A New Human Colonization Model for Nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae are small gram-negative bacteria that colonize the upper respiratory tract of humans, beginning at a very early age [1]. Although these organisms are normally commensals, when host defenses are compromised by underlying medical conditions such as malnutrition, immunodeficiency, chronic lung disease, or acute viral infection,H. influenzae–associated disease may ensue [2–4]. The contribution of nontypeable H. influenzae to the disease burden of children with otitis media is substantial. Among children in the developed world, this organism is currently responsible for an estimated 40%–50% of the cases of acute otitis media and an even higher percentage of cases of chronic and recurrent disease [5, 6]. In the adult population, particularly among patients with chronic obstructive pulmonary disease, nontypeable H. influenzae are major contributors to the ongoing disease process, particularly during the acute exacerbations that characterize the disease in many patients [4, 7]. Much has been learned about the molecular pathogenesis of disease caused by nontypeable H. influenzae in the past 2 decades, using a variety of in vitro models and in vivo models [8]. The long-term goal of much of this work has been to gain sufficient knowledge about the disease process, such that vaccines or other novel therapies can be developed to prevent nontypeable H. influenzae–associated disease in the future [9]. However, one criticism of this work has been that the findings may not be truly relevant to understanding human disease because many of these earlier studies have been conducted in nonhuman systems. Thus, the study reported by Winokur et al in this issue of the Journal, in which they describe the development of a new human model of nasopharyngeal colonization with nontypeable H. influenzae, is particularly timely and has the potential to be very important for the field [10]. In their newly described human colonization model, Winokur et al used a single well-characterized strain of nontypeable H. influenzae to intranasally inoculate a small group of human volunteers. The challenge strain was genetically modified to make it streptomycin resistant. This allowed the investigators to efficiently recover the organism from nasopharyngeal specimens collected from subjects over the course of the study. The investigators also used a carefully planned dosing algorithm that consisted of stepwise up-and-down dosing inocula, such that doses of bacteria for later volunteers were dependent on colonization success or failure in earlier study subjects. This approach allowed the investigators to quickly determine the dose that would be expected to lead to successful colonization of most study subjects and minimized the overall number of volunteers required for the project. Fifteen volunteers took part in the study and, depending on their place in the study sequence, were scheduled to receive 1000–100 000 colony-forming units (CFU) of the nontypeable H. influenzae test strain that was administered by nose drops to each nares. The actual inoculum received by each volunteer was monitored and in most instances reasonably approximated the intended dose. Nine of the fifteen study subjects were successfully colonized with the bacteria, and the chances of success generally correlated with increasing doses of the bacteria. The investigators estimated a human colonizing dose 50 (HCD50) of 1991 CFU and a HCD90 of 150 314 CFU, although each value was associated with relatively wide confidence intervals, reflecting the variability observed in the study group as a whole. Nasal wash specimens were collected from each volunteer on days 3–6 Received and accepted 23 April 2013; electronically published 28 May 2013. Correspondence: Stephen J. Barenkamp, MD, St. Louis University School of Medicine, Doisy Research Center, Rm 303, 1100 S Grand Blvd, Saint Louis, MO 63104 (barenksj@slu. edu). The Journal of Infectious Diseases 2013;208:717–9 © The Author 2013. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail: journals. permissions@oup.com. DOI: 10.1093/infdis/jit242