Two blaTEM-like genes were characterized that encoded IRT beta-lactamases (previously called TRI) in clinical isolates of Escherichia coli resistant to amoxycillin alone and to combinations of amoxycillin with beta-lactamase inhibitors. Plasmids carrying this resistance were isolated from E. coli K 12 transconjugants and the genes were sequenced after amplification of defined fragments, using TEM-1-specific primers. The gene for IRT-1 beta-lactamase resembled the blaTEM-1B gene, and that for IRT-2 resembled blaTEM-2. However, both IRT enzymes have a glutamine residue at position 37, which is characteristic of TEM-1. The unique nucleotide difference with parental genes corresponding to amino acid variation was observed at nucleotide position 929. The consequence of C to T transition in the blaIRT-1 gene and C to A transversion in the blaIRT-2 gene was the substitution of arginine 241 in the native protein by cysteine and serine, respectively, in the mutants. Thus, the nature of amino acid 241 is critical in conferring resistance or susceptibility to beta-lactamase inhibitors. Furthermore, these basic to neutral amino acid replacements explain the more acidic pI (pI = 5.2) of these IRT enzymes compared to that of TEM-1 (pI = 5.4). The presence of cysteine-241 in IRT-1 also explains the selective sensitivity of this beta-lactamase to inhibition by p-chloromercuribenzoate.
A report is given on two neutropenic patients with staphylococcal septicemia caused by Staphylococcus haemolyticus and Staphylococcus aureus (both strains methicillin-resistant) who failed to respond to therapy with teicoplanin. Both strains were resistant to teicoplanin (MIC 16 and 8 mg/l respectively), but remained sensitive to vancomycin (MIC 2 and 4 mg/l respectively). Replacement of teicoplanin with vancomycin led to full recovery of both patients and their discharge from hospital. These two cases emphasize the importance of clinical and microbiological monitoring of patients with staphylococcal septicemia, even when glycopeptides are used for treatment.
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