Investigation of serotype distribution and antibiotic susceptibility in β-lactamase-negative ampicillin-resistant Haemophilus influenzae strains from adult patients

Background: Haemophilus influenzae (H.influenzae) is an important bacterial pathogen which causing a wide variety of both invasive and respiratory infections. Antibiotic treatment may lead to development of resistance in H.influenzae strains, particularly in β-lactamase-negative ampicillin resistant (BLNAR) strains. The aim of the present study was to investigate the serotype distribution and antibiotic susceptibility of β-lactamase-negative ampicillin resistant (BLNAR) H.influenzae isolates from adult patients. Methods & Materials: One hundred and twenty-five BLNAR H. influenzae isolates were collected between the year of 2003 to 2012. The β-lactamase production of these isolates was examined by using DrySlide nitrocefin (BBL, Becton Dickinson, USA). All isolates were identified and characterized for capsular serotypes. Serotyping was performed by slide agglutination by using commercial antisera (BD Difco, H.influenzae antisera, USA). Antibiotic susceptibility to the following antimicrobial agents was determined with disc diffusion method according to CLSI: ampicillin (AMP), cefotaxime (CTX), cefuroxime (CXM), chloramphenicol (CHL) and trimethoprim-sulfamethoxazole (SXT). Results: Of the 125 H.influenzae isolates, 88 (70,40%), 13 (10,40%), 10 (8,00%), 7 (5,60%), 3 (2,40%), 2 (1,60%), 1 (0,80%) and 1 (0,80%) were from the sputum, bronchoalveolar lavage, tracheal aspirate, conjunctiva, nasofaringeal aspirate, blood, cerebrospinal fluid and pus, respectively. Serotype b 110 (88,00%) was most commonly observed among the BLNAR H.influenzae isolates. The rates of serotype e 6 (4,80%) and serotype d 3 (2,40%) were observed in follows. Five isolates were found as nontypeable (4,00%). Antimicrobial susceptibility results showed that 5 (4,00%) isolates were resistant to all studied antibiotics. All of these five isolates have serotype b, as well. Along with the AMP resistance, 53 (42.4%) isolates were susceptible to CTX, CXM, CHL and SXT in BLNAR. Conclusion: The rates of BLNAR H.influenzae infection is increasing worldwide which may cause severe important infection. This study may guide clinicians on appropriate use of antibiotics not only for reducing the morbidity and mortality in the patients but also to control the emergence and spread of antimicrobial resistance among BLNAR H.influenzae strains. This study emphasizes the continuous epidemiological monitoring of BLNAR H.influenzae isolates.