Abstract
Staphylococcus aureus is a Gram-positive bacterium responsible for a variety of mild to life-threatening infections including bone infections such as osteomyelitis. This bacterium is able to invade and persist within non-professional phagocytic cells such as osteoblasts. In the present study, four different S. aureus strains, namely, 2SA-ST239-III (ST239), 5SA-ST5-II (ST5), 10SA-ST228-I (ST228), and 14SA-ST22-IVh (ST22), were tested for their ability to modulate cell viability in MG-63 osteoblast-like cells following successful invasion and persistence. Methicillin-sensitive S. aureus (MSSA) ATCC-12598-ST30 (ST30) was used as control strain. Despite being proven that ST30, ST239, and ST22 have a similar ability to internalize and persist in MG-63 osteoblast-like cells under our experimental conditions, we demonstrated that the observed decrease in cell viability was due to the different behavior of the considered strains, rather than the number of intracellular bacteria. We focused our attention on different biochemical cell functions related to inflammation, cell metabolism, and oxidative stress during osteoblast infections. We were able to show the following: (1) ST30 and ST239 were the only two clones able to persist and maintain their number in the hostile environment of the cell during the entire period of infection; (2) ST239 was the only clone able to significantly increase gene expression (3 and 24 h post-infection (p.i.)) and protein secretion (24 h p.i.) of both interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) in MG-63 osteoblast-like cells; (3) the same clone determined a significant up-regulation of the transforming growth factorbeta 1 (TGF-β1) and of the metabolic marker glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNAs at 24 h p.i.; and (4) neither the MSSA nor the four methicillin-resistant S. aureus (MRSA) strains induced oxidative stress phenomena in MG-63 cells, although a high degree of variability was observed for the different clones with regard to the expression pattern of nuclear factor E2-related factor 2 (Nrf2) and its downstream gene heme oxygenase 1 (HO-1) activation. Our results may pave the way for an approach to S. aureus-induced damage, moving towards individualized therapeutic strategies that take into account the differences between MSSA and MRSA as well as the distinctive features of the different clones. This approach is based on a change of paradigm in antibiotic therapy involving a case-based use of molecules able to counteract pro-inflammatory cytokines activity such as selective cytokine signaling inhibitors (IL-6, TNF-α).
Highlights
The Colony-Forming Units (CFUs)/mL measured 3 h p.i. was quite similar for ST30, ST239, and ST22, ranging from 2.6 to 3 × 106 CFUs/mL, while it was higher for ST5 (4.2 × 106 ) and ST228
ST30 and ST239, the measured CFUs/mL was pretty similar to that observed at 3 h p.i., the amount of bacteria measured at 24 h p.i. strongly decreased for ST5, ST228, and ST22
Different S. aureus strains atand the amount of bacteria measured at 24 h p.i. strongly decreased for ST5, ST228, and ST22
Summary
Over the past fifty years, Staphylococcus aureus, representing one of the main adaptable human pathogens and responsible for community and nosocomial infections, has been able to acquire numerous resistance and virulence genes [3]. This bacterium has shown the ability to invade and persist within non-professional phagocytic cells such as osteoblasts, to remain alive in the intracellular environment, and to evade the immune system (the so-called “phagosomal escape”) [4]. This intracellular persistence has often been associated with a metabolic variant, known as small colony variant (SCV), able to persist inside human cells and refractory to antibiotic therapy [5]
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