Staphylococcus aureus ST228 and ST239 as models for expression studies of diverse markers during osteoblast infection and persistence

Abstract

The ability of S. aureus to infect bone and osteoblasts is correlated with its incredible virulence armamentarium that can mediate the invasion/internalization process, cytotoxicity, membrane damage, and intracellular persistence. We comparatively analyzed the interaction, persistence, and modulation of expression of selected genes and cell viability in an ex vivo model using human MG-63 osteoblasts of two previously studied and well-characterized S. aureus clinical strains belonging to the ST239-SCCmecIII-t037 and ST228-SCCmecI-t041 clones at 3 h and 24 h post-infection (p.i). S. aureus ATCC12598 ST30-t076 was used as a control strain. Using imaging flow cytometry (IFC), we found that these strains invaded and persisted in MG-63 osteoblasts to different extents. The invasion was evaluated at 3 h p.i and persistence at 24 h p.i., in particular: ATCC12598 internalized in 70% and persisted in 50% of MG-63 cells; ST239-SCCmecIII internalized in 50% and persisted in 45% of MG-63 cells; and ST228-SCCmecI internalized in 30% and persisted in 20% of MG-63 cells. During the infection period, ST239-III exerted significant cytotoxic activity resulting from overexpression of hla and psmA and increased expression of the genes involved in adhesion, probably due to the release and re-entry of bacteria inside MG-63 cells at 24 h p.i. The lower invasiveness of ST228-I was also associated with non-cytotoxic activity inside osteoblasts. This clone was unable to activate sufficient cellular reaction and succumbed inside MG-63 cells. Our findings support the idea of considering new strategies, based on a translational approach-eukaryotic host-pathogen interaction (EHPI)-and to be applied on a large scale, to predict S. aureus /osteoblast interaction and treat bone infections. Such strategies rely on the study of the genetic and biochemical basis of both pathogen and host.

Keywords: MRSA; ST228; ST239; crosstalk mechanism; osteoblast; virulence toxin.

FIGURE 1

Evaluation of the internalization frequency by IFC evaluation and cellular metabolism viability, at 3 h p.i. and 24 h p.i. at an MOI of 100:1. (a) The graph reports the percentage of spots at 3 h p.i. for cell ±SD for each strain for three different experiments. (b) The graph reports the percentage of spots at 24 h p.i. for cell ±SD for each strain for three different experiments. Statistical data were obtained using Student's t test versus ATCC12598: p‐value, 95% confidence interval, and R‐squared. Statistically significant p‐value: ≤0.05*; ≤0.01**; ≤0.001***; and ≤0.0001****

FIGURE 2

Evaluation of the relative mRNA expression of genes involved in regulation in ATCC12598, ST239, and ST228 strains at 3 h p.i. and 24 h p.i. versus baseline condition. Horizontal bars report the statistical analysis. Statistically significant p‐value ≤0.05 (*); ≤0.01 (**); and ≤0.001 (***)

FIGURE 3

Evaluation of the relative mRNA expression of genes involved in regulation in ATCC12598, ST239, and ST228 strains at 3 h p.i. and 24 h p.i. versus baseline condition. Horizontal bars report the statistical analysis. Statistically significant p‐value ≤0.05 (*); ≤0.01 (**); ≤0.001 (***)

FIGURE 4

Evaluation of the relative mRNA expression of toxin and metabolic genes in ATCC12598, ST239, and ST228 strains at 3 h p.i. and 24 h p.i. versus baseline condition. Horizontal bars report statistical analysis. Statistically significant p‐value ≤0.05 (*); ≤0.01 (**); and ≤0.001 (***)

FIGURE 5

Regulation of adhesion and virulence determinants in S. aureus by global regulatory loci. Arrows stand for activation; bars for repression. The molecules that act as activators or repressors (members of SarA protein family, SarA, SarS, and rot), that is, regulating protease expression, are represented in green ovals; the alternate sigma factor (SigB) in blue rectangles; the agr quorum‐sensing system (AgrA) in red hexagons; the toxins in pink rumbles (Hla, Hld, SdrE, and PsmA); and the adhesion factors in yellow circles (Bbp and FnaA/B) (Jenul & Horswill, ; Romilly et al., 2014).