Investigation of polymyxin B resistance in Stenotrophomonas maltophlila

• Main Authors: Hsing-Yu Chen, 陳星宇
• Other Authors: 廖淑貞
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/64808999445004224163

碩士 === 國立臺灣大學 === 醫學檢驗暨生物技術學研究所 === 94 === Stenotrophomonas maltophilia is an aerobic, nonfermentative gram-negative bacterium. The organism has increasingly emerged as an important nosocomial pathogen, particularly for immunocompromised patients. It is characterized by its hegh-level resistance to a variety of structurally unrelated antimicrobials. Polymyxin B (PB) is a potent antibacterial lipopeptide composed of a positively charged cyclic peptide ring and a fatty acid containing tail. This capacity is due to its relatively high-affinity binding to lipid A of LPS（lipopolysaccharide）. The polymyxin B has activity a wide variety of Gram-negative bacilli. We detected MIC of polymyxin B in seventy clinical isolates of S.maltophilia from NTUH, the susceptibility rate was lower than those found in prior studies（＞70% susceptible）. To unravel the resistance mechanisms of which S.maltophilia against polymyxin B, we isolated spontaneous polymyxin-resistant mutants and analyzed their difference from the wild type . The two-component regulatory system, PhoP-PhoQ, in many Gram-negative bacteria regulates resistance to cationic antimicrobial peptides, such as polymyxin B, in response to low Mg2+ conditions. Here we firstly have identified the PhoP/ PhoQ system, in S. maltophilia by bioinformatics and gene cloning. We in addition constructed the PhoP, PhoQ and PhoP/ PhoQ overexpression strains and investracted the effect of overexpression the polymyxin B resistance in S. maltophilia. Our result indicated that overexpression of PhoP affects polymyxin B resistance. For the first time, we identified the PhoP/ PhoQ system in S. maltophilia and demonstrated that the system is regulated by Mg2+ and involved in the polymyxin B resistance. The relative impermeability of outer membrane of S. maltophilia produces intrinsic resistance to many antibiotics. Certain polycationic substances such as polymyxin B are known to make the outer membrane of Gram-negative bacterial permeable to solutes that normally are unable to penetrate outer membrane. We tested the susceptibility of mutants and 42 clinical isolates to some antibiotics (erythromycin, rifampine, imipenem, cefepime, gentamicin, ciprofloxacin and cefotaxime) by agar diffusion method on plates containing 56U/mL of polymyxin B. We found polymyxin B increase the susceptibility of the mutants and clinical isolates to rifampin. The clinical utility of this combination remains to be established.