Department of Applied Microbiology, Faculty of Biology, University of Warsaw, Miecznikowa 1, 02-096 Warsaw, Poland.
BMC Microbiol. 2012 Nov 24;12:278. doi: 10.1186/1471-2180-12-278.
The food-borne pathogen Listeria monocytogenes is the causative agent of listeriosis. The β-lactam antibiotics penicillin G and ampicillin are the current drugs of choice for the treatment of listerial infections. While isolates of L. monocytogenes are susceptible to these antibiotics, their action is only bacteriostatic and consequently, this bacterium is regarded as tolerant to β-lactams. In addition, L. monocytogenes has a high level of innate resistance to the cephalosporin family of β-lactams frequently used to treat sepsis of unknown etiology. Given the high mortality rate of listeriosis despite rational antibiotic therapy, it is important to identify genes that play a role in the susceptibility and tolerance of L. monocytogenes to β-lactams.
The hly-based promoter trap system was applied to identify penicillin G-inducible genes of L. monocytogenes. The results of reporter system studies, verified by transcriptional analysis, identified ten penicillin G-inducible genes. The contribution of three of these genes, encoding a ferritin-like protein (fri), a two-component phosphate-response regulator (phoP) and an AraC/XylS family transcriptional regulator (axyR), to the susceptibility and tolerance of L. monocytogenes to β-lactams was examined by analysis of nonpolar deletion mutants. The absence of PhoP or AxyR resulted in more rapid growth of the strains in the presence of sublethal concentration of β-lactams, but had no effect on the MIC values or the ability to survive a lethal dose of these antibiotics. However, the Δfri strain showed impaired growth in the presence of sublethal concentrations of penicillin G and ampicillin and a significantly reduced ability to survive lethal concentrations of these β-lactams. A lack of Fri also caused a 2-fold increase in the sensitivity of L. monocytogenes to cefalotin and cephradine.
The present study has identified Fri as an important mediator of β-lactam tolerance and innate resistance to cephalosporins in L. monocytogenes. PhoP and AxyR are probably involved in transmitting signals to adjust the rate of growth of L. monocytogenes under β-lactam pressure, but these regulators do not play a significant role in susceptibility and tolerance to this class of antibiotics.
食源性病原体李斯特菌是李斯特菌病的病原体。β-内酰胺类抗生素青霉素 G 和氨苄西林是治疗李斯特菌感染的首选药物。虽然李斯特菌分离株对这些抗生素敏感，但它们的作用仅为抑菌作用，因此，这种细菌被认为对β-内酰胺类抗生素具有耐受性。此外，李斯特菌对头孢菌素类β-内酰胺类抗生素具有较高的固有耐药性，这些抗生素常用于治疗病因不明的败血症。鉴于李斯特菌病尽管采用合理的抗生素治疗，但死亡率仍然很高，因此，确定在李斯特菌对β-内酰胺类抗生素的敏感性和耐受性中起作用的基因非常重要。
应用 hly 启动子陷阱系统鉴定了李斯特菌的青霉素 G 诱导基因。通过报告系统研究的结果，经转录分析验证，鉴定了 10 个青霉素 G 诱导基因。通过分析非极性缺失突变体，研究了其中 3 个基因（编码铁蛋白样蛋白（fri）、双组分磷酸盐反应调节剂（phoP）和 AraC/XylS 家族转录调节剂（axyR））对李斯特菌对β-内酰胺类抗生素的敏感性和耐受性的贡献。PhoP 或 AxyR 的缺失导致菌株在亚致死浓度的β-内酰胺存在下生长更快，但对 MIC 值或这些抗生素的致死剂量的存活能力没有影响。然而，Δfri 株在亚致死浓度的青霉素 G 和氨苄西林存在下的生长受到损害，并且对这些β-内酰胺类抗生素的致死浓度的存活能力显著降低。Fri 的缺乏还导致李斯特菌对头孢噻肟和头孢菌素的敏感性增加了 2 倍。
本研究确定 Fri 是李斯特菌β-内酰胺类抗生素耐受性和对头孢菌素固有耐药性的重要介质。PhoP 和 AxyR 可能参与传递信号，以调节李斯特菌在β-内酰胺压力下的生长速度，但这些调节剂在对这类抗生素的敏感性和耐受性方面没有发挥重要作用。
