  Microbiologically influenced corrosion (MIC) compromises the integrity of
many technologically relevant metals. Protective coatings based on synthetic
materials pose potential environmental impacts. Here, we report a MIC resistant
coating based on a biofilm matrix of Citrobacter sp. strain MIC21 on underlying
copper (Cu) surfaces. Three identical corrosion cells varying in the type of
working electrode (annealed Cu, 29.5% coldworked, and 56.2% coldworked Cu) were
used. Graphite plate and Ag/AgCl served as counter and reference electrodes,
respectively. The working electrolyte was based on lactate-C media along with
an inocula consisting of Oleidesulfovibrio alaskensis strain G20 and
Citrobacter sp. strain MIC21. Passivating effect of the co-cultured biofilm
matrix was observed in the form of an ennoblement effect. Tests based on
sequencing, microscopy, and spectroscopy revealed the formation of a compact
biofilm matrix dominated by strain MIC21 cells, exopolymers, and insoluble
precipitates. This matrix displayed elastic modulus (a measure of rigidity) as
high as 0.8 Gpa and increased corrosion resistance by ~10-fold. Interestingly,
strain MIC21 has the capacity to inhibit the undesirable growth of aggressive
strain G20. Additional corrosion tests also substantiated the passivation
effects of strain MIC21. We provide mechanistic insight into the underlying
reasons responsible for corrosion prevention behavior of the biofilm matrix.
