| Aim: To study the resistant characteristics, the distribution and the resistant mechanisms of Gram-negative bacilli to β-lactam antibiotics in the local nosocomial infections. Methods: 57 PIP-resistant G - B were randomly isolated from February 2003 to January 2004 in the Affiliated Hospital of North Sichuan Medical College. MICs of antibacterials and the effects of efflux pump inhibitor CCCP on MICs were determined. The qualitative detection of β-lactamase and the enzyme activities were performed. Isoelectric points were determinded by IEF and the inhibitory effects of inhibitors of β-lactamases were observed. ESBLs, AmpC β-lactamase, MBL were tested. PCR detections of blaSHV, blaTEM, blaCTX-M, ampC, blaIMP-1 and blaVIM-1 genes were performed. The PCR products of ampC genes were sequenced. The nucleotide sequences and the deduced amino acid sequences were analyzed. Results: 57 PIP-resistant G-B included 39 Enterobacteriaceae (15 Enterobacter cloacae, 13 Escherichia coli and the other 11 Enterobacteriaceae) and 18 non-fermentative bacilli (8 Acinetobacter baumanii, 5 Pseudomonas aeruginosa, 3 Stenotrophomonas maltophilia and the other 2 non-fermentative bacilli). The resistance rates of G-B isolates were the following: PIP100.0%, CPZ82.4%, GTLX80.7%, CTX77.2%, CPLX75.4%, AZ71.9%, CAZ70.1%, FOX70.1%, CPE45.6%, CPZ/SB43.9%, AMK40.4%, PIP/TB26.3%, IMP5.3%. Non-fermentative bacilli were nearly resistant to all β-lactam antibiotics with the high MIC values except CPZ/SB, PIP/TB, IMP. Besides 6(10.5%) isolates with the efflux pumps for β-lactam antibiotics, all tested isolates produced β-lactamases, among which ESBLs, AmpC β-lactamase and MBL were responsible for 24(42.1%), 10(17.5%) and 4(7.0%) isolates, repectively. MBL's pI value was 9.6 or 8.0. β-lactamase activities of Enterobacter cloacae and Escherichia coli were higher than those of Pseudomonas aeruginosa and Acinetobacter baumanii. Of the 24 ESBLs-producing strains, 9(37.5%), 6(25.0%) and 3(12.5%) strains carried blaCTX-M, blaTEM and blaSHV genes, respectively. In addition, 4(16.7%) strains carried both blaTEM and blaCTX-M genes. The complete nucleotide sequences of the ampC genes in 8 Enterobacter cloacae isolates had very high homology with the ampC gene in Enterobacter cloacae ECLC074. Compared with the AmpC amino acid sequences of Enterobacter cloacae ECLC074, 2 isolates had one amino acid residue mutation. The ampC gene of Morganella Morganii 3-87 was highly homologous with ampC gene of 1 reported Morganella Morganii. Their deduced amino acid sequences showed one amino acid difference. The PCR product of ampC gene in 1 Pseudomonas aeruginosa isolate was negative. Amplifying blaIMP-1 and blaVIM-1 genes of 4 MBL-producing isolates failed. Conclusions:1.Of PIP-resistant G - B, Escherichia coli and Enterobacter cloacae are the most frequent G-B. 2. The production of β-lactamases is the main resistant mechanism of G-B to β-lactam antibiotics. ESBLs, AmpC β-lactamase and MBL contribute to the high resistance of G-B. ESBLs are the most frequent β-lactamase and CTX-M enzymes are the most frequent ESBLs. AmpC β-lactamases are the second frequent β-lactamases. In additon, A few strains produce MBL. Then the efflux pumps confer antibiotics resistance in a few isolates. 3. The production of ESBLs is the main resistant mechanism of Escherichia colito β-lactam antibiotics and the production of AmpC β-lactamase is the main resistant mechanism of Enterobacter cloacae to β-lactam antibiotics. All the ampC genes of AmpC β-lactamase-producing Enterobacter cloacae originate from the ampC gene in Enterobacter cloacae ECLC074 and some isolates have amino acid residue mutation. 4.β-lactam antibiotics resistance is accompanied by FQNS resistance frequently in G-B. 5. Imipenen is the best choice for the treatment of the infections caused by multidrug resistant G-B. PIP/TB and CPZ/SB combinations can be used to treat the infections caused by PIP-resistant non-fermentative bacilli. AMK is effective to treat the infections caused by AmpC β-lactamase-producing...
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