The genetic structures surrounding the plasmid-carried bla OXA-23 oxacillinase gene, encoding resistance to carbapenems, were studied in Acinetobacter baumannii. ISAba1 and the novel element ISAba4 were detected upstream of the bla OXA-23 gene, providing promoter sequences for its expression. These insertion elements were likely involved in transposition processes at the origin of acquisition of this -lactamase gene.Acinetobacter baumannii is a typical opportunistic pathogen, often involved in nosocomial outbreaks, for which resistance to carbapenems is increasingly reported (23) and may be linked to the production of Ambler class B metallo--lactamases (30) but also to the production of carbapenem-hydrolyzing class D -lactamases (CHDLs) (18,31). Although integrons are associated with many metallo--lactamase or oxacillinase genes, they are likely not the genetic vehicles for acquisition of CHDL genes since these genes are not in the form of gene cassettes (10,22,23). Three main acquired CHDL gene clusters have been described for A. baumannii, namely, the bla OXA-23 -, bla OXA-24 -, and bla OXA-58 -like genes, whereas the bla OXA-51 gene cluster is naturally occurring and chromosomally located in A. baumannii (5,6,11,16,23,29). The bla OXA-23 gene has been identified worldwide in A. baumannii (4,8,9,13,15,27) and in Proteus mirabilis in France (3). Whereas the bla OXA-24 -like genes have been identified as chromosomally encoded, the bla OXA-23 and bla OXA-58 genes are mostly found on plasmids (23). The acquisition of bla in an A. baumannii isolate from France has been associated with a homologous recombination process (21,22).ISAba1 has been found upstream of bla , bla OXA-51 -like, and bla ampC genes in A. baumannii and is involved in their expression (7,12,23,26,28). ISAba1 belongs to the IS4 family of insertion sequences, possesses two 16-bp imperfect inverted repeats (IRs), and generates a 9-bp target site duplication upon transposition. Its transposase is made of two open reading frames, encoding 189 and 178 amino acids, leading to a functional protein when a frameshift occurs during the translation process (12).The aim of this study was to analyze the genetics of acquisition and expression of the bla OXA-23 gene in unrelated A. baumannii isolates recovered from different countries (Table 1). Genes coding for CHDLs were searched for by PCR, using primers specific for the bla OXA-23 -like, bla OXA-24 -like, and bla OXA-58 genes (15). Two bla
Seventeen clinical isolates of Staphylococcus aureus (from the United States and Europe) selected for low (borderline)-level methicillin resistance (MIC of methicillin, 2 to 4 micrograms/ml; MIC of oxacillin, 0.5 to 8 micrograms/ml) were examined for their mechanisms of resistance. Five strains were typical of heterogeneous S. aureus: they gave positive reactions with a DNA probe specific for mec and contained a small fraction (10(-6] of highly resistant cells (MIC, greater than 100 micrograms/ml). The rest of the 12 strains were homogeneous with respect to their methicillin resistance: the MIC of methicillin for all cells was 2 to 4 micrograms/ml, and no cells for which MICs were 50 micrograms/ml or higher were detectable (less than 10(-9]. None of these strains reacted with the mec-specific DNA probe. One representative strain of each group was characterized in more detail. Strain CDC-1, prototype of heterogeneous methicillin-resistant S. aureus, contained penicillin-binding protein (PBP) 2a; its DNA could transform a methicillin-susceptible and novobiocin-resistant recipient to methicillin resistance with ca. 35% linkage to Novr. Introduction of the "factor X" determinant (K. Murakami and A. Tomasz, J. Bacteriol. 171:874-879, 1989) converted strain CDC-1 to high, homogeneous resistance. Strain CDC-6, prototype of the second group of isolates, showed completely homogeneous MICs of methicillin, oxacillin, and cefotaxime. The strain contained modified "normal" PBPs: PBPs 1 and 2 showed low drug reactivity (and/or cellular amounts), and PBP 4 was present in elevated amounts. No PBP 2a could be detected. DNA isolated from strain CDC-6 could transform the methicillin-susceptible and novobiocin-resistant strain to methicillin resistance in a multistep fashion, but this resistance showed no genetic linkage to the Nov marker. We suggest that staphylococci with borderline resistance may contain at least three different classes of mechanism: heterogeneous, methicillin-resistant S. aureus, PBPs of modified drug reactivities, and the previously reported hyperproduction of beta-lactamase (L.K. McDougal and C. Thornsberry, J. Clin Microbiol. 23:832-839, 1986).
