Combination effect of meropenem and antifungals against Escherichia coli-Candida albicans dual-species biofilms in vitro and in vivo using a silkworm model

<b>Introduction.</b> Biofilms are a primary form of device-associated infections and typically exhibit high tolerance to antimicrobial agents. In biofilms formed by multiple microbial species, microorganisms may show even greater tolerance, complicating treatment. There is evidence that meropenem (MEPM) tolerance in <i>Escherichia coli</i> is increased in dual-species biofilms with <i>Candida albicans</i>, and effective treatments have not been established.<b>Hypothesis/Gap Statement.</b> If the presence of viable <i>C. albicans</i> increases the MEPM tolerance of <i>E. coli</i> in mature biofilms, then the killing of <i>C. albicans</i> will attenuate the MEPM tolerance of <i>E. coli</i>.<b>Aim.</b> We evaluated the effectiveness of various antifungal combination treatments against dual-species biofilms of <i>E. coli</i> and <i>C. albicans in vitro</i> and <i>in vivo</i>.<b>Methodology.</b> The reduction in the number of viable cells in dual-species mature biofilms formed by <i>E. coli</i> and <i>C. albicans</i> was evaluated after treatment with a combination of antifungal drugs (fluconazole, amphotericin B and micafungin) and MEPM. In addition, the <i>in vivo</i> effects of combination therapy were assessed using a silkworm biofilm infection model.<b>Results.</b> The combination of amphotericin B and MEPM reduced the viable cell counts of both <i>E. coli</i> and <i>C. albicans</i> within dual-species biofilms. In contrast, the combination of fluconazole and MEPM did not reduce the viable cell count of either species, whereas the combination of micafungin and MEPM reduced <i>C. albicans</i> only. The reduction in viable <i>C. albicans</i> counts by micafungin was less than that by amphotericin B, suggesting that micafungin did not affect the tolerance of <i>E. coli</i>. The combination of amphotericin B and MEPM also reduced the viable cell counts of both <i>E. coli</i> and <i>C. albicans</i> in the <i>in vivo</i> model.<b>Conclusion.</b> These findings suggest that the combination of amphotericin B and antibacterial agents is a potential treatment option to reduce the <i>C. albicans</i>-induced bacterial tolerance for catheter-related infections involving <i>C. albicans</i> co-infection.