Molecules released by bacteria that harm other bacterial or host cells.

Exotoxins are secreted by pathogens and typically attach to specific receptors on host cells, ultimately modifying cell function in ways that lead to the symptoms of infection. Neurotoxins target and damage nerve cells (neurons); enterotoxins target cells of the gut, causing gastrointestinal symptoms. Exotoxins, which include botulinum toxin, cholera toxin, pertussis toxin and tetanus toxin, are among the deadliest molecules known.

Pore-forming toxins are the most common bacterial cytotoxic proteins. They are required for virulence in many important pathogens, including Streptococcus pneumoniae, group A and B streptococci, Staphylococcus aureus, Escherichia coli and Mycobacterium tuberculosis.

Endotoxins are components of the outer membrane of Gram-negative bacteria that are released when bacterial cells are damaged or die. They are composed of lipopolysaccharide–protein complexes. They tend to cause harm indirectly, for example by triggering sepsis.

Drugs that neutralize bacterial toxins (e.g. monoclonal antibodies) are in development or already marketed. Although they prevent treatment symptoms, they may not necessarily eradicate infection so they are likely to be used alongside antibiotics.