Selection of very small differences in bacterial evolution.

As the Science of Biology is constantly changing due to new discoveries and advanced techniques it is essential that a systematic study of the environmental causes of natural selection on microorganisms be conducted. Very small phenotypic differences among individuals within bacterial populations arise as a result of spontaneous genetic variation, but the evolutionary importance of these small changes is frequently considered to be non-significant. Recent in vitro experiments indicate that efficient selection of these very small differences may take place in environmental compartments where a particular intensity of the selective agent is exerted. Model studies based on competition between bacterial populations only differing in one or two amino acid changes of a detoxifying antibiotic enzyme (e.g. beta-lactamase) have shown that at a narrow range of antibiotic concentrations the variant population is strongly selected over the original type, despite the extremely low phenotypic differences in antibiotic susceptibility. These selective concentrations are expected to occur in precise environmental compartments (selective compartments). Due to the high frequency of structured habitats in natural environments, the intensity of selective agents is commonly exerted along certain gradients. Each one of the points forming these gradients (or intersection among gradients) may have a particular selective ability for a specific genetic variant. Considering the environment as a composition of an extremely high number of specific selective compartments may help to understand the existence of high levels of genetic variability in natural bacterial populations. This may be one of the clues towards the unraveling of bacterial evolution.     

Multifactorial determination of systemic invasivity in Escherichia coli

Abstract Studies of aerobactin excretion, production of antimicrobial agents, haemolysin production, serum resistance and presence of plasmids were made in 157 clinical isolates of Escherichia coli , 96 from blood and 61 from faeces. Only serum resistance and the presence of a small number of plasmids (< 4 per cell), appeared more frequently in strains from blood than in faecal isolates. It was noteworthy that combination of aerobactin, antimicrobial agents and haemolysin production was more frequent in blood isolates than in strains from faeces. A substantial number of strains from blood presented all 5 virulence factors (9.4%) while none of the faecal strains exhibited this phenotype. Definition of the systemic invasivity of E. coli as a multi-factorial phenomenon is suggested.     

Sewage dilution and loss of antibiotic resistance and virulence determinants in E. coli

The possession of potential virulence factors (serum resistance, aerobactin production, colicin production) and antibiotic resistance was evaluated in 418 E. coli strains isolated from river water. The strains were isolated from 11 points showing different levels of contamination. From the data obtained, it can be concluded that bacteria from less contaminated water present less antibiotic resistance and virulence factors than those isolated from highly contaminated water. This situation suggests that E. coli strains producing plasmid encoded antibiotic resistance and/or virulence factors, survive less well in aquatic environments without selective pressure) than the sensitive non-virulent ones.