We report the operation of etched GaAs/AlGaAs multiple quantum well microresonators as low threshold all-optical bistable devices. The studied samples are 2-dimensional arrays of cylindrical microresonators of $6~μ$m height, with diameters of $4~μ$m and $6.4~μ$m. They are realized by SiCl4 reactive ion etching of an epitaxial high finesse vertical microcavity structure. Due to the lateral carrier and light confinement, optical bistability is observed with a strongly reduced threshold power, below 100$~μ$W for the $4~μ$m diameter devices. The optical confinement allows to achieve a high cavity Q factor in a reduced volume, and leads to the observation of multiple hysteresis loops due to the transverse mode structure of $6.4~μ$m diameter microresonators. The low bistability threshold, obtained without post-etching surface treatment, is attributed to a selfpassivation occurring during the etching process, as evidenced by the observation of a thin coating film protecting the microresonator vertical surfaces.