(456) Staphylococcus aureus small colony variant infection pathology in the cystic fibrosis rat lung

Background: Cystic fibrosis (CF) is characterized by recurrent and chronic airway infections that lead to severe lung function decline and respiratory failure. Currently, Staphylococcus aureus is the most common microorganism found in the CF lung, but its diagnosis and treatment are complicated by a broad range of genetic diversity and subtypes amongst infectious strains. Small colony variants (SCVs) of S. aureus have been associated with worse respiratory outcomes in CF patients. However, it is unclear whether SCVs play a causal role in lung decline or are a result of more severe CF disease. To better understand the role that SCVs play in CF lung decline, we have developed an airway infection model in the CF rat using an SCV strain derived from a clinical isolate without direct genetic intervention.

Methods: A stable S. aureus SCV strain, derived from a CF clinical isolate by serially passaging with tobramycin, or its normal colony variant (NCV) parent strain, was used to intratracheally infect WT and CFTR^-/- (KO) rats 6 months of age. At 3 days post-infection, the rats were sacrificed to evaluate metrics of lung pathology, including histopathological evaluation of lung tissue, differential cell staining of bronchoalveolar lavage fluid (BALF), and enzyme-linked immunoassay (ELISA) for key inflammatory cytokines. Daily weight loss was monitored to determine disease severity. Bacterial colony-forming units (CFUs) recovered from lung tissue homogenate and BALF were evaluated as a metric of bacterial persistence.

Results: At 3 days post-infection, equal amounts of bacteria were recovered from lung cultures of NCV- and SCV-infected animals. When compared to NCV infections, SCV infections led to increased weight loss and higher levels of neutrophils in the BALF and lung tissue. In KO rats, SCV infections were also coupled with a lack of macrophage response that was otherwise seen with NCV infections, and significantly decreased levels of IL-6 in the BALF. Analysis of additional cytokines in the BALF showed no difference in levels of TNF-α and IL-1β between NCV and SCV infections in both WT and KO animals, and a slight increase in IL-10 for WT animals. Current studies using our SCV strain are ongoing to investigate disease pathology at additional chronic infection timepoints and the mechanisms that promote SCV persistence in the CF lung.

Conclusions: SCVs were equally able to persist in the lung compared to NCVs but led to worse outcomes including weight loss and neutrophilic inflammation. In SCV-infected KO rats, lower levels of IL-6 and reduced macrophages suggest a lack of macrophage response compared to NCV infections. However, pro-inflammatory cytokine levels did not increase, indicating inflammation in these animals may rely on a different pathway. Taken together, these findings indicate SCV infections as a potential cause of lung decline and highlight a further need for investigating SCV infection dynamics to better understand and predict these outcomes.

Acknowledgements:

References: