Bacteria containing a lot of peptidoglycan in their cell walls also tend to have less complex cell walls, and are called gram-positive bacteria. Conversely, gram-negative bacteria have less peptidoglycan, but have more complex cell walls overall. In particular, the gram-negative bacteria have an additional outer membrane with attached lipopolysaccharides. Most Gram-negative bacteria also have fimbriae; hair-like projections, external to the cell wall, that allow the bacteria to stick to the cells they infect.
Some of these lipopolysaccharides are toxic, serving to counteract the natural defenses of the host organism. Also, the additional membrane of gram-negative bacteria can make them more resistant to antibacterial medications (antibiotics). For these two reasons, a gram-negative bacterial infection can be far more severe than a gram-positive infection.
Many pathogenic diseases are caused by gram-negative bacteria. Do a Web search on Yersinia pestis. What disease does this bacterium cause in humans?