Phagosomes can engulf artificial low-density latex beads and then purified along a sucrose concentration gradient, allowing the structure and composition to be studied. By purifying phagosomes at different time points, the maturation process can also be characterised. Early phagosomes are characterised by Rab5, which transition into Rab7 as the vesicle matures into late phagosomes. The nascent phagosome is not inherently bactericidal. As it matures, it becomes more acidic from pH 6.5 to pH 4, and gains characteristic protein markers and hydrolytic enzymes. The different enzymes function at various optimal pH, forming a range so they each work in narrow stages of the maturation process. Enzyme activity can be fine-tuned by modifying the pH level, allowing for greater flexibility. The phagosome moves along microtubules of the cytoskeleton, fusing with endosomes and lysosomes sequentially in a dynamic "kiss-and-run" manner. This intracellular transport depends on the size of the phagosomes. Larger organelles (with a diameter of about 3 µm) are transported very persistently from the cell periphery towards the perinuclear region whereas smaller organelles (with a diameter of about 1 µm) are transported more bidirectionally back and forth between cell center and cell periphery. Vacuolar proton pumps (v-ATPase) are delivered to the phagosome to acidify the organelle compartment, creating a more hostile environment for pathogens and facilitating protein degradation. The bacterial proteins are denatured in low pH and become more accessible to the proteases, which are unaffected by the acidic environment. The enzymes are later recycled from the phagolysosome before egestion so they are not wasted. The composition of the phospholipid membrane also changes as the phagosome matures. Fusion may take minutes to hours depending on the contents of the phagosome; FcR or mannose receptor-mediated fusion last less than 30 minutes, but phagosomes containing latex beads may take several hours to fuse with lysosomes. It is suggested that the composition of the phagosome membrane affects the rate of maturation. "Mycobacterium tuberculosis" has a very hydrophobic cell wall, which is hypothesised to prevent membrane recycling and recruitment of fusion factors, so the phagosome does not fuse with lysosomes and the bacterium avoids degradation.