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PubMed1974

The pertinence of the periodic selection phenomenon to prokaryote evolution.

A quarter of a century ago, it was pointed out that evolution can act in an important conservative way, in addition to its normal progressive mode. Evolution to a fitter form via changes at one locus means that the descendants of an individual with an improved locus or set of loci will supplant the previous population and carry with them the bulk of the total genotype of that original individual in asexual populations. Inasmuch as that individual is most likely to be wild type at most other loci, neutral and even other positively selected mutations will be reduced or eliminated from the population, if they are rare at the time of the evolutionary advance. In the present paper this problem has been set up for a computer simulation. The computations show the limits within which this effect functions and the conditions under which it does not. The conclusion is that it is likely that evolution at a locus proceeds in the course of many population replacements or revolutions, mostly via the rare occasions when the revolution carries a previously infrequent mutational type into abundance.

Biological Evolution↗
PubMed1985

MLS-resistance determinants in Staphylococcus aureus and their molecular evolution.

This paper describes the genetic phenomenology of resistance to macrolide-lincosamide-streptogramin B antibiotics (MLSr) in Staphylococcus aureus and attempts to place this phenomenology in a broad evolutionary context. As antibiotic resistance in general and MLS resistance in particular are typical variable traits in bacteria of clinical interest, we shall begin by introducing the concept of variable genetic traits, as outlined in Figure 1. Variable traits are those that are expressed by some strains of a given species but not by others--in comparison to constant traits which are always present as part of the standard genetic make-up of the species and have constant chromosomal locations. Variable traits are often associated with variable and mobile genetic elements and it is suggested that, in general, they are not likely to have evolved as such in the species in which they are found. Rather, they will most probably have evolved as constant (chromosomal) traits in other species and acquired genetic mobility much later as a rare occurrence in that species. These rarely occurring mobile variants would then spread horizontally within a range of new species. The MLSr determinants in Gram-positive bacteria would appear to represent a classic example of this process. Their remarkable variability will be described as the extant end-point of the process and a probable evolutionary pathway will be traced back to the streptomycetes which are a likely primary source.

Anti-Bacterial Agents↗
PubMed1985

Evolution and epidemiology of MLS resistance.

Within the framework of this symposium, it is not feasible to present an exhaustive description of the present state of knowledge regarding the sensitivity and resistance of bacterial species to macrolides, lincosamides and streptogramins (MLS). This paper is limited to a description of the evolution of different types of resistance in the light of decisive factors described in previous papers, in order to deduce, if at all possible, trends in future strategy in therapeutics. Only acquired resistance lends itself to epidemiological study, in contrast to natural resistance which is, by definition, characteristic of a species or a genus, and not liable to change. Three groups will therefore be studied in turn: Staphylococcus aureus, streptococci and Bacteroides fragilis. There is as yet insufficient accumulated data to draw conclusions regarding the epidemiology and evolution of MLSB resistance observed in Clostridium perfringens and Corynebacterium diphtheriae, or regarding the high-level resistance to erythromycin due to enzymatic inactivation recently described in Escherichia coli.

Anti-Bacterial Agents↗
PubMed1986

Epidemiology of antibiotic resistance in Staphylococcus aureus.

The genetic equipment of Staphylococcus aureus is at least as comprehensive as other organisms. Transposons provide the potential for reassortment of genes between plasmids and the chromosome. At least six different mechanisms of gene transfer between cells are documented in vitro. Phage-mediated conjugation is the transfer mechanism most likely to occur between staphylococci in nature. MRSA have evolved from a single clone and are now heterogeneous in properties. Some may show decreased virulence. The origin of new resistant determinants is likely to be other human cultures of Staph. aureus rather than an animal staphylococcal reservoir.

Anti-Bacterial Agents↗
PubMed1986

New observations regarding evolution of trimethoprim resistance.

A clinically isolated strain of Escherichia coli, resistant to more than 1000 mg/l of trimethoprim, expressed chromosomal dihydrofolate reductase to a level 200-fold higher than that of drug sensitive E. coli K-12 strains, and this high cellular enzyme activity was found to increase further when the cells were cultured in the presence of trimethoprim. The induced increase in enzyme activity was dependent on the drug concentration. The increase was six-fold at 100 mg/l of trimethoprim. The aberrantly regulated dihydrofolate reductase gene mediating trimethoprim resistance could be transduced into E. coli K-12 or moved by recombination into an F' factor and then transferred into trans position in relation to the corresponding chromosomal gene. In either of these positions, the synthesis of dihydrofolate reductase could be induced to increase by adding trimethoprim to the culture medium. The observed induction was dependent on protein synthesis, since it could be abolished by chloramphenicol. No other folic acid analogue was found to induce increased expression of the dihydrofolate reductase gene. Also thymine starvation had no effect. Two further clinical isolates of E. coli, highly resistant to trimethoprim, were shown to produce drug resistant, plasmid-mediated dihydrofolate reductases, which were distinct from the earlier known enzyme types I and II.

Bacteria↗
PubMed1986

Evolution and transfer of aminoglycoside resistance genes under natural conditions.

