title: Viable group A streptococci in macrophages during acute soft tissue infection.
abstract: BACKGROUND
  Group A streptococcal severe soft tissue infections, such as necrotizing fasciitis, are rapidly progressive infections associated with high mortality. Group A streptococcus is typically considered an extracellular pathogen, but has been shown to reside intracellularly in host cells.
  
  
  RESULTS
  We characterized in vivo interactions between group A streptococci (GAS) and cells involved in innate immune responses, using human biopsies (n = 70) collected from 17 patients with soft tissue infections. Immunostaining and in situ image analysis revealed high amounts of bacteria in the biopsies, even in those collected after prolonged antibiotic therapy. Viability of the streptococci was assessed by use of a bacterial viability stain, which demonstrated viable bacteria in 74% of the biopsies. GAS were present both extracellularly and intracellularly within phagocytic cells, primarily within macrophages. Intracellular GAS were predominantly noted in biopsies from newly involved tissue characterized by lower inflammation and bacterial load, whereas purely extracellular GAS or a combination of intra- and extracellular GAS dominated in severely inflamed tissue. The latter tissue was also associated with a significantly increased amount of the cysteine protease streptococcal pyrogenic exotoxin SpeB. In vitro studies confirmed that macrophages serve as reservoirs for viable GAS, and infection with a speB-deletion mutant produced significantly lower frequencies of cells with viable GAS following infection as compared to the wild-type bacteria.
  
  
  CONCLUSIONS
  This is the first study to demonstrate that GAS survive intracellularly in macrophages during acute invasive infections. This intracellular presence may have evolved as a mechanism to avoid antibiotic eradication, which may explain our finding that high bacterial load is present even in tissue collected after prolonged intravenous antibiotic therapy. This new insight into the pathogenesis of streptococcal soft tissue infections highlights a need for alternative therapeutic strategies.
authors.affiliation: Center for Infectious Medicine, Karolinska Institute, Department of Medicine, Karolinska University Hospital, Stockholm, Sweden.
authors.forename: Pontus
authors.initials: P
authors.lastname: Thulin
authors.forename: Linda
authors.initials: L
authors.lastname: Johansson
authors.forename: Donald E
authors.initials: DE
authors.lastname: Low
authors.forename: Bing S
authors.initials: BS
authors.lastname: Gan
authors.forename: Malak
authors.initials: M
authors.lastname: Kotb
authors.forename: Allison
authors.initials: A
authors.lastname: McGeer
authors.forename: Anna
authors.initials: A
authors.lastname: Norrby-Teglund
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism; D005098:Exotoxins / Q000378:metabolism; D006801:Humans; D008264:Macrophages / Q000166:cytology / Q000382:microbiology*; D050296:Microbial Viability; D009000:Monocytes / Q000166:cytology / Q000382:microbiology; D010587:Phagocytosis / Q000502:physiology; D018461:Soft Tissue Infections / Q000276:immunology / Q000382:microbiology*; D013297:Streptococcus pyogenes / Q000472:pathogenicity / Q000502:physiology*
chemical_list: D001426:Bacterial Proteins; D005098:Exotoxins; C027951:erythrogenic toxin