Exceeding the reverse voltage of the LED can cause destructive breakdown, even if the current is limited. Far better to prevent the reverse voltage being given chance to do so.
Methods:
1. High voltage diode in series with the LED/resistor. Prevents risk of damage but the LED will still flicker at half cycle rate.
2. Low voltage diode across the LED. Prevents risk of damage but LED still flickers and overall power consumption is doubled without more light being produced.
3. Two LEDs in anti-parallel. Safe, the LEDs flicker alternately.
4. A bi-color LED. Safe, still one package, may be an intersting color!
5. LED wired across a bridge rectifier made of 4 low voltage diodes. Safe, LED flickers at twice line rate, will appear to be brighter.
Option 5 is probably best, still use the 100K resistor but wire it to a bridge rectifier made of 4 low voltage diodes (1N914, 1N4148 for example). Connect the LED across the '+' and '-' points on the bridge. The voltage is kept to a safe level and the LED will produce about twice the light level compared to your existing system.
PLEASE take note that without isolation, your circuit will be 'live' at all parts and extremely dangerous to touch. Keep fingers well away!
Brian.