EMB agar is considered selective because:

Coarse adjustment: The arrow points to the coarse adjustment knob, a large focusing knob on the microscope used to move the stage up or down quickly to bring the specimen into general focus. It is typically used with low-power objective lenses to locate the specimen and obtain an initial image. Using it with high-power lenses can risk damaging the slide or lens because it moves the stage in large increments.

Fine adjustment: A smaller focusing knob that moves the stage very slightly to sharpen the image after the specimen has been brought into general focus. It is mainly used with high-power or oil immersion objectives to achieve precise, clear visualization of cellular details without risking damage to the specimen.

Hemolysins are bacterial exotoxins that specifically target and lyse red blood cells, releasing hemoglobin into the surrounding environment. They are produced by certain pathogenic bacteria, such as Streptococcus and Staphylococcus species, and contribute to nutrient acquisition by providing iron from hemoglobin. Hemolysins can cause varying degrees of red blood cell destruction, resulting in different patterns on blood agar: alpha hemolysis (partial lysis with greenish discoloration), beta hemolysis (complete lysis with clear zones), and gamma hemolysis (no lysis). Their activity is an important virulence factor in bacterial infections.