▁there ▁is ▁a ▁t endency ▁to ▁adapt ▁to ▁culture ▁conditions , ▁which ▁is ▁rarely ▁precisely ▁like ▁the ▁micro organ ism ' s ▁natural ▁environment , ▁which ▁can ▁alter ▁their ▁bi ology . ▁ ▁Pro tocol s ▁for ▁pass aging ▁The ▁protocol ▁for ▁sub c ult uring ▁cells ▁depends ▁heavily ▁on ▁the ▁properties ▁of ▁the ▁cells ▁involved . ▁ ▁Non - ad her ent ▁cells ▁Many ▁cell ▁types , ▁in ▁particular , ▁many ▁micro organ isms , ▁grow ▁in ▁solution ▁and ▁not ▁attached ▁to ▁a ▁surface . ▁These ▁cell ▁types ▁can ▁be ▁sub c ult ured ▁by ▁simply ▁taking ▁a ▁small ▁volume ▁of ▁the ▁parent ▁culture ▁and ▁dil uting ▁it ▁in ▁fresh ▁growth ▁medium . ▁Cell ▁density ▁in ▁these ▁cult ures ▁is ▁normally ▁measured ▁in ▁cells ▁per ▁mill il iter ▁for ▁large ▁e uk ary otic ▁cells , ▁or ▁as ▁optical ▁density ▁for ▁ 6 0 0 nm ▁light ▁for ▁smaller ▁cells ▁like ▁b acter ia . ▁The ▁cells ▁will ▁often ▁have ▁a ▁preferred ▁range ▁of ▁dens ities ▁for ▁optimal ▁growth ▁and ▁sub c ulture ▁will ▁normally ▁try ▁to ▁keep ▁the ▁cells ▁in ▁this ▁range . ▁ ▁Ad her ent ▁cells ▁Ad her ent ▁cells , ▁for ▁example ▁many ▁m amm al ian ▁cell ▁lines , ▁grow ▁attached ▁to ▁a ▁surface ▁such ▁as ▁the ▁bottom ▁of ▁the ▁culture ▁flask . ▁These ▁cell ▁types ▁have ▁to ▁be ▁det ached ▁from ▁the ▁surface ▁before ▁they ▁can ▁be ▁sub c ult ured . ▁For ▁ad her ent ▁cells ▁cell ▁density ▁is ▁normally ▁measured ▁in ▁terms ▁of ▁confl