title: Invasion of solid culture media: a widespread phenotypic feature of clinical bacterial isolates.
abstract: OBJECTIVE
  The in-depth growth in solid culture media is a common feature in filamentous fungi and yeasts. However, there are very few bacterial species in which this phenomenon has been documented. The aim of this work was to assess the agar invasiveness of a wide range of Gram-positive and Gram-negative bacterial species of clinical interest.
  
  
  METHODS
  Three different clinical isolates for each of eleven bacterial species were plated onto Columbia blood agar and let grow up to 15 days. Isolated colonies were processed by histological methods, embedded in epoxy resin, and then, semithin sections were stained with toluidine blue and visualized by light microscopy.
  
  
  RESULTS
  Growth within the agar was observed in at least one strain in 9 of the 11 studied species. Invasions of Gram-negative rods were small, not plentiful, and round or triangle-shaped. In Gram-positive cocci, invasions were of big size, abundant and of variable shape (lentiform, globular, irregular, arrowhead) depending on the species.
  
  
  CONCLUSIONS
  We propose that the growth within the agar can indicate a survival strategy common to many bacterial species, and so far, not previously reported. This strategy could be either a nutrient gradient tropism or the spread and colonization of new ecological niches, with potential implications in pathogeny.
authors.affiliation: Department of Anatomical Pathology, Carlos III Hospital, Madrid, Spain. fgomez.hciii@salud.madrid.org
authors.forename: Fernando
authors.initials: F
authors.lastname: Gómez-Aguado
authors.forename: María Teresa
authors.initials: MT
authors.lastname: Corcuera
authors.forename: César
authors.initials: C
authors.lastname: García-Rey
authors.forename: María Luisa
authors.initials: ML
authors.lastname: Gómez-Lus
authors.forename: Carmen
authors.initials: C
authors.lastname: Ramos
authors.forename: María José
authors.initials: MJ
authors.lastname: Alonso
authors.forename: José
authors.initials: J
authors.lastname: Prieto
mesh_terms: D000362:Agar; D015169:Colony Count, Microbial; D004396:Coloring Agents; D003470:Culture Media; D006090:Gram-Negative Bacteria / Q000254:growth & development*; D006094:Gram-Positive Bacteria / Q000254:growth & development*; D010641:Phenotype; D013045:Species Specificity; D014048:Tolonium Chloride
chemical_list: D004396:Coloring Agents; D003470:Culture Media; D014048:Tolonium Chloride; D000362:Agar