Risk Factors for More Severe CF Lung Infections Caused by S. aureus Identified

Several factors may worsen lung infections caused by the bacteria Staphylococcus aureus in cystic fibrosis (CF) patients, from bacterial load to co-infections, according to a new study.

The study, “Factors Associated With Worse Lung Function In Cystic Fibrosis Patients With Persistent Staphylococcus aureus,” was published in the journal PLoS One.

S. aureus is one of the most common pathogens associated with lung infection in CF patients, and is prevalent in others. In fact, about 30 percent of the general public are believed to carry S. aureus in their nasal tissue, and these people are thought to have higher risk for infection, but a lower risk of developing lethal infections.

The characteristics of persistent S. aureus infection in people with CF, indicating a more severe infection, however, are not well identified.

Researchers at University Hospital Münster, in Germany, analyzed 195 CF patients (older than age 6) with chronic S. aureus airway presence as part of a multi-center study (NCT00669760). The team assessed lung function taking into account several parameters, such as S. aureus bacterial density in sputum and throat samples, co-infection with other CF-associated pathogens, presence of S. aureus in the nasal tissue, clinical status, antibiotic therapy, and the levels of IL-6- and IgG-levels, both of which serve as measures of the degree of immune response against S. aureus infection.

Patients were followed for 21 months with regular visits (usually every 3 months; 7 visits on average), in which researchers collected sputum (material from the airways) and throat samples. Blood samples were also taken once a year or at exacerbations.

Results indicated that the age at which patients became infected with S. aureus influenced the amount of bacteria present in sputum samples (but not throat samples). That is, older patients had significantly higher bacterial loads in the sputum than younger patients.

Bacterial density in throat samples (but not sputum samples) also significantly correlated with a decline in lung function each year. Loss of lung function was also associated with other parameters, such as exacerbations (60 patients), presence of small-colony variants of S. aureus (slow-growing subpopulations of the bacteria with distinctive traits; 84 patients), and co-infection with the bacteria Stenotrophomonas maltophilia (44 patients). Patients with increased blood levels of IL-6 had higher density of S. aureus in their sputum, and also showed lung function decline over time. Those with nasal cultures of S. aureus had better lung function over time.

“In CF-patients with chronic S. aureus cultures, independent risk factors for worse lung function are high bacterial density in throat cultures, exacerbations, elevated IL-6 levels, presence of S. aureus [small-colony variants] and co-infection with S. maltophilia,” the researchers wrote.

“Our findings may help to identify S. aureus patients at risk for more severe airway infections resulting in worse lung function and will be of value for designing future prospective studies to guide antibiotic therapy in patients chronically infected with S. aureus,” they concluded.