Streptococcus pneumoniae (the pneumococcus) is a predominant cause of pneumonia, meningitis,
and bacteremia. It is a leading killer of children under 5 years of age, responsible
for the deaths of up to 2 million children annually [1]. Most deaths occur in African
and Asian developing countries; however, pneumococcal disease is also a significant
problem in particular populations of developed countries, such as the North American
Indians, and indigenous Alaskans and Australians [1]–[3]. Although vaccination is
the most cost-effective method of protection against pneumococcal disease, cost remains
a barrier, as does vaccine delivery and efficacy. In this opinion piece, we discuss
the potential complementary role of probiotics to vaccines in preventing pneumococcal
disease through targeting the microbiome of the upper respiratory tract.
A prerequisite for pneumococcal disease is adherence of the bacterium to host nasopharyngeal
epithelium leading to colonization (carriage). The mucosal surface and the microbiome
of the nasopharynx are thought to protect against carriage [4]. Vaccination with pneumococcal
vaccines reduces carriage of the organism, and the risk of invasive disease caused
by vaccine serotypes and some cross-reactive non-vaccine serotypes. Moreover, vaccines
generate herd immunity that may protect unvaccinated individuals against infection
[5].
In North America and other developed regions, >80% of pediatric invasive pneumococcal
disease (IPD) is accounted for by serotypes contained within the first-generation
seven serotype conjugate vaccine (PCV7, Prevnar, Wyeth/Pfizer, United States). In
high-risk populations, several factors diminish the efficacy of pneumococcal vaccines.
For example, PCV7 protects against only ∼50% of serotypes causing IPD in developing
countries of Africa and Asia [6]. Pneumococcal conjugate vaccines are also too expensive
for resource-poor countries that experience the overwhelming burden of disease globally.
The GAVI Alliance has made significant inroads to this problem, providing access to
these and other life-saving vaccines to children most in need at a cost of US$1 billion
per year [7]. Nevertheless, complete vaccine delivery is another major public health
challenge. While GAVI is planning to implement pneumococcal conjugate vaccines in
19 developing countries over the next 2 years [8], vaccine uptake may be more difficult
in certain populations. Amongst indigenous Australians, <50% of infants aged 7 months
have received the full three-dose schedule (at 2, 4, and 6 months) [9], providing
suboptimal protection against colonization and disease. In many countries, the first
PCV7 dose is received after colonization has occurred—usually within the first 6 weeks
of life—which may further limit the efficacy of pneumococcal vaccination.
Furthermore, serotype replacement is considered the most significant problem in the
post-PCV7 era. Elimination of vaccine-serotype carriage has provided new niches for
colonization and subsequent rises in invasive disease with non-PCV7 serotypes [10].
Although licensure of higher valency PCVs containing ten or 13 serotypes would be
expected to reduce serotype replacement, the emergence of other invasive serotypes
is likely.
Other early life strategies to prevent pneumococcal disease are needed, particularly
for resource-poor settings. Maternal and neonatal immunization approaches are currently
under investigation for their impact on disease during the first weeks of life. Targeting
the microbiome to modulate colonization has been postulated as one mechanism to improve
the efficacy of a range of vaccines against multiple pathogens [11]. It has now been
demonstrated that in early infancy, colonization with pneumococci prior to conjugate
vaccination causes impaired immune responses to the carried serotype [12], [13]. Exploiting
the beneficial effects of probiotics on microbial colonization and immunity represents
a novel approach to prevent or reduce pneumococcal colonization and disease.
The World Health Organization (WHO) defines probiotics as live micro-organisms that
confer a health benefit to the host and are generally regarded as safe in humans [14].
