The genetic relatedness of Streptococcus milleri group isolates from the airways of cystic fibrosis patients was determined by using pulsed-field gel electrophoresis. This study reveals no evidence for patient-to-patient transmission in our patient population; however, within individual patients, complex inter-and intraspecies diversity and dynamics can be observed.In addition to their role in purulent infections (3,7,11), members of the Streptococcus milleri group (SMG), also known as the Streptococcus anginosus group, comprised of Streptococcus constellatus, Streptococcus intermedius, and Streptococcus anginosus, have emerged as clinically relevant in chronic airway infections in cystic fibrosis (CF) patients and have been implicated as etiologic agents of pulmonary exacerbation (6,10,13,14). We have recently described the isolation of a large number of SMG strains from a cohort of CF patients by using the semiselective medium McKay agar (12). This collection of SMG respiratory isolates was not recovered by conventional CF microbiology and enabled us to characterize the phenotypic properties of airway isolates and compare them to invasive strains (5). These results, in combination with analysis of the nucleotide sequence of the 16S rRNA gene of these strains, revealed clusters of isolates that included both CF and invasive isolates with indistinguishable phenotypic characteristics (5, 9).In this study, we evaluated whether patient-to-patient transmission was occurring in our CF patient cohort. The molecular epidemiological relationship of the SMG isolates was determined by using pulsed-field gel electrophoresis (PFGE) (4,8,15).PFGE was performed by modification of a protocol described by Bartie et al. (2). The isolates were cultured at 37°C for 48 h on brain heart infusion agar supplemented with colistin sulfate (10 g/ml) and oxolinic acid (5 g/ml) under anaerobic conditions. The cells were harvested and suspended to 20% transmittance (600 nm) in 100 mM Tris-HCl buffer (pH 7.6). Mutanolysin (100 U; Sigma-Aldrich, St. Louis, MO) was added to 500 l of cell suspension before an equal volume of molten 1% SeaKem Gold agarose (Lonza, Rockland, ME) was added. Plugs were cast at room temperature and then transferred to 1.5 ml of lysis solution (0.25 M EDTA [pH 9.0], 0.5% Brij 58, 2 g/liter sodium deoxycholate, 5 g/liter lauroyl sarcosine, 100 U/ml mutanolysin) and incubated at 37°C for 2 h. The lysis solution was replaced with 1.5 ml ESP solution (0.25 M EDTA [pH 9.5], 1% sodium lauroyl sarcosine, 0.5 mg/ml proteinase K) and incubated at 55°C for 2 h. The plugs were rinsed with 1 ml of distilled water and then washed for 10-min intervals, once with distilled water and three times with 1ϫ Tris-EDTA (TE; 10 mM Tris-Hcl, 1 mM EDTA [pH 8.0]) at room temperature.For restriction digestion, the plugs were preincubated in 300 l of 1ϫ reaction buffer (Invitrogen, Carlsbad, CA) at room temperature for 15 min and then replaced with fresh 1ϫ reaction buffer supplemented with 90 U of SmaI or ApaI (New England BioLabs, Beverly, MA)...