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Low level SZ-lactamase production in... » Isaúde
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BMC microbiology [electronic resource]
2012-05-09 03:31:36

Low level SZ-lactamase production in methicillinresistantstaphylococcus aureus strains with SZ-lactamantibiotics-induced vancomycin resistance

Descrição: Background:A class of methicillin-resistant Staphylococcus aureus (MRSA) shows resistance tovancomycin only in the presence of SZ-lactam antibiotics (BIVR). This type of vancomycinresistance is mainly attributable to the rapid depletion of free vancomycin in the presence ofSZ-lactam antibiotics. This means that SZ-lactam antibiotics remain active or intact in BIVRculture, although most MRSA cells are assumed to produce SZ-lactamase. We hypothesisedthat the BIVR cells either did not harbour the SZ-lactamase gene, blaZ, or the gene wasquiescent. We tested this hypothesis by determining SZ-lactamase activity and conducting PCRamplification of blaZ.Results:Five randomly selected laboratory stock BIVR strains showed an undetectable level of SZlactamaseactivity and were blaZ-negative. Five non-BIVR stock strains showed an averageSZ-lactamase activity of 2.59 +/- 0.35 U. To test freshly isolated MRSA, 353 clinical isolateswere collected from 11 regionally distant hospitals. Among 25 BIVR strains, only 16% and8% were blaZ positive and SZ-lactamase-positive, respectively. In contrast, 95% and 61% of328 non-BIVR strains had the blaZ gene and produced active SZ-lactamase, respectively. Toknow the mechanism of low SZ-lactamase activity in the BIVR cells, they were transformedwith the plasmid carrying the blaZ gene. The transformants still showed a low level of SZlactamaseactivity that was several orders of magnitude lower than that of blaZ-positive non-BIVR cells. Presence of the SZ-lactamase gene in the transformants was tested by PCRamplification of blaZ using 11 pairs of primers covering the entire blaZ sequence. Yield ofthe PCR products was consistently low compared with that using blaZ-positive non-BIVRcells. Nucleotide sequencing of blaZ in one of the BIVR transformants revealed 10 aminoacid substitutions. Thus, it is likely that the SZ-lactamase gene was modified in the BIVR cellsto downregulate active SZ-lactamase production.Conclusions:We concluded that BIVR cells gain vancomycin resistance by the elimination or inactivationof SZ-lactamase production, thereby preserving SZ-lactam antibiotics in milieu, stimulatingpeptidoglycan metabolism, and depleting free vancomycin to a level below the minimuminhibitory concentration of vancomycin.

Identificador: doi:10.1186/1471-2180-12-69
Volume: 0
Página: 6 a


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