Breakthrough Pediococcus acidilactici and Leuconostoc lactis bacteraemia with elevated meropenem minimum inhibitory concentrations during broad-spectrum antimicrobial therapy in a patient with advanced pancreatic cancer: A case report and review of the literature.

Pediococcus acidilactici and Leuconostoc lactis are bacteria commonly found in dairy and probiotic products that are intrinsically resistant to glycopeptides. Although historically regarded as organisms of low pathogenicity, there are increasing case reports with invasive infections, primarily in immunocompromised hosts with compromised bowel mucosa. However, concurrent bacteraemia with elevated meropenem minimum inhibitory concentrations (MICs) in both organisms has not been previously described. Here we present the first case, occurring in a patient with advanced pancreatic cancer, peritoneal and hepatic metastases, on a background of a polymicrobial liver abscess. Breakthrough bacteraemia with P. acidilactici and L. lactis developed despite treatment with meropenem and teicoplanin. Both organisms appeared as streptococci/enterococci on Gram stain, with definitive identification of culture isolates using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF-MS). Retrospective meropenem susceptibility testing demonstrated MICs of > 32 mg/L for P. acidilactici and 16 mg/L for L. lactis. The case highlights two key messages. First, although P. acidilactici and L. lactis mimic streptococci/enterococci on Gram stain, empirical glycopeptide therapy - usually reliable for these organisms - would be ineffective, and elevated meropenem MICs suggest carbapenems may also be unreliable. Therefore, early species-level identification by MALDI-TOF-MS and prompt susceptibility testing are essential. Second, prolonged and/or broad-spectrum antimicrobial exposure selects for pathogens with reduced susceptibility, particularly in immunocompromised hosts with disrupted gut mucosa and unachievable source control. As patients with advanced malignancy receive prolonged courses of broad-spectrum antimicrobials, infections with Pediococcus spp. and Leuconostoc spp. may be encountered more frequently and deserve consideration in this setting.