Multidrug resistance in clinical isolates of Stenotrophomonas maltophilia: roles of integrons, efflux pumps, phosphoglucomutase (SpgM), and melanin and biofilm formation.

Integrons, efflux pumps, phosphoglucomutase (SpgM), and melanin and biofilm formation were investigated in 40 multidrug-resistant (MDR) and 30 non-MDR Stenotrophomonas maltophilia isolates recovered from patients treated at National Taiwan University Hospital (Taipei, Taiwan). Class 1 integrons were clearly associated with multidrug resistance. Sequencing data revealed that aminoglycoside-modifying genes occurred most frequently in the integron and disclosed a new bla(IMP-8)/aac6-II/aadA5 gene cassette. Semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR) was performed to assess the expression of Sme efflux pumps and SpgM in S. maltophilia. MDR isolates exhibited hyperexpression of SmeABC, and SmeDEF and SpgM were more frequently found in MDR isolates than non-MDR isolates. In addition, the ability of MDR isolates to form melanin-like pigment and biofilm was also greater than that of the non-MDR isolates. The SmeABC or SmeDEF pump was shown to be associated with resistance to all agents tested. The presence of an integron as well as production of pigment and biofilm was also responsible for resistance against eight, six and six of the tested agents, respectively. High SpgM expression was associated with resistance to only three of the tested agents. These findings define the important roles of integrons, efflux pumps, and melanin-like pigment and biofilm formation in the multidrug resistance of S. maltophilia. MDR isolates possessed more resistance mechanisms than susceptible strains.