Increased Age, but Not Parity Predisposes to Higher Bacteriuria Burdens Due to Streptococcus Urinary Tract Infection and Influences Bladder Cytokine Responses, Which Develop Independent of Tissue Bacterial Loads

Abstract

Streptococcus agalactiae causes urinary tract infection (UTI) in pregnant adults, non-pregnant adults, immune-compromised individuals and the elderly. The pathogenesis of S. agalactiae UTI in distinct patient populations is poorly understood. In this study, we used murine models of UTI incorporating young mice, aged and dam mice to show that uropathogenic S. agalactiae causes bacteriuria at significantly higher levels in aged mice compared to young mice and this occurs coincident with equivalent levels of bladder tissue colonisation at 24 h post-infection (p.i.). In addition, aged mice exhibited significantly higher bacteriuria burdens at 48 h compared to young mice, confirming a divergent pattern of bacterial colonization in the urinary tract of aged and young mice. Multiparous mice, in contrast, exhibited significantly lower urinary titres of S. agalactiae compared to age-matched nulliparous mice suggesting that parity enhances the ability of the host to control S. agalactiae bacteriuria. Additionally, we show that both age and parity alter the expression levels of several key regulatory and pro-inflammatory cytokines, which are known to be important the immune response to UTI, including Interleukin (IL)-1β, IL-12(p40), and Monocyte Chemoattractant Protein-1 (MCP-1). Finally, we demonstrate that other cytokines, including IL-17 are induced significantly in the S. agalactiae-infected bladder regardless of age and parity status. Collectively, these findings show that the host environment plays an important role in influencing the severity of S. agalactiae UTI; infection dynamics, particularly in the context of bacteriuria, depend on age and parity, which also affect the nature of innate immune responses to infection.

Fig 1. Streptococcus agalactiae bacterial loads in the bladder in young, aged, and dam C57BL/6 mice at 24 and 48 h post-infection.

Data are a combination (n = 21–32) of 3 separate experiments each with 8–12 mice. Bars represent medians and are shown with interquartile ranges to illustrate the comparisons between groups based on Kruskal-Wallis ANOVA tests, and Dunn’s multiple comparison post-tests, with significance levels indicated (* P < 0.05; ** P < 0.01; **** P < 0.0001).

Fig 2. Streptococcus agalactiae bacterial loads in the urine in young, aged, and dam mice at 24 and 48 h post-infection.

Data are a combination (n = 29–42) of 3 separate experiments each with 8–12 mice. Bars represent medians and are shown with interquartile ranges to illustrate the comparisons between groups based on Kruskal-Wallis ANOVA tests, and Dunn’s multiple comparison post-tests, with significance levels indicated (* P < 0.05; ** P < 0.01; **** P < 0.0001).

Fig 3. Streptococcus agalactiae UTI induces distinctive patterns of cytokine secretion in young, aged, and dam mice at 24 h post-infection.

Pro-inflammatory cytokines and growth factors (blue), chemokines (red) and regulatory cytokines (green) are highlighted using coloured axes. Median bars and interquartile ranges are shown for 3 independent experiments combined (n = 6–13). The data were initially compared using Kruskal-Wallis ANOVA, and Dunn’s multiple comparison post-tests. Additional pair-wise comparisons were performed using Mann-Whitney U tests to compare PBS and infected conditions for cytokines across the same host background. Significance levels for the latter are indicated (* P < 0.05; ** P < 0.01; *** P < 0.001; **** P < 0.0001). The figure denotes S. agalactiae as group B streptococcus (GBS).

Fig 4. Streptococcus agalactiae UTI induces distinctive patterns of cytokine secretion in young, aged, and dam mice at 48 h post-infection.

Pro-inflammatory cytokines and growth factors (blue), chemokines (red) and regulatory cytokines (green) are highlighted using coloured axes. Median bars and interquartile ranges are shown for 3 independent experiments combined (n = 6–13). The data were initially compared using Kruskal-Wallis ANOVA, and Dunn’s multiple comparison post-tests. Additional pair-wise comparisons were performed using Mann-Whitney U tests to compare PBS and infected conditions for cytokines across the same host background. Significance levels for the latter are indicated (* P < 0.05; ** P < 0.01; *** P < 0.001; **** P < 0.0001). The figure denotes S. agalactiae as group B streptococcus (GBS).