Researchers have uncovered a crucial link between dietary zinc intake and protection against Streptococcus pneumoniae, the primary bacterial cause of pneumonia.
Globally, it is estimated that nearly two billion people suffer from zinc deficiency, but why this increases susceptibility to bacterial infection has not been well understood—until now.
University of Melbourne Associate Professor Christopher McDevitt, a laboratory head at the Doherty Institute, led an interdisciplinary team using state-of-the-art imaging techniques to reveal how the immune system uses zinc as an antimicrobial for protection during attack by Streptococcus pneumoniae.
Published today in PLOS Pathogens, the team which included University of Adelaide Research Fellow Dr. Bart Eijkelkamp, from the Research Centre for Infectious Diseases compared infections in mice fed with different levels of zinc.
They found that mice with lower zinc intake succumbed to infection up to three times faster because their immune systems had insufficient zinc to aid in killing the bacteria.
“Dietary zinc is associated with immune function and resistance to bacterial infection, but how it provides protection has remained elusive,” Dr. Eijkelkamp said.
“Our work shows that zinc is mobilised to sites of infection where it stresses the invading bacteria and helps specific immune cells kill Streptococcus pneumoniae.”
This work also translated its findings by showing that specific human immune cells could use zinc to enhance their killing of invading Streptococcus pneumoniae.
“The findings in this paper are a direct result of application of novel elemental imaging technology to uncover relationships that have previously been hidden to analysis, and a testament to cross-disciplinary collaboration,” said Professor Philip Doble, Director of the Elemental Bio-imaging Facility at the University of Technology Sydney, and a co-author of the study.
Pneumonia accounts for more than one million deaths every year, with the greatest health burden in countries where zinc deficiency frequently remains a major social challenge.
“Our findings highlight the importance of ensuring dietary zinc sufficiency as part of any population-wide strategy to control the burden of pneumococcal disease in conjunction with vaccination and other antimicrobial approaches,” Associate Professor McDevitt said.’
Streptococcus pneumoniae, or pneumococcus, asymptomatically colonizes the human nasopharynx and causes chronic otitis media through the formation of biofilms. Biofilms are highly structured bacterial communities that are protected from environmental stressors.
The ability to form biofilms is known to contribute to persisting colonization and survival of pneumococcus on nasopharyngeal cells and within the middle ear of the host (Honsa et al., 2013).
Colonization of the lung is also an important precursor for invasion into the bloodstream (Ogunniyi et al., 2010).
It has been shown that bacteria found growing within biofilms are less virulent than those grown planktonically; however, pneumococcal aggregates have also been shown to form in the myocardium of the heart during invasive pneumococcal disease (Lizcano et al., 2010; Sanchez et al., 2011; Brown et al., 2014; Gilley and Orihuela, 2014; Brown and Orihuela, 2015).
Pneumococcal biofilms are known to be regulated by a variety of factors including, quorum sensing molecules, choline availability, and exogenous iron (Moscoso et al., 2006; Trappetti et al., 2011a,b).
Since pneumococcal biofilms are essential for both colonization and persistence, it is important to continue to investigate the environmental stressors that affect their formation.
Metal concentrations vary from host to host, and pneumococcus encounters additional changes in metals upon aspiration from the upper respiratory tract into the lungs or following transmigration into the blood or cerebrospinal fluid (McDevitt et al., 2011). Extensive research has aimed to identify how metals alter the organism’s ability to cause invasive disease.
Metals such as zinc, copper, manganese, and magnesium are required as cofactors and structural components of many bacterial proteins, including those of S. pneumoniae, and are known to be essential for proper metabolism and cellular defenses (Waldron et al., 2009; Honsa et al., 2013).
Although there is vast knowledge on the importance of metals for bacterial cells, the effect of these metals on biofilm formation in the pneumococcus have been largely ignored.
Zinc is the second most abundant trace metal ion in the human body, and plays a role in a variety of cellular functions (Kehl-Fie and Skaar, 2010).
Though zinc availability in the body ranges from 5 to 300 μM (Bayle et al., 2011), McDevitt et al. have found zinc concentrations surpassing 600 μM in the bloodstream of mice infected with S. pneumoniae (McDevitt et al., 2011). Pneumococcus acquires zinc from the extracellular environment by zinc transport proteins, AdcA and AdcAII, and the Pht proteins, PhtA, PhtB, PhtD, and PhtE (Plumptre et al., 2014a,b; Eijkelkamp et al., 2016).
Mutants lacking the genes encoding AdcA and AdcAII were shown to be deficient in zinc uptake and cell growth; and less virulent in intranasal and intraperitoneal infection models (Bayle et al., 2011; Plumptre et al., 2014a; Brown et al., 2016).
Additionally, a previous study from our laboratory showed the importance of AdcAII, specifically, to adhesion in vitro and colonization in vivo, thus suggesting a role for zinc homeostasis in colonization and biofilm development of S. pneumoniae (Brown et al., 2016). Interestingly, evidence shows the importance of zinc for surface protein interactions that contribute to aggregation and biofilm formation of Staphylococcus aureus, another significant human pathogen (Conrady et al., 2008; Formosa-Dague et al., 2016). However, in contrast, high concentrations of zinc have also been shown to inhibit biofilm formation in the Gram-positive organism Streptococcus suis (Wu et al., 2013).
Since biofilms are an integral component of colonization, we hypothesized that zinc availability will affect the initial stages of biofilm formation. Our approach was to observe early stage construction of pneumococcal biofilms in a broad range of physiologically relevant zinc concentrations.
Here, we investigate the influence of zinc availability on cell-cell interactions, LytA-dependent autolysis, and initial biofilm formation. We have demonstrated that abundant zinc availability allows for the development of more substantial pneumococcal biofilms in vitro, modestly increases colonization in vivo, and delays the onset of autolysis.
These results contribute to our understanding of the role of zinc in pneumococcal aggregation and biofilm formation, suggesting a potential target for antimicrobials that aim to reduce bacterial burdens of S. pneumoniae and potentially other Gram-positive organisms.
More information:PLOS Pathogens (2019). DOI: 10.1371/journal.ppat.1007957
Journal information: PLoS Pathogens
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