Toast the bread using a toaster or under the broiler for 2-3 minutes each side. Spread each slice of bread with 1/4 of the mayonnaise, being sure to cover the entire surface. Divide the spinach between two slices of bread and arrange in a single layer. Top the spinach with two slices of tomato on each piece of bread. Arrange four slices of bacon on top of the tomatoes. Season with salt and pepper. Place the remaining bread on top of the bacon mayonnaise side down. Serve immediately.