Moreover, clinical studies have confirmed the safety and feasibility of oral administration
of probiotics in infancy [15], [16]. Lactobacillus and Bifidobacterium are the two
most widely studied genera of probiotic bacteria [17]. Probiotic activity is highly
species- and strain-specific [18], [19]. Principal amongst their pleiotropic effects
is the capacity to counteract microbiome disturbances, suggesting the potential to
modulate pneumococcal colonization [20]. Indeed, experimental data suggest that probiotics
can influence the profile of microbial species in the nasopharynx to reduce pneumococcal
colonization [21]–[24]. Probiotics also maintain epithelial barrier integrity and
modulate systemic and mucosal immune responses [14]. Furthermore, probiotic-microbiome
crosstalk is important, as intestinal microbiota can shape immune responses by controlling
the relative activity of regulatory T cells and Th17 cells [25], [26]. A paradigm
for the effects of probiotics in modulating host responses in the nasopharynx to protect
against pneumococcal infection is proposed in Figure 1. Importantly, while the mechanisms
of action proposed are largely supported by animal studies, more research is needed
to confirm these effects in humans.
10.1371/journal.ppat.1002652.g001
Figure 1
Paradigm for the proposed biological effects of probiotic bacteria in protection against
pneumococcal infection.
Commensal and/or probiotic bacteria can prevent pathogens (pneumococci) from attaching
to and colonizing the respiratory epithelium by associating with specific cell surface
receptors and by enhancing mucus secretion and the production of secretory IgA. Probiotic
bacteria interact with underlying dendritic cells (DCs) which signal to the adaptive
immune system to trigger a variety of effector cell types, including Th1, Th2, and
Th17 as well as regulatory T cells and B cells depending on the local cytokine/chemokine
microenvironment. Furthermore, probiotic bacteria also maintain the epithelial barrier
integrity by upregulating the expression of specific tight junction proteins on damaged
epithelium as a result of localized inflammatory responses following pathogen (pneumococcal)
encounter and invasion. Refer to references [49]–[52] for more detail on probiotic–host
effects. Th, T helper cell.
Probiotics show specificity in their effect on microbial patterns in the nasopharynx.
Most of the available data is based on animal models of colonization or disease. For
example, in a mouse model of pneumococcal pneumonia, Lactobacillus lactis lowered
lung colonization and increased specific IgG and IgA levels in bronchoalveolar secretions
after challenge with pneumococcus serotype 14 [21], while Lactobacillus fermentum
reduced nasopharyngeal colonization after challenge with pneumococcal serotype 6A
[22]. In humans, the potential for probiotics to have an impact on airway microbial
colonization is less clear. In 108 adult volunteers given a probiotic yogurt containing
Lactobacillus rhamnosus GG (LGG), Bifidobacterium sp. B420, Lactobacillus acidophilus
145, and Streptococcus thermophilus, a significant reduction in pathogenic bacteria
(including Staphylococcus aureus, S. pneumoniae, beta-hemolytic streptococci, and
Haemophilus influenzae) was observed compared to a standard yogurt [24]. Streptococcus
salivarius is suggested to be an appropriate probiotic species given that it is a
known colonizer of the upper respiratory tract in humans [27]. It has been shown to
produce bacteriocin-like substances with inhibitory activities against a number of
important airway pathogens in vitro and in vivo [27], [28] as well as possess immunomodulatory
properties in vitro [29], [30]. In otitis media-prone children given antibiotics prior
to oral treatment with a powdered S. salivarius K12 formula, 33% were newly colonized
with K12 while two of 19 children were shown to expand the pre-existing S. salivarius
population [31]. No impact on clinical outcomes was reported in this study, and the
small sample size used makes it difficult to draw meaningful conclusions. In contrast,
when otitis-prone children (n = 155) were given a daily probiotic mix containing LGG,
L. rhamnosus LC705, B. breve 99, and Propionibacterium freudenreichii JS for 24 weeks,
no effect on nasopharyngeal carriage of otitis pathogens was observed. Furthermore,
this probiotic formula did not prevent the occurrence of otitis media in these children,
although there was a trend of reduced recurrent respiratory infections [32]. Taken
together, the evidence of probiotic effects in human studies is more limited compared
to animal models and justifies the continued investigation of candidate probiotic
species such as S. salivarius and lactobacilli on airway microbial colonization and
their mechanisms of action.
