Rod shaped bacterium are hardly ever Gram positive. Gram positive are defined by their thick layer of peptidoglycan. Because it is a thick, strong structure,

Rod shaped bacterium are hardly ever Gram positive. Gram positive are defined by their thick layer of peptidoglycan. Because it is a thick, strong structure, it naturally sticks to round shapes. Gram negatives also have a layer of peptidoglycan, but much thinner. Therefore they can form rods that have bends and straight sections as they are not constrained by their thick structure.