Biochemical and genetic characterization of the beta-lactamases of Y. aldovae, Y. bercovieri, Y. frederiksenii and "Y. ruckeri" strains.

The beta-lactamases of five strains each of Y. aldovae and "Y. ruckeri", and 10 strains each of Y. bercovieri and Y. frederiksenii were examined phenotypically and genetically. Beta-lactamase activity and induction assays and SDS-PAGE were applied for phenotypic characterization of these enzymes. Genotypically, PCR experiments applying degenerated primer pairs for the detection of AmpC beta-lactamase genes were performed. All yersiniae yielded specific amplification products for ampC and all these strains expressed beta-lactamases. Each species produced its own, species-specific AmpC beta-lactamase. Inducibility of these enzymes was shown for Y. bercovieri, but not for the low-level enzyme producing species Y. aldovae and "Y. ruckeri". In contrast to these species, induction tests for Y. frederiksenii revealed heterogeneous results. Whereas the beta-lactamases of 6 of 10 strains were inducible, the enzyme activities after induction in the remaining four were similar to those measured without an inducer. In addition to the AmpC enzyme, all Y. frederiksenii strains expressed a second beta-lactamase belonging to Ambler class A. The present study enlarges the knowledge about the beta-lactamases of four novel Yersinia species that are likely to be involved in human disease. Beta-lactamases of Y. aldovae and "Y. ruckeri" have been characterized for the first time.

Natural antimicrobial susceptibility patterns and biochemical identification of Escherichia albertii and Hafnia alvei strains.

Bangladeshi diarrheagenic Hafnia alvei-like strains have been described recently as the new species Escherichia albertii (Int J Syst Evolut Microbiol. 2003;53:807-810). The natural susceptibility of 21 E. albertii and 76 H. alvei strains to 69 antimicrobial agents was examined, applying a microdilution procedure in IsoSensitest broth (for all the strains) and cation-adjusted Mueller-Hinton broth (for some strains). Examining the phenotypic features of both taxa with commercial identification systems and conventional tests, a database for an accurate biochemical separation of E. albertii from H. alvei was also established. Both taxa were naturally sensitive or sensitive and of intermediate susceptibility to aminoglycosides, acylureidopenicillins, ticarcillin, several cephalosporins, carbapenems, aztreonam, quinolones, folate pathway inhibitors, and nitrofurantoin. They were naturally resistant to tetracycline, penicillin G, oxacillin, all macrolides except for azithromycin, lincosamides, streptogramins, glycopeptides, rifampicin, and fusidic acid. Taxon-related differences in natural susceptibility affecting clinical assessment criteria were seen with doxycycline, minocycline, aminopenicillins, some cephalosporins, azithromycin, and fosfomycin. E. albertii was more susceptible than H. alvei to these agents and was naturally sensitive to all beta-lactams (except for penicillin G and oxacillin), azithromycin, and fosfomycin. H. alvei was naturally resistant or of intermediate susceptibility to all tetracyclines, amoxicillin, amoxicillin-clavulanate, ampicillin-sulbactam, narrow-spectrum cephalosporins, azithromycin, and fosfomycin. Motile malonate-negative Hafnia strains (indicating genospecies 2 of the H. alvei complex) were less susceptible to some cephalosporins than nonmotile, malonate-positive hafniae (indicating genospecies 1). Proline deaminase, hydroxyproline amidase, tripeptidase, chitinase, Voges-Proskauer reaction, and assimilation of histidine as well as acid production from glycerol, rhamnose, and xylose were suitable tests to separate strains of E. albertii from those of the H. alvei complex. Although out of the scope of this study, it should be noted that several strains of E. albertii showed acquired resistances to some penicillins and antifolates.

Natural antimicrobial susceptibilities of strains of 'unusual' Serratia species: S. ficaria, S. fonticola, S. odorifera, S. plymuthica and S. rubidaea.

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.