Pathogenicity refers to the ability of a pathogen to produce disease. Virulence is the ability of the pathogen to cause disease. The various bacterial pathogens, its pathogenesis clinical symptoms, laboratory diagnosis, control, prophylaxis and treatment with appropriate antibiotics are discussed below. S. aureus is a leading cause of hospital acquired infections. Cloxacillin is used against beta lactamase. Producing strains Streptococcus pyogenes is intrinsically a much more dangerous pathogen than Staphylococcus aureus and has a much greater tendency to spread in the tissues. Streptococcal pyrogenic exotoxin leads to streptococcal toxic shock syndrome (TSS). A common cross – reacting antigen exist in some group A streptococci and heart, therefore, antibodies produced in response to the streptococcal infection could cross react with myocardial and heart valve tissue, causing cellular destruction. N. meningitidis is the causative agent of meningococcal meningitis, also known as pyogenic or septic meningitis. Clostridium tetani is the causative organism of tetanus or lock jaw disease. The four important species of the genus Shigella are: Shigella dysenteriae, Shigella flexneri, Shigella sonnei and Shigella boydii. Several Leptospires are saprophytes, while many are potential pathogens of rodents, domestic animals and humans.