Drug-resistant strains of Mycobacterium tuberculosis, though not a novel phenomenon, are emerging worldwide. According to the latest figures of the World Health Organization and the International Union against Tuberculosis and Lung Diseases, drug resistance, in particular acquired resistance, has poured additional fuel on the fire of global tuberculosis (TB) (18). Several outbreaks of multidrug-resistant TB (7) were characterized by delayed diagnoses, inadequate treatment regimens, high rates of mortality, and significant rates of transmission and have taught us two lessons: first, the days are definitely gone where full susceptibility of TB bacilli to front-line drugs can be taken for granted. Second, rapid detection of drug resistance is paramount, not only for effective treatment of TB patients but also for initiating adequate public health measures.In the quest for new nonradiometric, culture-based strategies which allow both rapid detection of acid-fast bacilli and testing of susceptibility to antimicrobial agents, new liquid medium-based systems, such as the MB/BacT (Organon-Teknika, Durham, N.C.), ESP Culture System II (AccuMed International, Westlake, Ohio), MB Redox (Biotest, Dreieich, Germany), and the Mycobacteria Growth Indicator Tube 960 (MGIT 960, Becton Dickinson Microbiology Systems, Sparks, Md.), have become available. They all aim not only at recovering mycobacteria from clinical specimens but also at generating antimicrobial susceptibility testing (AST) data with a shorter turnaround time than that observed with the current "gold standard," the agar proportion method (11). The performance of a new system should be comparable with that of the BACTEC 460 TB system, with elimination of the two core problems associated with the old BACTEC 460 TB technology, i.e., the risk of needle punctures and disposal of radioactive waste. Preliminary studies utilizing those new systems report good overall agreement of AST results with those generated with established methods (1, 3-5, 8, 10, 12-13, 15).Recent automation of the MGIT 960 technology was another step forward, as it allows continuous monitoring of positive fluorescence, which is based on bacterial growth. It is noninvasive and eliminates potential reading difficulties during visual judging of the tubes, apart from saving labor. The threshold algorithms help in determining the susceptibility automatically.In this multicenter study we have evaluated the reproducibility and reliability of the BACTEC MGIT 960 instrument for testing of M. tuberculosis susceptibility to isoniazid (INH), rifampin (RIF) ethambutol (EMB), and streptomycin (STR) and have compared the results to those obtained by the radiometric procedure. Discordant results were resolved by testing the strains with the agar proportion method using Löwenstein-Jensen (LJ) medium (6). This was done by an additional site which thus acted as an independent arbiter. Last, in order to address safety, we performed drug susceptibility testing in plastic MGITs, in addition to the glass tubes.
Data surveillance using incidence rates stratified by cardiac procedure and type of infection is relevant to improving infection control efforts. Risk factors in patients who developed superficial infection were different from those in patients who developed mediastinitis. Coronary artery bypass graft using internal mammary artery was associated with a high risk of surgical-site infection, and independent factors such as reoperation for cardiac tamponade or pericard effusion increased the risk of infection.
13 episodes of bacteremia caused by Pasteurella multocida were seen in a general hospital during a 12-year period. All the patients had an underlying disease (77% had cirrhosis) and 2 were receiving chemotherapy for hematologic malignancy. There was a numerical preponderance of male patients (69%). In 5/13 cases a recent animal-derived trauma could be found. In the other cases the source of the infecting organism was thought to be endogenous (from patients' own pharyngeal commensal flora) or secondary to contact with secretions of a pet animal. The clinical presentation of sepsis caused by this organism was nonspecific. Hypotension was seen in 5 cases. Localized sites of infection were certain in 6 and only clinically suspected in 4 other cases. The overall mortality rate was 31%. The administration of ampicillin seems the appropriate therapy for Pasteurella multocida bacteremia.
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