title: Epidemic MRSA clone ST22-IV is more resistant to multiple host- and environment-related stresses compared with ST228-I.
abstract: BACKGROUND
  ST22-IV is a successful hospital-associated MRSA clone. Due to its known ability to replace other MRSA clones in hospitals, it became a dominant clone in Europe and beyond. So far, there are no studies investigating the relationship between the epidemiological success of MRSA clones and their capacity to withstand commonly encountered stresses.
  
  
  METHODS
  We investigated the fitness of ST22-IV in comparison with the replaced clone ST228-I, evaluating its resistance to oxidative stress, autolytic activity, growth at high osmolarity and in acid and alkaline environments and survival under desiccation and heat shock. We also compared their phenotypic characteristics and examined the impact of antibiotic consumption on epidemiological success.
  
  
  RESULTS
  Here we demonstrate that the dominance of ST22-IV is linked neither to changes in antibiotic consumption nor to acquisition of additional resistances over time. Strong α-haemolysin activity, the production of β-haemolysin and the presence of an active agr could partly explain the virulence of ST22-IV previously observed in a murine model of pneumonia. Most importantly, we show that ST22-IV compared with ST228-I, besides retaining susceptibility to most antibiotics over time, has a superior capacity to survive under all stress conditions tested, which bacteria commonly face during their life cycle.
  
  
  CONCLUSIONS
  Our results support our hypothesis that ST22-IV has a fitness advantage over ST228-I. This fitness advantage could have allowed ST22-IV to displace ST228-I without acquiring additional resistances and could help explain its epidemic success in hospital settings and its spread in Europe and beyond.
authors.affiliation: Emerging Bacterial Pathogens Unit, Division of Immunology, Transplantation and Infectious Diseases, IRCCS San Raffaele Scientific Institute, Milan, Italy.
authors.forename: Rossella
authors.initials: R
authors.lastname: Baldan
authors.affiliation: University Centre for Statistics in the Biomedical Sciences, Università Vita-Salute San Raffaele, Milan, Italy.
authors.forename: Paola Maria Vittoria
authors.initials: PM
authors.lastname: Rancoita
authors.affiliation: University Centre for Statistics in the Biomedical Sciences, Università Vita-Salute San Raffaele, Milan, Italy.
authors.forename: Clelia
authors.initials: C
authors.lastname: Di Serio
authors.affiliation: Department of Pharmacy, IRCCS Ospedale San Raffaele, Milan, Italy.
authors.forename: Maria
authors.initials: M
authors.lastname: Mazzotti
authors.affiliation: Department of Microbiology, IRCCS Ospedale San Raffaele, Milan, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Cichero
authors.affiliation: Department of Microbiology, IRCCS Ospedale San Raffaele, Milan, Italy.
authors.forename: Cristina
authors.initials: C
authors.lastname: Ossi
authors.affiliation: IRCCS Ospedale San Raffaele, Milan, Italy.
authors.forename: Anna
authors.initials: A
authors.lastname: Biancardi
authors.affiliation: IRCCS Ospedale San Raffaele, Milan, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Nizzero
authors.affiliation: IRCCS Ospedale San Raffaele, Milan, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Saracco
authors.affiliation: Department of Infectious Diseases, IRCCS Ospedale San Raffaele, Milan, Italy.
authors.forename: Paolo
authors.initials: P
authors.lastname: Scarpellini
authors.affiliation: Emerging Bacterial Pathogens Unit, Division of Immunology, Transplantation and Infectious Diseases, IRCCS San Raffaele Scientific Institute, Milan, Italy cirillo.daniela@hsr.it.
authors.forename: Daniela Maria
authors.initials: DM
authors.lastname: Cirillo
mesh_terms: D000143:Acids / Q000633:toxicity; D000468:Alkalies / Q000633:toxicity; D001433:Bacteriolysis; D005060:Europe / Q000453:epidemiology; D005838:Genotype; D006801:Humans; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects / Q000235:genetics / Q000254:growth & development / Q000502:physiology*; D009997:Osmotic Pressure; D018384:Oxidative Stress; D013203:Staphylococcal Infections / Q000453:epidemiology / Q000382:microbiology; D013312:Stress, Physiological; D037521:Virulence Factors / Q000235:genetics
chemical_list: D000143:Acids; D000468:Alkalies; D037521:Virulence Factors
keywords: S. aureus; fitness; stress resistance