3'-Aminoglycoside phosphotransferases [APH(3')] were chosen as a model to study the evolution and the transfer of aminoglycoside resistance genes under natural conditions. Comparison of the amino acid sequences of APH(3') enzymes from transposons Tn903 (type I) and Tn5 (type II) detected in Gram-negative bacteria, from the Gram-positive Staphylococcus and Streptococcus (type III), from the butirosin-producing Bacillus circulans (type IV) and from a neomycin-producing Streptomyces fradiae (type V) indicate that they have diverged from a common ancestor. These structural data support the hypothesis that the antibiotic-producing strains were the source of certain resistance determinants. We have shown that kanamycin resistance in Campylobacter coli BM2509 was due to the synthesis of an APH(3')-III, an enzyme not detected previously in a Gram-negative bacterium. The genes encoding APH(3')-III in Streptococcus and Campylobacter are identical. These findings constitute evidence for a recent in-vivo transfer of DNA between Gram-positive and Gram-negative bacteria.

Aminoglycosides↗
PubMed1987

Origin and evolution of genes specifying resistance to macrolide, lincosamide and streptogramin antibiotics: data and hypotheses.

Resistance to macrolide, lincosamide and streptogramin antibiotics is due to alteration of the target site or detoxification of the antibiotic. Postranscriptional methylation of 23S ribosomal rRNA confers resistance to macrolide (M), lincosamide (L) and streptogramin (S) B-type antibiotics, the so-called MLSB phenotype. Several classes of rRNA methylases conferring resistance to MLSB antibiotics have been characterized in Gram-positive cocci, in Bacillus spp, and in strains of actinomycetes producing erythromycin. The enzymes catalyze N6-dimethylation of an adenine residue situated in a highly conserved region of prokaryotic 23S rRNA. In this review, we compare the amino acid sequences of the rRNA methylases and analyze the codon usage in the corresponding erm (erythromycin resistance methylase) genes. The homology detected at the protein level is consistent with the notion that an ancestor of the erm genes was implicated in erythromycin resistance in a producing strain. However, the rRNA methylases of producers and non-producers present substantial sequence diversity. In Gram-positive bacteria the preferential codon usage in the erm genes reflects the guanosine plus cytosine content of the chromosome of the host. These observations suggest that the presence of erm genes in these micro-organisms is ancient. By contrast, it would appear that enterobacteria have acquired only recently an rRNA methylase gene of the ermB class from a Gram-positive coccus since the genes isolated in Escherichia coli and in Gram-positive cocci are highly homologous (homology greater than 98%) and present a codon usage typical of the latter micro-organisms. As opposed to the MLSB phenotype which results from a single biochemical mechanism, inactivation of structurally related antibiotics of the MLS group involves synthesis of various other enzymes. In enterobacteria, resistance to erythromycin and oleandomycin is due to production of erythromycin esterases which hydrolyze the lactone ring of the 14-membered macrolides. We recently reported the nucleotide sequence of ereA and ereB (erythromycin resistance esterase) genes which encode erythromycin esterases type I and II, respectively. The amino acid sequences of the two isozymes do not exhibit statistically significant homology. Analysis of codon usage in both genes suggests that esterase type I is indigenous to E. coli, whereas the type II enzyme was acquired by E. coli from a phylogenetically remote micro-organism. Inactivation of lincosamides, first reported in staphylococci and lactobacilli of animal origin, was also recently detected in Gram-positive cocci isolated from humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1992

On the evolution of functional secondary metabolites (natural products).

It is argued that organisms have evolved the ability to biosynthesize secondary metabolites (natural products) because of the selectional advantages they obtain as a result of the functions of the compounds. The clustering together of antibiotic biosynthesis, regulation, and resistance genes implies that these genes have been selected as a group and that the antibiotics function in antagonistic capacities in nature. Pleiotropic switching, the simultaneous expression of sporulation and antibiotic biosynthesis genes, is interpreted in terms of the defence roles of antibiotics. We suggest a general mechanism for the evolution of secondary metabolite biosynthesis pathways, and argue against the hypothesis that modern antibiotics had prebiotic effector functions, on the basis that it does not account for modern biosynthetic pathways.

Anti-Bacterial Agents↗
PubMed1987

Origin, evolution and dissemination of antibiotic resistance genes.

Comparison of resistance genes from different sources support the hypothesis that the antibiotic-producing microorganisms are the source of resistant determinants present in clinical isolates. There is also evidence that Gram-positive cocci (staphylococci and streptococci) can serve as a reservoir of resistance genes for Gram-negative bacteria.

Amino Acid Sequence↗
PubMed1988

[Evolution of bacterial resistance to five aminoglycosides. A study of 3,354 strains isolated in a hospital milieu].

The authors studied the susceptibility to 5 aminoglycosides (amikacin, dibekacin, gentamicin, netilmicin and tobramycin) of 3,354 strains isolated at the Centre Hospitalier Sud in Bordeaux during 1987. The results are compared to those obtained in 1984 on 2,818 strains. Amikacin remains the most active aminoside against the Enterobacteriaceae and Acinetobacter; against Pseudomonas, tobramycin has become the best one at that time, as well as netilmicin against Staphylococcus aureus. Evolution: no significative increase of Enterobacteriaceae resistance to aminoglycosides was observed during the last 3 years except for Providencia and Serratia. For Acinetobacter and Pseudomonas, percentage of resistant strains is respectively two-fold and three-fold higher. Although resistance increased in that species, netilmicin and amikacin showed a still good activity against Staphylococcus aureus.

Acinetobacter↗