Multidrug Resistance (MDR) and Collateral Sensitivity in Bacteria, with Special Attention to Genetic and Evolutionary Aspects and to the Perspectives of Antimicrobial Peptides-A Review

Antibiotic poly-resistance (multidrug-, extreme-, and pan-drug resistance) is controlled by adaptive evolution. Darwinian and Lamarckian interpretations of resistance evolution are discussed. Arguments for, and against, pessimistic forecasts on a fatal “post-antibiotic era” are evaluated. In commensal niches, the appearance of a new antibiotic resistance often reduces fitness, but compensatory mutations may counteract this tendency. The appearance of new antibiotic resistance is frequently accompanied by a collateral sensitivity to other resistances. Organisms with an expanding open pan-genome, such as Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae, can withstand an increased number of resistances by exploiting their evolutionary plasticity and disseminating clonally or poly-clonally. Multidrug-resistant pathogen clones can become predominant under antibiotic stress conditions but, under the influence of negative frequency-dependent selection, are prevented from rising to dominance in a population in a commensal niche. Antimicrobial peptides have a great potential to combat multidrug resistance, since antibiotic-resistant bacteria have shown a high frequency of collateral sensitivity to antimicrobial peptides. In addition, the mobility patterns of antibiotic resistance, and antimicrobial peptide resistance, genes are completely different. The integron trade in commensal niches is fortunately limited by the species-specificity of resistance genes. Hence, we theorize that the suggested post-antibiotic era has not yet come, and indeed might never come.

Rapid identification of Arabidopsis insertion mutants by non-radioactive detection of T-DNA tagged genes

To assist in the analysis of plant gene functions we have generated a new Arabidopsis insertion mutant collection of 90 000 lines that carry the T-DNA of Agrobacterium gene fusion vector pPCV6NFHyg. Segregation analysis indicates that the average frequency of insertion sites is 1.29 per line, predicting about 116 100 independent tagged loci in the collection. The average T-DNA copy number estimated by Southern DNA hybridization is 2.4, as over 50% of the insertion loci contain tandem T-DNA copies. The collection is pooled in two arrays providing 40 PCR templates, each containing DNA from either 4000 or 5000 individual plants. A rapid and sensitive PCR technique using high-quality template DNA accelerates the identification of T-DNA tagged genes without DNA hybridization. The PCR screening is performed by agarose gel electrophoresis followed by isolation and direct sequencing of DNA fragments of amplified T-DNA insert junctions. To estimate the mutation recovery rate, 39 700 lines have been screened for T-DNA tags in 154 genes yielding 87 confirmed mutations in 73 target genes. Screening the whole collection with both T-DNA border primers requires 170 PCR reactions that are expected to detect a mutation in a gene with at least twofold redundancy and an estimated probability of 77%. Using this technique, an M2 family segregating a characterized gene mutation can be identified within 4 weeks.