Staphylococcus aureus CC398: host adaptation and emergence of methicillin resistance in livestock

Identification of acquired antimicrobial resistance genes

Objectives

Identification of antimicrobial resistance genes is important for understanding the underlying mechanisms and the epidemiology of antimicrobial resistance. As the costs of whole-genome sequencing (WGS) continue to decline, it becomes increasingly available in routine diagnostic laboratories and is anticipated to substitute traditional methods for resistance gene identification. Thus, the current challenge is to extract the relevant information from the large amount of generated data.

Methods

We developed a web-based method, ResFinder that uses BLAST for identification of acquired antimicrobial resistance genes in whole-genome data. As input, the method can use both pre-assembled, complete or partial genomes, and short sequence reads from four different sequencing platforms. The method was evaluated on 1862 GenBank files containing 1411 different resistance genes, as well as on 23 de-novo-sequenced isolates.

Results

When testing the 1862 GenBank files, the method identified the resistance genes with an ID = 100% (100% identity) to the genes in ResFinder. Agreement between in silico predictions and phenotypic testing was found when the method was further tested on 23 isolates of five different bacterial species, with available phenotypes. Furthermore, ResFinder was evaluated on WGS chromosomes and plasmids of 30 isolates. Seven of these isolates were annotated to have antimicrobial resistance, and in all cases, annotations were compatible with the ResFinder results.

Conclusions

A web server providing a convenient way of identifying acquired antimicrobial resistance genes in completely sequenced isolates was created. ResFinder can be accessed at www.genomicepidemiology.org. ResFinder will continuously be updated as new resistance genes are identified.

On the shifting balance: the case of Staphylococcus aureus CC398

The recent study by L. Price et al. [mBio, 3(1):e00305-11, 2012] demonstrating the human origin of the livestock-associated CC398 Staphylococcus aureus provides an excellent example of how bacterial populations can explore a wide range of potential niches and acquire optimal adaptations for life in alternative hosts. The shifting balance theory proposed by Sewall Wright 80 years ago serves as a perfect model for understanding the observed facts: a possibly large CC398 population optimally positioned on the top of a fitness peak for living in a human host probably began evolving long ago by variation, drift, and migration, until it reached the low fitness edge of a neighboring alternative peak in the fitness landscape, one in a livestock host. The bacterial population then evolved again, moving uphill to reach a novel optimal "top of the peak" position in livestock. In the case of CC398, it is worrisome to think that it might readapt to human hosts without losing fitness in livestock; a double-host-adapted organism could certainly be in an optimal position for increasing its virulence and antibiotic resistance.

Human origin for livestock-associated methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of human morbidity and mortality worldwide. The emergence in the last decade of a livestock-associated MRSA (LA-MRSA) clone which also has the capacity to cause zoonotic infections in humans has raised important questions regarding its origin and its potential to cause human epidemics. An important study by L. B. Price et al. [mBio 3(1):e00305-11, 2012] provides evidence for a human ancestral origin for LA-MRSA, raising concerns about agricultural practices that may have contributed to its emergence and expansion. The study highlights the potential for comparative whole-genome sequencing of closely related strains to provide valuable insights into the evolutionary history of bacterial pathogens.