BACTERIAL EMPIRE 20xx, VOL. x, NO. x, e354 1 GROWTH EVALUATION OF SULPHATE-REDUCING BACTERIA (SRB) ASSOCIATED WITH BURIED MILD STEEL RODS Ifeoluwaposi Oluwabunmi Ogundana 1* , Adewale Oluwasogo Olalemi 1 , Daniel Juwon Arotupin 1 Address (es): 1 Department of Microbiology, Federal University of Technology, P.M.B. 704, Akure, Nigeria *Corresponding author: ogundana.ifeoluwaposi@gmail.com https://doi.org/10.36547/be.354 ABSTRACT Introduction: Sulphate-reducing bacteria are the major cause of corrosion of underground structure corrosion of underground structures made of mild steel. Objective: This study set out to evaluate the growth pattern of sulphate-reducing bacteria associated with buried mild steel rods in Akure metropolis, Nigeria Methodology: Mild steel rods (12 mm diameter, area 39.97 cm 2 ), were collected from Ondo State Water Corporation (ODSWC), Akure, Nigeria and were buried and monitored for a period of three months. Percentage composition of the mild steel rods was determined. Isolation of sulphate-reducing bacteria was carried out using compounded Postgate C medium. Turbidimetric growth pattern and biochemical characteristics of the isolates were determined using standard protocols. Results: Citrobacter freundii, Citrobacter gilleni, Enterobacter aerogenes, Pseudomonas aeruginosa, Desulfovibrio vulgaris and Bacillus megaterium were the bacterial consortia isolated from the buried mild steel rods. Turbidity was observed to be at low level on the 5 th day of sulphate-reducing bacteria (SRB) growth, but increased to optimum on the 10 th day of incubation and reduced drastically on the 15 th day. The growth outline of sulphate-reducing bacteria (SRB) was confirmed by the swelling turbid periodic growth for all four mediums. Iron (Fe) had the highest percentage mineral composition of 98.7 % whereas lanthanum had the least composition at 0.00031 %. Conclusion: Findings of this study revealed the presence of sulphate-reducing bacteria (SRB) on buried mild steel rods of a water distribution system suggesting induction of corrosion of the underground mild steel pipes. Keywords: Turbidity; sulphate-reducing bacteria; mild steel rods; mineral composition; bacteria; corrosion INTRODUCTION Metal corrosion is caused by (bio) chemical processes in which the metal releases electrons or ions. Anodic reactions are triggered by corrosive substances such as acids. Microbes increase cathodic processes by utilizing hydrogen, and they may also stimulate corrosion by secreting enzymes and acidic metabolites (Procópio, 2020). The main types of bacteria associated with the corrosion of iron and steel are sulfate-reducing bacteria (SRB), sulfur-oxidizing bacteria, iron oxidizers, iron reducers, manganese oxidizers and microbes that secrete organic acids and produce extracellular polymeric substances (EPS) (Hamilton, 2003). Microbial corrosion, primarily caused by sulphate-reducing bacteria, is a major cause of underground structure corrosion, particularly pipeline corrosion. Although various microorganisms, primarily bacteria, may contribute to corrosion, the sulphate-reducing bacteria are mainly responsible for corrosion of iron and steel (Okabe et al., 1995; Sass and Cypionka, 2004; Zhang et al., 2011). The bacterial species enhances the corrosion of pipeline material via either reduction or oxidation of the metal; SRB species generate H2S which accelerates the corrosion of mild steel (Wang et al., 2015). Sulfur-oxidizing bacteria convert sulphide to sulphate using a sulphide-oxidizing enzyme, and iron- and sulfur- oxidizing bacteria of the genus Thiobacillus are only active in acidic environments (El-Shamy et al., 2015). Sulphate-reducing bacteria are anaerobic and non- pathogenic bacteria, but they are capable of causing economic and environmental problems (Lopes et al., 2018). Due to the emission of corrosive H2S, sulphate-reducing bacteria (SRB) have been shown to be the most aggressive bacterium in bio-corrosion induction (Mansour and Elshafei, 2016). In many industries, SRB have a negative impact by forming toxic biofilms on metal surfaces that may affect anodic and cathodic processes, thereby creating ideal circumstances for microbial corrosion induction (Mansour and Elshafei, 2016). Many additional bacteria, including sulphate-reducing bacteria like Desulphovibrio and Desulfotomaculum, may corrode iron anaerobically. The generation of sulphuric acid, sulfate, and H2S by these bacteria to oxidize iron to iron oxide suggests their very corrosive action (Coetser and Cloete, 2005). Corrosion of ferrous metals in soil have significant negative impact on oil and water pipelines, storage tanks, and cooling water pipeline distribution systems (Cole and Marney, 2012). These materials are subjected to various types of corrosion, including localized, pitting, crevice, and uniform corrosion (Wu et al., 2013). Corrosion is the primary source of material degradation and destruction in both natural and man-made materials. It is estimated that approximately 25 % of annual steel production is destroyed by corrosion. Soil as a corrosive environment is probably of greater complexity than any other environment. The corrosion process of buried metal structures is extremely variable and may range from rapid to negligible (Arriba-Rodriguez et al., 2018). The aim of this study was to evaluate the growth pattern of sulphate-reducing bacteria (SRB) on buried iron rods collected from a tropical vegetation cover in the vicinity of Ondo State water corporation, Akure, Nigeria. . This is to gain a better understanding of how sulphate-reducing bacteria (SRB) induce corrosion on underground mild steel pipes of a water distribution system. METHODOLOGY Sample collection Mild steel rods (12 mm diameter, area 39.97 cm 2 ), used for this study were obtained from Ondo State Water Corporation (ODSWC), Alagbaka, Akure, Ondo State. They were buried for a period of three months and closely monitored. Determination of percentage composition of mild steel Mild steel (12 mm) commercially obtained were analyzed at the SPECTRO, AMETEK Materials analysis division, New Jersey, United States. Cylindrical mild steel samples were cut with an average length of 10 mm with their exposed surface ends metallo-graphically prepared in accordance with American society for Testing and Materials (ASTM G1-03, 2011). Media preparation Postgate C medium for Sulphate-reducing bacteria (Yahaya et al., 2011) was prepared according to the chemical composition as shown in Table 1. The prepared medium was adjusted to pH 7.5, thereafter, dispensed into test tubes, then surged with nitrogen free oxygen gas for 2 minutes before being clamped and sterilized in autoclave at 121℃ for 15 minutes. Isolation of sulphate-reducing bacteria SRB were isolated from the buried mild steel rod samples periodically on a monthly basis. Samples were inoculated in the freshly prepared Postgate C medium in test tubes and were incubated under anaerobic condition at 37°C for 7- 22days (Yahaya et al., 2011).