its AAC(6')-Ic enzyme, a chromosomal 6'-N-acetyltransferase,70,71 expressed naturally at low levels.72
In conclusion, the data presented in this study comprise information about the natural susceptibility to a wide range of antimicrobial agents of S. ficaria, S. fonticola, S. odorifera, S. plymuthica and S. rubidaea. These data can be applied to validate forthcoming susceptibility data of these serratiae and might contribute to their reliable identification, supported by a number of species-related differences in antibiotic susceptibility. Evidence of novel mechanisms of resistance, in particular species-specific ß-lactamases that have been described in the present study for the first time, should also direct the interest in research on unusual Serratia spp. for which isolation from clinical specimens can no longer be regarded as anecdotal1 and which are likely to be important human pathogens.
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Acknowledgements |
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Footnotes |
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References |
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