Staphylococcus aureus in Non-Cystic Fibrosis Bronchiectasis: Prevalence and Genomic Basis of High Inoculum β-Lactam Resistance

Abstract

Rationale: The pathobiology of Staphylococcus aureus in non-cystic fibrosis bronchiectasis (nCFB) is poorly defined. When present at high density or "inoculum," some methicillin-sensitive S. aureus (MSSA) can inefficiently degrade antistaphylococcal β-lactam antibiotics via BlaZ penicillinases (termed the "inoculum effect" [IE]). Given the high burden of organisms in bronchiectatic airways, this is particularly relevant. Objectives: Drawing from a prospectively collected biobank, we sought to understand the prevalence, natural history, potential for transmission, and antibiotic resistance profiles among nCFB-derived MSSA isolates. Methods: All individuals attending a regional consultancy nCFB clinic with sputum collected between 1981 and 2017 were considered, and those with one or more S. aureus-positive cultures composed the cohort. Each individual's most recent biobank isolate was subjected to whole-genome sequencing (including the blaZ gene), antibacterial susceptibility testing, and comparative β-lactam testing at standard (5 × 10⁵ colony-forming unit [cfu]/ml) and high (5 × 10⁷ cfu/ml) inocula to assess for the IE and pronounced IE. Results: Seventy-four (35.4%) of 209 individuals had one or more sputum samples with S. aureus (68 MSSA, 6 methicillin-resistant S. aureus). Those with S. aureus infection were more likely to be female. Among 60 of 74 MSSA isolates subjected to whole-genome sequencing, no evidence of transmission was identified, although specific multilocus sequence typing types were prevalent, including ST-1, ST-15, ST-30, and ST-45. Antibiotic resistance was uncommon, except for macrolides (∼20%). Among the 60 MSSA samples, the prevalence of IE and pronounced IE was observed to be drug specific: meropenem (0% and 0%, respectively), cefepime (3% and 5%, respectively), ceftazidime (8% and 0%, respectively), cloxacillin (12% and 0%, respectively), cefazolin (23% and 0%, respectively), and piperacillin-tazobactam (37% and 17%, respectively). The cefazolin IE was associated with blaZ type A (P < 0.01) and ST-30 (P < 0.01), whereas the piperacillin-tazobactam IE was associated with type C blaZ (P < 0.001) and ST-15 (P < 0.05). Conclusions:S. aureus infection was common, although no evidence of transmission was apparent in our nCFB cohort. Although routine susceptibility testing did not identify significant resistance, inoculum-related resistance was found to be relevant for commonly used nCFB antibiotics, including cefazolin and piperacillin-tazobactam. Given previous associations between IEs and negative patient outcomes, further work is warranted to understand how this phenotype impacts nCFB disease progression.

Keywords: blaZ; cefazolin; infection transmission; inoculum effect; piperacillin-tazobactam.