JAC Advance Access originally published online on November 24, 2004
Journal of Antimicrobial Chemotherapy 2005 55(1):45-50; doi:10.1093/jac/dkh479
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JAC vol.55 no.1 © The British Society for Antimicrobial Chemotherapy 2004; all rights reserved
Viridans streptococci in endocarditis and neutropenic sepsis: biofilm formation and effects of antibiotics
1 Department of Medicine I, Division of Infectious Diseases, Medical University of Vienna, Waehringer Guertel 18–20, 1090 Vienna; 2 Institute of Hygiene, Medical University of Graz, Universitätsplatz 4, 8010 Graz; 3 Division of Clinical Microbiology, Institute of Hygiene and Medical Microbiology, University of Vienna, Allgemeines Krankenhaus, Waehringer Guertel 18–20, 1090 Vienna, Austria
* Corresponding author. Tel: +43-1-40400-4440; Fax: +43-1-40400-4418; Email: elisabeth.presterl{at}meduniwien.ac.at
Objectives: Viridans group streptococci (VGS) are a frequent cause of bacterial endocarditis or sepsis in patients with neutropenia. Endocarditis in particular, is associated with plaque formation on the endocardium and valve leaflets whereas VGS septicaemia in neutropenic patients is caused by the influx of oral flora bacteria through mucositic lesions. This study examined the in vitro potency for biofilm formation of clinical VGS bloodstream isolates, and the effects of antibiotics on these biofilms.
Methods: During the years 1998–2000, 40 VGS bloodstream isolates from 18 patients with endocarditis and 22 patients with severe sepsis and neutropenia were collected. The MICs of penicillin, teicoplanin and moxifloxacin were determined using the microdilution broth method according to NCCLS criteria. Biofilms were grown in microtitre plates, dyed with Crystal Violet, and the mean optical density (OD) was used for quantification. Biofilms were incubated with penicillin, teicoplanin and moxifloxacin at various concentrations starting with the MICs for the respective isolates tested.
Results: Isolates from eight out of 18 patients with endocarditis and six out of 22 patients with neutropenia formed biofilms (not significant). For the 14 isolates, the MIC90s (range) of penicillin, teicoplanin and moxifloxacin were 0.5 mg/L (0.001–0.5), 0.125 mg/L (0.025–0.125) and 0.5 mg/L (0.05–0.5), respectively. Generally, biofilms persisted although incubated with the antibiotics up to concentrations of 128x MIC. However, the ODs of biofilms after incubation with an antibiotic were significantly lower than the ODs of biofilms without antibiotic (P<0.05). A significant decrease in the biofilms with increasing antibiotic concentrations was observed for teicoplanin and moxifloxacin, but not for penicillin G.
Conclusions: VGS isolated from patients with endocarditis and patients with sepsis and neutropenia form biofilms. Biofilms persist even when exposed to antibiotics at concentrations up to 128x MIC. Nevertheless, teicoplanin and moxifloxacin reduced the density of the biofilms at concentrations 
16xMIC. Thus, testing the effects of antibiotics on biofilms may supply useful information in addition to standard in vitro testing, particularly in diseases where biofilm formation is involved in the pathogenesis.
Keywords: neutropenia , penicillin , teicoplanin , moxifloxacin
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