ModA2 Phasevarion Switching in NontypeableHaemophilus influenzaeIncreases the Severity of Experimental Otitis Media

Abstract

Several human-adapted bacterial pathogens use a phasevarion (ie, a phase-variable regulon) to rapidly and reversibly regulate the expression of many genes, which include known virulence factors, yet the influence of phasevarion-mediated regulation in pathogenesis remains poorly understood. Here we examine the impact of the nontypeable Haemophilus influenzae (NTHI) ModA2 phasevarion on pathogenesis and disease severity in a chinchilla model of experimental otitis media. Chinchillas were challenged with NTHI variant populations that were either inoculated ON and remained ON, inoculated OFF and shifted ON, or inoculated OFF and remained OFF, within the middle ear. We show that populations that shift from OFF to ON within the middle ear induce significantly greater disease severity than populations that are unable to shift. These observations support the importance of phasevarion switching in NTHI pathogenesis and the necessity to considered phasevarion regulation when developing methods to treat and prevent infection.

Panel 7 – Pathogenesis of otitis media – a review of the literature between 2015 and 2019

“…Brockman et al subsequently found that the phasevarion impacted disease progression and severity in a chinchilla model of NTHi-induced experimental OM. A shift in the phasevarion status, or an epigenetic switch, significantly increased disease severity compared to a genetically identical population that did not shift phasevarion status [61]. The phasevarion was also shown to regulate NTHi biofilm formation under various disease specific conditions.…”

“…The chinchilla has been utilized in studies of in-host microbial adaptation with NTHi with respect to the phasevarion [48,61] or nutritional limitation [63]. The infectivity of wild type and mutant NTHi [44] and pneumococcal [18,25] strains have been compared effectively in the chinchilla model.…”

Subversion of host immune responses by otopathogens during otitis media

“…Examples of such evasion mechanisms include coexisting viral infections resulting in a negative pressure buildup in the Eustachian tube, exaggerated cytokine production leading to increased inflammation, and a build‐up of mucin, which decreases mucociliary clearance and hence results in OM infections that are less likely to be cleared (Table ) . Bacteria are even able to manipulate their gene expression through phase variation and interact with other pathogens, increasing their virulence and decreasing the chances of host clearance . Immunologic evasion is multifactorial, and often depends on damage to the host mucociliary tract, genetic regulatory changes within otopathogens, the number as well as a variety of different otopathogens that have colonized the nasopharynx, and finally, the delicate interplay between the host's adaptive and innate immune response (Table ).…”

“…Besides mucin, many bacterial pathogens are also able to use phase variation to regulate gene expression and evade potent host immune responses (Fig. ), though researchers are still attempting to better elucidate this phenomenon . NTHi, in particular, has various forms of phase variation such as rearrangement and modification of glycosyltransferase genes, allele on/off switching of N 6 ‐adenine DNA methyltransferase (ModA), and manipulation of the polythymidine (poly‐T) tract in the hia promoter .…”

“…NTHi ModA allele phaseversion was studied in animal models that specifically investigated OM. It has been demonstrated that animals with the “on” modA2 phaseversion had a higher burden of disease . Tetranucleotide repeats that influence phase variation in lipo‐oligosaccharide genes have also been studied.…”