Microbial contamination — commonly called 'diesel bug' — occurs when bacteria, fungi, and algae grow at the fuel-water interface in diesel storage tanks. These microbes form biofilms and sludge that clog filters, corrode tanks, and degrade fuel quality. Biodiesel blends are particularly susceptible. Fuel additives with biocidal properties and demulsifiers help prevent microbial growth by removing the water that microbes need to survive.