Corticosteroid and antimicrobial therapy in macrolide-resistant pneumococcal pneumonia porcine model

Abstract

Background: Streptococcus pneumoniae, a primary cause of community-acquired pneumonia (CAP), is typically treated with β-lactams and macrolides or quinolones. Corticosteroids are now recommended as adjunctive therapy in severe CAP to improve outcomes. In this prospective randomized animal study, we evaluated the bactericidal efficacy of various antibiotic regimens combined with corticosteroids using a porcine pneumococcal pneumonia model.

Results: In 30 White-Landrace female pigs, pneumonia was induced by intrabronchial inoculation of macrolide-resistant S. pneumoniae 19A isolate. Animals were randomized to receive saline, ceftriaxone (CRO) with levofloxacin (LVX), CRO with azithromycin (AZM), or combinations of these with methylprednisolone (MP). The primary outcome, S. pneumoniae concentrations in lung tissue after 48 h of treatment, showed that the CRO + LVX, CRO + AZM, CRO + LVX + MP, and CRO + AZM + MP groups were equally effective in reducing bacterial load. However, complete bacterial eradication from lung tissue was achieved only in the CRO + AZM + MP group. Secondary outcomes, including bacterial burden in tracheal aspirates and bronchoalveolar lavage (BAL) samples, showed similar bactericidal activity across all treatment groups. The CRO + AZM + MP group demonstrated the most controlled inflammatory response, achieving baseline levels of inflammation, while other groups exhibited elevated inflammatory markers.

Conclusions: Despite using a macrolide-resistant S. pneumoniae isolate, the combination of CRO, AZM, and MP achieves similar or even superior results compared to other antibiotic combinations. This regimen provides both bactericidal and immunomodulatory benefits, suggesting its effectiveness in treating macrolide-resistant S. pneumoniae pneumonia.

Keywords: Streptococcus pneumoniae; Animal model; Combination antibiotics therapy; Community-acquired pneumonia; Corticosteroids; Mechanical ventilation; Severe pneumonia.

Fig. 1

A Boxplot displays S. pneumoniae concentration (log₁₀ CFU/g) in lung tissue among study groups. Horizontal bars represent the median, boxes represent the interquartile range, and whiskers represent the range. Dots represent each of pulmonary lobe burden. There was a notable post hoc FRD-controlled difference in bacterial burden between the control and treatment groups (p < 0.0001) but without differences between treated animals. B Bars display the percentage of lung tissue concentration of S. pneumoniae score among study groups, defined as no growth; S. pneumoniae colonization for < 3 log₁₀ CFU/g, and pneumonia when S. pneumoniae concentration was ≥ 3 log10 CFU/g. The percentage of colonization and pneumonia differed markedly between the control group and treatment groups (p < 0.0001), while neither colonization nor pneumonia was found in the CRO + AZM + MP group. C Boxplot displays lung injury score following ATS recommendations. No significant differences were observed among study groups (p > 0.99). D Bars display the percentage of histological features among analyzed microscopic fields (score 1 means 1–5 neutrophils in the alveolar airspaces, twice to four times normal septal or one airspace filled by proteinaceous debris; score two means > 5 neutrophils in the alveolar airspaces, > 4 × septal thickening or > one airspace filled by proteinaceous debris). E Bars display the percentage of predominant histological pneumonia features in each pulmonary lobe among study groups. Repeated measures two-way ANOVA with FDR-controlled multiple post hoc comparisons. P-value < 0.05 is flagged with one asterisk, < 0.01 with two asterisks, and < 0.001 with three. CFU colony-forming unit, CRO ceftriaxone, LVX levofloxacin, AZM azithromycin, MP methylprednisolone

Fig. 2

S. pneumoniae concentrations (log₁₀ CFU/mL) in tracheal secretions (A) and bronchoalveolar lavage fluids (B) among the study groups. Data are expressed as the mean and standard error of the mean. A S. pneumoniae concentration in tracheal secretions varied among study groups (p = 0.001) and study time (p < 0.001). Compared with the control group, treatments have shown a crucial impact on the decrease of S. pneumoniae colonization in tracheal secretions. At the same time, only statistical significance was reached for CRO + AZM vs CRO + LVX + MP at 48 h among different antimicrobial combinations. B Similarly, S. pneumoniae concentration in BAL fluid varied among the study groups (p < 0.001) and the study time (p < 0.001). All antimicrobial combinations significantly decrease S. pneumoniae BAL burden compared with the control group, control vs. CRO + LVX (p = 0.002), control vs. CRO + AZM (p < 0.001), control vs. CRO + LVX + MP (p = 0.001), control vs. CRO + AZM + MP (p = 0.001). Repeated measures two-way ANOVA with FDR-controlled multiple post hoc comparisons. P-value < 0.05 is flagged with one asterisk, < 0.01 with two asterisks, and < 0.001 with three. CFU colony-forming unit, CRO ceftriaxone, LVX levofloxacin, AZM azithromycin, MP methylprednisolone

Fig. 3

Serum inflammatory markers among study groups. Bars represent the mean fold-change from baseline (log2), and whiskers are the standard deviation. Repeated measures two-way ANOVA without correction for multiple post hoc comparisons. P-value < 0.05 is flagged with one asterisk, < 0.01 with 2 asterisks, and < 0.001 with three asterisks. CRO, ceftriaxone; LVX, levofloxacin; AZM, azithromycin; MP, methylprednisolone

Fig. 4

ELF inflammatory markers among study groups. Bars represent the mean fold-change from baseline (log2), and whiskers are the standard deviation. Repeated measures two-way ANOVA without correction for multiple post hoc comparisons. P-value < 0.05 is flagged with one asterisk, < 0.01 with two asterisks, and < 0.001 with three. CRO ceftriaxone, LVX levofloxacin, AZM azithromycin, MP methylprednisolone