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. 2014 Jul 9;16(1):55-67.
doi: 10.1016/j.chom.2014.06.005.

Influenza promotes pneumococcal growth during coinfection by providing host sialylated substrates as a nutrient source

Steven J Siegel  1 , Aoife M Roche  1 , Jeffrey N Weiser  2
Affiliations

Influenza promotes pneumococcal growth during coinfection by providing host sialylated substrates as a nutrient source

Steven J Siegel et al. Cell Host Microbe. .

Abstract

Much of the mortality attributed to influenza virus is due to secondary bacterial pneumonia, particularly from Streptococcus pneumoniae. However, mechanisms underlying this coinfection are incompletely understood. We find that prior influenza infection enhances pneumococcal colonization of the murine nasopharynx, which in turn promotes bacterial spread to the lungs. Influenza accelerates bacterial replication in vivo, and sialic acid, a major component of airway glycoconjugates, is identified as the host-derived metabolite that stimulates pneumococcal proliferation. Influenza infection increases sialic acid and sialylated mucin availability and enhances desialylation of host glycoconjugates. Pneumococcal genes for sialic acid catabolism are required for influenza to promote bacterial growth. Decreasing sialic acid availability in vivo by genetic deletion of the major airway mucin Muc5ac or mucolytic treatment limits influenza-induced pneumococcal replication. Our findings suggest that higher rates of disease during coinfection could stem from influenza-provided sialic acid, which increases pneumococcal proliferation, colonization, and aspiration.

