β-Lactam Antibiotics Enhance the Pathogenicity of Methicillin-Resistant Staphylococcus aureus via SarA-Controlled Lipoprotein-Like Cluster Expression

Abstract

Methicillin-resistant <i>Staphylococcus aureus</i> (MRSA) resists nearly all β-lactam antibiotics that have a bactericidal activity. However, whether the empirically used β-lactams enhance MRSA pathogenicity <i>in vivo</i> remains unclear. In this study, we showed that a cluster of lipoprotein-like genes (<i>lpl</i>, <i>sa2275</i> to <i>sa2273</i> [<i>sa2275</i>-<i>sa2273</i>]) was upregulated in MRSA in response to subinhibitory concentrations of β-lactam induction. The increasing expression of <i>lpl</i> by β-lactams was directly controlled by the global regulator SarA. The β-lactam-induced Lpls stimulated the production of interleukin-6 and tumor necrosis factor alpha in RAW 264.7 macrophages. The <i>lpl</i> deletion mutants (N315Δ<i>lpl</i> and USA300Δ<i>lpl</i>) decreased the proinflammatory cytokine levels <i>in vitro</i> and <i>in vivo</i> Purified lipidated SA2275-his proteins could trigger a Toll-like-receptor-2 (TLR2)-dependent immune response in primary mouse bone marrow-derived macrophages and C57BL/6 mice. The bacterial loads of N315Δ<i>lpl</i> in the mouse kidney were lower than those of the wild-type N315. The β-lactam-treated MRSA exacerbated cutaneous infections in both BALB/c and C57BL/6 mice, presenting increased lesion size; destroyed skin structure; and easily promoted abscess formation compared with those of the untreated MRSA. However, the size of abscesses caused by the β-lactam-treated N315 was negligibly different from those caused by the untreated N315Δ<i>lpl</i> in C57BL/6 TLR2^-/- mice. Our findings suggest that β-lactams must be used carefully because they might aggravate the outcome of MRSA infection compared to inaction in treatment.<b>IMPORTANCE</b> β-Lactam antibiotics are widely applied to treat infectious diseases. However, certain poor disease outcomes caused by β-lactams remain poorly understood. In this study, we have identified a cluster of lipoprotein-like genes (<i>lpl</i>, <i>sa2275</i>-<i>sa2273</i>) that is upregulated in the major clinically prevalent MRSA clones in response to subinhibitory concentrations of β-lactam induction. The major highlight of this work is that β-lactams stimulate the expression of SarA, which directly binds to the <i>lpl</i> cluster promoter region and upregulates <i>lpl</i> expression in MRSA. Deletion of <i>lpl</i> significantly decreases proinflammatory cytokine levels <i>in vitro</i> and <i>in vivo</i> The β-lactam-induced Lpls enhance host inflammatory responses by triggering the Toll-like-receptor-2-mediated expressions of interleukin-6 and tumor necrosis factor alpha. The β-lactam-induced Lpls are important virulence factors that enhance MRSA pathogenicity. These data elucidate that subinhibitory concentrations of β-lactams can exacerbate the outcomes of MRSA infection through induction of <i>lpl</i> controlled by the global regulator SarA.