Viral upper respiratory tract infections (URI), either preceding or concurrent, is the most common predisposing factor for bacterial OM and as such protection against viral URI has been a long-held strategy for protection against OM. To date, evidence in support of this assertion is available for vaccines against influenza A virus only [30], however multiple respiratory tract viruses are associated with the development of bacterial OM. In addition to influenza A virus, the most common are respiratory syncytial virus (RSV), human rhinovirus, and adenovirus. There are active efforts to develop vaccines against RSV as well as other viral co-pathogens of OM, however antigenic diversity amongst strains and the lack of appropriate animal models are considered major barriers to progress in this regard. In addition to predisposing to bacterial invasion of the middle ear via a variety of mechanisms, prolonged AOM and antibiotic treatment failure are associated with concurrent viral infection. Thus, the targeting of viral coinfections for vaccine development efforts to prevent OM remains an active and important area of investigation.

Despite the success of conjugated polysaccharide vaccines which have greatly reduced the global burden of pneumococcal diseases, including pneumococcal OM, the observations of increasing disease due to replacement serotypes and differences in distribution of serotypes responsible for disease worldwide [31] strongly suggest that any vaccine that provides coverage for a limited number of serotypes will not provide the needed long-term solution for protection against pneumococcal OM. As such, there is an effort to identify and test broadly conserved pneumococcal protein antigens that could, in theory, provide serotype-independent protection. Moreover, it is likely that these vaccines would not induce serotype replacement, and further, believed to be much less costly to produce than the current conjugated capsular polysaccharide vaccines. Several candidates are currently being investigated in this regard, including: pneumococcal surface protein A (PspA), histidine triad family (Pht), pneumococcal surface adhesin A (PsaA), pneumococcal type 1 and type 2 pilus subunits, pneumolysin, pneumococcal serine-rich repeat protein (PsrP), pneumococcal choline binding protein (PcpA), heat shock protein caseinolytic protease (Clp), sortase A (SrtA), polyamine transport operon (potD), pneumococcal protective protein (PppA), a protein analogous to the cell wall separation protein of group B streptococcus (PcsB), and a serine/threonine protein kinase (StkP), among others. These candidates cannot be discussed fully here, however several excellent reviews are available for interested readers [29, 32]. Recently, Berglund et al. [25] reported the results of a phase I clinical trial in adults, in which a protein-based NTHI and pneumococcal vaccine that contained pneumococcal histidine triad D (PhtD), detoxified pneumolysin (dPly), and NTHI protein D was tested, thereby demonstrating significant forward momentum in continued vaccine development wherein two predominant pathogens of OM, as well as multiple other diseases of the airway are targeted.

M. catarrhalis has always been considered the third ranking bacterial agent of OM, after S. pneumoniae and NTHI, however the recent shift in the microbiology of OM resulting from the broad use of pneumococcal conjugate vaccines has now resulted in an increase in the relative role of both NTHI and M. catarrhalis in OM. As a result, there has been much progress recently in attempts to identify potential vaccine antigens that target M. catarrhalis. A genome mining approach has been particularly fruitful in this regard and as such, several new and promising candidates have emerged. Currently, the following potential antigens are under development: MID/Hag; MchA1 & MchA2; MhaB1 & MhaB2; McmA; OppA, UspA2, Msp75; McaP; OMP E; OMP CD; M35; OMP G1a & OMP G1b; OlpA; Msp 22, Type IV pili; and lipooligosaccharide [29, 33]. Although limitations in availability of relevant animal models in which to test M. catarrhalis-derived vaccine candidates has slowed progress in the past, use of the murine pulmonary clearance model [34] and a newly developed chinchilla polymicrobial model wherein RSV predisposes to invasion of the middle ear by both NTHI and M. catarrhalis [35] are being used to move these candidates forward.

