Macrolide and clindamycin resistance in Streptococcus milleri group isolates from the airways of cystic fibrosis patients

Abstract

Organisms belonging to the Streptococcus milleri group (SMG) are known for their role in pyogenic infections but have recently been implicated as etiological agents of pulmonary exacerbation in adult patients with cystic fibrosis (CF). The prolonged exposure of CF patients to antibiotics prompted us to investigate the susceptibility profiles of 118 SMG isolates from the airways of CF patients to 12 antibiotics compared to 43 SMG isolates from patients with invasive infections. We found that approximately 60% of all isolates failed to grow using the standard medium for disc diffusion, Mueller-Hinton blood agar (MHBA), so we explored the usefulness of brain heart infusion (BHI) agar for susceptibility testing. Zone-of-inhibition comparisons between BHI and MHBA showed strong correlations for six antibiotics, and interpretations were similar for both medium types. For ceftriaxone and cefepime, both groups of isolates were highly susceptible. Tetracycline resistance levels were comparable between the two groups (22% in CF isolates and 17.4% in invasive isolates). However, more than half of the CF isolates were not susceptible to azithromycin, erythromycin, and clindamycin, compared to 11%, 13%, and 6.5% of invasive isolates, respectively. There were 5-fold and 8-fold increased risks of azithromycin and clindamycin resistance, respectively, for the isolates from the airways of CF patients relative to the invasive isolates. Macrolide resistance was strongly linked to chronic azithromycin therapy in CF patients. This study shows that BHI agar is a suitable alternative for antimicrobial susceptibility testing for the SMG and that SMG isolates from the airways of CF patients are more resistant to macrolides and clindamycin than strains isolated from patients with invasive infections.

FIG. 1.

(A) Percentage of S. anginosus, S. constellatus, and S. intermedius isolates used in this study. (B) Percentage of species cultivated on the standard medium for susceptibility testing, MHBA, and BHI.

FIG. 2.

The relationship of the zone-of-inhibition sizes for SMG isolates that were cultivated on MHBA and BHI is illustrated with a simple linear regression model. For each antibiotic (indicated on the bottom right of each panel), the coefficient of determination (R²) and the equation of the least-squares line are shown within the corresponding panel. Antibiotics on the right have predicted MHBA breakpoints.

FIG. 3.

A box-and-whisker plot depicting the comparison of the spread in sizes of the zones of inhibitions for SMG isolates recovered from CF airways and invasive infection. Boxes delineate the first (75%) and third (25%) quartiles, with the median shown as an internal black line; whiskers represent the 95th and 5th percentiles. Statistically significant differences between the median zone sizes of CF airway and invasive isolates are indicated with an asterisk (P < 0.05 and > 0.0001) and double asterisk (P < 0.0001).

FIG. 4.

Chronic macrolide exposure results in high-level resistance in SMG isolates recovered from the airways of CF patients treated with prolonged azithromycin therapy. The vast majority of azithromycin-resistant SMG isolates, as determined by the size of the zone of inhibition on BHI, were isolated from patients that were chronically exposed to the antibiotic during suppressive therapy. Likewise, the majority of sensitive isolates were recovered from naive patients. Acute exposure to azithromycin does not appear to be associated with increased resistance.