| Summary: | <p>Abstract</p> <p>Background</p> <p><it>Haemophilus influenzae </it>is one of the main aetiological agents of community-acquired respiratory tract infections. The primary aim of this study was to evaluate the antibacterial activity of telithromycin against <it>H. influenzae </it>clinical isolates showing different pattern of resistance in comparison with azithromycin and clarithromycin at 1/4 ×, 1/2 ×, 1 ×, 2 ×, 4 × minimum inhibitory concentration (MIC) and to peak concentrations in epithelial lining fluid (ELF). The secondary aim was to determine the influence of CO<sub>2 </sub>enriched atmosphere on bacterial susceptibility.</p> <p>Results</p> <p>Telithromycin showed high activity against <it>H. influenzae</it>, including strains susceptible to β-lactams (n = 200), β-lactamase producer (n = 50) and β-lactamase negative ampicillin resistant (BLNAR) (n = 10), with MIC from ≤0.03 to 4 mg/L, and MIC<sub>50</sub>/MIC<sub>90 </sub>of 1/2 mg/L with susceptibility rate of 100%, and minimum bactericidal concentrations (MBC) from 2 to 4-fold higher than the MIC. Azithromycin was the most active tested macrolide (range: 0.25 – 4 mg/L; MIC<sub>50</sub>/MIC<sub>90</sub>: 1/2 mg/L), comparable to telithromycin, while clarithromycin showed the highest MICs and MBCs (range: 0.25 – 8 mg/L; MIC<sub>50</sub>/MIC<sub>90</sub>: 2/8 mg/L). In time-kill studies, telithromycin showed a bactericidal activity at the higher concentrations (4 – 2 × MIC and ELF) against all the strains, being complete after 12 – 24 hours from drug exposition. At MIC concentrations, at ambient air, bactericidal activity of telithromycin and azithromycin was quite similar at 12 hours, and better than that of clarithromycin. Besides, telithromycin and clarithromycin at ELF concentrations were bactericidal after 12 hours of incubation for most strains, while 24 hours were needed to azithromycin to be bactericidal. Incubation in CO<sub>2 </sub>significantly influenced the MICs and MBCs, and only slightly the <it>in vitro </it>killing curves.</p> <p>Conclusion</p> <p>Telithromycin showed an <it>in-vitro </it>potency against <it>H. influenzae </it>comparable to azithromycin, with an <it>in-vitro </it>killing rate more rapid and superior to clarithromycin at 2X-MIC against β-lactamase producers and BLNAR strains, and to azithromycin at ELF concentrations against β-lactamase negative strains. Against all strains, MICs and MBCs were lower in the absence of CO<sub>2 </sub>for the tested antibiotics, showing an adverse effect of incubation in a CO<sub>2 </sub>environment. The <it>in-vitro </it>potency together with the tissue concentrations of the antimicrobial, should be considered in predicting efficacy.</p>
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