Increased resistance to beta-lactams in a Klebsiella pneumoniae strain: role of a deletion downstream of the Pribnow box in the blaSHV-1 promoter

Relative Strengths and Regulation of Different Promoter-Associated Sequences for Various blaSHV Genes and Their Relationships to β-Lactam Resistance

AIMS

This work investigated the relative strengths of different blaSHV promoter-associated sequences and their regulation function in blaSHV expression and β-lactam resistance.

METHODS

Recombinant plasmids with the promoter-associated sequences (P-W, P-S, P-IS, and P-WPD), tac promoter, and combined fragments of promoter and blaSHV were separately constructed and transformed into Escherichia coli DH5α. The relative strengths of the promoters indicated by the intensities of green fluorescent protein and the mRNA expression levels of blaSHV were compared. The minimum inhibitory concentration and extended spectrum β-lactamase phenotypes were evaluated.

RESULTS

The relative strengths were ranked as P-tac > P-WPD > P-IS > P-S > P-W. The mRNA expression and β-lactam resistance levels of the different promoter-associated sequence groups were generally consistent with the strength rank, but the extent of gfp and blaSHV mRNA levels varied significantly in each group. The β-lactam resistance levels were inconsistent with the strength rank in certain blaSHV groups. In relation to the different promoter-associated sequences, blaSHV-ESBLs displayed significantly different change modes of β-lactam resistance compared with blaSHV-non-ESBLs.

CONCLUSION

The mRNA expression and β-lactam resistance of the blaSHV showed consistencies and inconsistencies with the strengths of the promoter-associated sequences. The mechanisms accounting for these discrepancies need further investigation.

Differential CTX-M Expression from a Conserved Promoter: Role of Promoter-Associated Spacer Sequences Downstream of the blaCTX-M Regulon

AIMS

The aim of this project was to explore the different CTX-M expression levels occurring from a single conserved promoter with different spacer sequences, the variation of which is hypothesized to be a key factor in fluctuating levels of CTX-M.

METHODS

The blaCTX-M promoter fragments with five different spacer sequences were amplified, sequenced and cloned into the pUA66 expression vector carrying the green fluorescent protein (GFP) gene. The expression of blaCTX-M in the transconjugants was analyzed using fluorescence microscopy, flow cytometry and qRT-PCR.

RESULTS

The promoters of all the blaCTX-M genes were provided by ISEcp1 and were extremely conserved. The promoter-associated spacer sequences varied from 42 to 127 bp and variations in GFP expression in the five transconjugants were observed. A nucleic acid deletion and point mutation were detected in the spacer sequences by variations in which the expression of blaCTX-M was influenced.

CONCLUSION

The different spacer sequences have a significant impact on the activity of the conserved promoter. The shorter spacer sequence between the conserved promoter and the blaCTX-M gene does not specifically enhance the expression of blaCTX-M, contrary to previous reports. The expression of blaCTX-M may be regulated by changes in promoter activity caused by diverse spacer sequences.

TEM-187, a new extended-spectrum β-lactamase with weak activity in a Proteus mirabilis clinical strain

A Proteus mirabilis clinical strain (7001324) was isolated from urine sample of a patient hospitalized in a long-term-care facility. PCR and cloning experiments performed with this strain identified a novel TEM-type β-lactamase (TEM-187) differing by four amino acid substitutions (Leu21Phe, Arg164His, Ala184Val, and Thr265Met) from TEM-1. This characterization provides further evidence for the diversity of extended-spectrum β-lactamases (ESBL) produced by P. mirabilis and for their potential spread to other Enterobacteriaceae due to a lack of sensitive detection methods used in daily practice.

Selection of SHV extended-spectrum-beta-lactamase-dependent cefotaxime and ceftazidime resistance in Klebsiella pneumoniae requires a plasmid-borne blaSHV gene

In Klebsiella pneumoniae, it is common for plasmid-located and chromosome-located bla(SHV) copies to coexist within single cells. The plasmid-borne genes are mainly derived from two separate IS26-mediated mobilizations of bla(SHV). The objective of this study was to test the hypothesis that the presence of a non-extended-spectrum beta-lactamase (non-ESBL) encoding plasmid-borne form of bla(SHV) facilitates the cefotaxime (CTX)-mediated selection of ESBL-expressing mutants, even when there is a chromosomal copy of the same gene. Twenty-one diverse ESBL-negative, bla(TEM)-negative K. pneumoniae clinical isolates were tested for the IS26 insertions characteristic of the two mobilization events. The isolates were then tested for their ability to be selected for ESBL-mediated CTX resistance by serial subculturing with a doubling of the CTX concentration at every subculture. Fourteen isolates possessed neither of the IS26 insertions. None of these became ESBL positive, and all died during the course of the experiment, despite possessing chromosomal bla(SHV) copies. The other isolates all became ESBL positive and grew abundantly up to a CTX concentration of 128 microg/ml. Similar results were obtained with ceftazidime. ESBL expression was associated with the appearance of the expected G-->A mutation at position 1 of codon 238 and also with bla(SHV) copy number amplification. It was concluded that plasmid-borne bla(SHV) greatly facilitates the selection of ESBL expression, even when the same gene is on the chromosome, and that gene dosage effects are likely to contribute to this phenomenon.