Molecular Diagnostics of Staphylococcus aureus

The bacterial species of the genus Staphylococcus are important human and animal pathogens which cause severe infectious diseases. The most pathogenic species Staphylococcus aureus is an extraordinary versatile pathogen and the major causative agent of numerous hospital- and community acquired infections. The disease spectrum includes abscesses, bacteremia, central nervous system infections, endocarditis, osteomyelitis, pneumonia, urinary tract infections, and syndromes caused by exotoxins, including staphylococcal scaled skin syndrome, toxic shock syndrome and food poisoning. The spread, survival and prevalence of antibiotic resistant clones of S. aureus are immensely important problems for human health. Apart from the major pathogen S. aureus, other species of the genus Staphylococcus may also be involved in serious infections. The rapid and accurate identification of the disease causing agent is therefore a prerequisite for disease control as well as for epidemiological surveillance. Modern molecular methods for the identification and typing of bacterial species are based on their genomic and proteomic analysis, which is very advantageous compared to standard biotyping methods. For species identification of staphylococcal isolates, the most frequently used molecular method is amplification of conserved gene sequences by PCR. Genotypic methods focus on the characterization of chromosomal, plasmid, or total genomic DNA. The objectives of these methods are to assess relevant parameters of the staphylococcal genome and to detect polymorphism of DNA sequences with either direct or indirect methods. All the genotypic methods are aimed in generation of DNA fingerprints specific for individual species and/or isolates of distinct clonal lineages. S. aureus genomes comprise a core component consisting of genes present in all of the strains, and approximately 22% of the genome comprises an "accessory genome", containing genes that encode a diverse array of non-essential functions, ranging from virulence, drug and metal resistance to substrate utilization and miscellaneous metabolism. The accessory genome is made up of a mobile genetic element (MGE), e.g. bacteriophages, pathogenicity islands, staphylococcal cassette chromosomes, plasmids and transposons. Given the steadily varying content of MGE and of their individual types in the S. aureus strains, these elements are the sources of genetic polymorphism and are one of the most important targets of molecular typing methods for S. aureus. Some S. aureus strains produce one or more exoproteins, which include toxic shock syndrome toxin-1, the staphylococcal enterotoxins, the exfoliative toxins and Panton-Valentine leukocidin. PCR for detection of relevant genes in S. aureus strains is the most commonly used method for the diagnostics of toxigenic staphylococci.