The natural susceptibility to 71 antibiotics of 104 Serratia strains of Serratia ficaria (n = 15), Serratia fonticola (n = 18), Serratia odorifera (n = 16), Serratia plymuthica (n = 32) and Serratia rubidaea (n = 23) was examined. MICs were determined using a microdilution procedure in IsoSensitest broth for all the strains and in cation-adjusted Mueller-Hinton broth for some strains. With few exceptions, all species tested were uniformly naturally resistant to penicillin G, oxacillin, cefazolin, cefuroxime, all tested macrolides, lincosamides, streptogramins, glycopeptides, fusidic acid and rifampicin, and naturally sensitive to several aminoglycosides, piperacillin, piperacillin/tazobactam, carbapenems, some cephalosporins, fluoroquinolones and folate-pathway inhibitors. Major species-related differences in natural susceptibility affecting clinical assessment criteria were seen with tetracyclines, some aminoglycosides, aminopenicillins, ticarcillin, cefaclor, loracarbef, cefoxitin, pipemidic acid, chloramphenicol, nitrofurantoin and fosfomycin. Differences in susceptibility dependent on the medium were seen with macrolides, tetracycline, fosfomycin and some beta-lactams. The natural antibiotic susceptibility patterns suggest novel species-specific mechanisms of antibiotic resistance. Uncharacterized species-specific aminoglycoside-modifying enzymes and multidrug efflux systems affecting tetracyclines, quinolones and chloramphenicol are probably responsible for some of the phenotypes observed. The natural amoxicillin sensitivity of several strains of some species combined with natural resistance to some narrow-spectrum cephalosporins indicate the expression of naturally occurring beta-lactamases with unique substrate profiles. beta-Lactamases of representative strains of each species were characterized phenotypically and genotypically. It was shown that all species expressed naturally occurring AmpC beta-lactamases and, with respect to S. fonticola, also a species-specific class A beta-lactamase. Inducibility of these enzymes was shown in all species with the exception of S. rubidaea and four of five strains of S. plymuthica.

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The emergence of metallo-␤-lactamase (MBL)-producing pathogens is an increasing therapeutic problem. These enzymes have a broad-substrate spectrum; they hydrolyze all ␤-lactams except for the monobactam aztreonam. At the present time, there is no clinically useful inhibitor available. Four distinct types of MBLs-IMP, VIM, SPM, and GIM enzymes-are known in Pseudomonas aeruginosa (3,5,9). Of the VIM MBL, 11 variants have been identified up to the present, constituting three main clusters, represented by VIM-1, VIM-2, and VIM-7 (http://www.lahey.org/Studies/). bla VIM genes are either chromosomally or plasmid located and have been described as parts of the variable region of class 1 integrons (8).The multiresistant P. aeruginosa strain B63230 with resistance to carbapenems was investigated for the presence of MBLs. The strain was isolated in Berlin, Germany, in October 2003, from a blood culture of a 70-year-old male cancer patient during an episode of febrile neutropenia that followed a course of anticancer therapy. Antimicrobial susceptibility testing was performed using the broth microdilution method recommended by the NCCLS. The MICs of imipenem, meropenem, ceftazidime, cefepime, piperacillin, piperacillin-tazobactam, aztreonam, amikacin, gentamicin, ciprofloxacin, and levofloxacin were 128 mg/liter, Ն32 mg/ liter, Ն64 mg/liter, Ն32 mg/liter, Ն128 mg/liter, Ն128 mg/ liter, 8 mg/liter, 16 mg/liter, Ն64 mg/liter, Ն16 mg/liter, and 32 mg/liter, respectively. Sequential treatment with piperacillin-tazobactam and gentamicin for 4 days, meropenemvancomycin for 1 day, ceftazidime-ciprofloxacin intravenous for 4 days, and ciprofloxacin given orally for 4 days was carried out. During the therapy with ceftazidime-ciprofloxacin intravenous, regeneration of the leukocytes was detected and the fever diminished. Considering the order of events and the susceptibility pattern of the isolate, the recovery of the

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