To date, the effect of probiotics on the gastrointestinal microbiome have provided
the best evidence for host–microbe interactions such as pathogen exclusion, enhanced
mucus secretion, production of anti-bacterial factors, and modulation of host immunity
[14]. Probiotics can restore aberrant microbiota patterns associated with inflammatory
diseases such as Crohn's Disease [33] and allergy [17]. Several clinical studies have
shown that infants who later develop atopic dermatitis have altered microbiota, with
greater numbers of pathogenic clostridial and staphylococcal species and fewer beneficial
bifidobacteria [34], [35]. Importantly, dysbiosis precedes clinical symptoms of allergy
[36], indicating a causal relationship between altered microbiota and disease. Administration
of LGG modulates the composition of the intestinal microbiota in allergic infants,
and reduced by half the incidence of atopic dermatitis in high-risk infants by age
2 [36], [37]. LGG also corrected dysbiosis and reduced disease severity in a mouse
model of colitis [38].
These data have implications for pneumococcal disease. Importantly, lung immunity
is affected by the intestinal microbiome, which induces Th1 and IgA responses via
specific inflammasomes [39]. Therefore, modulation of inflammasome activity by probiotics
represents a key biological target. The balance between microbiome status and health
are also linked to the production of potent anti-inflammatory short-chain fatty acids
such as butyrate and acetate [40]. Probiotics restore short-chain fatty acid levels,
and the protective effects of Bifidobacteria species against enterohemorrhagic E.
coli infection was shown to be dependent on acetate production [41].
Probiotics also appear to play an important role in facilitating mucosal immunity
against infection [42]. Specifically, probiotics are demonstrated to be effective
vaccine adjuvants, enhancing IgG- and IgA-specific responses to parenteral and mucosal
vaccines such as influenza [43], H. influenzae type b (Hib) [44], polio [45], rotavirus
[46], and Salmonella typhi
[47] in humans. More studies on the adjuvant properties of probiotics in humans are
needed, as the effects reported are often variable and have been based on clinical
trials involving small sample sizes. For example, in the study by Fang et al. [47],
treatment with LGG or L. lactis did not significantly enhance the IgG or IgA response
to an oral S. typhi Ty21a vaccine despite LGG increasing S. typhi–specific IgA antibody
secreting cells in a greater number of subjects than L. lactis or placebo. Similarly,
while supplementation with a Bifidobacterium longum BL999 and L. rhamnosus LPR mix
to infants doubled the anti-HBsAg IgG levels following vaccination compared to placebo,
this was not statistically significant [48]. In a study by Kukkonen et al. [44], daily
administration of a LGG, L. rhamnosus LC705, B. breve Bbi99, and Propionibacterium
freudenreichii combination to mothers in the last 4 weeks of pregnancy, and to their
infants for the first 6 months of life, increased the Hib-specific IgG response in
infants. However, no change in diphtheria toxoid or tetanus toxoid IgG levels was
observed, suggesting that the effects of probiotics may vary depending on the vaccine
antigen used. Recently, Lactobacillus casei was reported to significantly enhance
the pneumococcal protective protein A (PppA)-specific IgG and IgA response in the
serum and mucosa following nasal vaccination with PppA and was associated with a significantly
reduced pathogen load in the nasal lavage by day 42 post-immunization [42]. Despite
this, the adjuvant activity of probiotics following pneumococcal vaccination in humans
is unknown and remains an intriguing prospect for further research.
The promising findings of these studies has made it increasingly clear that significant
research emphasis on reducing pneumococcal colonization during the neonatal period
is warranted, ideally involving human clinical trials. Novel early life strategies
that reduce infection with S. pneumoniae may have important health benefits, especially
in high-risk populations. The combined effects of modulating the nasopharyngeal microbiome
and enhanced mucosal immunity justify the continued investigation of probiotics for
protection against pneumococcal infection.