Microevolution in response to transient heme-iron restriction enhances intracellular bacterial community development and persistence

Bacterial pathogens must sense, respond and adapt to a myriad of dynamic microenvironmental stressors to survive. Adaptation is key for colonization and long-term ability to endure fluctuations in nutrient availability and inflammatory processes. We hypothesize that strains adapted to survive nutrient deprivation are more adept for colonization and establishment of chronic infection. In this study, we detected microevolution in response to transient nutrient limitation through mutation of icc. The mutation results in decreased 3',5'-cyclic adenosine monophosphate phosphodiesterase activity in nontypeable Haemophilus influenzae (NTHI). In a preclinical model of NTHI-induced otitis media (OM), we observed a significant decrease in the recovery of effusion from ears infected with the icc mutant strain. Clinically, resolution of OM coincides with the clearance of middle ear fluid. In contrast to this clinical paradigm, we observed that the icc mutant strain formed significantly more intracellular bacterial communities (IBCs) than the parental strain early during experimental OM. Although the number of IBCs formed by the parental strain was low at early stages of OM, we observed a significant increase at later stages that coincided with absence of recoverable effusion, suggesting the presence of a mucosal reservoir following resolution of clinical disease. These data provide the first insight into NTHI microevolution during nutritional limitation and provide the first demonstration of IBCs in a preclinical model of chronic OM.
Author summary Nontypeable Haemophilus influenzae (NTHI) inhabits diverse niches in the host. The ability to adapt to new microenvironments is consistent with the predominance of NTHI as a causative agent of otitis media (OM) in children. We evaluated the microevolution of NTHI associated with adaptation and persistence in response to nutrient limitation. We identified a naturally occurring mutation that enhances NTHI persistence and formation of intracellular bacterial communities (IBCs) in a pre-clinical model of OM. The presence of IBCs during OM provides the first opportunity to evaluate the role of intracellular populations in chronicity and quiescence as a new paradigm for recurrent OM. This model provides a new platform to identify novel therapeutics for this highly prevalent and costly infectious disease.