Microbiologically influenced corrosion (MIC) is a term used to describe corrosive damage to metals caused by microbes including bacteria archaea and fungi. MIC affects many industries such as power generation oil production transportation and water storage and distribution. The costs inferred by corrosion across all industrial sectors are staggering. Annual corrosion damage is estimated at 3% of gross domestic product (GDP) rendering corrosion a 1 trillion-dollar problem for the U.S. alone. MIC is implicated at least 20% of corrosion cases. As such monitoring for and diagnosing MIC as part of a complete corrosion mitigation strategy is of paramount importance.Traditional MIC diagnostic techniques employ culture-based methods aimed at detecting and enumerating specific groups of bacteria presumed to be associated with MIC. Culture-based diagnostics are slow and incapable of producing a truly representative environmental community associated with corrosion products due to limitations imposed by culture techniques and growth media – 99% of microbes resist cultivation in the laboratory. Moreover microbes mediating corrosion typically exhibit a sessile or ‘biofilm’ lifestyle yet traditional culture techniques employ liquid media designed to grow microbes in a suspended ‘planktonic’ lifestyle thus leading to a disconnect between the assay and reality.As such culture-independent biochemical and genetics-based assays have been developed to allow for the enumeration and identification of microbes associated with MIC which are summarized in the NACE Standard TM0212-2012. These tests range from simple field tests such as ATP and the hydrogenase test to complex lab tests such as fluorescence microscopy and polymerase chain reaction (PCR) based assays. An additional test not described in the above standard is 16s metagenomic sequencing which is a genetic (culture independent) test which provides the profile of all microbes within a given sample.As mentioned the majority of corrosion is mediated by sessile microbes residing in a biofilm in direct contact with the material surface and underneath corrosion deposits. However despite the variety of highly accurate testing methods the data collected is still not representative as planktonic sampling is still far more common than sessile likely due to sample access. As planktonic and sessile counts vary widely only sampling the planktonic population can lead to over or underestimates of microbial abundance and subsequent mitigation plans may not be designed appropriately. This paper compares planktonic and sessile counts of samples using a variety of testing methods (including culture media ATP assay and nucleic acid analysis) emphasizing the importance of appropriate sampling methods and analysis. Data will be presented from a series of case studies including both lab work and field assessments.

Key words: Molecular methods, ATP, Culture media, qPCR, Microbiologically Influenced Corrosion, MIC, Sessile, Planktonic, Microbes.