Increased antimicrobial resistance in bacteria has caused a global crisis in human health care. Strains of pathogenic bacteria resistant to all known antibiotics have been found.

Antimicrobial resistance is a trait that can spread within a bacterial species or even across the species barrier. Resistance will expand and become increasingly prevalent when bacteria that survive in an environment containing antibiotics genetically pass on or otherwise distribute this trait to other bacteria.

Recent studies indicate that even weak antibiotic concentrations are sufficient to cause a rise in the prevalence of antimicrobial resistance in bacterial populations cultured in laboratory conditions. Such conditions are brought about when antibiotics used to treat both human and animal infections end up in sewage or elsewhere in the environment.

"However, it should be noted that in natural conditions bacteria are a part of multispecies communities and exist in interaction with viruses and protozoa that hunt for bacteria. That's why the matter requires more study," says Johannes Cairns, who defended his dissertation at the Faculty of Agriculture and Forestry, University of Helsinki, Finland.

Low antibiotic concentrations increased antimicrobial resistance in laboratory-cultured microbial communities equally well if not better compared to single species settings. Surprisingly, no antibiotics at all were necessary to see a rise in the prevalence of antimicrobial resistance.

"We found that antimicrobial resistance can also become more prevalent due to the presence of protozoa that prey on bacteria or in conditions where the bacteria had settled on a surface as a biofilm, or a compact community," notes Cairns.