The county commissioners of each county in this State are declared to be a corporation and shall have full power to appoint road supervisors, collectors of taxes, trustees of the poor, a clerk to their board, and all other officers, agents and employees required for any county purpose not otherwise provided for by law or by the Constitution. The county commissioners shall have charge of and control over the property owned by the county, and over county roads and bridges. Whenever in their opinion the public interests require or when the public interests will be advanced, the county commissioners may commit the whole matter of grading and constructing public roads and the repairs of public roads, and the construction and repair of public bridges to the charge of competent and scientifically educated civil engineers. The civil engineers shall direct and manage all such public works under the immediate control of the county commissioners. The civil engineers shall hold office for the time, with the salary, under a bond and subject to regulations as may be directed by the county commissioners, and calculated to secure competent officers and a faithful discharge of duty.