Microbes colonize nearly every biotic and abiotic niche on our planet. This also includes our human body, which is densely populated with microbes, the majority of which interact with us in a commensal, sometimes even mutualistic, relationship. Only a minority of our microbiota are pathogenic organisms with the ability to cause infection. This book covers various aspects of the interplay between commensal and pathogenic bacteria with their hosts. The chapters summarize recent findings on the geno- and phenotypic traits of opportunistic bacterial pathogens, such as Escherichia coli, staphylococci or Pseudomonas aeruginosa, as well as the impact of commensal and probiotic bacteria on intestinal physiology and health. The differential interaction of pathogenic, commensal and probiotic bacteria with their host is reviewed from both the bacterial and the host's perspective to round out this compilation of articles on the differences and similarities of pathogenic and commensal microorganisms.