Plasmid-mediated sulfanilamide resistance

Dynamics of the IncW genetic backbone imply general trends in conjugative plasmid evolution

Plasmids cannot be understood as mere tools for genetic exchange: they are themselves subject to the forces of evolution. Their genomic and phylogenetic features have been less studied in this respect. Focusing on the IncW incompatibility group, which includes the smallest known conjugative plasmids, we attempt to unveil some common trends in plasmid evolution. The functional modules of IncW genetic backbone are described, with emphasis on their architecture and relationships to other plasmid groups. Some plasmid regions exhibit strong phylogenetic mosaicism, in striking contrast to others of unusual synteny conservation. The presence of genes of unknown function that are widely distributed in plasmid genomes is also emphasized, exposing the existence of ill-defined yet conserved plasmid functions. Conjugation is an essential hallmark of IncW plasmid biology and special attention is given to the organization and evolution of its transfer modules. Genetic exchange between plasmids and their hosts is analysed by following the evolution of the type IV secretion system. Adaptation of the trw conjugative machinery to pathogenicity functions in Bartonella is discussed as an example of how plasmids can change their host modus vivendi. Starting from the phage paradigm, our analysis articulates novel concepts that apply to plasmid evolution.

Overexpression of inhA, but not kasA, confers resistance to isoniazid and ethionamide in Mycobacterium smegmatis, M. bovis BCG and M. tuberculosis

The inhA and kasA genes of Mycobacterium tuberculosis have each been proposed to encode the primary target of the antibiotic isoniazid (INH). Previous studies investigating whether overexpressed inhA or kasA could confer resistance to INH yielded disparate results. In this work, multicopy plasmids expressing either inhA or kasA genes were transformed into M. smegmatis, M. bovis BCG and three different M. tuberculosis strains. The resulting transformants, as well as previously published M. tuberculosis strains with multicopy inhA or kasAB plasmids, were tested for their resistance to INH, ethionamide (ETH) or thiolactomycin (TLM). Mycobacteria containing inhA plasmids uniformly exhibited 20-fold or greater increased resistance to INH and 10-fold or greater increased resistance to ETH. In contrast, the kasA plasmid conferred no increased resistance to INH or ETH in any of the five strains, but it did confer resistance to thiolactomycin, a known KasA inhibitor. INH is known to increase the expression of kasA in INH-susceptible M. tuberculosis strains. Using molecular beacons, quantified inhA and kasA mRNA levels showed that increased inhA mRNA levels corre--lated with INH resistance, whereas kasA mRNA levels did not. In summary, analysis of strains harbouring inhA or kasA plasmids yielded the same conclusion: overexpressed inhA, but not kasA, confers INH and ETH resistance to M. smegmatis, M. bovis BCG and M. tuberculosis. Therefore, InhA is the primary target of action of INH and ETH in all three species.

A correlation with type of sulfonamide resistance gene in Escherichia coli and synergy between trimethoprim and sulfamethoxazole

Plasmids carrying type I or II sulfonamide-resistance (Sur) genes were evaluated for their effect on synergy between trimethoprim (Tmp) and sulfamethoxyzole (Smx) in E. coli. Strain J53 containing each of three plasmids (R1, pSa, and R388) with the type I Sur gene displayed a synergistic response to Tmp/Smx; strain LE392 containing a plasmid (RSF1010) with the type II Sur gene displayed no synergy. The difference in synergy between type I and type II Sur genes might be explained by the difference in amount of resistant enzyme produced.

Plasmid-specified aminoglycoside-modifying enzymes in clinical isolates of Klebsiella pneumoniae

In 106 clinical isolates of multiresistant Klebsiella pneumoniae strains, we found that aminoglycoside-resistance was due mostly to two adenylating enzymes: AAD (2") (56.6%), that modifies gentamicin, kanamycin, tobramycin and sissomicin, and AAD (3") 9 (56.6% + 19.8%) that modifies streptomycin and spectinomycin. The identification of these enzymes was possible by MICs determination against a set of aminoglycosides antibiotics. AAD (2") + AAD (3")9 were coded by conjugative plasmid of about 120 Md.

Plasmid-borne sulfonamide resistance determinants studied by restriction enzyme analysis

The relationship between sulfonamide resistance genes carried on different plasmids was investigated by restriction enzyme analysis and DNA-DNA hybridization. The results showed that sulfonamide resistance mediated by different plasmids is determined by the production of at least two different types of drug-resistant dihydropteroate synthase. Plasmids pGS01, pGS02, and R22259, found in bacteria isolated from patients in Swedish hospitals, contained identical sulfonamide resistance genes, which were also identical to those of plasmids R1, R100, R6, and R388. These latter plasmids, which have been well studied in different laboratories, were originally from clinical isolates from different parts of the world. Two other clinically isolated plasmids, pGS04 and pGS05, were shown to contain sulfonamide resistance determinants of a completely different type.

Characterization of different plasmid-borne dihydropteroate synthases mediating bacterial resistance to sulfonamides

Plasmid-borne resistance to sulfonamides was studied in both newly isolated and earlier characterized R plasmids. Two different classes of drug-resistant dihydropteroate synthases were found to be responsible for most cases of plasmid-mediated sulfonamide resistance. The plasmid-coded enzymes could be completely separated from their chromosomal counterpart and also showed differences in heat stability and molecular size. The resistant and chromosomal enzymes could bind the normal substrate, p-aminobenzoic acid, with equal efficiency. In contrast, sulfonamide binding was about 10,000 times lower with the plasmid-coded enzymes than with the chromosomal enzyme. Another substrate analog, p-aminosalicylic acid, on the other hand, inhibited chromosomal and plasmid-mediated enzymes to a similar extent. Evidence was also found for the existence of a plasmid-borne resistance mechanism independent of drug-insensitive enzymes.