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Figures

Figure 1
Figure 1. Influenza promotes pneumococcal colonization, growth and aspiration
(A) Mice were infected with influenza for the indicated number of days, nasal lavages with RLT RNA lysis buffer obtained, and qRT-PCR performed to measure the relative expression of x31 viral nucleoprotein (NP). (B) Mice given PBS (mock, open symbols) or influenza (closed symbols) were weighed daily for 7 days, and the percent change in weight from baseline graphed. (C) Mice were infected with influenza or PBS, followed 7 days later by intranasal inoculation with bacteria of the indicated serotype. After 24 hrs post-bacterial inoculation (hpi), nasal lavages were obtained and plated for quantitative culture of colonizing pneumococci. (D) Mice were inoculated with influenza or PBS, followed 7 days later by challenge with strain P1121. Within 1 hr, nasal lavages were obtained and plated for quantitative culture (CFUs). (E) After 7 days of mock (black line) or influenza (grey shaded) infection, mice were inoculated with CFSE-labeled pneumococci for 8 hrs. Nasal lavages were obtained, fixed and stained for pneumococcal capsule, and flow cytometry performed to compare bacterial replication. (F) The median fluorescence intensity (MFI) of CFSE per bacterial cell was calculated for each condition, and displayed as 1/MFI, or number of divisions per cell (division index). (G) Mock or influenza-infected mice were challenged 7 days later with WT pneumococci for 24 hrs. Bronchoalveolar lavages were obtained and plated for quantitative culture. Horizontal lines indicate median values, and data in F are represented as mean +/− SD. n.s. = not significant, * = p < 0.05, ** = p < 0.01, *** = p < 0.001. All experiments were performed at least twice, with 4-11 mice per group. See also Figure S1.
Figure 2
Figure 2. Influenza increases sialic acid availability in the nasopharynx
(A-D) Mice were intranasally inoculated with influenza (closed bars) or mock (PBS, open bars) for 7 days, then nasal lavages obtained to measure levels of protein (A), reducing sugars (B) and sialic acid (C, D) in the lavages. Sialic acid measurements were performed without (C) or with (D) mild acid hydrolysis to measure free and total sialic acid, respectively, in lavages from mice infected with influenza for the indicated number of days (dpi). Data are represented as mean +/− SD. ND = not detected (below the limit of detection, indicated with a dotted line). n.s. = not significant, ** = p < 0.01. Experiments were performed at least twice, with 3-14 mice per group. See also Figure S2.
Figure 3
Figure 3. Pneumococci exploit host glycoconjugates for growth during co-infection
(A) The genetic locus in strain P1121 containing the neuraminidase (nanA, nanB) and sialic acid transport (satABC) genes is displayed. (B) Seven days after mock (open symbols) or influenza infection (closed symbols), mice were inoculated with CFSE-labeled pneumococci of the indicated genotypes for 8 hrs. Nasal lavages were obtained and plated for quantitative culture. (C) Lavages were fixed, stained for pneumococcal capsule, and flow cytometry performed to measure bacterial replication (division index). (D) Mice were infected with influenza for 7 days, followed by 24 hrs of colonization with the indicated bacterial strains. Nasal lavages were plated for quantitative culture. (E) Seven days after mock or influenza infection, mice were given a mixed inoculum of sialic acid-catabolizing and ΔsatABC pneumococci. One day later, nasal lavages were obtained and plated for quantitative culture. Colonies from the plated inoculum and the lavages were patched onto antibiotic-containing media to determine the relative advantage in vivo of sialic acid catabolism and calculate competitive indices. CI > 1 indicates outcompetition of the streptomycin-resistant ΔsatABC pneumococci, and was defined as the ratio of WT/mutant bacteria in the output (nasal lavage), divided by the ratio of WT/mutant bacteria in the input (inoculum). Data in C are represented as mean +/− SD. Horizontal lines indicate median values. n.s. = not significant, * = p < 0.05, ** = p < 0.01, *** = p < 0.001. Experiments were performed at least twice, with 3-15 mice per group.
Figure 4
Figure 4. Sialylated airway mucins are required for influenza-induced pneumococcal growth
(A) Mouse heads were obtained after 7 days of mock or influenza infection, fixed, decalcified and sectioned. URT sections were stained with alcian blue and nuclear fast red. Scale bars, 50 μm. (B) Nasal lavages were obtained from mice infected with influenza or PBS for 7 days, then analyzed by Western blot for the presence of Muc5ac. (C) Nasal lavages with RLT RNA lysis buffer were obtained from mice influenza or mock-infected for 7 days. qRT-PCR was performed, and relative expression of Muc5ac measured. (D) Mice of indicated genotype were mock or influenza infected for 7 days, followed by inoculation with CFSE-labeled pneumococci for 8 hrs. Nasal lavages were obtained and plated for quantitative culture. (E) Lavages were also fixed, stained for pneumococcal capsule, and flow cytometry performed to measure bacterial replication (division index). (F-G) Wildtype mice were infected with influenza or mock, followed by daily treatment for 7 days with vehicle (PBS) or 0.5 M N-acetylcysteine (NAc). CFSE-labeled pneumococci were inoculated for 8 hrs, then nasal lavages obtained and used for quantitative culture (F) and flow cytometric analysis of cell division (G). (H) Total sialic acid content was measured by thiobarbituric acid assay on samples from D-G. Data are represented as mean +/− SD. Horizontal lines indicate median values. * = p < 0.05, ** = p < 0.01. Experiments were performed at least twice, with 3-13 mice per group.
Figure 5
Figure 5. Influenza and bacterial neuraminidases desialylate host substrates
(A-B) Mouse heads (A) and nasal lavages (B) were obtained from separate mice after 7 days of influenza or mock infection, followed by 24 hrs of colonization with WT or ΔnanAΔnanB pneumococci, or PBS. Heads were fixed, decalcified and sectioned. URT sections were stained with DAPI and FITC-labeled lectin from E. cristagalli that binds galactose residues exposed when sialic acid is removed. All images were taken with the same optical settings. Scale bars, 50 μm. Lavages were stained with FITC-labeled E. cristagalli lectin. Flow cytometry was performed by gating on neutrophils and comparing desialylation. Experiments were performed at least twice, with 3-10 mice per group.
Figure 6
Figure 6. Sialic acid catabolism contributes to pneumococcal colonization in the absence of influenza
(A) Mice were colonized with pneumococci for the indicated number of days. Sialic acid concentrations in lavages were measured by thiobarbituric acid assay, with or without acid hydrolysis to measure total (solid bars) and free sialic acid (open bars), respectively. (B) WT and ΔsatABC pneumococci were competed in vivo by colonizing naive mice with mixed inocula. Nasal lavages were plated for quantitative culture and colonies from the inocula and lavages patched onto antibiotic-selective media to determine the relative advantage of sialic acid catabolism in vivo and calculate competitive indices. CI > 1 indicates WT outcompeted ΔsatABC pneumococci. (C) Nasal lavages were obtained with RLT RNA lysis buffer from mice colonized with pneumococci for the indicated number of days. qRT-PCR was performed to measure relative expression of Muc5ac. Data are represented as mean +/− SD. ND = not detected (below the limit of detection, indicated with a dotted line), * = p < 0.05. Experiments were performed at least twice, with 3-20 mice per group.
Figure 7
Figure 7. Nutrient-poor conditions on the human mucosal surface favor growth of sialic acid catabolizing pneumococci
(A) Bacteria of the indicated genotypes were grown for 8 hrs in mixed inocula in tryptic soy nutrient broth, or in nutrient-limited human nasal airway surface fluid (hNASF). Aliquots were plated at the beginning and end of the growth period, and colonies patched onto antibiotic-selective media to determine the relative advantage of sialic acid catabolism in vivo and calculate competitive indices. CI > 1 indicates WT outcompeted the indicated mutant strain. (B) Total sialic acid in the growth media used in A was measured by thiobarbituric acid assay after acid hydrolysis. (C) WT (solid bars) and ΔsatABC (open bars) pneumococci were grown for 24 hrs in chemically defined medium with the indicated concentrations of sialic acid as the sole carbon source. Data show fold-growth compared to inoculum. Data are represented as mean +/− SD. n.s. = not significant, * = p < 0.05, ** = p < 0.01, *** = p < 0.001. Experiments were performed at least twice.
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