Aerobes require oxygen to grow and cannot survive without it. Microaerophiles grow below the surface in shake tubes due to the presence of some oxygen. Some cultures showed more growth at the surface of the medium, likely due to oxygen levels decreasing farther down, while others were equally distributed, maintaining sufficient oxygen throughout. The C. sporogenes culture separation or elevation from the bottom is probably because of gas production. Streptococcus species that are catalase-negative can grow with oxygen because they have other defenses besides the catalase enzyme to protect against oxygen's harmful effects.