Differences in biofilm formation and aggregative adherence between β-lactam susceptible and β-lactamases producing P. mirabilis clinical isolates

Biofilm formation of multidrug resistant (MDR) extended-spectrum β-lactamase (ES βL) producing Proteus mirabilis isolates from a long-term care and rehabilitation facility (LTCRF) in Northern Italy was evaluated. A total of 10 strains, 4/10 producing the acquired AmpC β-lactamase CMY-16, 3/10 producing the ES βL TEM-92 and the remaining negative for the presence of β-lactamase genes, were studied using standard adherence assays on titer plates. Tests were performed in three different media, including Luria Bertani (LB), LB diluted and urine. Three rep resentative strains were also tested for biofilm production in microtiter in presence of β-lactam sub-MIC concentra tions. The same isolates were screened for aggregative adherence onto monkey kidney cells (LLC-MK 2 ). All strains stud ied were capable of biofilm formation, though at different levels. The β-lactamase positive strains were statistically bet ter significant in biofilm formation than negative ones regardless of growth medium. Cellular adherence assays showed a preferential ability of all isolates, regardless of β-lactamase production, to adhere to inert surfaces rather than to cells. Although the results did not fully support a direct correlation between β-lactamase production and biofilm formation, both mechanisms can greatly contribute to bacterial persistence in the urinary tract. SUMMARY