By definition, a biofilm is a highly organized, multicellular community encased in an extracellular polymeric matrix or substance (often referred to as the EPS) that is affixed to a surface. Biofilms are the preferred state of all bacterial lifestyles in nature. Bacteria populations within a biofilm, as opposed to their planktonic or free-living counterparts, have a reduced growth rate (due to a nutrient limited environment), and a distinct transcriptome [36, 37]. They also exchange genetic material at an increased frequency. Bacteria in a biofilm have substantially increased resistance not only to effectors of innate and acquired immunity but also to the action of antibiotics [38–40]. Moreover, the EPS presents a formidable physical barrier to cellular effectors of immunity and is highly recalcitrant to removal [41]. Diseases wherein there is a biofilm component as part of the disease course, such as OM, thus require novel methods for diagnosis, treatment, and prevention. The biofilm paradigm was originally put forth because OM is a spectrum of diseases that are very difficult to treat with antibiotics and are often chronic and recurrent in nature. Moreover, effusions recovered from middle ears are often bacteriologically sterile. However, although bacteria cannot be cultured from these effusions, they are nonetheless typically PCR-positive for bacterial DNA [42]. Moreover, Rayner et al. [42] demonstrated that, in addition to bacterial DNA, there was also bacterial messenger RNA present in middle ear fluids. The presence of this short-lived message suggested the existence of metabolically active bacteria within those fluids, despite an inability to culture them. To date, all three major otopathogens—S. pneumoniae, NTHI, and M. catarrhalis—have been shown to form biofilms both in vitro and in vivo [43–47]. Direct detection of bacterial biofilms in association with mucosa samples recovered from the middle ears of children with chronic and recurrent OM has been shown [48]. Current data support the role of biofilms in recurrent and chronic OM, however, it would be counterintuitive to not consider the possibility that biofilms also contribute to AOM as bacteria require only minutes to begin building a biofilm in a favorable environment.

Due to the unique and highly resistant of bacteria resident within a biofilm, many in the research community are attempting to better characterize these biofilms, as well as understand the molecular mechanisms and microenvironmental cues that trigger their development/dispersal so that novel methods to target them for either disruption or immune intervention can be developed [49–55].

Another significant challenge to development of vaccines for OM is the already extremely crowded recommended pediatric immunization schedule [56]. An infant in developed countries receives no less than 8–11 injections in the first year of life [57], with typically 4–5 injections per visit. Worldwide, there are concerns about reduced compliance due to parental anxiety over the “pin-cushion” status of their newborns, as well as scientific concerns about the potential for immune interference, particularly when multiple vaccines are formulated with common carriers. This state of pediatric immunization practice is leading to two significant developments in the pediatric vaccine industry. The first is an emphasis on the development of combination vaccines, to reduce the total number of injections received [58, 59]. However, there is also tremendous interest and emphasis being put on the development of alternative delivery strategies, and particularly the use of noninvasive or “needle-free” routes of immunization [60–63]. There are multiple advantages to noninvasive routes of immunization in general, including the fact that they eliminate the pain, anxiety, and aversion associated with injection, thus yielding better compliance; they eliminate the use of needles and thereby both increase safety and eradicate the need to dispose of medical “sharps” waste; they are typically much cheaper to produce and less likely to require a “cold chain” due to their greater stability and longer shelf life; and the fact that for many of these delivery regimes, trained medical personnel are not required. With regard to mucosal diseases such as OM, the primary and perhaps critical advantage of noninvasive vaccination routes is that unlike parenteral immunization, these approaches induce the formation of both mucosal and systemic immunity, thereby facilitating the availability of a robust, protective response at the exact site of bacterial colonization/infection, and thus precisely where it is needed the most.

Mucosal immunization has become one area of great developing interest in the OM community [64, 65]. Further, recent publications report efficacy against experimental OM due to NTHI after immunizing transcutaneously (by rubbing the vaccine candidate onto the skin of the chinchilla ear) [66, 67]. Not only was this approach protective against the development of experimental OM but was also efficacious as a therapeutic vaccine, mediating significantly more rapid resolution of existing disease. Collectively, these efforts will not only help de-crowd the pediatric immunization schedule but they also have the potential for even greater efficacy than can be achieved by traditional immunization routes. Fostering a local immune response may also prove to provide greater protection to those targeted groups wherein there are genetic and/or anatomical risk factors for proneness to OM (i.e., Native Americans, Alaskan Natives, Aboriginal peoples, those with Down’s syndrome, or with cleft lip/palate, among others) [68–70].