Growth in glucose-based medium and exposure to subinhibitory concentrations of imipenem induce biofilm formation in a multidrug-resistant clinical isolate of Acinetobacter baumannii.
BACKGROUND
Acinetobacter baumannii is emerging as an important nosocomial pathogen. Multidrug resistance, as well as ability to withstand environmental stresses, makes eradication of A. baumannii difficult, particularly from hospital settings.


RESULTS
Over a six-year period, 73 isolates of A. baumannii were collected from infected patients in two hospitals in Italy. While 69 out of the 73 isolates displayed identical multidrug antibiotic resistance pattern, they were susceptible to carbapenems. Genetic profiles of these 69 isolates, determined by Pulsed Field Gel Electrophoresis (PFGE), indicated that they were genetically related and could be clustered in a specific clone, called SMAL. We tested the ability of the SMAL clone to form biofilm, an important determinant for bacterial colonization of the human host and for persistence in the hospital environment. Biofilm formation by A. baumannii SMAL, measured as surface adhesion to polystyrene, is strongly affected by growth conditions, being impaired in rich growth media such as LB, while being favoured in glucose-based medium. Surface adhesion in glucose-based media is inhibited by treatment with cellulase, suggesting that it depends on production of cellulose or of a chemically related extracellular polysaccharide. Exposure of A. baumannii SMAL to subinhibitory concentrations of imipenem resulted in biofilm stimulation and increased production of iron uptake proteins. Growth in iron-supplemented medium also stimulated surface adhesion, thus suggesting that increased intracellular iron concentrations might act as an environmental signal for biofilm formation in A. baumannii SMAL.


CONCLUSIONS
Our results indicate that exposure to subinhibitory concentrations of imipenem can stimulate biofilm formation and induce iron uptake in a pathogenic strain of A. baumannii, with potential implications on antibiotic susceptibility and ability to persist in the human host.
Nucleo
Elisabetta
E
Department of Biomolecular Sciences and Biotechnology, Università degli Studi di Milano, Milano, Italy. elisabetta.nucleo@unipv.it
Steffanoni
Laura
L
Fugazza
Giulia
G
Migliavacca
Roberta
R
Giacobone
Ernesto
E
Navarra
Antonella
A
Pagani
Laura
L
Landini
Paolo
P
D000151:Acinetobacter Infections / Q000382:microbiology; D040981:Acinetobacter baumannii / Q000187:drug effects / Q000235:genetics / Q000254:growth & development* / Q000302:isolation & purification; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001422:Bacterial Adhesion; D018441:Biofilms / Q000187:drug effects / Q000254:growth & development*; D003428:Cross Infection / Q000382:microbiology; D003470:Culture Media; D004269:DNA, Bacterial / Q000235:genetics; D024901:Drug Resistance, Multiple, Bacterial; D016521:Electrophoresis, Gel, Pulsed-Field; D020869:Gene Expression Profiling; D005947:Glucose / Q000378:metabolism; D006801:Humans; D015378:Imipenem / Q000494:pharmacology*; D007501:Iron / Q000378:metabolism; D011135:Polysaccharides, Bacterial / Q000096:biosynthesis
D000900:Anti-Bacterial Agents; D003470:Culture Media; D004269:DNA, Bacterial; D011135:Polysaccharides, Bacterial; D015378:Imipenem; D007501:Iron; D005947:Glucose