Interspecies interactions in mixed-species biofilms formed by <i>Enterococcus faecalis</i> and gram-negative bacteria isolated from polymicrobial diabetic foot ulcers

Diabetic foot ulcers (DFU) are exacerbated by bacterial colonisation. Here, a high prevalence of <i>Enterococcus faecalis</i> was observed in DFU patients from an Argentinean hospital. <i>E. faecalis</i> was frequently co-isolated with <i>Escherichia coli</i>, <i>Morganella morganii</i>, and <i>Pseudomonas aeruginosa</i>. The effect of interspecies interactions on bacterial growth was investigated in mixed-species macrocolony biofilms developed in Lubbock-Glc-agar. Similar cell counts were found for <i>E. faecalis</i> and <i>M. morganii</i> growing in mixed and single-species biofilms. An <i>E. faecalis</i> strain showed 1 Log higher cell counts in mixed biofilms with <i>E. coli</i>. Remarkably, <i>E. faecalis</i> strains showed 2 to 4 Log higher cell counts in mixed biofilms with <i>P. aeruginosa</i>. This effect was not observed in planktonic growth or biofilms developed in tryptic soy agar. The present findings reveal bacterial interactions that benefit <i>E. faecalis</i> in mixed-species biofilms, mainly with <i>P. aeruginosa</i>, in a medium that partially mimics the nutrients found in DFU.