Escherichia coli & Staphylococcus aureus are among the most frequent pathogens responsible for a wide variety of infections. While S. aureus may cause several diseases, ranging from moderately severe skin infections to fatal pneumonia and sepsis, E. coli pathotypes, which are basically divided into two groups: extraintestinal pathogenic E. coli (ExPEC) and intestinal pathogenic E. coli (InPEC), may lead to serious systemic infections, such as hemorrhagic colitis and hemolytic uremic syndrome. This book describes the major virulence factors in both pathogens.