  We model the atmospheres and spectra of Earth-like planets orbiting the
entire grid of M dwarfs for active and inactive stellar models with $T_{eff}$ =
2300K to $T_{eff}$ = 3800K and for six observed MUSCLES M dwarfs with UV
radiation data. We set the Earth-like planets at the 1AU equivalent distance
and show spectra from the VIS to IR (0.4$\mu$m - 20$\mu$m) to compare
detectability of features in different wavelength ranges with JWST and other
future ground- and spaced-based missions to characterize exo-Earths. We focus
on the effect of UV activity levels on detectable atmospheric features that
indicate habitability on Earth, namely: H$_2$O, O$_3$, CH$_4$, N$_2$O and
CH$_3$Cl.
  To observe signatures of life - O$_2$/O$_3$ in combination with reducing
species like CH$_4$, we find that early and active M dwarfs are the best
targets of the M star grid for future telescopes. The O$_2$ spectral feature at
0.76$\mu$m is increasingly difficult to detect in reflected light of later M
dwarfs due to low stellar flux in that wavelength region. N$_2$O, another
biosignature detectable in the IR, builds up to observable concentrations in
our planetary models around M dwarfs with low UV flux. CH$_3$Cl could become
detectable, depending on the depth of the overlapping N$_2$O feature.
  We present a spectral database of Earth-like planets around cool stars for
directly imaged planets as a framework for interpreting future lightcurves,
direct imaging, and secondary eclipse measurements of the atmospheres of
terrestrial planets in the HZ to design and assess future telescope
capabilities.
