Complex sentence: The organisms are hardy due to their cell wall, which is neither truly Gram negative nor positive, and unique to the family, they are naturally resistant to a number of antibiotics that work by destroying cell walls, such as penicillin.
Split into simpler sentences: The organisms are hardy due to their cell wall, which is neither truly Gram negative nor positive. Additionally, they are naturally resistant to a number of antibiotics that disrupt cell-wall biosynthesis, such as penicillin.