Relationship Between Planktonic and Sessile Cells as They Relate to Biofilm Growth

Abstract

Microbially influenced corrosion (MIC) is a process whereby microbial cells living in an exopolymer matrix, known as a biofilm, induce corrosion of the associated surface. The risk of MIC within a system is usually assessed by measuring planktonic (free-floating) cells, which is assumed to approximate the biofilm-associated cells (sessile) cells. This work aimed to determine the accuracy of this industry-practiced approach of assessing MIC risk from planktonic cell counts. Planktonic and sessile cell counts of two single species cultures, one of an aerobe (Pseudomonas fluorescens) and one of an anaerobe (Geoalkalibacter subterraneus), both of which have been associated with MIC, were monitored in a growth curve test to determine how planktonic and sessile cell counts relate during the initial stages of biofilm formation. The results indicate two factors govern biofilm initiation of the tested species. Firstly, a minimum planktonic cell density is required and secondly, a minimum exposure time of the surface are both required prior to biofilm initiation and the onset of internal corrosion of carbon-steel pipelines. Both these factors affect biofilm formation in a species specific manner.

Introduction

Microbially influenced corrosion (MIC) is a process by which microbes, both archaea and bacteria, deteriorate metals through their metabolic activities. As of 2012, the total annual cost of corrosion in the United States was estimated at $1 trillion per year¹. MIC is known to be caused by numerous types of microbes, ranging from iron-oxidizing, iron-reducing, sulfate-reducing, acid-producing and exopolymer-producing bacteria^2,3. Some types of MIC are dependent on a microbial physical interaction with the metal surface, wherein the microbes exist within a biofilm. This biofilm is a 3D structure of bacteria in an extracellular matrix protecting them from external environmental threats such as biocides while keeping them in a niche environment that provides nutrients. Biofilms can exist as both single- or multi-species communities with higher MIC rates being more associated with mixed microbial communities^3,4. This is due to the formation of oxic/anoxic environments within the biofilm, where oxygen is depleted on the metal surface (beneath the biofilm), facilitating anaerobic metabolisms by bacteria such as sulfate-reducing bacteria (SRB) ^5,6.