  We investigate the emission properties of polycyclic aromatic hydrocarbons
(PAHs) in various metallicity environments with the Infrared Spectrograph on
board Spitzer. Local giant HII regions are used as references as they enable
access to the distinct interstellar medium components that contribute to the
mid-infrared spectrum of star-forming galaxies: photodissociation regions
(PDRs), photoionized gas, stellar clusters, and embedded regions. Three objects
are considered, NGC3603 in the Milky Way, 30Doradus in the Large Magellanic
Cloud, and N66 in the Small Magellanic Cloud. From the variations of the
PAH/14um ratio, we find that PAHs are destroyed in the ionized gas for a
radiation field such that [NeIII]/[NeII]>3. From the variations of the
PAH/Hu-alpha ratio, we find that the PAH emission sources in the giant HII
regions follow the same photodestruction law regardless of metallicity. We then
compare these results with observations of starburst galaxies, HII galaxies,
and blue compact dwarf galaxies (BCDs). While the integrated mid-infrared
spectra of BCDs are reminiscent of a warm dusty ionized gas, we observe a
significant contribution to the PAH emission in starburst galaxies that is not
arising from PDRs.
