Abstract
RationaleAlthough frequently retrieved in tracheal secretions of critically ill patients on mechanical ventilation, the existence of pneumonia caused by coagulase-negative staphylococci (CoNS) remains controversial.ObjectiveTo assess whether Staphylococcus haemolyticus (S. haemolyticus) inoculated in mice’s trachea can infect normal lung parenchyma, increasing concentrations of S. haemolyticus were intratracheally administered in 221 immunocompetent mice.MethodsEach animal received intratracheally phosphate-buffered saline (PBS) (n = 43) or live (n = 141) or inactivated (n = 37) S. haemolyticus at increasing load: 1.0 × 106, 1.0 × 107, and 1.0 × 108 colony forming units (CFU). Forty-three animals were sacrificed at 12 h and 178 were sacrificed at 36 h; 64 served for post-mortem lung histology, 157 served for pre-mortem bronchoalveolar lavage (BAL) analysis, and 42 served for post-mortem quantitative bacteriology of lung tissue. The distribution of biofilm-associated genes was investigated in the S. haemolyticus strain used in our in vivo experiment as well as among 19 other clinical S. haemolyticus strains collected from hospitals or nursing houses.Measurements and Main ResultsIntratracheal inoculation of 1.0 × 108 CFU live S. haemolyticus caused macroscopic and histological confluent pneumonia with significant increase in BAL white cell count, tumor necrosis factor-α (TNF-α), and macrophage inflammatory protein (MIP)-2. At 12 h, high concentrations of S. haemolyticus were identified in BAL. At 36 h, lung injury and BAL inflammation were less severe than at 12 h and moderate concentrations of species belonging to the oropharyngeal flora were identified in lung tissue. The inoculation of 1.0 × 106 and 1.0 × 107 CFU live S. haemolyticus caused histologic interstitial pneumonia and moderate BAL inflammation. Similar results were observed after inoculation of inactivated S. haemolyticus. Moreover, biofilm formation was a common phenotype in S. haemolyticus isolates. The low prevalence of the ica operon in our clinical S. haemolyticus strain collection indicated icaA and icaD independent-biofilm formation.ConclusionIn immunocompetent spontaneously breathing mice, inoculation of S. haemolyticus causes concentration-dependent lung infection that spontaneously recovers over time. icaA and icaD independent biofilm formation is a common phenotype in S. haemolyticus isolates.
Highlights
Coagulase-negative staphylococci (CoNS), ubiquitous colonizers of healthy skin and mucosal surfaces, are among the most frequently isolated bacteria in the microbiology laboratory
As shown in the Supplementary Videos available on the online data supplement, mice behavior was deeply affected only in animals inoculated with 108 colony forming units (CFU) live S. haemolyticus
Our study demonstrates for the first time that intratracheal inoculation of S. haemolyticus induces histological interstitial and confluent pneumonia in immunocompetent mice with influx of inflammatory cells and cytokines in bronchoalveolar lavage (BAL)
Summary
Coagulase-negative staphylococci (CoNS), ubiquitous colonizers of healthy skin and mucosal surfaces, are among the most frequently isolated bacteria in the microbiology laboratory. Considered as non-pathogenic, CoNS has been recognized over the past three decades as nosocomial pathogens causing significant morbidity, mortality, and healthcare costs worldwide (Becker et al, 2014), in adults (Wisplinghoff et al, 2004; Rogers et al, 2009) and newborns (Cunha Mde et al, 2002; Venkatesh et al, 2006; Rogers et al, 2009) Because of their propensity to form biofilms on device materials (John and Harvin, 2007), they frequently infect catheters inserted in critically ill patients (Rogers et al, 2009; Becker et al, 2014; Grasselli et al, 2017) and prosthesis implemented during orthopedic surgery (Wisplinghoff et al, 2004; Becker et al, 2014). Whether the phenotype of biofilm formation, especially ica locus, is critical for the virulence of CoNS remains unclear
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