Molecular methods for the epidemiological analysis of methicillin-resistant Staphylococcus aureus (MRSA) and Streptococcus pneumoniae

Anni Vainio. Molecular Methods for the Epidemiological Analysis of MethicillinResistant Staphylococcus aureus (MRSA) and Streptococcus pneumoniae. National Institute for Health and Welfare (THL), Research 71, 164 pages. Tampere, Finland 2012. ISBN 978-952-245-554-3 (printed); ISBN 978-952-245-555-0 (pdf). Methicillin-resistant Staphylococcus aureus (MRSA) and Streptococcus pneumoniae are major health problems worldwide, both found in symptomless carriage but also causing even life-threatening infections. The aim of this thesis was to characterise MRSA and S. pneumoniae in detail by using several molecular typing methods for various epidemiological purposes: clonality analysis, epidemiological surveillance, outbreak investigation, and virulence factor analysis. The characteristics of MRSA isolates from the strain collection of the Finnish National Infectious Disease Register (NIDR) and pneumococcal isolates collected from military recruits and children with acute otitis media (AOM) were analysed using various typing techniques. Antimicrobial susceptibility testing, pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (MLST), spa typing, staphylococcal cassette chromosome mec (SCCmec) typing, and the detection of Panton-Valentine leukocidin (PVL) genes were performed for MRSA isolates. Pneumococcal isolates were analysed using antimicrobial susceptibility testing, serotyping, MLST, and by detecting pilus islet 1 (PI-1) and 2 (PI-2) genes. The epidemiological background data for the pneumococcal pneumonia outbreak investigation were retrieved by interviewing all the military recruits involved in the outbreak. In addition, the medical records of five hospitalised recruits were reviewed. Forty-four previously recognised epidemic MRSA strains (EMRSA) in Finland were grouped into 26 PFGE clusters, 20 MLST sequence types (ST) belonging to 12 clonal complexes (CC), and 27 spa types divided into four spa clonal complexes (spa CC). The CC8, spa CC 051/008, and SCCmec type IV were the most prevalent among these strains. In addition, 52% of the strains were multiresistant to antimicrobials. FIN-4 and FIN-16 isolates mainly associated with spa type t172 (2006: 81% and 1997–2006: 92%) and t067 (2006: 93% and 1997–2006: 80%), respectively. The 124 MRSA blood isolates from 1997–2006 were divided into 19 PFGE types and 38 spa types. The most prevalent spa types were t067 (32%), t041 (13%) and t172 (10%). Typeability, discriminatory power, concordance, Wallace coefficient, and the consumption of time and money was compared between MRSA typing methods. The overall typeability of the methods was high, above 89%. PFGE clusters, MLST and spa typing showed a high discriminatory power. The concordance was highest THL –Research 71/2012 5 Molecular Methods for the Epidemiological Analysis of MRSA and S. pneumoniae between MLST and PFGE clusters. spa typing demonstrated a high probability to predict PFGE, MLST ST and MLST CC, as well as SCCmec types, depending on the study material. Compared to PFGE, spa typing was found to be more expensive but approximately four times faster to perform. S. pneumoniae serotype 7F and genotype ST2331 associated with an outbreak of pneumonia and nasopharyngeal carriage among 43 military recruits in Finland in 2006. Of these recruits, five (12%) were hospitalised due to pneumonia and two of them were found to have positive blood cultures for S. pneumoniae. Forty-two per cent of the military recruits carried S. pneumoniae in their nasopharynx. Half of these isolates had the same serotype 7F, ST2331. Among the 75 pneumococcal isolates from middle ear fluid (MEF) and/or nasopharyngeal aspirate (NPA) samples of children with acute otitis media (AOM), 14 different serotypes were detected. PI-1 genes were present among 20% of the isolates. PI-2 genes were not found. The 52 isolates from 33 children genotyped by MLST belonged to 30 different STs and 18 known CCs. PI-1 was associated with genotype CC490 and serotypes 6A, 6B and 9V. Several international communityand hospital-associated MRSA clones were recognised in Finland. The genetic diversity among MRSA FIN-4 isolates and among FIN-16 isolates was low. Overall, MRSA blood isolates from 1997 to 2006 were genetically diverse. spa typing was found to be a highly discriminatory, rapid and accurate typing method and it also qualifies as the primary typing method in countries with a long history of PFGE-based MRSA strain nomenclature. However, additional typing by another method, e.g. PFGE, is needed in certain situations to be able to provide adequate discrimination for epidemiological surveillance and outbreak investigation. An outbreak of pneumonia was associated with one pneumococcal strain among military recruits, previously healthy young men living in a crowded setting. The pneumococcal carriage rate after the outbreak was found to be exceptionally high. PI-1 genes were detected at a rather low prevalence among pneumococcal isolates from children with AOM. However, the study demonstrated that PI-1 has existed among pneumococcal isolates prior to pneumococcal conjugate vaccine and the increased antimicrobial resistance era. Moreover, PI-1 was found to associate with the serotype rather than the genotype.